Technical glossary



To wear away by friction.

An abradable material, such as a coating, is intended to wear to protect the component beneath, for example, between moving jet engine blade tips and engine shrouds. When operating at temperatures above 900°C, only ceramic abradables are suitable.

See also flame spray, HVOF, plasma spray.


Acetone is a colourless, highly-flammable liquid hydrocarbon with a sweet smell and the formula CH3COCH3.

It is widely used as a solvent in laboratories and is readily soluble in water, ethanol, and other common solvents. Residues vaporise quickly leaving a dry surface. The most familiar household use of acetone is as the active ingredient in nail polish remover.

Extremely flammable in both liquid and vapour form. Harmful if swallowed or inhaled and causes irritation to skin and eyes.

Properties: Melting point -95°C
Boiling point 56°C
Relative density 0.819 (at 0°C, Water = 1)
Flash point -20°C
Auto-ignition temperature 465°C
Explosive limits 2 to 13% in air


A substance which releases hydrogen ions when dissolved in water, and has a sour taste.

An acid is the opposite of an alkali, has a pH lower than 7.0 and turns litmus paper red. Most acids will dissolve the common metals and will react with a base to form a neutral salt and water.

Acidic means having the properties of an acid.


The active gasses, introduced into a furnace atmosphere, which cause the required reaction (carburising or carbonitriding) to take place.


A binding force that holds together molecules of substances whose surfaces are in contact or near proximity.

Age hardening

A low-temperature heat treatment which increases the hardness and strength of a material by causing the precipitation of sub-microscopic particles.

Originally, age hardening was the process and precipitation hardening was the phenomenon. Nowadays, the terms tend to be used interchangeably.


A change in properties that may occur gradually at atmospheric temperature (natural ageing) and more rapidly at higher temperatures (artificial ageing).

Alkali (Alkaline)

A chemical which neutralises acids.

Alkalis are the hydroxides of the alkali and alkaline earth metals and also ammonia solution. Apart from ammonia, the most common alkalis are derived from sodium (caustic soda), potassium (caustic potash) and calcium (slaked lime). In solution, they have a pH greater than 7 and turn litmus paper blue.

Solutions containing an alkali (alkaline solutions) can dissolve oils and greases on metals and also on skin. They are therefore frequently the active ingredient in metal washing chemicals. Very strong alkaline solutions (caustic solutions) can cause severe skin damage, which looks very much like a burn when it has been cleaned, and is therefore called a chemical burn.

Alkaline means having the properties of an alkali.


A metal to which has been added one or more elements to improve its properties.

The elements added may be metals or non-metals and are called alloy elements. Steel is an alloy of iron and carbon. However, other metals such as chromium and nickel can be added to improve its properties further. It is then known as an alloy steel.

Alloy element

An element which has been added to a metal to form an alloy.

Alloy steel

Steel, to which metal alloy elements have been added to improve its properties.

Alloy steels are often called after the main alloy elements they contain: chromium-nickel steels (Cr-Ni); nickel-chromium-molybdenum steels (Ni-Cr-Mo). The names are often shortened for convenience, for example, the latter is more-commonly known as nickel-chrome-moly steel.

See also carbon steel, low alloy steel, high alloy steel.


A hard white ceramic formed by the reaction of aluminium with oxygen, having the formula Al2O3.

Used as a refractory for making small, high temperature parts for furnaces or as a component of other refractories such as mullite.


A thermal spray coating method using aluminium. The aluminium is usually sprayed onto substrates of steel or nickel chromium alloys which are subsequently heat treated to aluminise the surface. Thermally sprayed aluminium is typically used as part of a galvanic protection system.

Aluminium (Al)

A silvery-coloured, soft, light metal element with the symbol Al.

Aluminium is an abundant, soft, lightweight metal with appearance ranging from silvery to dull gray, depending on the surface roughness. It is non-toxic, non-magnetic, and non-sparking. Aluminium has about one-third the density and stiffness of steel. It is ductile and easily machined, cast, and extruded. Its corrosion resistance is excellent due to a thin surface layer of aluminium oxide that forms rapidly when the metal is exposed to air, effectively preventing further oxidation.

In 1886, American Charles Martin Hall patented an electrolytic process to extract aluminium, forming a company for its production which later became Alcoa. Americans adopted the name aluminium for most of the 19th century, as did Hall in all of his patents. However, in 1892, Hall used the aluminum spelling in an advertising handbill and the name was adopted in America owing to his domination of the aluminium business in that country.

Properties: Melting point 660°C
Density 2.70 g/cm3 (Water = 1)

Identified in 1808 by Sir Humphrey Davy and named after alumina, the mineral from which he was trying to isolate it.


Anhydrous ammonia is a colourless, gaseous compound (which is easily liquefied under pressure) with a pungent smell and the formula NH3.

It reacts with steel at temperatures above 450°C and imparts nitrogen into its surface. Ammonia is the main reactant gas in nitriding and nitrocarburising.

When broken down (dissociated) into its constituent gases, it provides a reducing gas that is frequently used in furnace atmospheres for bright processing. See, for example, bright annealing.

Anhydrous simply means without water. Ammonia is so hydroscopic (water loving) that one cubic foot of water will dissolve 1300 cubic feet of ammonia. When ammonia reacts with water the alkaline compound ammonium hydroxide (NH4OH) will form.

Ammonia gas is much lighter than air and leaks in the open air normally disperse readily into the atmosphere. Under situations of high humidity however, the gas from a leak may absorb water from the atmosphere and hug the ground appearing as a white cloud.

Ammonia is extremely toxic in high concentrations and is highly irritant to the respiratory tract, eyes and skin, even in low concentrations.

Properties: Melting point -77°C
Boiling point -33°C
Vapour density 0.6 (Air = 1)
Vapour pressure 8.6 bar at 20°C
Flash point 11°C
Auto-ignition temperature 651°C
Explosive limits 15 to 27% in air


Annealing involves heating steel to a high temperature (above 750ºC) followed by very slow cooling in order to make the metal as soft as possible.

This, very time-consuming process is also known as full annealing since there are many types of intermediate or quicker annealing processes which make the material soft enough for a particular purpose but not as soft as possible. Annealing is also applied to many other non-ferrous metals and alloys.

Softening processes are used to improve hot and cold working characteristics, to increase machinability, to reduce internal stress due to working, welding etc, and also to condition components for subsequent hardening treatments. Occasionally they are used to impart particular final properties, as with low carbon transformer core material, which is annealed to optimise its magnetic characteristics.

The control of furnace atmosphere is vitally important since the prolonged treatment times required for many annealing processes would produce significant surface deterioration due to scaling if oxygen ingress were to occur. Atmospheres used for the annealing of steel include inert gases such as nitrogen and argon, cracked ammonia, exothermic gas mixtures and vacuum.

The use of continuous furnaces greatly improves the cost effectiveness when large volumes of small to medium sized components are to be annealed. The speed of throughput is variable and is the mechanism used to control time at annealing temperature. The uniformity of loading of the furnace belt or trays is another critical factor and sufficient even spacing of components and weight across the belt is vital.

When batch furnaces are used it is often a requirement, particularly with large components, that contact thermocouples are used, strategically placed over the surfaces of the component to provide a permanent trace record of the thermal history of the annealing process.

See also full annealing, process annealing, recrystallisation annealing, sub-critical annealing.


The electrode maintained at a positive electrical potential. The opposite to cathode.


The treatment of a metal part, commonly aluminium alloys, using an electrolytic passivation process.

The part being treated forms the anode of the electrolytic cell, increasing the thickness of the surface oxide layer of the part and forming an anodic film which provides improved corrosion and wear resistance. Anodising can also be used to produce cosmetic effects such as coloured films and is non-conductive.


A luminous discharge of electrical current crossing the gap between two electrodes.

Arc plasma

A gas that has been heated by an electric arc to at least a partially ionised condition enabling it to conduct an electric current.

Arc spraying

A thermal spray process using an arc between two consumable electrodes of surfacing materials as a heat source and a compressed gas to atomise and propel droplets of the surfacing material on to the substrate.

Arc welding

Arc welding uses electricity as the power source to create an electric arc between an electrode and the base materials to melt the base materials and cause them to join as the metal solidifies. The region being welded is sometimes protected by an inert gas such as argon, known as a shielding gas. Arc welding can create joints by introducing additional metal, called filler metal or by simply melting the parent metals, called autogenous welding.

See also electron beam welding, metal joining, TIG welding.

Argon (Ar)

A colourless and odourless gaseous element that makes up 0.94% of the Earth’s atmosphere.

It will not support life or combustion, is very inert and is not known to form true chemical compounds. For that reason, it is widely used as an atmosphere for working with materials which are reactive when heated in air.

Argon is heavier than air and is obtained as a by-product of the liquefaction and separation of air.

Properties Boiling Point: -186.0ºC
Relative density 1.38 (Air = 1)
Classification: Noble gas

Discovered in 1894 by Sir William Ramsay and named after the Greek word for inert: Argon.

See also liquid argon.

AS 9100

The standard quality management system for the aerospace industry which, although linked to ISO 9001, is an industry standard controlled by the International Aerospace Quality Group (IAQG) which is part of SAE (Society of Automotive Engineers). The majority of worldwide aerospace manufacturers specify compliance with AS 9100 as a condition of doing business with their suppliers. AS 9100 supersedes the earlier AS 9000 standard.

See also Nadcap.


The abbreviation for the American Society for Testing and Materials.

Now known as ASTM International. Based in the USA, it is one of the largest voluntary standards development organisations in the world.


The gas or mixture of gasses in a furnace surrounding components during heat treatment.

The nature of the heat treatment atmosphere varies according to the process being carried out and can be inert (completely unreactive, e.g. argon); neutral (does not change the composition of the component but may protect it from oxidation or other unwanted reactions, e.g. hydrogen) or reactive (plays an important part in the heat treatment by controlling or altering the composition of the component surface, e.g. endothermic atmosphere).

Atmosphere cooling

Involves cooling components fairly slowly after heat treatment whilst keeping them under the heat treatment atmosphere to protect them from oxidation.


The smallest particle of an element that has all of the chemical properties of that element.

Atoms are the basic component of all matter and consist of a nucleus of protons and neutrons surrounded by electrons.


1. In thermal spray coating, atomisation is the division of molten material at the end of the wire into fine particles.

2. The process used in the manufacture of metal powder.

Atomised powder

A powder produced by the dispersion of molten material into particles by a rapidly moving gas or liquid stream or by mechanical dispersion.


Steels having more than 0.5% carbon content are capable of being hardened without a drastic quenching operation, by the mechanism known as austempering, which is particularly used in hardening springs and involves isothermal transformation to the hard phase, martensite.


A high-temperature phase of iron, stable above 911ºC.

Austenite has a face-centred cubic crystal structure and is commonly denoted both in writings and on phase diagrams by the use of the Greek letter gamma (γ). Austenite is a very soft, non-magnetic form of iron.

It is the ability of austenite to absorb just over 2% carbon that makes the carburising and carbonitriding processes possible. The addition of carbon makes austenite stable at temperatures as low as 723ºC. However, if significant quantities of chromium and nickel are added, the austenite becomes stable at room temperature. These steels are the well-known austenitic stainless steels containing 18% chromium and 8% or 10% nickel.

Austenite was named after the British metallurgist, Sir William Chandler Roberts-Austen (1843-1902). Roberts-Austen published the first iron-carbon phase diagram.

See also austenitic, retained austenite.


A steel whose structure is, essentially, completely made of austenite.

Austenitic nitrocarburising

Austenitic nitrocarburising is carried out at 650/720°C. It provides additional load bearing capacity as deeper case depths can be achieved. The core remains ferritic.


Top of page


A decomposition product of austenite formed at cooling rates slightly slower than those required to form martensite.

Bainite was named after the American metallurgist, Edgar C. Bain.


A water-soluble compound capable of turning litmus paper blue and reacting with an acid to form a salt and water.

Bases include oxides and hydroxides of metals and also ammonia. Any solution with a pH greater than 7 is known as a basic solution.

Batch furnace

A furnace that heat treats one load at a time.

Furnaces which carry out more than one process, such as sealed quench furnaces with their heating and cooling chambers, may have a batch in each chamber. These are sometimes referred to as semi-continuous furnaces.


A section of metal, produced by casting, and used to form bars and rods which are often the basis for the manufacture of components.

Blended powder

A powder consisting of two or more different materials which are thoroughly mixed in order to provide a material capable of producing an alloyed deposit.


See engineering drawing.

Body-centred cubic

A cubic crystal structure containing one atom at each corner of the cube and another in the centre of the cube.


See ‘Mechanical bond’ and ‘Metallurgical bond’.

Bond coat

The initial thermal spray coating layer that is applied for the purpose of optimising the bond strength between the thermal spray coating and the substrate.

Bond strength

The strength of the adhesion between the coating and substrate or, in some cases, between coating layers. A number of test methods can be used to measure the bond strength of coatings. A typical test would be to ASTM C633.


The absorption and diffusion of boron into the surface of steel to give an extremely hard surface.

Also referred to as boronising.

Boron (B)

From the Arabic word buraq or the Persian word burah.


See boriding.

Box carburising

An obsolete name for pack carburising, based on the practice of enclosing the components to be carburised in a box packed with the carburising compound.

See also pack carburising.


An alloy of copper and zinc.

Brass is a copper-based alloy containing between 5 and 50% zinc, to which small amounts of other elements may be added to produce specific properties. The greater the zinc content, the more yellow the colour of the brass.

Owing to the perceived superiority of bronze over brass, some brasses have been called bronzes, for example, manganese bronze and architectural bronze.


A versatile metal joining method which is available for a range of alloys, including steels, cast iron, and nickel alloys. Despite the increasing use of modern adhesives and automated welding processes, it remains an economical and efficient method for fabricating a wide variety of parts, ranging from automotive components to parts for gas turbines.

See also metal joining.

Bright annealing

Annealing steel in a protective atmosphere to keep it free from oxidation after processing.

Following processing, the components should be as bright and clean as they were before the treatment.


A solution of common salt (sodium chloride) and water.

See also quenching.


An alloy of copper and tin.

Bronze is any of a broad range of copper alloys usually with tin as the main additive, but sometimes with other elements such as phosphorus, manganese, aluminium, or silicon. It is strong, tough and has many uses in industry. It was particularly significant in antiquity, giving its name to the Bronze Age. The word bronze is perhaps derived from the Persian word birinj, meaning copper.

See also brass.


Abbreviation for British Standard.

British Standards are produced by the British Standards Institution, now known as BSI International, the national standards body of the UK.


The act of making a surface smooth by rubbing it with a tool. This cold-works the skin or surface of the material.


A rough edge or area remaining on material, such as metal, after it has been cut, drilled or machined.


Top of page


A carbide is a compound formed from carbon and another, more electropositive, element.

Tungsten carbide is used frequently for thermal spray coating processes and produces a very hard wearing coating. Other examples of carbides include silicon carbide, calcium carbide and cementite.

Carbon (C)

From the Latin word carbo meaning charcoal.

Carbon dioxide

A colourless, odourless and non-flammable gas with the formula CO2.

Carbon dioxide is formed during respiration in animals, photosynthesis in plants and whenever carbon-containing material decomposes or is burnt. It reacts with carbon at temperatures above about 500oC and produces carbon monoxide. Accordingly, it is an important, if small, constituent of most heat treatment carrier gases and carburising atmospheres.

Carbon dioxide will not support combustion and is frequently used in fire extinguishers for use on electrical equipment. It should never be used in a confined space since it can cause asphyxiation. It is slightly soluble in water and is the cause of the fizz in lemonade and sparkling water.

Properties: Melting point -56.6°C
Boiling point -78.5°C
Relative density 1.53 (Air = 1)
Flash point Non-flammable

Widely used in its solid form as a refrigerant.

See also dry ice.

Carbon monoxide

A colourless, odourless, toxic and highly flammable gas with the formula CO.

It reacts with steel at temperatures above 800oC and imparts carbon into its surface. Accordingly, it is an important constituent of most carrier gases and carburising atmospheres.

Toxic if inhaled.

Properties: Melting point -205°C
Boiling point -192°C
Relative density 1 (Air = 1)
Flash point Flammable at all temperatures
Auto-ignition temperature 620°C
Explosive limits 12 to 74% in air

Carbon potential

A measure of the capability of a furnace atmosphere to impart carbon into a steel during heat treatment.

The carbon potential of an atmosphere is defined as the carbon content of a thin sheet of pure iron in equilibrium with the atmosphere.

Carbon restoration

Restoration of partially decarburised surfaces of a component can sometimes be achieved by the application of a controlled re-carburisation cycle in a sealed quench furnace.

Carbon steel

An alloy of iron and carbon with no metal alloy elements deliberately added.

Also known as plain carbon steel. Carbon steels may contain small amounts of a wide range of residual elements from the manufacturing process. It is often loosely classified according to its carbon content:

Low carbon steel Less than 0.2% carbon (Also known as mild steel)
Medium carbon steel 0.2-0.6% carbon
High carbon steel more than 0.6% carbon

See also alloy steel.


Carbonitriding is the absorption and diffusion of carbon and nitrogen into the surface of steel to give a hard surface and softer core after hardening by quenching. Carbonitriding is a surface heat treatment, a form of case hardening, for plain low carbon and low alloy steels and cast irons, which provides wear resistance and moderate load bearing capability.

It is found with plain carbon steels, that the use of gas carburising is restricted to small section sizes if the case is to be fully hardened by oil quenching. The addition of nitrogen (provided by adding ammonia as well as propane to the furnace atmosphere in a sealed quench furnace), increases surface hardenability by allowing both carbon and nitrogen diffusion to occur. Carbonitriding can therefore be considered as a gaseous equivalent to cyanide salt bath hardening. The normal range of temperature employed is 820/910°C, with 870°C being the optimum temperature for best case hardening conditions with the majority of suitable steels. Single quench treatments are generally employed and the process is mainly used for case depths up to 0.75mm (0.030"). For deeper cases in plain carbon steels it is useful to carburise only at 930/950°C and then reduce the furnace temperature to 870°C and complete the process by carbonitriding followed by oil quenching.

Fluid bed furnaces may also be employed to provide carbonitriding heat treatment. This method is particularly suited to the treatment of small components and those whose geometry would be prone to masking and attendant uneven hardening, were the sealed quench method to be used. Cyanide salt bath treatments have now largely been superseded by fluid bed treatments which do not have the health and safety and environmental risks associated with the operational and disposal aspects of cyanide salt baths.

