Confused by a word or phrase you see here or on other related sites? We've put together a comprehensive list of industry terms and definitions.

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Acicular Ferrite

A highly substructed nonequaixed ferrite formed upon continuous cooling by a mixed diffusion and shear mode of transformation that begins at a temperature slightly higher than the transformation temperature range for upper bainite in that it has a limited amount of carbon available; thus, there is only a small amount of carbide present.

Acid Embrittlement

A form of hydrogen embrittlement that may be induced in some metals by acid treatment.

Acid Steel

Steel produced in a furnace with an acid lining, i.e. consisting of a siliceous refractory and under a siliceous slag. With an acid slag, carbon, silicon and manganese only are removed so that the pig iron must not contain sulfur and phosphorus in percentages exceeding those permissible for the specification being made. Most steel manufactured today is in furnaces with basic linings.

Age Hardening

Hardening by aging, usually after rapid cooling or cold working.

Age Softening

Spontaneous decrease of strength and hardness that takes place at room temperature in certain strain-hardened alloys, especially those of aluminum.


Aging is a structural change, usually by precipitation, that occurs in some alloys after a preliminary heat treatment or cold working operation. Aging may take place in some alloys at room temperature in moderate time (days) or in others may be done in shorter time at furnace temperatures. Over-aging may be done at a temperature above normal to produce some desirable modification of physical properties.

Air-Hardening Steel

An alloy steel which will form martensite and develop a high hardness when cooled in air from its proper hardening temperature. Sometimes referred to as self- hardening steel. A steel that becomes fully hardened when cooled in air from above its critical point and does not require rapid quenching by oil or water. The risk of distortion is greatly reduced by air hardening. High Speed Steel was one of the earliest examples of this type of steel.


The property possessed by certain elements to exist in two or more distinct forms that are chemically identical but have different physical properties. In the case of iron the crystal structure has one form at room temperature and another at high temperature. When heated above 910°C the atomic structure changes from body centered cubic to face centered cubic but reverts again when cooled. The allotropy of iron modifies the solubility of carbon, and it is because of this that steel can be hardened.

Alloy Steel

A steel to which one or more alloying elements other than carbon have been deliberately added (e.g. chromium, nickel, molybdenum) to achieve a particular physical property.

Alpha Iron

The body centered cubic form of iron which, in pure iron, exists up to 910°C.


Forming a corrosion and oxidation-resistant coating on a metal by coating with aluminum and usually diffusing to form an aluminum-rich alloy.

Anneal to Temper

A final partial anneal that softens a cold worked nonferrous alloy to a specified level of hardness or tensile strength.


Heating steel to, and holding at a suitable temperature, followed by relatively slow cooling. The purpose of annealing may be to remove stresses, to soften the steel, to improve machinability, to improve cold working properties, to obtain a desired structure. The annealing process usually involves allowing the steel to cool slowly in the furnace.

Annealing, Finish

sub-critical annealing treatment applied to cold-worked low-or medium-carbon steel. Finish annealing, which is a compromise treatment, lowers residual stresses, thereby minimizing the risk of distortion in machining while retaining most of the benefits to machinability contributed by cold working.

Annealing, Intermediate

Annealing wrought metals at one or more stages during manufacture and before final treatment.

Annealing, Partial

An imprecise term used to denote a treatment given to cold-worked material to reduce their strength to a controlled level or to affect stress relief. To be meaningful, the type of material, the degree of cold work, and the time-temperature schedule must be stated.

Arc Furnace

A steel melting furnace in which heat is generated by an arc between graphite electrodes and the metal. Both carbon and alloy steels are produced in electric arc furnaces and scrap rather than molten metal is used as the base material. Furnaces with capacities up to 200 tons are now in use.


The gaseous environment in which the metal being treated is heated for processing. Atmospheres are used to protect from chemical change or to alter the surface chemistry of steel through the addition or removal of carbon, nitrogen, hydrogen, and oxygen and to add certain metallic elements as chromium, silicon, sulfur, etc.