As with all hardening processes, it is good practice to finish with a tempering treatment to reduce brittleness and impart optimum strength. Irrespective of which carbonitriding method is used, a tempering temperature of 150°C is generally suitable.

Carbonitriding should not be confused with its lower temperature partner, nitrocarburising.


Carburising is the absorption and diffusion of carbon alone into the surface of steel to give a hard surface and softer core after hardening by quenching.

Carburising is the oldest of the case hardening methods. Case hardening, as the name implies, produces a hard surface to the treated component whilst at the same time producing a softer, more ductile core, which provides support for the harder case. It has been known from pre-history that it is possible to increase the quenched hardness of steel by first increasing its carbon content. This fact was utilised to produce hard and therefore keen cutting edges by heating the articles in a carbonaceous material such as charcoal, prior to quenching.

If carburising has been correctly carried out, the core material will have an unchanged carbon content, whilst that of the surface or 'case' material should be in the region of 0.8%. The precise case carbon content for optimum results varies slightly with the steel analysis. Higher carbon contents than this produce the cementite phase at grain boundaries which, if not subsequently rectified, would lead to case embrittlement with attendant dangers of spalling. Lower carbon contents can lead to 'lean' case compositions which fail to harden properly on quenching. Also, due to the prolonged heating in the austenitic range during carburising, the grain size of the steel can increase, leading to a reduction in strength and increased brittleness.

In order to obtain the optimum combination of case and core properties, carburised parts are subjected to a sequence of post carburising treatments, culminating in the quenching operation to induce hardening. The grain size of the core material can be refined by heating to above the transformation austenitising temperature, which for the low carbon material of the core is approximately 870°C, and quenching. It is then necessary to refine the grain size of the case structure. This is achieved during the hardening stage by heating to approximately 760° C, which is just above the transformation austenitising temperature for the case material. This procedure is known as the 'double quench' treatment and is normal practice in pack carburising.

With grain refined steel it is possible to achieve satisfactory hardening with acceptable grain size and microstructures by using a ‘single quench’ treatment. Although this can be done by quenching straight from the carburising temperature, it is common to carburise at 900/950°C, furnace cool to 840/850°C and equalise at this temperature (to give some case diffusion and core refinement).

As an alternative to furnace quenching, previously carburised components may be hardened by induction hardening or flame hardening, where their geometry dictates that a localised surface heating method is preferable.

Carrier gas

A carrier gas is the basic atmosphere in a furnace to which is added the active gases which impart the carbon or nitrogen into the surface of the steel.

Carrier gas is normally neutral with regard to the surface carbon content of the steels being treated, i.e. it neither increases nor decreases the surface carbon content. The active gases which actually carry out the case hardening are called the additions.


The surface region of a component, the properties of which have been deliberately modified by heat treatment.

The properties can be modified by heat treatment alone, for example by induction hardening, or by a change in composition, for example by nitriding.

Case diffusion

See diffusion.

Case hardening

A general term for any heat treatment process which is used to harden the surface of steel.

It is, however, most-commonly used as a synonym for carburising and nowadays, for carbonitriding also.


A solidification process used for manufacturing metal shapes by the pouring of molten metal into sand or metal moulds. The subsequent solidified shape is known as a casting.

Defects associated with the casting process include shrinkage porosity and gas porosity, which can be eliminated effectively by carrying out hot isostatic pressing.


A substance that speeds up a chemical reaction but which remains unchanged at the end of the reaction.


The electrode maintained at a negative electrical potential. The opposite to anode.

Cavitation erosion

A form of erosion causing material to be removed by the action of vapour bubbles in a very turbulent liquid. Effects can be reduced via the application of ceramic coatings.

Cemented carbide

An obsolete name for tungsten carbide.


A hard and brittle compound formed by the reaction of iron with carbon, having the formula Fe3C.

A major constituent of pearlite, it is also known as iron carbide.

Cementite was named after the early steel-making process of cementation, which increased the carbon content of iron to make it into steel.


A non-metallic solid material, commonly crystalline in structure, formed by a process of heating and cooling. Ceramics are generally very hard and have high abrasion and temperature resistant properties. This makes them ideal for coating components which operate in high temperature environments for extended periods of time, such as turbine blades.

See also Ceramic coating, K-Tech.

Ceramic coating

Coating the surface of steel components with a ceramic slurry and then firing it, to provide a high-temperature, hard, wear-resistant and corrosion-resistant coating.


A cermet is a combination of ceramic and metallic materials, therefore exhibiting the properties of both, such as high strength and temperature resistance. A cermet is typically applied as a sprayed coating.

See also Thermal spray.

Chain dogs

Specially-shaped blocks attached to the transfer chain in a straight-through design sealed quench furnace, that push the load from the heating chamber into the cooling chamber.

Chemical symbols

Chemical symbols provide an internationally recognised, shorthand means of identifying chemical elements.

Symbols normally consist of one or two letters which are normally easily recognised as relating to the name of the element. Some of the earliest-known elements have symbols relating to Latin or Arabic origins of their names.

Chlorinated hydrocarbons

Organic chemicals made of carbon and hydrogen which have one or more of the hydrogen atoms replaced by a chlorine atom.

Chromium (Cr)

From the Greek word chrome, meaning colour.

The full name of the metal, chromium, is often shortened to ‘chrome’ and used to describe the finish obtained after plating with chromium – i.e. chrome plate.


See diffusion bonding.

Cold die quenching

Involves quenching thin, flat components between water-cooled plates or dies under high pressure.

The water-cooled dies are simply flat plates which have a large contact area with the component and extract the heat fast enough to cause full hardening.

See also press quenching.

Cold gas dynamic spray

Cold Gas Dynamic Spraying (CGDS) is an emerging coating deposition process where high pressure, low temperature gas is utilised to accelerate the coating material particles to supersonic velocities (400 – 1000 m/s), which at impact generate sufficient energy for plastic deformation and cold welding of the coating and substrate materials. This allows efficient deposition of layers with exceptionally low oxide and porosity levels.

Moreover, due to the minimised influence of thermally induced stresses in the coating and the high deposition efficiency of the process, cold spray can deliver very thick coatings (several mm) on complex geometries. A range of materials can be successfully sprayed with cold spray, such as:

  • Pure metals (copper, aluminium, zinc, silver, nickel, niobium, tantalum)
  • Alloys (Steels, Ni-alloys, Ti- alloys, MCrAlY’s)
  • (Cu-W, Al-SiC, Al-Al2O3)

Cold Isostatic Pressing

Cold isostatic pressing (CIP) is a forming technique in which high fluid pressure is applied to a powder, normally encapsulated in an elastomeric mould, at ambient temperature in order to form a green part. Water or oil is used as the pressure medium.

Cold treatment

See sub-zero treating.

Cold working

Mechanically forming material at about room temperature.

Cold working processes include rolling, drawing, spinning, hammering, etc. As the amount of cold work increases the material gets harder owing to the deformation of the crystal structure, a process called work hardening. The original properties can be completely restored by full annealing or partially restored by other heat treatment processes such as normalising and process annealing.


A combination of two or more materials, either naturally occurring or engineered to produce optimum properties.

Cermets and metal matrix composites are examples of metallurgical composites.

Composite coating

A thermal spray coating consisting of two or more dissimilar spray materials which may or may not be layered.


A substance made from two or more elements and having a definite chemical formula. See for example, alumina or cementite, which are both compounds.

Compound layer

The surface of nitrocarburised steel which has been converted to a complex iron-carbon-nitrogen mixture.

The equivalent layer on nitrided steel is known as the white layer.

Constitution diagram

An alternative name for phase diagram or equilibrium diagram.

Continuous annealing

Continuous mesh belt furnaces are employed for the sub-critical annealing of steel components, such as pressings and small machined parts, with section thickness of up to 1 inch. The temperature of the components is gradually raised as the batch progresses through the tunnel furnace. The belt speed is variable and is set to provide the necessary time within the high temperature region of the furnace to produce the required softening, based upon the section thickness of the component. Uniform spacing of the components on the belt is vital to ensure uniformity of heating and the spread of the load governs the efficiency of the soak at temperature. Although somewhat labour intensive as operators are needed at the entry and discharge ends of the furnace when there are a variety of components to be annealed, it is possible to automate the process when large volumes of very similar parts are involved. The energy efficiency of the process is good if sufficient product is available to run the equipment on a 24 hour basis. Endothermic gas generators coupled to the furnace provide an efficient supply of protective atmosphere economically.

Control thermocouple

A thermocouple used to control the temperature of a furnace.

Compare with load thermocouple and probe thermocouple.

See also load thermocouple, probe thermocouple.

Controlled atmosphere

A mixture of gases, the composition of which can be varied to match the required surface carbon content of the material being treated.

Controlled atmospheres normally consist of a neutral or inert carrier gas, which can be used for hardening, and can have additions of active gasses which produce carburising or carbonitriding as required.

Since, when heated in air, steel readily scales and the sub-surface region can suffer decarburisation due to the oxidation of the steel surface, and the loss of oxygen atoms from the sub-surface, hardening must be carried out in a protective or controlled environment, if expensive finishing operations are to be avoided. There are many protective ‘atmospheres’ available, ranging from endothermic and exothermic gas mixtures, to inert gases, such as nitrogen or argon, and molten salt or treatment in vacuum may be employed. Carburising conditions can be obtained when required by the addition of a hydrocarbon gas such as propane to a carrier gas, generally an endothermic gas mixture. Carbonitriding or nitrocarburising conditions can be obtained by the extra addition of ammonia gas to the carburising gas mixture.

Controlled atmosphere furnace

Controlled atmosphere furnaces have now largely superseded box (pack carburising) and salt bath furnaces on the basis that they exhibit better furnace control, more efficient throughput and are less labour demanding.

They also offer much improved environmental conditions of operation, without the serious problems of land contamination with toxic salts (cyanides) and the difficulties of disposing of waste salts, contaminated jigs and fixtures and pack carburising waste.

Controlled atmosphere furnaces fall into two major categories:

(a) Batch type furnaces – where the workload is charged and discharged as a single unit or batch.

(b) Continuous furnaces – where the work enters and leaves the furnace in a continuous stream. These furnaces are favoured for high volume production of similar parts.

See also sealed quench.

Copper (Cu)

From cuprum, the Latin name for the island of Cyprus, the Roman source for copper.


The unaltered centre of a component after case hardening.

Core refinement

The process of improving or optimising the properties of a material and/or its microstructure, below the case hardened surface layer, by using heat treatment.


Corr-I-Dur® is a proprietary Bodycote process which enhances wear properties and significantly improves corrosion resistance. The process is a combination of various thermochemical process steps, including gas nitrocarburising and oxidising. Wear and corrosion resistant layers are created which show a dark grey to black colour.

Corr-I-Dur® has very little effect on distortion and dimensional changes of components. Compared to carburising and carbonitriding, dimensional changes are significantly lower. Dimensional changes can further be positively influenced by varying the process parameters. By diffusion of carbon and nitrogen into the surface, a diffusion zone and compound layer are created. The compound layer determines the component's wear properties, while the diffusion zone influences the mechanical and dynamic properties. The achievable surface hardness mainly depends on the base material.

Applications range from single components to serial products, including a wide range of materials such as unalloyed construction and case hardened steels. Quenched and tempered steel can also be treated. For many components from the automotive and hydraulic industries, engineering and mining industries, Corr-I-Dur® is an excellent alternative to salt bath nitriding with oxidation.


The chemical reaction which occurs on the exposed surface of a metal caused by exposure to substances such as air, water and salt, which causes the surface to deteriorate. Rust is the most common example of electrochemical corrosion.

Surface treatments such as thermal spray and ceramic coatings may be applied to provide a barrier which protects the metal from corrosion.


An automotive industry specific process of self assessment against check sheets which cover quality systems, process audits and job audits in a similar manner to that used by PRI (Performance Review Institute) for Nadcap special process audits. In some instances automotive customers prefer the CQI-9 approach to that of TS 16949.


The deformation of a metal under the action of a continuous level of stress at a high temperature.


Any activity involving very low temperatures or material at such temperatures.

The term low temperature is generally taken to mean temperatures below -40ºC.

Cryogenic is derived from the Greek words kryos, which means very cold or freezing and genes, meaning created.

Cryogenic treating

See sub-zero treating.

Crystal structure

Most materials form crystals when they cool down from the molten state. In metals, this crystal structure can normally only be seen clearly using a high-power microscope, the individual crystals then being called grains.

Crystals are most-commonly found when a hot, concentrated, liquid solution of a suitable crystalline chemical (e.g. sugar) cools down slowly. However, some minerals occur naturally in the form of large crystals.

Some metals can have more than one crystal structure and this is what enables iron to be heat treated. At room temperature pure iron crystals are body-centred cubic (bcc) and are called ferrite. Above 911ºC they are face-centred cubic (fcc) and are called austenite.

Components which are cast in such a way that they consist of a single crystal only are extremely strong and are used for arduous duties such as high temperature turbine blades.

See also grain.


See salt bath hardening.


Top of page

Deep freezing

See sub-zero treating.


The removal of carbon from the surface of a component.

Decarburisation can either be a deliberate action or, more often, the accidental result of a material being exposed at high temperature to an atmosphere which removes carbon from its surface.


A heat treatment process which is subsequently applied after electroplating where hydrogen embrittlement is likely to take place.


Deformation is a change is shape due to applied force, such as heat, pressure, or stresses. When an object’s change in shape is temporary and reversible, it is called elastic deformation. Plastic deformation involves the breaking of atomic bonds and results in permanent deformation.

See also elastic limit, strain, stress, Hooke’s law, plastic limit, young’s modulus, fatigue.


The removal of grease and oil from a surface. Degreasing by immersion in liquid organic solvents or by solvent vapours condensing on the parts to be cleaned.

Denatured alcohol

Ethyl alcohol to which has been added chemicals that make it useless for drinking but still useful for industrial processes.

This is done in order to make it undrinkable and therefore exempt from taxes that apply to drinking alcohol. It is also known as industrial alcohol.


Densal is a specialised proprietary hot isostatic pressing technique, trademarked by Bodycote, which allows the most cost effective HIP processing of most aluminium alloy castings.


Densification is the consolidation of metal powders into a single unit, or the consolidation of components (e.g. castings, PM parts) to increase density by eliminating internal voids and porosity.


A physical property of all materials, defined as mass per unit volume. Density can be measured by total mass divided by total volume.


An insulated flask used for carrying cryogenic liquids.

Originally made from glass in the same way as vacuum flasks, industrial dewars are usually made of metal insulated with expanded polystyrene to make them more robust.

Dewar flasks are named after Sir Edward Dewar who discovered in the late 1800s how to make liquid gases and store them.


A crystalline form of carbon, widely used as a precious stone in jewellery.

Diamonds are the hardest natural substance known, rating 10 on the Mohs scale of hardness. They are widely used in engineering owing to their great hardness and form the tips of the indenters in many types of hardness testing machines.


Diffusion refers to the movement of atoms in solid metals at elevated temperatures.

Without diffusion, there would be no heat treatment. During the heat treatment of steel, it is the smaller atoms, especially carbon and nitrogen, which move readily through the iron crystal structure. When the carbon content at the surface increases, it changes the composition of the steel and therefore its properties after hardening.

Atoms move very slowly in solid metals and it therefore requires treatment for a long time to give a very deep case. For example, a case depth of 6mm would typically require carburising for five days.

Diffusion bonding

Diffusion bonding is a solid-state process between two or more materials in contact with each other where inter-diffusion occurs between the various components on an atomic level. The materials weld together without melting, coalescing by the simultaneous application of heat and pressure. A zone of intermediate composition is created between the two materials being bonded. An additional interlayer material may be used to promote bonding between the two base materials.


Dissociation means breaking a gaseous compound down into its constituent elements.

The term is most-commonly found in relation to ammonia, which is frequently used in heat treatment atmospheres.


The unwanted change in shape of components during heat treatment.

Although distortion may be caused by the heat treatment processing, it may also be a result of residual stress left in the material by the earlier machining or forming operations.

Distortion during quenching may be minimised by plug quenching or avoided by press quenching.

Dry ice

Carbon dioxide gas that has been cooled to below -78.5ºC and converted to a solid.

It is called dry ice because of its similar appearance and low temperature. However, unlike ice, which melts to give the liquid water, dry ice does not melt but goes directly from a solid to a gas. This process is called sublimation and produces 845 volumes of gas for each volume of solid.

Properties: Boiling point -78.5°C
Density 1564kg/m3
Relative density 1.56 (Water = 1)
Ratio to volume of gas 1 : 845 (At room temperature)


The ability of a material to be deformed without breaking.

Duplex coating

A term used to denote that two or more coating systems are used in conjunction, in order to create superior properties for the combined coating.


Top of page

Eddy currents

Electric currents created in a steel component held in an alternating electro-magnetic field.

When an electric current passes through a wire, a magnetic field is created around the wire. If the electric current is alternating, the magnetic field collapses and grows in the opposite direction with each cycle. If the wire is made into a coil and a steel bar is inserted into it, the continually growing and collapsing field causes (or induces – hence induction heating) eddy currents to flow in the bar and thereby heat it up.

See also induction heat treatment.

Elastic deformation

See Deformation.

Elastic limit

The maximum stress that a material will withstand before permanent deformation occurs.

A material which has not reached its elastic limit will return to its original shape once the applied load has been removed.


A component for the electrical circuit through which current is conducted, and the means by which an electric current enters or leaves a substance. In an electrolytic cell, an electrode can be either an anode or a cathode.


The smallest of the three particles which make up atoms and the one which carries the negative charge.

Electric current consists of the flow of electrons through a conductor. It follows that electrical conductors have their electrons loosely tied to the atoms – a characteristic of metals, whereas non-conductors, or insulators, have their electrons tightly bound to the atoms.

Electron Beam Welding (EBW)

A method of welding in which the energy required to melt the area to be welded is provided by a focussed stream of electrons.

The fabrication of distortion prone assemblies can be achieved by electron beam welding, a method which uses a focussed stream of high energy electrons generated by a filament and directed to the joint requiring to be welded. The heating is very localised and the bulk of the assembly therefore remains cold and stable. This results in a very narrow weld with a minimal heat affected zone. There is no need to use filler metal as the parent metal of the assembly is melted. As this is a line of sight method it is not possible to weld around corners or re-entrant angles. Weld depths of up to 30mm can be produced and computer controls ensure minimal operator dependence, thus providing good reproducibility throughout a batch of components, even though this is a piece part process. Since the heat input is very localised it is possible to weld together previously heat treated components, which is a very economical method for producing composite gear shafts, with for example a case hardened gear on a hardened and tempered shaft. Generally, electron beam welded assemblies require very little finishing after welding and are mostly used in the as-welded condition.

The materials to be electron beam welded must be electrically conductive and the method is very versatile, being suitable for steels, cast irons, titanium and nickel alloys, copper alloys and most pure metals.


An electrodeposition process used to plate metal with a layer of material in order to produce a component part with improved properties, such as wear and corrosion protection. The plating process uses an electric circuit, immersed in an electrolyte solution of dissolved metal ions, where the anode is the metal plating material and the cathode is the part awaiting plating. The anode dissolves metal ions into the electrolytic solution, which are then transferred by the electric circuit to be deposited as a plated metal layer onto the cathode.


A substance composed of a single kind of atom.

Elements can not be broken down into other substances or made by combining other substances together.


The change in length of a tensile test piece as a percentage of its original length.

% elongation = change in length (e) x 100 divided by the original length (L)
Elongation = e x 100/L %


The prefix formerly given to steels used for general ENgineering purposes in the UK.

Such steels were covered by British Standard BS970. However, in 1983, the designations were all revised and the EN steel designations are now obsolete.


The process of enclosing free flowing or green compacted metal powders in a sheet metal canister. Canister materials are typically mild or stainless steel. Canister shapes can be simple to very complex, referred to as near-net shape. Encapsulation can also be used to bond powders or solids to specific regions of a part, often for the purpose of increasing corrosion and/or wear resistance preferentially (HIP cladding).

Endothermic atmosphere

An atmosphere which is manufactured by passing a hydrocarbon gas and air mixture through a converter or generator containing a catalyst, at a high temperature.

The advantage of endothermic atmospheres is that they are very flexible and can be tailored to suit the particular heat treatment process being carried out. A typical composition of an endothermic atmosphere generated from methane would be: approximately 39% nitrogen; 20% carbon monoxide and 39% hydrogen, together with small amounts of water vapour, carbon dioxide and residual methane.

The name is derived from endothermic, the term for a chemical reaction in which heat is absorbed.

Engineering drawing

A blueprint.


A designation generally given metal-metalloid, metal-nonmetallic and intermetallic compounds found in ferrous alloy systems, for example, Fe3Mo2, FeSi, and Fe3P.

Epsilon carbide

A carbide with a composition which corresponds to the empirical formula Fe2.4C. It has a hexagonal close-packed lattice that precipitates during the first stage of the tempering of martensite.

Equilibrium diagram

A graph showing the temperature and composition ranges within which each of the phases of a particular alloy exist under equilibrium conditions.

More-accurately known as an equilibrium phase diagram or constitution diagram. Steel is normally shown as the simple iron-carbon equilibrium diagram since the low metal alloy content of up to 1.5%, found in the most common steels used in engineering, has little effect on the diagram. High alloy contents can have a significant effect and require very complicated diagrams to explain their phases.

Where the diagram involves the base metal with one alloy element, such as iron-carbon, it is known as a binary phase diagram. If an additional alloy element is added, it is called a ternary phase diagram – for three constituents, such as iron-carbon-nitrogen.


Erosion is the wearing away of a surface over a period of time, generally by fluid, gas, or other abrasive particles. Coatings can help to protect metals from erosion.


The common name for ethyl alcohol.

Ethyl alcohol

A pleasant-smelling, colourless liquid compound of carbon, hydrogen and oxygen with the formula C2H5OH.

Commonly known as ethanol, it is the alcohol found in beer and spirits. Although it is the major constituent of industrial alcohol, the latter is not pure and is harmful if drunk. To prevent it being consumed, nauseous chemicals are added and it is called denatured alcohol.

Alcohol is widely used in industry as a solvent, a weak degreaser and a drying agent to remove water, with which it mixes completely in all proportions. Its freezing point is -144ºC which is why it is used in low-temperature thermometers (mercury freezes at -39ºC). It vaporises easily and is highly flammable.

Properties: Melting point -144°C
Boiling point 78°C
Relative density 0.789 (Water = 1)
Flash point 14°C
Auto-ignition temperature 363°C
Explosive limits 3 to 25% in air

See also denatured alcohol, industrial alcohol.

Eutectoid transformation

The breaking down of a single solid phase into two different solid phases as it cools.

Eutectoid transformations take place at a single temperature and composition and generally give rise to a distinctive structure. For example, pearlite is formed by the eutectoid transformation of austenite containing 0.8% carbon, at a temperature of 723ºC.


Exothermic refers to a form of chemical reaction or process where energy is released, usually in the form of heat and light.


See thermal expansion.


Extrusion is used to manufacture cross-sectional parts by drawing or pushing hot or cold material through a die.


Top of page

Face-centred cubic

A cubic crystal structure containing one atom at each corner of the cube and another in the centre of each of the six faces of the cube.


The tendency of a metal component to break when it is subjected to a great number of repeated stress cycles, even when the applied stress is considerably below the tensile strength of the material.

Failure normally occurs after a large number of stress cycles – usually several million – and therefore rotating parts, such as shafts which rotate at high speed, are the most-commonly affected components.

When a load is applied to a metal part, the maximum stress is usually located at the surface. Therefore, any treatment that increases the surface strength, such as carburising, nitriding and shot peening will increase the fatigue life of the component.

Fatigue strength

The point, expressed as a value of stress, at which component failure occurs following repeated stress cycles.


A steel whose structure is, essentially, completely made of ferrite.

Ferritic nitrocarburising

Ferritic nitrocarburising is carried out at 550/580°C. Treatment consists of heating the component in an environment consisting of around 50% endothermic gas and 50% ammonia, such that the epsilon phase is produced at the component surface as a compound layer. This phase has a hexagonal close packed crystal structure which imparts very good tribological (sliding wear resistance) properties. The core remains ferritic.


A low-temperature phase of iron, stable up to 911ºC, which has a body-centred cubic crystal structure and is commonly denoted by the Greek letter alpha (α). Ferrite is the magnetic form of iron.

Ferrite is unable to absorb any significant amount of carbon - about 0.01% maximum. Ferrite was named after the Latin word for iron, ferrum.

See also ferritic.


Relating to iron (Fe).

The word ferrous was created from the Latin word for iron, ferrum.

See also iron.


The process carried out after casting to remove mould material, such as sand, and feeder parts from a component. This is generally done by grinding and machining.


Special jigging that has been made or adapted to support (i.e. fix the position of) a specific component.

Flame hardening

As an alternative to induction hardening this process is also used for surface hardening similar materials. The surface to be hardened is traversed by an oxy-gas flame 'head' followed closely by the quench spray. Either oil mixtures or polymer quenchants may be used. Although it is not capable of quite the same degree of control or automation as the induction method, it has the advantage of being applicable to a wider range of geometrical shapes and sizes. Early flame hardening equipment was developed from standard oxy-gas metal cutting torches. Modern sets incorporate gas volume control, temperature and time controls.

Heating times are longer than by induction, and there is more likelihood of variations in hardening response across the surface traversed. In many instances both induction and flame hardening are applied to previously hardened and tempered parts. This combination provides optimum results in terms of wear resistance and fatigue life improvement.

Flame spray

A thermal spray process in which an oxyfuel gas flame is the source of heat for melting thermal spray materials in wire or powder form. Compressed air may or not be used to atomise the molten particles and propel them on to the substrate to form a thermal sprayed coating.

Fluid bed

This method is particularly suited to the treatment of small components and those whose geometry would be prone to masking and attendant uneven hardening, were the sealed quench method to be used. Cyanide salt bath treatments have now largely been superseded by fluid bed treatments which do not have the health and safety and environmental risks associated with the operational and disposal aspects of cyanide salt baths.

The use of gas activated (hence ‘fluidised’) and heated powder, such as alumina or silica, as a means to transfer heat to components being heat treated is increasingly used instead of molten salts. Its benefits include rapid heat transfer, the ability to add process gases to alter surface chemistry, and hence case harden or nitride components, in an environmentally friendly manner.

Fluid bed hardening

The use of fluidised beds, consisting of a suitable solid inert medium, such as silica or alumina powder, agitated by flow of the heating gas medium through the bed, has largely replaced salt bath hardening. As with salt baths, heat input to the work piece is equally rapid and the method is similarly labour intensive, although health and safety and environmental risks are negligible. The heating gas can be supplemented with the controlled addition of a hydrocarbon gas for carburising and ammonia for nitriding or in combination with a hydrocarbon gas for carbonitriding or nitrocarburising. Small components, particularly those with geometries which are difficult to treat in batch furnaces, due to the risk of masking, may be treated very effectively in fluid beds.


A very old metalworking process, traditionally carried out by a blacksmith with a hammer and anvil, and used to shape metal under compressive force. In modern industry, forging is carried out by powered presses or hammers. Metals are generally hot forged, but can also be cold forged. Because of the effect on grain flow, which is compressed to follow the part shape, forged components are generally strong and tough.

See also cold working.


Fretting is surface wear resulting from relative motion between surfaces in contact under pressure.

Full annealing

A synonym for annealing.

It is used to avoid confusion with the many other types of annealing such as recrystallisation annealing, process annealing, etc.

Full annealing consists of heating steel to above the upper critical temperature, and slow cooling, usually in the furnace. It is generally only necessary to apply full annealing cycles to the higher alloy or higher carbon steels. In some instances a special form of full annealing called isothermal annealing is used, to obtain maximum softening response. This consists of holding the steel at a selected temperature above the upper critical temperature for sufficient time to allow transformation to pearlite before cooling the steel. Long cycle times are required to do this with many high alloy steels and it is therefore expensive.

Where it is considered desirable to fully austenitise a steel during a softening process, (e.g. to refine forged structures etc.) but economy is important, a normalising treatment is often used rather than a time consuming full anneal. This consists of heating above the upper critical temperature and air cooling. This process is only applicable to plain carbon and low alloy steels.


Top of page


Dipping steel components into a bath of liquid zinc in order to obtain a surface covering of the metal.

Galvanising protects the surface of the steel against corrosion.

Gas carburising

One of the most used industrial methods, having superseded the pack and salt bath processes. Furnaces suitable for this method are expensive but are economical in operation since large payloads are possible, and their automatic operation allows very efficient manning levels; two operators being able to manage three or more furnaces, depending on the process cycle times employed. Efficiency has been further enhanced by the development of automated material handling systems and linked computer control of all furnace process parameters and work movement between furnaces. Both batch type and continuous furnaces have been developed for gas carburising. Pit furnaces were amongst the first to be modified for gas carburising, but they require separate quench tanks, with attendant process control and safety risks.

Gas nitriding

There are several methods of nitriding available, the first to be developed and still the industrial leader is gas nitriding. The gas nitriding process consists of heating the components in a furnace with a retort in which air has been replaced with ammonia gas. The process is controlled by monitoring the dissociation of the ammonia gas and controlling the gas flow and process temperature and time. A dissociation burette is used for this, on the basis that the undissociated ammonia gas in a sample of the furnace atmosphere can be dissolved in water and hence give a measure of the volume of atomic nitrogen available for nitriding. It is also now possible to monitor and control the process using a modified infra red gas analysis method, similar to that used in gas carburising control.

Gas porosity

See Porosity.

Graded coating

A thermal spray coating composed of mixed materials in successive layers which progressively change in composition from the constituent material of the substrate to the surface of the thermal sprayed deposit. Also referred to as graduated or gradated coating.


A crystal formed during the solidification of a metal or its subsequent heat treatment.

Crystals formed in this manner are generally deformed owing to the nearby solid crystals restricting their growth.

See also crystal structure.

Grain boundary

The area where grains meet.

It shows up as a line on micrographs but, since grains exist in three dimensions, it is actually a surface where two solid objects meet. The simplest way to visualise grain boundaries is to press two transparent balloons together and you will be able to see the surface where they come together.

As two adjacent crystals or grains solidify, the orientation of their layers of atoms differ. When they meet, there is a misalignment between the grains, which forms the grain boundary just a few atoms thick.


Compacted powder, held together by mechanical means only, prior to sintering or firing.


The removal of material by the use of fixed abrasives. Examples include the diamond grinding of HVOF sprayed carbide containing coatings.


Top of page


A measure of how easy it is to fully harden a steel is known as its hardenability. The higher the hardenability, the easier it is to harden and the slower the quenching rate can be. It is the quantity and type of alloy in the steel that determines its hardenability.

Steels with high hardenability can be fully hardened easily, by quenching in air for example. Those with low hardenability are difficult to fully harden and need to be quenched in water.

Another way to consider hardenability is in terms of how large a diameter of bar can be fully hardened to its centre by a given quenching method. For example, after oil quenching, a low hardenability steel might only fully harden in a bar 2cm thick whereas a high hardenability steel might fully harden in a bar 15 cm thick.

A steel’s hardenability is determined by its alloy content. The maximum hardness of a steel after it has been fully hardened is determined by its carbon content, not its hardenability.


Hardening processes are used to impart specific mechanical properties to a component in order to render it fit for use. Hardening occurs when a steel component is heated to the austenitic range and cooled rapidly by quenching in a suitable medium such as water, oil or an inert gas. The choice of quenchant is determined by the composition of the steel and the geometry and application of the component being treated.

Steel must be in its austenite phase before it can be hardened. The temperature from which a steel can be hardened (called the hardening temperature) depends on its composition and can be determined from the equilibrium diagram. The rapid cooling during quenching causes the structure of the steel to change to martensite, which is very hard. Slow cooling would cause the austenite to transform to the much-softer ferrite.

The main points to be considered when selecting a hardening treatment are the application for which the component has been designed, its geometry and the steel composition which has been selected to provide the required mechanical properties. These will, to a large extent, dictate the hardening treatments which are suitable and the choices available. All stages of the manufacture of the component can affect the efficiency of the hardening treatment and the overall manufacturing economics can be greatly influenced by the choice of heat treatment. All methods of manufacture, every steel composition and each hardening treatment has its advantages and disadvantages. Care is required if the optimum choice is to be made and advice from heat treatment specialists, such as Bodycote, should be sought at an early stage of component design.

Various designs of heat treatment furnaces are available, including gas fired or electric continuous furnaces or sealed quench furnaces with integral oil quenching chambers, electrically heated vacuum furnaces with inert gas cooling facilities and gas or electrically heated pit furnaces. Other heat treatment equipment including fluid beds, salt baths, flame hardening and induction heat treatment sets offer a wide selection for the economic heat treatment of components of various sizes and in quantities from one-offs to mass production volumes.

The heating and cooling regimes required for hardening must be closely controlled if optimum results are to be obtained. There is a risk of component distortion occurring, due to a combination of factors, including the relief of stresses induced by the prior manufacturing history, the production of stresses due to the volume changes accompanying the crystallographic changes during hardening and temperature gradients set up by variations in the cross section of the treated component.

Hardening colours

The colour of steel when it is held at its hardening temperature.

As any metal is heated it changes colour, depending on its temperature. During the early days of heat treatment, before there were reliable temperature measuring systems, the temperature from which steels had to be quenched were judged by eye.

See also tempering colours.

Hardening temperature

The temperature from which a steel should be quenched to give the best mechanical properties after hardening.

Hardening temperatures vary from steel to steel and depend on the composition of the steel and the properties required after hardening.


An obsolete name for tungsten carbide.


The ability of a material to resist indentation by an applied load.

Hardness test

A test which determines the resistance of the material to deformation.

In the most-common tests, a hard indenter is forced into the surface of the material under a known load for a certain time. When the indenter is removed, the volume of the indentation can be determined and used to produce a hardness number. The three main tests are Brinell, which uses a hard steel or tungsten carbide ball as the indenter; Rockwell, which uses a diamond cone for hard material and a steel or tungsten carbide ball for soft material; Vickers, which uses a diamond pyramid. Generally, steel balls are being replaced as standard by tungsten carbide balls owing to the latter’s reduced likelihood of distortion.

There are many more methods of hardness testing, such as the scratch test, rebound test (scleroscope) and file tests.

Heat treatment

Heat treatment is a controlled process, performed by metallurgists and engineers, which is used to alter the microstructure of materials such as metals and alloys to impart properties which benefit the working life of a component, for example increased surface hardness, temperature resistance, ductility and strength.

Although modern techniques are scientifically advanced processes, mankind has used heat treatment to improve the properties of metals for thousands of years. In many instances, heat treatment is a vital part of the manufacture of a component and is generally used as either an intermediate process, for example to improve machinability or hot and cold working properties, or a finishing process where the treatment is required to impart final specific properties such as wear and corrosion resistance.

Heat treatment includes an extensive range of heating and cooling processes, each with the purpose of manipulating the material microstructure to achieve desired mechanical or metallurgical properties. Modern furnaces are capable of very precise temperature and atmosphere controls which, in turn, enable the experienced metallurgist to optimise treatments.


A monatomic noble gas and the most inert element, atomic number 2, represented by the symbol He. Used as a plasma gas in plasma spraying.

High alloy steel

Steel containing over 10% of metal alloying elements.

See also alloy steel, carbon steel, low alloy steel.


Generally, industrial gas-fired burners do not switch on and off but switch from low-fire, whilst idling, to a greater heat input (called high-fire) when heating the furnace up.

High Velocity Oxy Fuel


High speed steel

A type of tool steel, with high temperature and hardness properties, generally used for tooling parts such as drill bits and cutting tools. Named for its ability to cut quickly, high speed steel (HSS) can contain various combinations of alloys including molybdenum and tungsten to name a couple. Heat treatment and thermal spray coatings are also used to improve the hardness and abrasion resistance of high speed steel.


see Hot Isostatic Pressing

HIP brazing

HIP-assisted brazing utilises the fabrication method of encapsulation and hot isostatic pressing to form a superior braze bond. The braze material will be in the liquid state for at least part of this process to ‘wet’ the parts to be joined and fill in gaps. Some alloying occurs with the materials to be joined although they remain in the solid state. Some brazes are transient liquid phase meaning that their composition changes during the brazing process as they alloy with the parts to be joined; this results in a bond that is more stable at higher temperatures than the original braze material was.