Quenching from a temperature above the transformation range to a temperature above the upper limit of martensite formation, and holding at this temperature until the austenite is completely transformed to the desired intermediate structure, for the purpose of conferring certain mechanical properties.


This is the name for any solid solution in which gamma iron is the solvent. It is a phase in steel where it consists of face-centered cubic iron with carbon in solid solution. It is non-magnetic and unstable at room temperature. Austenite is a structure name and means nothing as to composition. It is the structure from which all quenching heat treatments must start.

Austenitic Grain Size

The size attained by the grains of steel when heated to the austenitic region; may be revealed by appropriate etching of cross sections after cooling to room temperature.

Austenitic Steels

Steels containing high percentages of certain alloying elements such as manganese and nickel which are austenitic at room temperature and cannot be hardened by normal heat-treatment but do work harden. They are also non-magnetic. Typical examples of austenitic steels include the 18/8 stainless steels and 14% manganese steel.


Forming austenite by heating a ferrous alloy into the transformation range (partial austenitizing) or above the transformation range (complete austenitizing). When used without qualification, the term implies complete austenitizing.

Austenitizing Temperature

The temperature at which a steel is substantially all austenite.


Chemical symbol for Boron.


An acicular aggregate of ferrite and carbide particles formed when austenite is transformed on cooling at temperatures in the intermediate (200-450°C) range, i.e. above the martensite and below the pearlite range.

Balanced Steel

Steels in which the deoxidisation is controlled to produce an intermediate structure between a rimmed and killed steel. Sometimes referred to as semi- killed steels, they possess uniform properties throughout the ingot and amongst their applications are boiler plate and structural sections.

Banded Structure

A layering effect that is sometimes developed during the hot rolling of steel.


An older term used to describe the decarburized skin that develops on steel bars heated in a non-protective atmosphere.

Base Metal

A metal which oxidizes when heated in air, e.g. lead, copper, tin, zinc, as opposed to noble metals such as gold and platinum.

Basic Steel

Steel produced in a furnace in which the hearth consists of a basic refractory such as dolomite or magnesite, as opposed to steel melted in a furnace with an acid lining. The basic process permits the removal of sulfur and phosphorous and in this respect is superior. Present day BOS and electric arc furnaces use basic linings.

Batch Furnace

A furnace used to heat-treat a single load at a time. Batch-type furnaces are necessary for large parts such as heavy forgings and are preferred for complex alloy grades requiring long cycles.


Chemical symbol for Beryllium.

Belt Furnace

A continuous-type furnace that uses a mesh-type or cast-link belt to carry parts through the furnace.

Bend Test

Bending tests are carried out to ensure that a metal has sufficient ductility to stand bending without fracturing. A standard specimen is bent through a specified arc and in the case of strip, the direction of grain flow is noted and whether the bend is with or across the grain.

Bessemer Process

A method of producing steel, first introduced in the last century, where air is blown under pressure through molten iron to remove the impurities by oxidation. The development of the process has led to the present day Basic Oxygen Steel-making plants that account for bulk production of commercial quality steels in the UK.

Beta Annealing

Producing a beta phase by heating certain titanium alloys in the temperature range of which this phase forms followed by cooling at an appropriate rate to prevent its decomposition.


Chemical symbol for Bismuth.


A section of steel used for rolling into bars, rods and sections. It can be a product of the ingot route, or increasingly today produced directly by continuous casting.

Binary Alloy

An alloy containing only two component elements.

Black Oxide

A black finish on a metal produced by immersing it in hot oxidizing salts or salt solutions.

Blast Furnace

A tall, cylindrical, refractory lined furnace for the production of pig iron or hot metal for direct conversion into steel.


A large square section of steel intermediate in the rolling process between an ingot and a billet. Blooms are now also being produced by the continuous casting process eliminating the necessity of first producing an ingot.