HIP cladding

A specialized diffusion bonding where a premium powder or solid material is selectively bonded to a more economical substrate surface, providing premium properties such as corrosion and wear resistance only where they are needed on the component.


Homogeneous materials and substances are uniform in composition. Hot isostatic pressing (HIP) is an example of a process by which the homogeneity of a material’s microstructure can be improved.

Hooke’s Law

The amount by which a material is stretched is directly related to the applied force.

This law is only applicable provided the elastic limit of the material is not exceeded. A spring balance is a simple application of this Law. Thus, during a tensile test, the extension of the test piece is linear until the yield point is reached.

This Law is named after the English physicist and mathematician Robert Hooke (1653-1703).

Hot Isostatic Pressing

Hot isostatic pressing (HIP) takes various forms:

  • 1. A solid-state PM process for simultaneously heating and forming a fully dense part either by:
    a. Encapsulating powder in an evacuated and hermetically sealed sheet metal canister, or
    b. Sintering a pressed or CIPed compact to high enough density to allow unencapsulated HIP to achieve full density. Equal pressure is applied in all directions (isostatic) at a temperature high enough for plastic deformation and sintering to occur to reach theoretical density.
  • 2. A process that subjects a casting, MIM component, part created by additive manufacturing or powder forging to both elevated temperature and isostatic gas pressure in an autoclave. The most widely used pressurising gas is argon. When these components are HIPed, the simultaneous application of heat and pressure eliminates internal porosity through a combination of plastic deformation, creep and diffusion leading to densification.
  • 3. A process that allows diffusion bonding to occur to fuse two or more materials, either in solid or powder form, together on an atomic level.


A thermal spray process in which a fuel gas is mixed with oxygen and delivered at high pressure to the HVOF gun and ignited to form a high velocity oxygen/fuel gas stream in to which thermal spray powders are introduced and propelled on to the substrate.


An organic chemical compound consisting only of hydrogen and carbon.

The molecular structure of hydrocarbon compounds varies from the simplest, methane (CH4) to very heavy and very complex structures such as that of octane (C8H18), for example, a constituent of crude oil which is one of the heavier and more-complex hydrocarbons.

Hydrogen (H)

A colourless, odourless and tasteless gaseous element with the chemical symbol H.

Hydrogen is the lightest known substance, being fourteen and a half times lighter than air (hence its use in filling balloons), and over eleven thousand times lighter than water. It is very abundant, being an ingredient of water and of many other substances, especially those of animal or vegetable origin. It is highly inflammable.

Properties Melting Point: -259.2ºC
Boiling Point: -252.8ºC
Relative density: 0.07 (Air = 1)
Auto-ignition temperature: 565ºC
Explosive limits 4-74% in air

Used as a secondary plasma gas in the plasma spraying process. Used as a fuel gas in combustion thermal spray processes.

Discovered in 1766 by Henry Cavendish and named after the Greek words hydro and genes meaning water and generator. In its natural form it has two atoms combined: H2.

Hydrogen annealing

An annealing process using a hydrogen atmosphere which relieves mechanical stresses and imparts magnetic properties.

Hydrogen embrittlement

The unintentional diffusion of hydrogen into metal, particularly at elevated temperatures, which causes brittleness and cracking. More common with ferritic materials.

Hydrogen damage can take a number of forms, including blistering, stress cracking and loss of tensile ductility.

See also de-embrittlement.

Hydrogen Induced Stress Corrosion

See Hydrogen embrittlement.

Hydrogen potential

See pH.


A composition containing more alloying element than the eutectoid composition, for example hypereutectoid steel contains more carbon than the eutectoid composition.

See also hypoeutectoid.


A composition containing less alloying element than the eutectoid composition, for example hypoeutectoid steel contains less carbon than the eutectoid composition.

See also hypereutectoid.


Top of page

Impact test

A test which determines the energy required to break a test piece when it is struck suddenly.

The two most-common tests are the Charpy and the Izod tests. Both use a notched test piece of standard dimensions, which is struck by a pendulum.

Impact tests are carried out to determine the ductility of the material after heat treatment. In reality, the results obtained are very variable and they best serve to identify whether a material has a tendency to behave in a brittle manner when a notch is present.


Non-metallic particles, usually compounds, introduced into steel during its manufacture.

Usually considered undesirable, in some cases, such as in free-machining steels, inclusions may be deliberately introduced to improve their machinability.


The part of the hardness testing machine which contacts the piece being tested and creates the indentation.

Indenters are subject to harsh conditions and are removable to enable easy replacement when necessary.


Rotating a circular table, which holds a number of components in set positions around its outside edge, one position at a time so that each component is presented to an induction coil at each movement.


A chemical which shows whether a solution is acid or alkaline by changing colour when it is added to the solution.

See also litmus paper.


See eddy currents.

Induction coil

See induction heat treatment.

Induction hardening

Heating a component by induction, followed by oil or water quenching.

With steels having carbon contents of 0.4/ 0.5%, it is possible to obtain a hard case for wear resistance or to increase fatigue strength by means of induction hardening. A copper induction coil is made to surround the workpiece and the surface temperature is raised to above the upper critical temperature in a few seconds, by the heating effect of the induced electromagnetic current in the surface of the workpiece. A quenching spray follows the inductor and provides rapid cooling to produce full transformation of the heated surface.

The depth of heat penetration and therefore the hardening effect is proportional to the frequency of current in the inductor, the power generated, the steel composition of the workpiece and the heating or dwell time. Thus, for one generator operating at a given frequency it is possible to obtain various 'case' depths. Considerable skill is required in 'setting up' the workpiece to be hardened, in order to obtain the ideal combination of dwell time and quench delay, such that an optimum case to core hardness profile is produced. Once programmed, modern handling units are capable of operation by less skilled staff.

There are two main methods of induction hardening, 'single shot' hardening, where the whole area to be hardened is heated at once e.g. small gears or shafts, spun inside the induction coil and the complete periphery heated and quenched. Alternatively, the workpiece can be traversed, as with long shafts, where the area to be hardened is progressively heated and quenched by a moving coil followed by a quench ring, or in the case of gears the tooth by tooth method of hardening. Surface hardnesses between 50 and 6ORc are achievable by this process, depending upon the steel composition of the work piece.

Since induction hardening only uses electrical energy to heat the surface zone of a component, it is the most energy and therefore cost effective method for surface hardening many components. It has, as a piece part operation, the disadvantage that for small volumes of components it can be labour intensive. High frequency (HF) induction sets are used to heat treat small components up to 2 inches in diameter or localised flank hardening of areas of larger components and medium frequency (MF) induction sets are used to heat treat larger components. The HF method is particularly suitable when large volumes of components of relatively simple shape, such as pins, bushes, studs, and camshafts are required to be hardened. Automated handling equipment can be readily applied and the resulting hardening facility may be easily incorporated into a manufacturing line next to machining and finishing stations. The effectiveness of induction hardening depends upon the manufacture of a close fitting copper induction coil, which involves considerable product knowledge and skill. Electronic control of the power input provides control of the temperature regime but the induction method has the disadvantage that the point effect of sharp edges causes local overheating and may even lead to localised melting. Therefore care has to taken when components have sharp edges or contain details such as threads or circlip grooves. Quenching is achieved by a linked quenchant spray system, which closely follows the heating coil as they both traverse the component surface, normally using proprietary oil mixtures or polymer quenchant. Hardened depths of up to 1mm are usually achieved with the HF method, whereas MF sets can economically provide hardened depths of up to 5mm. The latter process is applied to large components such as shafts and gears, which may be flank hardened tooth by tooth.

Induction heat treatment

Heating a metal by holding it in an alternating electric field which induces an electric current in it.

A medium-frequency or high-frequency alternating current is passed through an induction coil and creates a magnetic field around the coil. When a conducting material, such as steel, is held in the centre of the coil, the magnetic field causes a current to flow in the surface of the steel, which heats it up. The temperature to which the steel is heated can be controlled readily and so induction heating can be used to harden or soften steel, as required.

See also eddy currents.

Induction softening

Heating a component by induction, followed by slow cooling.

Industrial alcohol

An impure form of ethanol used in industry, generally as a solvent, but not suitable for human consumption.

To avoid unauthorised consumption, industrial alcohol is also sold with a nauseous substance added to make it undrinkable. Such alcohol is also known as denatured alcohol.

See also isopropyl alcohol.


Inert refers to a material or substance that is not chemically reactive.

Integral quench furnace

The American term for a sealed quench furnace.

Interference fit

Mating components where the outside diameter of one is equal to or greater than the inside diameter of the other.

If the diameters are equal, the parts may be forced together in a press. If the outside diameter of the inner part is larger than the inside diameter of the outer part, then they must be assembled by shrink-fitting.


A compound, intermediate phase or solid solution, containing two or more metals, having physical properties and crystal structures different from those of the pure metals and their solid solutions.

Investment casting

One of the oldest metal-forming methods, also known as lost-wax or precision casting, used for casting metal into a mould produced by surrounding, or ‘investing’, an expendable pattern with a refractory slurry coating that sets at room temperature. After setting, the wax or plastic pattern is removed through the use of heat prior to filling the mould with liquid metal. Investment casting is commonly used to produce complex components such as turbine blades.

Ion implantation

The process of embedding ions into a solid substrate by using a beam of ionised particles to alter the physical and chemical properties of the substrate. This produces an alloyed surface where embedded ions are surrounded by the atoms of the substrate.

The underlying principle of plasma nitriding.

Ion nitriding

See plasma nitriding.

Iron (Fe)

From isarn, the old Saxon word for iron.

The atoms in metals are arranged in a regular three-dimensional pattern called a crystal structure. In the case of iron it can be visualised as a series of cubes stacked side by side, and one on top of the other. The corners of the cube are atoms and each corner is shared by eight adjoining cubes or cells. As well as corner atoms each unit cell contains additional atoms, with one atom at the centre of the cell, it is termed a body-centred cubic structure, (bcc), with atoms at the centre of each face of the cell it is termed a face centred cubic structure, (fcc).

Pure iron is capable of existing in three forms, all of which are stable within different temperature ranges. Between room temperature and 911°C iron has a body-centred cubic, bcc crystal structure and is termed ά (Alpha) iron, (commonly known as ferrite). At 91I°C a crystalline transformation occurs and the bcc structure changes to face- centred cubic, fcc. This form is termed γ (Gamma) iron (austenite) and exists up to 1392°C, at which temperature the structure again changes to bcc, the high temperature, δ (Delta-ferrite) form.

Other metallic elements, when added to iron, have their atoms interspersed in the gaps between the iron atoms and in this way alloys are formed. The addition of carbon to iron, as in the case of steel, causes alterations to the crystal structure by the imposition of carbon atoms into the gaps between iron atoms; e.g. in gamma-iron, austenite. Rapid cooling of steel by quenching from the austenitic temperature range produces crystallographic transformation to the meta-stable hard phase, martensite.

See also ferrous.

Iron carbide

See cementite.


Abbreviation for International Standards Organisation.

ISO does not create standards but provides a means of verifying that a proposed standard has met certain requirements for due process, consensus, and other criteria by those developing the standard.

ISO 14001

A globally accepted standard related to Environmental Management Systems, the ISO 14000 family of standards exists to help organisations to identify and minimise any negative effects of their operation on the environment. Related to the ISO 9001:2008 family of standards, ISO 14001 is process rather than product driven.

ISO 9001

A globally accepted standard related to Quality Management Systems which are designed to ensure that organisations are focused on meeting customer needs and expectations. The ISO 9001:2000 family of standards, which are improvement and process driven, were a radical departure from the previous clause driven versions. The current version, ISO 9001:2008, is more of a standard for business systems rather than just quality management systems. ISO 9001:2008 is a common base for the linking of related standards such as ISO 14001, TS 16949 and AS 9100.

Isopropyl alcohol

A colourless liquid compound of carbon, hydrogen and oxygen with the formula (CH3)2CHOH and a pleasant smell.

Isopropyl alcohol (also known as isopropanol and rubbing alcohol) is widely used in industry as a solvent, a weak degreaser and a drying agent to remove water, with which it mixes completely. Its freezing point is -89ºC which is why it is used in sub-zero treating baths with dry ice. It vaporises easily and is highly flammable.

Properties: Melting point -89°C
Boiling point 82°C
Relative density 2.1 (at 0°C, Water = 1)
Flash point 12°C
Auto-ignition temperature 425°C
Explosive limits 2 to 12% in air


Balanced forces acting equally in all directions; in hot isostatic pressing it pertains to omnidirectional equal pressure.

Isothermal annealing

See annealing.

Isothermal transformation

A phase transformation which occurs at a constant temperature (isothermal). The time required for transformation to be completed, and in some instances the time delay before transformation begins, depends on the temperature of the transformation and the composition of the alloy being treated.

Izod test

See impact test.


Top of page


Any material used to hold or support components during heat treatment. (It is also used to describe the activity of assembling a furnace load using jigging.)

Normally, specially-made, general-purpose heat-resisting jigging is used but it can equally well be made out of ceramic, stainless steel or even mild steel, depending on the application.

Jig tempering

Tempering carried out on components which are fitted into fixturing that restrains their movement during processing.

The intention of the fixturing is to control the shape or dimensional tolerances of the components which may have suffered distortion during hardening.


Top of page


Kolsterising® is a proprietary Bodycote diffusion process which enhances the mechanical properties of austenitic and duplex stainless steel, nickel base and cobalt chromium alloys without affecting the good corrosion resistance of the base material. The process introduces massive amounts of carbon into the austenitic structure and leads to the formation of what is called S-phase or expanded austenite.

As a result, the steel surface gets 4-5 times harder (900-1200HV) than the untreated material and finally leads to an increase in wear resistance, fatigue life, erosion and cavitation resistance. Diffusion depth up to 50µm can be obtained without lowering the corrosion resistance or affecting the magnetic properties of the base material. As a positive side effect the tendency of galling or fretting will be eliminated.

The Kolsterising® process has no effect on the size, shape, colour or roughness of the treated parts.

Numerous applications for Kolsterised steel components and parts can be found in the food & beverage industry, in chemical production equipment, medical devices, offshore drilling equipment, oil & gas and automotive industry.


Bodycote K-Tech ceramics are a unique range of high quality thermochemically formed ceramic coatings for the prevention of corrosion and wear in a wide variety of industrial applications. They can be applied to most ferrous and some non-ferrous metals and are fundamentally different to all other techniques for depositing ceramics.

What distinguishes Bodycote K-Tech technology from almost all other deposition techniques for ceramics, carbides and metals is their unique corrosion barrier performance. All other techniques e.g. HVOF, plasma, airpsray, thermospray and electroplating result in coatings with inherent porosity. Micro-cracking can, and will, allow corrosion products to penetrate the coating, corrode the substrate at the interface and result in de-bonding and spalling of the coating. Even stainless steel substrates are not immune as the passive layer that provides the stainless properties can be interrupted during the coating process and not allowed to reform as it would in atmosphere.

The K-Tech range produces coatings that are chemically, not mechanically, bonded and have absolutely dense, pore-free corrosion barriers. They have a wide operating temperature capability - from cryogenic pump applications to gas turbine compressors. Due to the application process, geometries such as internal bores can be coated effectively. K-Tech coatings exhibit extreme hardness which significantly improves the life of mechanical components. They have a smooth, low friction surface and are anti-galling.

The K-Tech ceramic densification process significantly improves the corrosion resistance of other coatings. A composite ceramic material is thermochemically bonded to customer-specified areas on a part, including OD's and ID's and some out-of-sight holes and ports. Individual ceramic particles are sub-micron in size and consist of mixtures of selected ceramic materials bonded together and to the substrate. Porous after the initial formation of the ceramic, the K-Tech application is densified using ceramic precursor chemicals plus corrosion resisting chemicals. When thermochemically converted into ceramic and corrosion protection in situ, the densification processes form additional bonds and mass within the initial ceramic body. Each densification cycle fills some of the remaining porosity until a fully dense, non porous, corrosion-resistant ceramic coating has been created.

The K-Tech coating develops a bond into the substrate through the formation of a spinel-like interface between the ceramic coating and the metal surface. Part of the thermochemical reaction causes the substrate metal atoms to migrate into the ceramic coating during initial processing resulting in an extremely high bond strength to the substrate, in excess of 10,000 psi.

The unique combination of particle hardness, chemical bonding, and lack of porosity results in a coating which is unparalleled in wear resistance in corrosive environments. This has been proven in the field by the use of K-Tech coatings in downhole applications, resulting in life expectancies of components now being measured in years instead of days and weeks.


Top of page


Rubbing two surfaces together with, or without abrasives, for the purposes of obtaining extreme dimensional accuracy or superior surface finish.

Lead (Pb)

From the Anglo-Saxon word Lead and the Latin Plumbum, meaning soft white metal.


The iron-cementite eutectic found in cast irons.

Ledeburite was named after the German metallurgist, Professor Adolf Ledebur (1837-1916).

See also eutectoid transformation.


Lindure® is a proprietary Bodycote diffusion treatment which is an alternative to conventional heat treatment when improved dimensional control is desired. During the Lindure® process nitrogen, carbon and oxygen are diffused into the surface of a work piece. The colour of a Lindure® processed part is typically matte grey. The actual surface finish will not change if the finish is greater than 32 RMS. For finishes finer than 32 RMS, the surface will roughen slightly. Lindure® surfaces can be polished to produce a surface that is both cosmetically appealing and durable.

The Lindure® process produces a solid, primarily, single epsilon phase nitride surface layer commonly referred to as the compound layer, which results in a high integrity metallurgical bond not subject to flaking or peeling. Below this layer, nitrogen is at lower concentrations and can be found in solid solution; this region is referred to as the diffusion zone. Nitrogen in solid solution imparts compressive stress into the surface of a work piece resulting in improved fatigue properties. While tempering is not required as part of the Lindure® process it can be used to enhance ductility by precipitating out nitrogen in the diffusion zone.

Lindure® has been successfully applied to a wide range of parts from a single plastic injection tool through to high-volume automotive gearing. In most cases, Lindure® is selected as a cost-effective engineering alternative to conventional heat treat processes that create unacceptable distortion or growth. While growth and distortion are not entirely eliminated, they are reduced by an order of magnitude. Dimensional changes are typically controlled to less than .0005” per surface. In some applications, post grinding and plating operations have been eliminated.