Boron Steels

The addition of boron in the range 0.0005- 0.005% to certain steels increases the hardenability. A range of boron steels is now listed in the current BS 970 and are widely used for the production of cold headed fastenings.


Brazing is a method of joining metal parts together by fusing a layer of brass between the adjoining surfaces. A red heat is necessary and a flux is used to protect the metal from oxidation.


Creases or ridges usually in “untempered” or in aged material where the yield point has been exceeded. Depending on the origin of the break, it may be termed a cross break, a coil break, an edge break, or a sticker break.

Bright Annealing

An annealing process that is carried out in a controlled atmosphere furnace or vacuum in order that oxidation is reduced to a minimum and the surface remains relatively bright.

Bright Drawing

The process of drawing hot-rolled steel through a die to impart close dimensional tolerances, a bright, scale-free surface, and improved mechanical properties. The product is termed bright steel.

Brinell Hardness Test

The Brinell hardness test for steel, involves impressing a ball 10 mm diameter, of hard steel or tungsten carbide, with a loading of 3000 kilograms into the steel surface. The hardness of the steel is then determined by measurement of the indentation. For steels with a hardness over 500 BHN the Vickers test is more reliable.

Brittle Tempering Range

Some hardened steels show an increase in brittleness when tempered in the range of about 450°F to 700°F even though some tempering causes some softening.


Chemical symbol for Carbon.


Chemical symbol for Calcium.


In the form of calcium silicide acts as a deoxidizer and degasifier when added to steel. Recent developments have found that carbon and alloy steels modified with small amounts of calcium show improved machinability and longer tool life. Transverse ductility and toughness are also enhanced.

Car Furnace

A batch-type furnace using a car on rails to enter and leave the furnace area. Car furnaces are used for lower stress relieving ranges.


A case-hardening process in which steel components are heated in an atmosphere containing both carbon and nitrogen.


A nonmetallic element that is the principal hardening element present in all steels. In general, increased carbon content reduces ductility but increases tensile strength and the ability of the steel to harden when cooled rapidly from elevated temperatures. At temperatures below 700°C, carbon is present in steel as iron carbide, cementite, Fe3C. The cementite forms lamellae, which “reinforces” the iron. This explains why a steel with a high-carbon content is harder than one with a low-carbon content. Generally, 0.05 to 0.25% is considered low-carbon steel, 0.25 to 0.5% is medium-carbon and 0.5 to 0.9% is high-carbon. The mixture of ferrite and cementite in a grain is called pearlite. Ductility and weldability decrease with increasing carbon content. Austenite can dissolve up to 2% carbon. When cooled in water or oil, carbon stays in a supersaturated solution of martensite.

Carbon Potential

A measure of the ability of an environment containing active carbon to alter or maintain, under prescribed conditions, the carbon level of the steel. NOTE: In any particular environment, the carbon level attained will depend on such factors as temperature, time and steel composition.

Carbon Restoration

Replacing the carbon lost in the surface layer from previous processing by carburizing this layer to substantially the original carbon level. Sometimes called recarburizing.

Carbon Steel

A steel whose properties are determined primarily by the amount of carbon present. Apart from iron and carbon, manganese up to 1.5% may be present as well as residual amounts of alloying elements such as nickel, chromium, molybdenum, etc. It is when one or more alloying elements are added in sufficient amount that it is classed as an alloy steel. Also known as ordinary steel, straight carbon steel, or plain carbon steel.


The introduction of carbon into the surface layer of a steel that has a low carbon content. The process is carried out by heating the components in a solid liquid, or gaseous carbon containing medium. The depth of penetration of carbon into the surface is controlled by the time and temperature of the treatment. After carburising it is necessary to harden the components by heating to a suitable temperature and quenching.


The surface layer of a steel whose composition has been changed by the addition of carbon, nitrogen, chromium, or other material at high temperature.


The process of hardening the surface of steel whilst leaving the interior unchanged. Both carbon and alloy steels are suitable for case-hardening providing their carbon content is low, usually up to a maximum of 0.2%. Components subject to this process, particularly in the case of alloy steels, have a hard, wear-resistant surface with a tough core.

Cast Iron

A definition can be applied that Cast Iron is an alloy of iron and carbon in which the carbon is in excess of the amount that can be retained in solid solution in austenite at the eutectic temperature. Carbon is usually present in the range of 1.8% to 4.5%, in addition, silicon, manganese, sulfur and phosphorus are contained in varying amounts. Various types of cast iron are covered by a British Standard classification and includes grey, malleable and white irons. Elements such as nickel, chromium, molybdenum, vanadium can be added to produce alloy cast irons.

Cast Steel

A term originally applied to crucible steel and sometimes today used to describe tool steels. The term is misleading and is falling into misuse. It can also be applied to steel castings made by pouring molten steel into a mould but which are not subject to further forging or rolling.


Chemical symbol for Columbium.


Chemical symbol for Cerium.


An iron carbide (Fe3C) constituent of steel. It is hard, brittle and crystalline. Steel which has cooled slowly from a high temperature contains ferrite and pearlite in relative proportions varying with the chemical composition of the steel. Pearlite is a lamellar structure of ferrite and cementite.

Charpy Test

A test to measure the impact properties of steel. A prepared test piece, usually notched, is broken by a swinging pendulum. The energy consumed in breaking the test piece is measured in Joules. The more brittle the steel the lower the impact strength. Izod is a similar and more widely used impact test in this country. Both are quoted in the current edition of BS 970.


When used as an alloying element, chromium increases the hardenability of steel and in association with high carbon gives resistance to wear and abrasion. Chromium has an important effect on corrosion resistance and is present in stainless steels in amounts of 12% to 20%. It is also used in heat-resisting steels and high duty cast irons.


Chemical symbol for Cobalt.


An alloying element used in tool, magnet and heat resisting steels. Together with tungsten and molybdenum, cobalt is used to form the super high speed steels. It improves the red hardness value of the steel, that is, it enables the steel to resist softening at a high temperature or in the case of a cutting tool to hold its edge under severe conditions.

Coefficient of Expansion

The ratio of change in length, area, or volume per degree to the corresponding value at a standard temperature.


An intermediate rolling process when a hot ingot is reduced to a bloom or slab in a cogging mill.

Cold Drawing

The process of reducing the cross sectional area of wire, bar or tube by drawing the material through a die without any pre-heating. Cold drawing is used for the production of bright steel bar in round square, hexagonal and flat section. The process changes the mechanical properties of the steel and the finished product is accurate to size, free from scale with a bright surface finish.

Cold Treatment

Exposing to suitable subzero temperatures for the purpose of obtaining desired conditions or properties such as dimensional or microstructural stability. When the treatment involves the transformation of retained austenite, it is usually followed by tempering.

Cold Working

Altering the shape or size of a metal by plastic deformation. Processes include rolling, drawing, pressing, spinning, extruding and heading, it is carried out below the recrystallisation point usually at room temperature. Hardness and tensile strength are increased with the degree of cold work whilst ductility and impact values are lowered. The cold rolling and cold drawing of steel significantly improves surface finish.

Compressive Strength

The maximum compressive stress that a material is capable of developing, based on original area of cross section. If a material fails in compression by a shattering fracture, the compressive strength has a very definite value. If a material does not fail in compression by shattering fracture, the value obtained for compressive strength is an arbitrary value depending upon the degree of distortion that is regarded as indicating complete failure of the material.

Contact Corrosion

When two dissimilar metals are in contact without a protective barrier between them and they are in the presence of liquid, an electrolytic cell is created. The degree of corrosion is dependent on the area in contact and the electro-potential voltage of the metals concerned. The less noble of the metals is liable to be attacked, i.e. zinc will act as a protector of steel in sea water whereas copper or brass will attack the steel in the same environment.