Liquid argon

Argon gas that has been converted to a colourless liquid by cooling it to a temperature below 186ºC.

Liquid is the highest purity form in which argon is supplied. It is also far more efficient to store argon as a liquid rather than as a compressed gas since each volume of liquid will give 822 volumes of gas when it is converted to gas at room temperature and atmospheric pressure.

Liquid argon is frequently used as a source of very pure argon gas for use in hot isostatic pressing and heat treatment atmospheres.

Properties: Boiling point -186°C
Density 1394kg/m3
Relative density 1.39 (Water = 1)
Ratio to volume of gas 1 : 822 (At room temperature)

Liquid nitrogen

Nitrogen gas that has been converted to a colourless liquid by cooling it to a temperature below 196ºC.

Liquid is the highest purity form in which nitrogen is supplied. It is also far more efficient to store nitrogen as a liquid rather than as a compressed gas since each volume of liquid will give 682 volumes of gas when it is converted to gas at room temperature and atmospheric pressure.

Liquid nitrogen is frequently used as a refrigerant in sub-zero treating and as a source of very pure nitrogen gas. Nitrogen derived from liquid is very much heavier than air owing to its very low temperature.

Properties: Boiling point -196°C
Density 808kg/m3
Relative density 0.8 (Water = 1)
Ratio to volume of gas 1 : 682 (At room temperature)

Liquid oxygen

Oxygen gas that has been converted to a pale blue liquid by cooling it to a temperature below 183ºC.

Liquid is the highest purity form in which oxygen is supplied It is also far more efficient to store oxygen as a liquid rather than as a compressed gas since each volume of liquid will give more than 500 volumes of gas when it is converted to gas at room temperature and atmospheric pressure.

Properties: Boiling point -183°C
Density 1142kg/m3
Relative density 1.14 (Water = 1)
Ratio to volume of gas 1 : 842 (At room temperature)

Litmus paper

A paper, normally purple in colour, which turns red in an acid solution and blue in an alkaline solution.

Litmus is a water-soluble mixture of different dyes extracted from certain lichens which is available as a solution or can be absorbed on to a porous paper. The resulting solution or piece of paper becomes a pH indicator which is used to determine whether a solution is acid or alkaline.

Litmus paper turns red under acidic conditions at a pH of 4.5 or below and turns blue under alkaline conditions at a pH of greater than 8.3. Weak acids and alkalis with a pH of between 4.5 and 8.3, appear to be neutral.

Load cell

A device that converts an applied load into an electrical signal.

Load thermocouple

A thermocouple placed within a furnace load at a position that represents the average temperature of the load.

Compare with control thermocouple and probe thermocouple.

Low alloy steel

Steels containing less than 2% of metal alloying elements.

See also alloy steel, carbon steel, high alloy steel, mild steel.

Low carbon steel

See carbon steel.

Low Pressure Carburising (LPC)

LPC has reached industrial maturity with the development of vacuum furnaces and controls capable of gas carburising and quenching the carburised components using oil or pressurised inert gas. Due to their highly controllable heating rates and the availability of high carburising temperatures (950/1030°C), they are finding an economic application for medium and deep case treatments. These methods have the advantage that treated components remain stationary throughout the process and the risks of component damage due to movement of hot components are eliminated. The surface and case chemistry can be very closely controlled, as can case depths, to within very tight limits and, as with all vacuum processes, treated components are kept clean. Savings can therefore be made in post heat treatment finishing operations, which more than offset the slightly higher treatment costs of these carburising methods. Whilst there is a need for careful tailoring of the process parameters for each design of component to be treated, the vacuum methods provide for much closer control of case depth range, uniformity and case chemistry than the other case hardening methods.

See also vacuum carburising.

Low Pressure Plasma Spraying (LPPS)

A thermal spray process variation in which the process is carried out under controlled atmosphere conditions. The process is carried out in a vacuum chamber and the thermal spray gun is normally operating in a low pressure environment of an inert gas, normally argon.


Top of page


Severe porosity, visible to the naked eye.


The property of a metal which allows it to be formed into various shapes without breaking.

Manganese (Mn)

From the Latin word magnes, meaning magnet.


A hardening method, which is particularly employed to minimise distortion. Martempering involves quenching the component to just above the transformation temperature and holding the heated component to allow equalistion of temperature throughout the component, followed by cooling to ambient temperature.


A steel whose structure is essentially, completely made of martensite.


The structure of steel after quenching (or hardening).

Martensite is an acicular (needle-like) type of ferrite. It is formed when austenite is cooled too rapidly for ferrite to form normally, in accordance with the equilibrium diagram. Since martensite is not an equilibrium phase, it is never shown on phase diagrams.

Martensite is very hard and brittle but can be made tougher (and softer) by tempering. Upon tempering, martensite breaks down into ferrite containing a fine precipitate of cementite. The structure obtained after tempering is today simply called tempered martensite. However, in the past the structures obtained from tempering martensite at different temperatures were called troostite (low temperature tempering) and sorbite (high temperature tempering).

Martensite was named after the German engineer, Adolf Martens (1850-1914).

See also martensitic.


See stopping-off.

Mechanical bond

In thermal spraying, mechanical bonding refers to the adherence of a thermal spray deposit to a roughened surface by the mechanism of mechanical interlocking.

Mechanical interlocking

In the context of metallurgy, mechanical interlocking refers to the first stage in the process of adhesion where adhesives are used to join two or more materials. In order to result in good adhesion, adhesive must penetrate any pores and irregularities.

Mechanical properties

Those properties of a material which are determined by mechanical means.

Mechanical properties are determined by tests involving the deformation or destruction of the piece being tested. Typical tests used are tensile, impact, bend, stress rupture, creep, hardness and fatigue tests.

Since all of these tests damage or destroy material, they are often carried out on test pieces representative of the components, rather than the expensive components themselves. Hardness tests can be carried out on components that have a suitable area which will not be damaged by the impression left by the test.

Mechanical testing

Tests which are used to determine the mechanical properties of a material used for manufacturing components.

There are a great many tests that can be carried out but those most-commonly used following heat treatment are the tensile test, impact test (called Charpy or Izod, depending on the test piece being used) and hardness test. Since these tests are destructive, they are generally carried out on test pieces representative of the components, to avoid the expense of destroying a component. Hardness tests can be carried out on components that have a suitable area which will not be damaged by the impression left by the test.

Medium carbon steel

See carbon steel.

Metal/oxygen cell

A small chemical reactor in which a metal is slowly reacted with the oxygen in the air.

Widely used in oxygen monitoring devices.

Metal injection moulding

Metal injection moulding (MIM) is a high volume, small dimension forming technique in which a mixture of fine metal powders (~60 vol. %) and a binder are forced into dies at high pressure. After forming, the parts are subjected to debinding and sintering processes to achieve high density.


See Thermal spray.

Metal joining

Metal joining covers a variety of techniques for obtaining a mechanical bond between separate components when they are joined together.

See also brazing, electron beam welding, vacuum brazing.


The study of the physical properties of metals, using metallurgical techniques such as microscopy. Metallographic samples are prepared by grinding, polishing and etching and are typically set into resin to aid examination and storage. The samples are then examined under a microscope where analysis of microstructure, material properties and quality can be carried out.


A metalloid is an element of the periodic table which has intermediate physical and chemical properties, meaning that it can be defined neither as a metal nor a nonmetal. Some metalloids demonstrate semi-conductive properties.

Metallurgical bond

Also referred to as a metallic bond, a metallurgical bond is the primary bond that holds metal together. This bond is formed during welding processes between base and filler metals.


The field of metallurgy encompasses the science, technology and related processes involving metals and alloys.

Metal matrix

Composed of a continuous metal or alloy phase which contains additional phase(s) within.

Metal matrix composite (MMC)

A composite consisting of a non-metallic reinforcement incorporated into a metal matrix. The reinforcements may be either continuous (e.g. carbon fibres) or discontinuous (e.g. silicon carbide whiskers). MMCs can be produced by chemical vapour deposition, liquid metal infiltration, diffusion bonding, direct casting or near-net shape techniques. The composite receives the metallic nature of thermal and electrical conductivity with higher-temperature operating limits and improved mechanical properties than the base metal.

Metal powder

An aggregate of discrete metal and/or alloy particles that are usually in the size range of 1 to 1000 µm. Powder can either be pre-alloyed or an elemental blend, or a mixture of both, to achieve a final composition.


A colourless and odourless gas with the formula CH4.

It is widely known as natural gas because it is the main constituent (80/95%) of the naturally occurring hydrocarbon gasses often found in association with crude oil and is also emitted by swamps owing to the decomposition of vegetation under the water.

Methane reacts with steel at temperatures above 800oC and imparts carbon into its surface and so is frequently used as one of the additions to heat treatment atmospheres to control their carbon potential. Being highly flammable, it is also used as the fuel for heating furnaces.

Properties: Melting point -182°C
Boiling point -164°C
Relative density 0.6 (Air = 1)
Flash point -221°C
Auto-ignition temperature 537°C
Explosive limits 5 to 15% in air

Mf temperature

The temperature at which the transformation of austenite to martensite will be complete (finished).

Mf simply means martensite finish. In low carbon, low alloy steels the Mf temperature is about 250ºC.

The Mf temperature varies, depending upon the carbon and alloy content of the steel, reducing as the carbon and alloy content increases. If the Mf temperature lies below room temperature, some austenite will be retained in the structure (retained austenite).

See also Ms temperature.


Short for photomicrograph.


Porosity not visible without magnification. Often microporosity is simply referred to as porosity.


The physical properties of a material’s microstructure strongly influence its use in an industrial environment. Thermal processing is used to alter and improve material microstructure to obtain desirable properties such as strength, hardness, corrosion resistance, and so on. The microstructure of materials can be revealed by a microscope at magnifications greater than 25×.

See also metallography.

Mild steel

A steel with no metal alloy elements and less than about 0.2% carbon.

Also known as low carbon steel, mild steel is the simplest and cheapest steel and is generally used in the un-heat treated condition.

See also alloy steel, carbon steel, high alloy steel, low alloy steel.


Milling is a machining technique used for cutting and shaping solid material. It is performed by milling machines using rotating cutters which can be operated manually or by automation. Digitally automated machining is called computer numerical control (CNC). Milling machines are capable of providing simple to very complex machining operations.

Moly (Mo)

A colloquial name for the metal molybdenum (Mo).

From the Greek word molybdos meaning lead.

Ms temperature

The temperature at which the transformation of austenite to martensite begins (starts).

Ms simply means martensite start. In low carbon, low alloy steels the Ms temperature is about 350ºC.

The Ms temperature varies, depending upon the carbon and alloy content of the steel, reducing as the carbon and alloy content increases.

See also Mf temperature.


A chamber within a furnace that prevents direct radiation from the heaters striking the workload and can also serve to direct the gasses through the load.

With early gas-fired furnaces, the products of combustion went into the furnace and effectively formed the atmosphere. This was no problem where the materials or components were not in the finished condition. However, for precision heat treatment using controlled atmospheres, mixing the products of combustion with the atmosphere was not permissible. Accordingly, the muffle was originally an inner, gas-tight chamber which segregated the products of combustion and the controlled atmosphere.

Modern gas-fired furnaces enclose the burners in tubes (radiant tubes) to keep the products of combustion separate from the furnace atmosphere. Accordingly, the muffle not only serves to prevent direct radiation from the radiant tubes, which are at a much higher temperature than the workload, but also directs the atmosphere over the radiant tubes and through the load to ensure even heating and atmosphere distribution.


A hard, brown-coloured refractory formed by the combination of alumina with silica in the approximate ratio three parts alumina to two parts silica.

Mullite is widely used for making high temperature, refractory parts for furnaces.

Originally found as a naturally-occurring mineral on the island of Mull in Scotland, from which its name derives. It is now produced synthetically and used as a refractory.


Top of page


Originally an acronym (National Aeronautical and Defense Contractors Accreditation Program), Nadcap is now the global brand name of a system developed in the early 1990s by US-based Aerospace Prime Contractors who convened to co-operate in developing a system of industry wide ‘standards’ to control the activities of providers of ‘Special Processes’ to aerospace and related industries. Run by the Performance Review Institute (PRI), which is part of SAE (Society of Automotive Engineers), their mission is to "provide international, unbiased, independent manufacturing process and product assessments and certification services for the purpose of adding value, reducing total cost, and facilitating relationships between primes and suppliers."

See also AS 9100.

Natural gas

See methane.

Near-net-shape (NNS)

The shape of a PM part, casting or forging that conforms fairly closely with specified dimensions. Such a part requires finish machining on some or all surfaces to achieve final dimensions. The closeness to finish dimensions depends on the economy of raw material savings versus machining expense versus the design and fabrication complexity.


The thinning of the centre of the sample during a tensile test.


The most commonly used unit of force.

One newton is defined as the force needed to produce an acceleration of 1 m/s2 on a mass of 1 kg. (force = mass x acceleration).

It is named after the English scientist and mathematician Sir Isaac Newton (1643-1727).

Nickel (Ni)

From the German word kupfernickel, meaning devil’s copper.

Nickel silver

Named for its silvery appearance, rather than any elemental content, nickel silver, also known as German silver, is an alloy of copper and nickel and often includes zinc. The usual composition is 60% copper, 20% nickel and 20% zinc. Modern nickel silvers generally all contain significant amounts of zinc.


Nitriding is the diffusion of nitrogen into the surface of special alloy steel to give a hard surface and soft core without the need for further treatment. Processing is generally carried out in the temperature range 470ºC to 530ºC in an atmosphere of ammonia, although other processing media can be used, such as salt baths and plasma.

Nitriding is only carried out on special alloy steels containing chromium or aluminium. It is the reaction of the nitrogen with these alloy elements that causes the hardening, so that, unlike carburising and carbonitriding, quenching is not required after processing. The nascent nitrogen is obtained from an atmosphere of ammonia gas, which at 500°C dissociates into its constituent elements, nitrogen and hydrogen. The nitrogen, which is in solution in the iron, diffuses inward and forms aluminium or chromium nitrides producing high hardness in the surface of the nitrided component. A layer of iron nitride and alloy nitrides forms on the surface (the 'white layer'). Since this is brittle it is normally removed from bearing surfaces before service. As with carburising the case depth is time and temperature dependent.

Owing to the fact that nitriding is a low-temperature treatment, it is carried out on steels that have already been hardened and tempered. Final tempering must have been carried out at least 50ºC above the nitriding temperature. All steels for nitriding have to contain molybdenum in order to avoid temper brittleness caused by holding the steel for a long time at about 500ºC.

Nitriding produces advantages in addition to a freedom from distortion, which is due to the low treatment temperature and the fact that quenching is not required. The hardening response is due to the dislocation blocking capability of the alloy nitrides dispersed throughout the nitrided layer. Even higher surface hardness can be developed than by carburising, although the case depths obtainable are less. Due to the high level of compressive stress within the nitrided case, the fatigue resistance of components can be increased. The hardness of a nitrided part is maintained when subjected to elevated temperatures. Whereas temperatures of 200°C are sufficient to cause a carburised case to begin to soften, it requires temperatures above that of nitriding or very extended exposure to cause softening of a nitrided case.

Whilst the nitriding process itself is virtually 'distortionless', it does cause a small, predictable amount of growth of the nitrided component, and it is necessary to ensure that a component is in a stress free condition prior to nitriding, otherwise distortion can result. Thus, it is preferable to include a stabilisation treatment after the rough machining stage. Since in most component applications core strength is important, the usual planning sequence is as follows:

  • 1. Oil harden and temper to produce specified core properties
  • 2. Rough machine
  • 3. Stabilise at 550/580°C for a time suitable for the section size
  • 4. Finish machine
  • 5. Nitride
  • 6. Polish to remove 'white layer'.

Selective nitriding can be achieved by the use of electroplated tin, or copper or using tin-based protective paint to blank off areas to be kept soft, thus preventing the diffusion of nitrogen from taking place there.

See also plasma nitriding, gas nitriding, Corr-I-Dur®.


Nitrocarburising is carried out at sub-critical temperatures and involves the diffusion of nitrogen and carbon into the surface of carbon steel to give a somewhat harder case and soft core with a very thin compound layer on the surface.

The compound layer is wear- and corrosion-resistant and yet is not brittle, unlike its counterpart in the nitriding process. Since it provides an essential part of the properties required from the process it must not be removed by subsequent machining. Below the compound layer, the thin case significantly enhances the fatigue resistance of the component.

Although nitrocarburising can be used with most steels that can be nitrided, it is most-commonly applied to mild steel and low alloy steels, the properties of which it improves dramatically.

Salt baths were initially used for nitrocarburising, using a variety of salt mixtures, generally sold under proprietary names. Fluidised beds are nowadays often used when small components require nitrocarburising. They have the benefit of ensuring even treatment throughout the load and across each component.

As with all gaseous processes, control is better than with the salt bath and the quality of the compound layer, in particular its freedom from porosity and evenness, is far superior. Longer treatment times are also possible than with salt baths, since the shortcomings of the compound layer (porosity and spalling problems) do not exist to cause limitations as in the salt processes. Hence gaseous nitrocarburising is applied to a wide range of materials and components.

Nitrocarburising can be used in place of cyaniding and carbonitriding for distortion prone parts e.g. clutch plates, retaining washers etc. Many parts, such as camshafts, crankshafts, torsion bars, benefit from nitrocarburising after hardening and tempering and increases in fatigue life of between 30 and 130% are usual.

All nitrocarburising treatments have the benefit of freedom from component distortion due to the low treatment temperature and the fact that quenching is only necessary if optimum fatigue resistance required. The use of nitrocarburising as an alternative to conventional shallow case nitriding with suitable alloy steels containing chromium or aluminium is also practicable, with great savings in processing time.

See also austenitic nitrocarburising, ferritic nitrocarburising, plasma nitrocarburising, Corr-I-Dur®.

Nitrogen (N)

A colourless and odourless gaseous element that makes up 78.1% of the Earth’s atmosphere.

It will not support life or combustion and is generally considered to be unreactive (inert), except at very high temperatures. For that reason, it is widely used as a protective gas in heat treatment.

Nitrogen is obtained as a by-product of the liquefaction and separation of air.

Properties Boiling Point: -195.8ºC
Relative density 0.967 (Air = 1)

Used as a primary and secondary gas in plasma spraying.

Discovered in 1772 by Daniel Rutherford and subsequently (1790) named after nitre (saltpetre – KNO3) and gennan (forming). In its natural form it has two atoms combined: N2.

See also liquid nitrogen.


Nivox® processes represent a group of Bodycote patented plasma based diffusion treatments such as nitriding or nitrocarburising for various steel grades, in particular stainless steel, as well as nickel base and titanium alloys. The treatment significantly improves surface hardness and resistance against abrasive wear. The gentle process prevents distortion and dimensional changes. Depending on the process, pure nitriding - with or without compound layer - or nitrocarburising to improve the component properties can be applied.

The special process technique of Nivox® also allows for the surface hardening of corrosion resistant materials by nitriding or nitrocarburising, creating the so called S-Phase that can be mainly found in the nuclear power industry, as well as in mechanical engineering and aeronautics. The corrosion resistance of the treated components is mainly unaffected and guarantees optimal mechanical, wear and corrosion properties.


Any metal or alloy which does not contain a deliberate addition of iron.


See Nonmetal.


All elements in the periodic table can be considered either a metal or a non-metal, given their physical and chemical properties. Elements with intermediate properties are called metalloids.


Heat treatment followed by air cooling, of heavily forged and cold formed steel, intended to return the structure to “normal”.

When plain carbon or low alloy steels are required to be softened sufficiently to allow a moderate degree of cold forming or machining or to homogenise the crystal structure, normalising may be employed. This treatment involves heating the work piece to above the upper critical temperature and holding at temperature for sufficient time to allow full austenitisation to occur, then air cooling or cooling in a controlled atmosphere to ambient temperature. Whilst not producing the same degree of softening as the annealing treatments, normalising has a lower cost and is a far quicker method.


Nucleation, in a metallurgical sense, refers to the beginning of a phase transformation at distinct sites, where the nucleus is the first stable particle allowing a matrix interface and the initiation of a new phase or a phase recrystallisation.

The seeding of clouds with carbon dioxide to nucleate rain droplets is an example of the introduction of a foreign particle to effect nucleation.


Top of page

Organic coating

Organic coatings are non-metallic and are used to protect metal from detrimental environmental conditions and chemical attack. They are generally applied by spray or dip spin, which is ideal for small components such as fasteners. Organic coatings are able to provide excellent salt spray resistance for relatively small coating thicknesses and are perfect for corrosion protection systems.


The result of a chemical reaction in which an element is oxidised by combining with oxygen. A common example of an oxide is rust – the low temperature oxide formed when oxygen reacts with iron.


Converting the surface of a component into its oxide by reaction with oxygen at high temperatures.

Iron and steel can also be oxidised by water and the reddish, powdery oxide produced is called rust.

Oxygen (O)

A colourless and odourless gaseous element that makes up 20.9% of the Earth’s atmosphere.

Oxygen is highly reactive and readily oxidises many materials at both ambient and elevated temperatures. Oxide films can sometimes be an advantage on metals, imparting corrosion resistance or a cosmetic finish to a component, so some heat treatment atmospheres contain oxygen-bearing constituents such as water vapour.

Oxygen is the only gas capable of supporting life and oxygen deficiency is a danger to life, especially in confined spaces. Symptoms or effects of reduced oxygen levels are:

20.9-18% oxygen Normal breathing
18-14% oxygen Breathing and pulse rate increased, muscular coordination slightly disturbed
14-10% oxygen Emotional upset, abnormal fatigue, disturbed respiration
10-6% oxygen Nausea and vomiting, collapse or loss of consciousness
Below 6% oxygen Convulsions, respiratory collapse and rapid death

Properties: Boiling Point: -183.0ºC
Relative density 1.1 (Air = 1)

Discovered in 1774 by Joseph Priestly and named from the Greek words oxus (acid) and gennan (forming). In its natural form it has two atoms combined: O2.

Ozone (O3) is another form of oxygen, containing three oxygen atoms combined together. It is formed naturally in the atmosphere by the action of ultraviolet light on oxygen and during electrical discharges. It is the acrid smell, noticeable after using a child’s electric train set for some time.

See also liquid oxygen.

Oxygen probe

A device used to monitor and control the carbon potential of the atmosphere in a furnace or atmosphere generator.


See oxygen.


Top of page

Pack carburising

The earliest method of case hardening, in which components were packed into a suitable box, together with carbon-bearing materials such as charcoal, hooves, hide, animal fat and horn, and heated to the carburising temperature.

Modern pack carburising is normally carried out using a less-variable carburising agent, such as charcoal, and an energiser, such as barium carbonate.

Pack carburising is very inefficient in that close control of the case depth and quality is difficult and quenching from the carburising temperature is not possible. It is really only good for one-off components, where controlled industrial processes are not available or are too expensive.

Occasionally called box carburising.


A passivation process is used to make the chemically active surface of a metal passive, and therefore more resistant to corrosion. The formation of a chemically inert, or passive, oxide layer on the surface of the metal can be achieved by various methods, depending upon the metal itself. Pure aluminium will naturally form a protective aluminium oxide layer when it reacts with air which prevents further reactions occurring. Ferrous metals are generally passivated by using acid to form the protective oxide layer.


A phase in the iron-carbon system consisting of alternate plates of ferrite and cementite.

It is the product of the eutectoid transformation of austenite as it cools down below 723ºC and contains 0.8% carbon.

It was called pearlite owing to its mother of pearl (pearlescent) appearance when viewed under a microscope.


A liquid chlorinated hydrocarbon with the formula CHCl:CCl2.

Long-known as a dry cleaning solvent for clothes, it is becoming more popular with industry owing to the problems associated with using trichloroethylene now that it has been re-classified as a carcinogen. Insoluble in water.

Properties: Melting point -19°C
Boiling point 121°C
Relative density 1.62 (Water = 1)
Vapour density 5.7 (Air = 1)


It is a measure of the activity of hydrogen ions (H+) in a solution and, therefore, defines whether it is an acid or alkali.

The term pH means hydrogen potential and it has a numerical value between 1 and 14, without units. Solutions with a pH less than seven are acidic, while those with a pH greater than seven are alkaline. pH 7 is considered neutral because it is the accepted pH of pure water at 25°C, although strictly, pure water cannot be assigned a pH value since it is non-ionic.


A distinct crystal structure of a metal or alloy.

The structure can either be simple, for example ferrite – pure iron, or complex, for example pearlite – alternate platelets (small plates) of cementite and ferrite. To qualify as a phase, the structure must exist within a certain temperature range and composition limits.

A graph which shows the temperature and composition limits of phases is called a phase diagram.

Phase diagram

A graph showing the temperature and composition ranges within which each of the phases of a particular alloy exist.

These temperature and composition ranges vary according to the heating and cooling rates used because the phases are solid and take time to form and change. Where the diagram shows the ranges obtained under infinitely slow cooling and heating rates, it is known as an equilibrium diagram.

Also known as a constitution diagram.


A toxic chemical produced when chlorinated hydrocarbons are burned at high temperatures.

Phosgene is widely used in the manufacture of many organic chemicals as well as insecticides, and pharmaceuticals. It was also used as a chemical warfare agent in World War I. Great care must be taken not to carry any solvents, remaining on components after degreasing, into furnaces.


A compound of phosphorous and another metallic element, phosphides have a variety of physical and chemical properties.

Phosphorus (P)

From the Greek word phospheros, meaning bringer of light.


A photograph of the structure of a metal as viewed through a microscope.

Often shortened to micrograph.

Pit furnace

A furnace set into the ground with the top at about waist height, in order to facilitate loading and unloading.

Plain carbon steel

See carbon steel.


Often referred to as the fourth state of matter, plasma contains a mixture of dissociated molecules, heated to form ionised particles: positive ions and negative electrons. Plasma can be controlled by the use of electromagnetic fields to perform in certain ways.

Naturally occurring examples of plasma include lightning and St Elmo’s Fire.

Plasma nitriding

A more modern development of the nitriding process, also known as ion nitriding. In this process the component is made cathodic in relation to the furnace shell and ammonia gas is fed into the evacuated chamber. The glow discharge at the surface of the steel part produces atomic nitrogen by ionisation of the ammonia gas.

Although this process uses more expensive equipment, it has the advantage of being extremely controllable. Also it has a time advantage compared with conventional gas nitriding and lower nitriding temperatures can be used (450/590°C). Nitriding commences as soon as the surface ionisation occurs, and since it is not necessary to wait until the whole cross section of the component reaches nitriding temperature, cycle times are shorter. Also the improved reaction conditions existing in vacuo provide cleaner treated components. A major benefit is the lack of white layer, due to the surface reactivity of the glow discharge. This same characteristic makes the process better suited to the nitriding of stainless steels and other high alloy steels, as their surface passive layers are broken down by the glow discharge, allowing uniform nitriding to be produced.

Plasma nitrocarburising

Plasma nitrocarburising is an alternative nitrocarburising method which provides additional treatment benefits arising from the catalytic effect of the glow discharge and its ability to remove the protective surface layers present on stainless steels and other high alloy steels and alloy cast irons. As a result it is the preferred method for these materials.

Plasma spray

A thermal spray process in which a non-transferred arc is produced by ionising an inert gas to form plasma that then forms the heat source into which thermal spray materials, such as metal powder, are injected which are then subsequently propelled to the substrate to form a thermal spray coating.

Plastic deformation

See Deformation.

Plastic limit

The maximum amount of tensile stress that a test piece can withstand before it breaks.


Depositing a metal from a solution onto a component by passing an electric current through the solution.

See also electroplating.

Plug quenching

Quenching a component whilst its bore is restrained by having a plug inserted into it, in order to control the final bore dimensions after hardening.

Plug quenching is generally used on small batches of simple-shaped rings such as gears, the bores of which must be free from distortion after hardening.


Acronym used for powder metallurgy processing or powdered metal.


Porosity refers to the void spaces in a material. These voids often occur as defects in cast metal components, due to shrinkage and gas bubbles as the liquid metal cools and solidifies, and present possibilities for component failure, such as fatigue, if they are not treated.

Metals generally shrink as they solidify; if there is not enough metal to compensate for the shrinkage then defects can form. Shrinkage defects can be closed or open, meaning that closed defects are contained within the metal (shrinkage porosity) or form on the surface of the metal. Another type of porosity, gas porosity, occurs after the metal has cooled due to the release of dissolved gas from the liquid metal.

Porosity can be detected using non-destructive testing techniques such as radiography (x-ray) or ultrasonic inspection and can be effectively eliminated by hot isostatic pressing.

See also macroporosity, microporosity.

Powder encapsulation

See encapsulation.


Term trademarked by Bodycote that refers to fabricating parts via hot isostatic pressing of metal powders, or powder metallurgy in general.

Powder metal

Referring to a process or product thereof that involves fabrication by densifying metal powders.

Powder metallurgy

Powder metallurgy (PM) is the technology of producing and utilising metal and alloy powders for the fabrication of shaped parts, varying in size from grams to tonnes and in shapes from simple to highly complex (near-net shape).


The solid substance ejected from solution during precipitation.


The ejection of a solid, called a precipitate, from a concentrated solution in which it was dissolved, as the solution cools down.

Precipitation also occurs in some solid metals, called solid solutions, as they cool down.

Precipitation hardening

The hardening which occurs when a second phase is precipitated from a supersaturated solid solution.

Originally, age hardening was the process and precipitation hardening was the phenomenon. Nowadays, these two terms tend to be used interchangeably.

Press quenching

Quenching a component whilst it is restrained in a jig clamped shut by a press in order to control its final dimensions after hardening. Press quenching is generally used on simple-shaped, flat components which are prone to distortion, particularly gears and thin rings.

After being heated to the hardening temperature, the component is taken from the furnace and placed in a die on a quench press. As the press closes, it clamps the component between two specially-made dies and immediately, oil flows over the component and hardens it. The component keeps its dimensions because it is clamped under very high pressure between the dies.

The geometry of some components, such as clutch plates, synchromesh sleeves, and helical, worm, ring and spur gears, present increased risks of component distortion at the quenching stage, if free quenching is applied, even when optimum controls are employed. Press quenching provides an effective solution. Very close fitting dies can be manufactured and the austenitised component transferred to them prior to quenching. This is carried out with the dies pressed together in a suitable press quench apparatus and the constrained component cooled by either immersion in, or spray cooling with, the quenchant, usually oil or a polymer mixture. Press quenching allows precise control of finished dimensions and can greatly improve the yield by reducing scrap due to distortion, as well as reducing or removing the need for expensive finish grinding. Simple shapes such as rings can be plug quenched when it is required to inhibit bore shrinkage or increase compressive stresses to enhance fatigue resistance. The method is a piece part process and may also be labour intensive but it is nonetheless an economic proposition for precision components. When large production volumes are available it is possible to automate the process and thus reduce unit costs.

See also cold die quenching.

Probe thermocouple

A thermocouple used to measure the temperature at a specific point within a furnace.

Probe thermocouples are generally used to check that the temperature distribution within a furnace is uniform. Load thermocouples are often erroneously called probe thermocouples.

Compare with control thermocouple and load thermocouple.

Process annealing

A heat treatment used to soften material in preparation for further cold working, without significantly changing its structure.

Process annealing is carried out at a temperature just below the transformation temperature. It is generally used in the production of thin sheet and wire where cold working is used to produce material to very close tolerances. Full annealing results in material which is too soft to produce the close tolerances required.


Coating the surface of components with a metal vaporised from a target by an electric discharge.

The initials stand for Physical Vapour Deposition.


Top of page


The medium in which a metal is cooled rapidly, usually in order to harden it.

Vacuum furnaces use rapid circulation of gas (gas quenching) to cool loads down, often to shorten cycle times rather than to harden the parts.

Quenchants can be liquids, such as oil and water, or gasses, such as nitrogen or air.


Quenching is the rapid cooling of steel after heat treatment by immersion in water, or oil.

Water is used for plain carbon steels and very low alloy steels. Where the fastest possible quench is required, salt can be added to the water which is then called brine. Oil is used for higher-alloy steel to cool more gently and minimise distortion. It is possible to quench very high alloy steels using air or another suitable gas, such as nitrogen or even argon.

The ease with which a steel can be hardened is known as its hardenability. The higher the hardenability, the easier it is to harden and the slower the quenching rate can be. It is the quantity and type of alloy in the steel that determines its hardenability.

With most steels, quenching causes a great increase in hardness. In general, the higher the carbon content, the higher the hardness that can be achieved. Typically, the hardness of a fully-hardened steel varies from 40Rc for 0.1% carbon to 60Rc for 0.8% carbon.


Top of page

Radiant tubes

A ceramic or metal tube which separates the gas burner (or electric element) from the furnace atmosphere.

A method of heating a furnace without contaminating the gas atmosphere with the products of combustion from the heating gas. The gas is burnt inside the tube, which heats up and in turn heats the furnace by radiation. Modern radiant tubes use a recuperator to save energy.

Electric elements may also be used inside radiant tubes in order to protect them from the atmosphere gasses.

Recrystallisation annealing

A low temperature annealing treatment carried out on cold-worked material in order to develop a new, fine, crystalline microstructure (known as a fine-grained structure) without changing its phase.

The new crystal structure is free from the strains caused by cold working and responds in a predictable manner to subsequent processing. If too high a temperature is used, it can result in a coarse crystalline structure (known as a coarse-grained structure) with unpredictable properties.

Cold working followed by recrystallisation annealing is the only way to obtain a smaller grain size in pure metals and alloys which only have a single phase.

Rectification treatments

Some of the unwelcome adverse effects of heat treatment can be rectified by other thermal processes, the most common of which is cryogenic treatment used to remove retained austenite. Another less common rectification is carbon restoration, whereby surface decarburisation is re-carburised by means of exposure to carburising atmosphere. Control is difficult and this rectification is best carried out by gas carburising in sealed quench furnaces. It has also proved possible to de-nitride components by use of vacuum heat treatment. Again, control is difficult and the process time required is lengthy and therefore cost considerations are generally the deciding factor whether rectification is worthwhile.


A device fitted to radiant tubes that uses the waste gas (products of combustion) to heat the incoming air and so improve the efficiency of the burner.

Reducing atmosphere

A reducing atmosphere is one whose constituent gases will remove oxygen from the metal oxides on the surface of the components during heat treatment.

The most common reducing gases used in heat treatment are hydrogen and carbon monoxide.

Hydrogen is converted to water by reaction with metal oxides. (M stands for any metal.)

MO + H2 → M + H2O

Carbon monoxide is converted to carbon dioxide by reaction with metal oxides.

MO + 2CO → M + 2CO2

Reduction in area

The change in cross-sectional area of a tensile test piece as a percentage of its original cross sectional area.

% reduction in area = change in area (a) x 100 divided by the original area (A)

Reduction in area = (A-a) x 100/L %


A material manufactured from one or more ceramics and intended to resist high temperatures.

Typical examples used in furnaces are: alumina; silica; silicon carbide and mulllite.


See sub-zero treating.

Residual stress

Stress that remains inside a component following heat treatment, machining or forming operations.

Residual stresses can either be compressive stresses (act as if they are trying to crush the component) or tensile stresses (act as if they are trying to pull the component apart).

Retained austenite

The austenite which has not transformed to martensite after certain steels have been hardened and cooled to room temperature.

Generally it is the high-carbon, high-alloy steels that suffer from retained austenite. The faster a steel is quenched, the less austenite will be retained. High alloy steels tend to be oil quenched rather than water quenched, which is required for hardening plain carbon steels.

Retained austenite can be transformed by sub-zero treating or tempering at temperatures above about 570ºC.

See also Mf temperature.

Rockwell test

See hardness test.

Rotary hearth furnace

A circular furnace with a revolving hearth.

Rotary hearth furnaces are ideal for presenting heated components one at a time to a subsequent process, such as press quenching. They have a single door through which components are both loaded and unloaded. The speed of rotation is controlled to ensure that the components are thoroughly heated. Once they have rotated though 360º they will be at the required temperature and have returned to the door for unloading.


A powdery red oxide of iron formed on steel when it is exposed to moisture and air.

The oxide consists of hydrated ferric oxide (Fe2O3).


Top of page

Salt bath hardening

Molten salt baths have the benefit of very rapid heat transfer into the work piece and although a labour intensive method of heat treatment salt bath hardening is economical for treating small components. Capital cost is low but the cost of safe disposal of the used salt is expensive. Salt compositions are available for case hardening of low carbon steels and the neutral hardening of higher carbon and alloy steels, including tool steels. The method has greatly reduced in use due to environmental and health and safety considerations, as the operator is exposed to contact with the salt.

In order to provide a quicker alternative to the long cycle times required to develop gas or plasma nitrided case depths and to extend the range of ferrous alloys which may be treated, several salt bath treatments have been developed. Slightly higher temperatures are used, (550/ 570° C), and cycle times are mainly in the range of 2 to 4 hours. Although these processes can be applied to alloy nitriding steels with similar results as gas or plasma nitriding, they are generally applied to plain carbon and low alloy steels and cast irons.

Sealed quench

A furnace in which the heating chamber is attached to the quenching chamber so that the workload remains within the protective atmosphere throughout processing.

Secondary hardening

A further increase in hardening which is possible to achieve during the tempering cycle, due to the sub-microscopic precipitation of fine alloy carbide particles. In some alloys, where a phase transformation does not take place, secondary hardening may be the only method by which the alloy can be hardened.


In the field of metallurgy, the term segregation refers to the non uniform distribution or concentration of alloying elements, impurities, or microphases. For example, segregation in castings is a defect whereby alloying elements are concentrated in specific areas, such as surfaces or grain boundaries. Segregation can be micro- or macroscopic in nature.

Segregation can be a problematic occurrence, resulting in embrittlement, stress cracking and fatigue.

Selective case hardening.

Selective case hardening involves case hardening only the required portion of a component.

Most components are designed so that they can be case hardened all over. However, some components must be case hardened in certain areas only, the remainder being left soft to permit subsequent processing such as machining or welding. The method used to achieve this is called stopping-off

Set point

The temperature at which the furnace is intended to be controlled and to which the temperature controller is adjusted.

Selective Surface Net Shape

Application of hot isostatic pressing where one or more surfaces of a component are designed to have net shape (final) dimensions after encapsulation and densification of metal powders

Set point

The temperature at which the furnace is intended to be controlled and to which the temperature controller is adjusted.


SheraCote® is a proprietary Bodycote coating process based on tried and tested thermal diffusion coating, and represents a major advance in corrosion protection for iron and steel components. In many applications, SheraCote® provides a technically superior alternative to epoxy coatings and even stainless steel, having a higher surface hardness, in respect of both impact and abrasion, than any other zinc-based protection – and more than most other coating systems.

SheraCote® comfortably exceeds the performance of most other coatings in salt spray tests, consistently achieving in excess of 1000 hours, with a 3000 hours version available for alternatives to stainless steel components. Self-healing and spot weldable, SheraCote® is particularly beneficial for the coating of threaded components with internal threads and recesses and complex shapes, as it demonstrates excellent coating uniformity without ‘recess fill’. SheraCote® is suitable for high tensile components and does not cause hydrogen embrittlement or affect mechanical properties.


Sheraplex is a proprietary duplex coating system, trademarked by Bodycote, which utilises the excellent sacrificial corrosion protection afforded by the Sherardizing process combined with an organic barrier layer.


A proprietary diffusion coating process for alloying the surface of steel components with zinc. The process is normally carried out in a slowly rotating closed container at temperatures ranging from 320-500°C.

A competitor to galvanising.


Small cast iron or steel balls used in shot blasting and shot peening.

Cast iron is generally used for shot blasting because it breaks up in use and the broken shot cuts the surface contamination and removes it quicker. For heavy scale removal, pre-crushed shot can be obtained.

Steel shot is manufactured by cutting wire into short lengths and rolling it between plates to make it round. It is more-expensive than cast iron shot but it is ductile and does not break up in use giving sharp edges. Accordingly, it is ideal for shot peening which requires surface impacts without a cutting action.

After extended use, the shot does break up into very small pieces and these are then removed by the dust extractors fitted to all shot blasters.

Shot blasting

A method of cleaning the surface of metals by shooting small cast iron pellets (called shot) at it using a special machine called a shot blaster.

The brittle cast iron breaks up to form abrasive particles.

See also shot peening, for comparison.

Shot peening

A method of work hardening the surface of metals by shooting small steel balls (called shot) at it using a special machine similar to a shot blaster.

Steel shot is used since it is ductile and less-likely to break up than cast iron shot. The shot must be spherical and of a size selected for the application. It is therefore carefully filtered to remove any small or broken shot which could damage the surface.

The objective is to strengthen the surface by developing compressive stresses (residual stress) in the surface layers and thereby improve the fatigue properties.

See also shot blasting, for comparison.

Shrinkage porosity

See Porosity.


Shrink-fitting is a procedure which is used to fit together two parts, at least one of which is metal, with an interference fit.

The fitting can be carried out by expanding the outer metal part and allowing it to shrink onto the other part as it cools. Alternatively, an inner metal part can be shrunk by sub-zero treating and then allowed to expand into the other part as it heats up to room temperature.


A hard, transparent or frosted glassy ceramic formed by the reaction of silicon with oxygen and having the formula SiO2.

Used for making high-temperature, transparent furnace tubes or as a component of other refractories.

Silicon (Si)

From the Latin word silicis meaning flint.

Silicon carbide

A hard white ceramic formed by the reaction of silicon with carbon, having the formula SiC.

Silicon carbide is available in several forms including a slurry that can be cast into the required shape. Accordingly, it is used for making large, high temperature parts for furnaces.

See also carbide.


The absorption and diffusion of silicon into the surface of steel to give a heat-resistant surface.

Single crystal

See crystal structure.


A typically solid state process where adjacent surfaces of particles in a mass of powder, or a green compact, become bonded by heating. Sintering increases strength and produces densification. In addition to bonding, sintering decreases the pore volume and leads to rounding of pores and the formation of grain boundaries where particles are in contact. Recrystallization often occurs in PM. Liquid phase sintering is similar, except one of the components is present as a liquid during part of the process.

S-N curve

A graph on which is plotted the Stress against the Number of cycles to failure, displaying the results of fatigue tests.


Time held at a selected temperature to effect homogenisation of structure or composition.


Softening processes are mainly utilised as intermediate heat treatments. They are used to improve hot and cold working characteristics, to increase machinability, to reduce internal stress due to working, welding etc, and also to condition components for subsequent hardening treatments.

Occasionally they are used to impart particular final properties as with low carbon transformer core material, which is annealed to optimise its magnetic characteristics. Softening occurs when the steel component is heated to the austenitic range and slow cooled.

See also annealing.

Solid solution

A solid metal in which an alloying element is dissolved, for example carbon dissolved in iron.

A solid solution behaves in a similar way to a liquid solution except that the reactions are generally much slower and are therefore carried out at higher temperatures to speed them up.

In general, as the temperature is increased, more of the alloying element can be dissolved. As the temperature is reduced, the solid solution can not hold as much alloying element and it is ejected from the solution as a precipitate. The precipitate can be the pure alloying element but is more often a compound of the alloying element and the base metal.

In iron-carbon alloys, the precipitate is cementite or iron carbide (Fe3C).

Soluble oil

An oil that has had special chemicals (emulsifiers) added to allow it to form a mixture with water, called an emulsion, to produce a fluid with a mixture of their properties.

Soluble oil combines the lubricating properties of oil with the cooling ability of the water. It will not catch fire and is also relatively cheap, owing to the high water content – usually 80/90%.

It is used to cool components after tempering and gives a black, adherent oxide finish which is both attractive and corrosion-resistant.

Emulsions are liquids containing small particles of oil suspended in water which do not separate out. Normally, oil and water mixtures will rapidly separate, forming a layer of oil on the surface of the water.

Solution treatment

Heating an alloy to a suitable temperature, holding at that temperature long enough to cause one or more constituents to enter into a solid solution and then cooling rapidly enough to hold these constituents in solution. Subsequent precipitation heat treatments allow controlled release of these constituents either naturally (at room temperature) or artificially (at higher temperatures).


An obsolete term, formerly used to describe the structure obtained (cementite precipitated in ferrite) when martensite is heavily tempered.

At the time the term was created, this structure was thought to be a distinct phase. However, it is now recognised that the same structure can be obtained in numerous different ways.

Sorbite was named after the British metallurgist H. C. Sorby.


Spalling is a variety of surface failure identified by the flaking of particles from a surface, and is usually the result of material fatigue, rolling pressure or corrosion.


The S-phase, also called expanded austenite, is a structure that can be obtained on austenitic or duplex stainless steel by interstitial super saturation of the metal lattice with carbon or nitrogen. The solution of massive amounts of carbon/nitrogen leads to compressive stresses that can be measured as increased hardness on the surface. The typical layer thickness, depending on material and the hardening process, ranges from 5 to 40 microns. Resulting benefits include an increase in abrasion resistance, improved fatigue life and the prevention of galling for austenitic material pairings.

See also Kolsterising.


This treatment involves subjecting steel to a selected temperature cycle usually within or near the transformation range in order to produce a suitable globular form of carbides for such purposes as:
(a) Improved machinability
(b) Facilitating subsequent cold working
(c) Obtaining a desired structure for hardening the steel

These treatments are frequently used on hypereutectoid steels to overcome grain boundary carbide networks, which are brittle and unsuitable for subsequent hardening of these high carbon steels (i.e. hypereutectoid steels contain more than 0.80% carbon.


Heat treatment carried out to prevent changes in structure and size with time. Classic examples include thermal stabilisation of nitriding steels and cryogenic (sub-zero) treatments to remove retained austenite on quenched hardenable steels.

Stainless steel

An alloy of iron containing at least 13% chromium, which will not rust under normal circumstances.

For optimum corrosion resistance, stainless steels should contain at least 18% chromium and 8% nickel.


Although containing many other elements in combination with iron, it is the carbon content of steel which is the most important, and is largely responsible for the wide range of properties which can be obtained. Steel heat treatments fall into two broad categories, namely softening processes which are mainly utilised as intermediate heat treatments and hardening processes applied as part of the finishing operations given to a component.

The majority of steel is hardened by heat treatments that involve quenching of the product from the austenitising temperature. Oil remains the most common quenchant and has associated risks, beyond its inherent flammability. The main one involves the ingress of water (possibly from a leaking cooling system). In small quantities, water in the oil can lead to component cracking. Larger volumes can produce foaming of the oil bath and the attached risk that the mixture can overflow and cause a fire. In extreme situations, sufficient water can explosively form water vapour within the oil and be a source for a major fire or explosion.


The ability of a material to resist changes to its shape when it is subject to a load.


Masking an area of a component to prevent case hardening or surface contamination during heat treatment.

Areas of components that must not be case hardened can have a special coating applied to prevent the controlled atmosphere from contacting the surface. Thus, no carbon or nitrogen is absorbed in those areas, which remain soft.

Stopping-off is usually carried out in either of two ways:
Plating the area to remain soft with copper (Cu), to a depth of 20µm to 25µm.
Painting the area to remain soft with a proprietary stopping-off paint.


The removal of distortion in heat treated components.

There are many straightening techniques but the most common is by means of a straightening press.

Sometimes, even with the most diligent care and the application of component distortion control, it is necessary to mechanically straighten heat treated components.


The ratio of the increase in length of a material under load to its original length.

Strain has no units because it is the extension divided by the original length.


The ability of a material to absorb an applied load without breaking.


The forces within a body (internal or residual stress) or external forces on a body (applied stress).

Stress is defined as the load per unit area and the normal units are newtons per square millimetre (N/mm2) or Megapascals (1 MPa = 1 N/mm2)

See also stress relieving.

Stress relief annealing

A low-temperature stress relieving process in which the time at temperature is followed by very slow cooling.

Some large components and those with thick and thin sections would cool at varying rates during rapid or uncontrolled cooling. This could result in too high a level of residual stress, even after the stress relieving operation. Controlled, slow cooling gives the lowest level of residual stress.

The term is sometimes used as a synonym for stress relieving.

Stress relieving

Heating below the transformation temperature in order to reduce or eliminate residual stresses in a component. Because no transformation has taken place, the cooling rate is not critical and is generally fairly rapid.

Castings and welded fabrications generally contain complex internal stress distributions, which arise from the thermal and material transformations which take place during the foundry and welding operations. If they are not rectified, such stress distributions may be disturbed during further manufacturing operations, leading to distortion or cracking of the components produced. With higher alloy steels and cast irons internal stress can cause distortion or cracking even before any further manufacturing operations are commenced. It is possible, by means of a thermal cycle, generally within the temperature range 550-650°C to reduce or remove the internal stress and render the work piece suitable for further manufacturing operations. Close control of the thermal cycle, ensuring temperature uniformity within the furnace and temperature distribution throughout the work piece is vital and multi-point probe thermocouples are routinely used for this.

Sometimes called stress relief annealing.


The arrangement of the various phases within a metal.

Short for crystal structure.

Sub-critical annealing

Sub-critical annealing consists of heating the steel to below the lower critical temperature. This type of annealing is mainly carried out in the temperature range 630° - 700°C to reduce hardness by allowing recrystallisation of the microstructure to occur. Alternatively, if a temperature in the range 690°C to 719°C is used, it is possible to spheroidise the cementite phase instead of forming lamellar pearlite consisting of platelets of ferrite and cementite. This technique is particularly useful with high carbon steels to optimise machinability.

The lower temperature sub-critical annealing treatments (550° - 600°C), are specifically used for stress relief of welded fabrications and to stabilise rough machined components which are to be ultimately hardened and tempered, case hardened, or nitrided and whose dimensional stability is critical.

Sub-zero treating

Holding steel components at a temperature below zero degrees Centigrade to obtain the required structure. The temperature used is usually between -70ºC and -196ºC and the process is always followed by tempering.

Sub-zero treatment is carried out in order to complete the transformation of retained austenite to martensite after hardening and before tempering. It is usually applied to high carbon, high alloy steels such as tool steels but is more-widely applied by aerospace companies to guarantee complete transformation.

In the early days of sub-zero treating, when large, low-temperature refrigerators were not available, the problem was how to get reproducible low temperature processing equipment. The answer was to add dry ice to a bath containing a suitable liquid such as industrial alcohol or trichloroethylene. With sufficient dry ice, the temperature of the liquid could be maintained at a temperature of -78.5ºC. Accordingly, most specifications require a temperature between -70ºC and -80ºC. Nowadays, with the ready availability of liquid nitrogen at -196ºC, many companies have based their sub-zero treating requirements on that lower temperature.

An unwelcome outcome of the hardening of some steels, which becomes more likely as the carbon and alloy content increases, is incomplete transformation to martensite during quenching. The resulting crystal structure contains retained austenite which renders the steel unstable as this austenite is able to transform over time leading to component distortion, as well as an increased risk of cracking. Cryogenic, or sub-zero treatments at temperatures down to -150°C are necessary, following hardening and tempering to cause the retained austenite to transform to martensite. A further tempering treatment at a temperature of 150- 180°C is then required to provide complete stabilisation. Cryogenic treatment is cost effective and regularly employed in the manufacturing cycle of critical components required for demanding applications.


Superalloys are alloys which have a number of properties that enable them to operate in high-performance environments such as the hot zones of turbine engines. They typically exhibit high temperature creep resistance, mechanical strength, phase stability and excellent fatigue life. Additionally, superalloys form a protective oxide layer when exposed to oxygen which gives oxidation and corrosion resistance.

The crystal structure of superalloys is typically austenitic face-centred cubic, and they are generally categorised into three main groups: cobalt-based, nickel-based and iron-based superalloys.

Superplastic forming

The controlled tensile deformation of a solid crystalline material, such as metal or ceramic, at elevated temperature, to form shape. For superplastic forming to take place, materials must have a fine grain structure and the ability to maintain this grain structure at higher temperatures. During forming, a superplastic sheet is subjected to gas pressure, to form a shape using a die.

See also superplasticity.


A property of some materials with fine grain structures, enabling high tensile deformation at elevated temperatures.

See also superplastic forming.

Surface engineering

The use of surface treatments to design a surface and core which together possess properties unachievable in either the core or surface materials alone.

Surface hardening

Several methods are available to provide surface hardening of components. When steels having a carbon content of 0.45%C and above are involved, surface hardening can be achieved by the use of induction or flame hardening methods. Low carbon steels, having carbon contents around 0.15%C may be case hardened by carburising and hardening, carbonitriding, nitrocarburising or nitriding.

When it is necessary to restrict surface hardening to a localised portion of a component’s surface it is possible to choose from several methods. If the end of a shaft or similar shaped component is the only area to be surface hardened, flame or induction methods can be employed with steels having 0.45% C and above. Case hardening steels can be treated in salt baths by immersion of the end only. Alternatively, the component can be carburised all over, annealed for machinability and then the surface to be kept soft can be re-machined to remove the carburised case, leaving the remaining carburised area to be hardened by re-austenitising and quenching. Another method involves carburising the component all over and induction or flame hardening the restricted area required to be hard. Another technique involves the use of electroplating (a fine grained copper deposit is necessary) to prevent carburising, or alternatively proprietary ‘stop-off’ paints containing copper salts may be used, which inhibit the diffusion of carbon into the steel, or those containing tin salts for similar use in nitriding.


Particles of metal produced during machining, drilling and grinding operations.


Top of page

Temper brittleness

The loss of ductility caused in certain steels when they are held in or slowly cooled through the temperature range 300º to 600ºC.

This effect is commonly seen in nickel-chromium steels and is due to the precipitation of carbides in the areas between the crystals in their structure (grain boundaries). It can be overcome by adding 0.2% to 0.3% molybdenum.

This effect is called temper brittleness since it occurs in the normal tempering range of steels.

See also nitriding.


A low temperature (150ºC to 650ºC) heat treatment intended to remove the stresses and brittleness caused by quenching and to develop the required mechanical properties.

Tempering high carbon, high alloy steels at temperatures about 550ºC will transform any retained austenite in their structure and must generally be followed by a second temper.

Tempering colours

The colour of a piece of polished steel after it has been tempered in air.

When steel is heated in air, a thin layer of iron oxide forms on its surface. The colour of this oxide varies with the temperature at which the steel is held and was previously used to judge the tempering temperature of tools.

See also hardening colours.


When it is necessary to harden and temper long thin components, such as blades for example for hedge-trimmers, it is possible to harden in furnace loads in which the blades are suspended vertically but are not restrained. The resulting slight bowing of the blades can be rectified by clamping them together between holding plates and tightening the pack to a precise pre-determined level of torque and then tempering the pack in the normal way. This technique is termed temper-setting and is often used for hardening and tempering clutch plates, washers and similar thin components.

Tensile strength

The maximum stress withstood by a material during a tensile test.

During a tensile test, the applied stress is continually increased until the test piece breaks. In practice, the stress rises to a maximum and then falls as the test piece begins to stretch prior to failing. This maximum value is used to determine the tensile strength. This is also known as the ultimate tensile strength.

The tensile strength of a metal can be improved by through hardening.

Tensile test

A mechanical test in which a test piece of the material is held in two jaws which are pulled apart until the test piece breaks.

The test determines both the strength of the material, based on the load required to break it, and the ductility, based on how much it stretches before it breaks.

See also tensile strength.

Test piece

One or more samples of the same material from which the component was made, and of comparable section with the component.

These are heat treated, together with the component, to provide specimens with properties that represent those of the component, which can be used for mechanical testing.

Theoretical density

The maximum achievable density of a particular element, compound or alloy, assuming no internal voids or contaminants. It is calculated from the number of atoms per unit cell and measurement of the lattice parameters.

Thermal barrier

A thermal barrier coating is a type of thermal spray coating used to reduce the rate of heat transfer to allow a coated component to operate at a higher temperature. An example of a component requiring this type of coating would be a gas turbine combustor.

Thermal deburring

A process using intense heat energy to remove small, often inaccessible burrs, originating as a result of machining. Parts are placed inside a sealed cylindrical chamber which is pressurised with a mixture of combustible gases including pure oxygen.

The gaseous mixture completely encloses the parts, reaching into even the most confined areas. When the mixture is ignited, a powerful combustion takes place generating intense heat which oxidises the burrs. Only the burrs are removed because the heat attacks areas with large surface area and very little mass.

Thermal diffusion

In the context of metallic coating, thermal diffusion describes the process of heating components in an airtight container in the presence of zinc powder. The zinc diffuses into the metal component forming a protective zinc-iron alloyed coating.

See also sherardizing.

Thermal expansion

An increase in the dimensions of a material caused by heating.

The material returns to its initial dimensions when it cools to its original temperature.

A bar of low alloy steel will increase by about 1% in length and about 3%, in volume, when it is heated from room temperature to its hardening temperature of about 900ºC.

Thermal processing

In the field of metallurgy, thermal processing is the collective name given to a variety of techniques and specialised engineering processes which use heat, pressure and applied materials to improve the properties of metals and alloys and extend the life of components.

Thermal spray

A group of processes in which finely divided metallic or non-metallic materials, usually in the form of powder, are deposited in a semi molten state to a substrate to form a thermal spray coating.

Metals, alloys, ceramics and composites can all be thermally sprayed, providing varying coating thicknesses from a few micrometres to millimetres thick.

See also plasma spray, arc spray, flame spray, HVOF, cold gas dynamic spray.


A chemical reaction or physical transformation involving heat and energy.


A device made by joining two different metals together, and used to measure the temperature in a furnace.

It consists of two wires of different metals or alloys, joined together at one end and enclosed in a protective sleeve. The junction of the wires is placed at the temperature which needs measuring and the wires produce a small voltage which is proportional to the difference between the temperature to be measured and room temperature. From the voltage measured, the actual temperature can be determined. The combination of wires determines the voltage produced and the maximum operating temperature of the thermocouple.

See also control thermocouple, load thermocouple and probe thermocouple.

Through hardening

Steels having a carbon content between 0.3% and 0.8% are capable of being through hardened. As the carbon content increases, so does the degree of hardness achievable. The depth to which a steel grade will fully harden depends upon the rate of quenching, with faster quenches in brine or water producing a deeper hardening effect than with oil, air or inert gas. The addition of alloying elements, such as manganese, nickel, chromium and molybdenum, increase the achievable hardening depth, i.e. the hardenability of the steel is thereby increased.

For every steel composition there is a limiting ruling section at which the specified combination of properties can be achieved. In parallel with hardening, the brittleness of the steel increases. This is the reason for the secondary treatment which follows hardening, which is termed tempering. The instability of steel in the as-hardened condition, due to the high level of internal stresses present, is prone to induce cracking. The cracking tendency increases with increasing hardenability and with the severity of the quenchant used in hardening. In order to relieve the internal stress produced in the microstructural change causing hardening (the formation of martensite), it is necessary to re-heat the quenched steel to a temperature below the martensite transformation finish temperature, suitable for the particular steel in question.

The cracking propensity increases with increasing hardness i.e. with increasing carbon and alloy content. Thus, tempering has to be carried out with as short a time delay as possible following hardening, particularly for tool steels. During tempering, in addition to stress relief, many steels undergo a further sub-microscopic structural change consisting of the precipitation of carbide particles from the martensite. Tempering produces a reduction in hardness and a corresponding improvement in ductility. The effect is both time and temperature dependant, with higher temperatures and longer soak times producing maximum reduction in hardness and increase in ductility. Ultimately with some steels, over-tempering can produce a breakdown of the martensite structure and the formation of a spheroidised carbide structure.

Low alloy steels are usually tempered in the range 450-650°C for the most useful combination of mechanical properties. Some high alloy tool steels exhibit secondary hardening during the tempering treatment, due to the precipitation of hard alloy carbides.

TIG welding

An arc welding process, Tungsten Inert Gas welding, also known as Gas Tungsten Arc welding, uses a tungsten electrode which is not consumed during the welding process. An inert shielding gas (commonly argon) is used to protect the weld area from atmospheric contamination, resulting in a clean weld. Filler metal may or may not be required.

Tin (Sn)

From the Anglo-Saxon word tin and Stannum, the Latin word for tin.

Titanium (Ti)

A silvery-coloured, strong but light metal element with the symbol Ti.

Titanium is a light, strong and corrosion-resistant transition metal. Its low density (60% as dense as steel) and ductility make it easy to work. Titanium is as strong as steel, but 43% lighter. Although it is 60% heavier than aluminium, it is twice as strong. Owing to its high strength-to-weight ratio and corrosion-resistance, it is used to make strong, light-weight alloys, usually by alloying with aluminium and vanadium, for use in aerospace and other critical applications.

Titanium forms a wide range of colourful, passive and protective oxide coatings when exposed to air at elevated temperatures but at room temperatures it resists tarnishing. The metal, which burns when heated in air 610°C or higher (forming titanium dioxide), is one of the few elements that will burn in pure nitrogen gas (at 800°C, forming titanium nitride). It is paramagnetic (weakly attracted to magnets) and has a very low electrical and thermal conductivity.

The metal is a dimorphic allotrope with the hexagonal alpha form changing into the cubic beta form very slowly at around 880°C. When it is hot the metal will absorb nitrogen, hydrogen and oxygen.

Properties: Melting point 1668°C
Density 4.506 g/cm3 (Water = 1)

Discovered in 1871 by the Reverend William Gregor and named after the Titans, the sons of the Earth goddess Gaea, in Greek and Roman mythology.


The capacity of a material to withstand a load without breaking.

Toughness is generally measured in terms of the energy it absorbs before it breaks.


A change from one phase to another as the temperature increases or reduces.

Some metals have different crystal structures (also known as phases) at different temperatures, even though they remain solid at these temperatures. The change from one structure to another is called a transformation. The temperature at which the transformation takes place is called the transformation temperature.

It is this property of iron, with its ferrite and austenite phases, which allows steel to be heat treated so readily. At high temperatures, the steel is transformed to its austenite phase. When austenite is quenched rapidly, it forms very hard martensite.

Certain transformations occur at a single temperature and composition and give a particular transformation product. These have specific names such as eutectoid transformation.

Transformation temperature

The temperature at which a solid metal changes from one phase to another.

In alloys, steel for example, this change generally occurs over a range of temperatures (known as the transformation range) rather than at a single temperature. The upper and lower transformation temperatures denote the limits of the transformation range.

Only the named transformations, such as the eutectoid transformation, take place at a single temperature and composition.


A liquid chlorinated hydrocarbon with the chemical formula CHCl:CCl2.

Trichloroethylene (often shortened to trike) was the most widely-used degreasing solvent but has recently been classified as a carcinogen. It is now being replaced by other, less harmful solvents or completely different cleaning systems. Insoluble in water and non-inflammable.

Properties: Melting point -85°C
Boiling point 87°C
Relative density 1.46 (Water = 1)
Vapour density 4.5 (Air = 1)


A short form of trichloroethylene.


An obsolete term, formerly used to describe the structure obtained when martensite is lightly tempered.

At the time the term was created, this structure was thought to be a distinct phase. The structure is now known to be cementite precipitated in ferrite, however, the precipitate is so fine that it can not be seen clearly using an optical microscope.

Troostite was named after the French chemist Louis J. Troost.

TS 16949

An automotive industry standard developed by the larger automotive OEM’s (Original Equipment Manufacturers), which is linked to ISO 9001:2008. TS 16949 addresses automotive requirements through a specifically focused approach to process and improvement, as they affect the automotive industry. TS 16949 is controlled by the Automotive Industry Action Group (AIAG), which is part of SAE (Society of Automotive Engineers).

See also CQI-9.

Tungsten (W)

A pale gray metal, found only in chemical compounds, with the chemical symbol W. Tungsten has the second highest melting point, after carbon, of any element. It also has excellent tensile strength. These properties make tungsten particularly useful for high temperature applications and in superalloys.

See also tungsten carbide.

Tungsten carbide

A very hard carbide of tungsten with the formula WC.

Tungsten carbide was also known as cemented carbide, or hardmetal. Tools of the material are made by "cementing" the very hard tungsten carbide particles together using a binder of tough cobalt metal, thus giving rise to its former name cemented carbide.


Turning is a machining process, which can be carried out manually or by an automated CNC lathe. Turning uses a single point cutting tool to cut and shape a rotating workpiece, either on an external or internal surface.


Top of page

Ultimate tensile strength

See tensile strength.

Ultrasonic cleaning

Cleaning in a solvent through which very high frequency vibrations are being passed.

Ultrasonic means that the vibrations are at a frequency above the level normally audible to humans. In fact, a high-pitched buzzing sound can normally be heard.

The ultrasonic vibrations are transmitted very well by liquids and act by vibrating the particles of dirt off the surface of the components.

Ultrasonic inspection

A non-destructive testing method used to detect surface and subsurface defects or to characterise materials. The technique uses high-frequency sound waves which travel through the material and reflect beams when they encounter flaws or irregularities.

Upper critical temperature

The temperature at which, on cooling, an alloy undergoes a solid state transformation from austenite to ferrite. The upper critical temperature varies between alloys, and is dependent on the carbon content.


Top of page

Vacuum brazing

The use of vacuum furnaces for brazing is very well established, especially for brazing complex assemblies in stainless steels or nickel alloys. The method allows flux free brazing and produces ultra clean assemblies which require no post braze cleaning. A variety of braze alloys are used, including copper based, gold based and nickel based alloys. These enable a range of the higher temperature materials to be brazed, using brazing temperatures between 1000°C and 1200°C. The vacuum environment provides ideal conditions for the braze alloy to wet the surfaces of the joint and allow capillary action to draw the braze into filling the whole joint. Care and expertise is required to calculate the effect of thermal expansion of the mating parts upon the joint gap. Each braze alloy has an optimum gap filling capability. If the gap is too wide it encourages the formation of shrinkage voids and the precipitation of inter-metallic compounds at the centre of the cooling joint, thus weakening it. If the gap is too narrow, capillary action will not be able to fill the joint, resulting in a dry joint and, again, a weak result.

The close control of the heating cycle and temperature uniformity, which is provided by the radiation heating under vacuum conditions, ensures that all the assembly reaches braze temperature at the same time and thus prevents uneven stress distribution and hence results in a high integrity joint with minimal internal stress. This temperature uniformity, which can be as close as +/- 2°C throughout the furnace chamber, also allows batches of similar assemblies to be brazed together, thus capitalising upon the economic benefits of using large vacuum furnaces. Hence, this high capital cost method has become cost effective for a wide variety of parts.

As with other methods of brazing, the fixturing of the assemblies prior to brazing is important and in some cases a precision engineered jig is used to hold the assembly throughout the braze cycle. Such jigs may be made from ceramics, graphite or heat resisting alloys. Positional TIG welding is also routinely employed for positioning the elements of the assembly to be brazed. The braze alloy may be applied as a paste, powder, foil or wire, depending upon the joint design used.

Vacuum carburising

Vacuum carburising has reached industrial maturity with the development of vacuum furnaces and controls capable of gas carburising and quenching the carburised components using oil or pressurised inert gas. Due to their highly controllable heating rates and the availability of high carburising temperatures (950/1030°C), vacuum processes are finding an economic application for medium and deep case treatments. These methods have the advantage that treated components remain stationary throughout the process and the risks of component damage due to movement of hot components are eliminated. The surface and case chemistry can be very closely controlled, as can case depths, to within very tight limits and, as with all vacuum processes, treated components are kept clean. Savings can therefore be made in post heat treatment finishing operations, which more than offset the slightly higher treatment costs of these carburising methods. Whilst there is a need for careful tailoring of the process parameters for each design of component to be treated, the vacuum methods provide for much closer control of case depth range, uniformity and case chemistry than the other case hardening methods.

See also low pressure carburising.

Vacuum heat treatment

A theoretical or ideal vacuum is an empty space that does not contain either vapours, particles, gases or other matter, and as a consequence has no absolute pressure. Because this condition does not exist, even in outer space, an ideal vacuum cannot be achieved.

Normally when the term vacuum is used it refers to an absolute pressure below that of a normal atmosphere. Normal atmospheric pressure is 14.7 lb/sq in, commonly termed 1 Bar. Nowadays vacuum gauges measure pressures in millibars (mbar) where 1000 mbar = 1 Bar. For use in vacuum heat treatment operating pressures are classified as:

  • Rough vacuum: 100mbar to 10-1mbar
  • Fine vacuum: 10-1 to 10-4mbar
  • High vacuum: less than 10-4mbar

Most vacuum heat treatment is carried out in fine to high vacuum.

With the development of vacuum technology it has become possible, by means of an array of roughing pumps, rotary pumps and diffusion pumps, to progressively evacuate a furnace chamber to high vacuum conditions, reducing the available oxygen to miniscule levels. The resulting environment is unreactive, even to alloys of titanium which are especially prone to oxidation. For all grades of steel, including those requiring high temperature austenitisation, such as high speed steels at 1320°C and all nickel alloys, vacuum heat treatment is the optimum method.

For those alloys which require quenching for hardening, such as steels, or quenching during solution treatment, such as some nickel alloys and stainless steels, integral quench systems have been developed based upon oil or inert gas. Various quenching rates can be obtained by delivering the inert gas to the furnace chamber at a pressure of up to 20 bar. Provision is made in some furnaces for alternating the direction of flow of the quenching gas from top to bottom of the furnace load and the reverse. Thus, steels of relatively low hardenability, such as low alloy engineering steels can be fully hardened. Since the work pieces remain stationary in the furnace chamber throughout heating and quenching there is no risk of component damage due to work movement at high temperatures.

Multi zone heating is provided by electrically heated elements surrounding the furnace chamber. The elements are made of graphite or high nickel alloys and the furnace chamber is surrounded by heat shields made from molybdenum and backed by stainless steels and insulating media such as ceramics. Temperature uniformity throughout the furnace chamber can be controlled to very tight limits, +/- 2°C at temperatures of 1300 - 1350°C.

Vacuum heat treatment is the cleanest and most environmentally friendly of all hardening methods and, as the size of furnaces has increased and computerised process controls are now standard, treatment economics are increasingly attractive. Tempering following hardening can be carried out in vacuum furnaces evacuated to low pressures, using roughing and rotary pumps only, since the risk of oxidation is less due to the lower temperatures employed.

Vacuum nitrocarburising

Vacuum nitrocarburising and low pressure nitrocarburising are alternative nitrocarburising treatment methods which have the advantages of superior process control and cleanliness, typical of the vacuum option.

Vapour degreasing

Cleaning material by submersing it in the hot vapour blanket formed above the boiling solvent in a specially-designed plant.

The principal involved is that the hot vapour condenses on the cold surface of the component, dissolving any soluble contaminants and flushing the insoluble ones off. Once the component reaches the temperature of the vapour, condensation stops and the cleaning process is at an end.

Vickers test

See hardness test.


Top of page

Water quenching

Hardening of plain carbon steels demands a very fast quench from the austenitising temperature and water, (or brine solution when an even more drastic quench is required, such as with heavier sections), provides an economic method. Large components, many up to several tons in weight, such as pipeline fittings and housings for the oil & gas industry are routinely water quenched. The external cooling systems required are a significant aspect of this method nowadays, when environmental and cost considerations have resulted in the early ‘throw away’ water discharge systems being no longer used. Quench agitation and flow controls are also critical to ensure that uniform hardening is achieved.


See also arc welding, electron beam welding, TIG welding.

White layer

The surface of nitrided steel which has been converted to a complex iron-nitrogen compound.

It is called white layer because it does not etch (i.e. remains white) when a nitrided microstructure is prepared.

During the nitriding cycle (the length of which is dictated by the case depth required), a surface coating is produced on the component, known as the 'white layer', Fe4N. This tends to be brittle and is often better removed after nitriding by polishing, an allowance of 0.002˝ per surface is usually sufficient for this.

Work hardening

The increase in strength (and therefore hardness) that occurs when metals are deformed in the cold state.

This is generally an unavoidable by-product of certain cold working processes such as rolling, drawing, pressing and spinning. However, it is produced intentionally by shot peening.

This effect can be completely removed by full annealing or partially removed by process annealing or normalising.


Top of page


An organic barrier coating developed in multiple colours and coating thicknesses for extreme salt spray resistance in environments such as those experienced by automotive components and offshore pipelines.


Top of page

Yield point

The point at which elongation (strain) increases without an accompanying increase in stress.

Only a few materials (notably steels) have a yield point and generally only under tensile loading.

Young’s Modulus

The resistance of a material to elastic deformation.

Also known as the modulus of elasticity. It is the applied tensile stress to the resulting strain. Young’s Modulus (E) = Stress/Strain N/mm2


Top of page


From the German word zink.