Contact Fatigue

Cracking and subsequent pitting of a surface subjected to alternating Hertzian stresses such as those produced under rolling contact or combined rolling and sliding. The phenomenon of contact fatigue is encountered most often in rolling-element bearings or in gears, where the surface stresses are high due to the concentrated loads and are repeated many times during normal operation.

Continuous Casting

A method of producing blooms, billets and slabs in long lengths using water cooled moulds. The castings are continuously withdrawn through the bottom of the caster whilst the teeming of the metal is proceeding. The need for primary and intermediate mills and the storage and use of large numbers of ingot moulds is eliminated. The continuous casting process is also used in the production of cast iron, aluminium and copper alloys.

Continuous Type Furnace

A furnace used for heat-treating materials that progress continuously throughout the furnace, entering one door and being discharged from another.

Control Zone

A portion of the working zone of a piece of thermal processing equipment having a separate sensor/instrument/heat input or output mechanism to control its temperature.

Controlled Atmosphere

A gas or mixture of gases in which steel is heated to produce or maintain a specific surface condition. Controlled atmosphere furnaces are widely used in the heat treatment of steel as scaling and decarburisation of components is minimized by this process.

Controlled Cooling

Cooling from an elevated temperature in a pre-determined manner, to avoid hardening, cracking, or internal damage, or to produce desired microstructure or mechanical properties.

Cooling Curve

A curve showing the relation between time and temperature during the cooling of a material.

Cooling Stresses

Residual stresses resulting from non-uniform distribution of temperature during cooling.


In the case of steel this refers to a component that has been case-hardened where the centre is softer than the hard surface layer or case. It can also be applied to the central part of a rolled rimming steel.

Correction Factor

That number of degrees, determined from the most recent calibration, which must be added to, or subtracted from, the temperature reading of a sensor, or an instrument, or a combination thereof (system) to obtain NIST true temperature. When expressed as a percent, it means percent of reading. The correction factors of sensors and instruments are usually kept separately and added together algebraically when a combination is used.

Corrosion Embrittlement

The severe loss of ductility of a metal resulting from corrosive attack, usually intergranular and often not visually apparent.

Corrosion Fatigue

Fatigue that arises when alternating or repeated stress combines with corrosion. The severity of the action depends on the range and frequency of the stress, the nature of the corroding condition and the time under stress.


Chemical symbol for Chromium.


The dimensional change with time of a material under a mechanical load. The flow or plastic deformation of metals held for long periods of time at stresses lower than the normal yield strength. Plastic deformation that proceeds slowly and continuously when stress is applied at elevated temperatures. In steel, creep is negligible below about 300°C (572°F). Methods of creep testing involve the determination of strain/time curves under constant tensile load and at constant temperature.

Creep Strength

The constant nominal stress that will cause a specified quantity of creep or secondary creep in a given time at a constant temperature. The resulting rate of continuous deformation is often expressed as PSI to produce 0.1 percent elongation in 10,000 hours at the temperature indicated.

Critical Cooling Rate

The slowest rate of cooling required to suppress phase changes. The term is most usually applied to the rate of quenching various products that are heated or “worked.” For example, the rate required to produce a martensic structure in the hardening of steel. The critical cooling rate in patenting is selected to provide minimum pearlitic spacing.

Critical Diameter

(D) Diameter of the bar that can be fully hardened with 50% martensite at its center.

Critical Point

This generally refers to a temperature at which some chemical or physical change takes place. These transformations cause evolution of heat on cooling or absorption of heat on heating and appear as discontinuities or arrest points in the heating and cooling curves. The temperatures vary with the carbon content of the steel and the rate of cooling.

Critical Range

The temperature range between an upper and lower critical point for a given material.

Critical Temperature

The temperature at which some phase change occurs in a metal during heating or cooling, i.e. the temperature at which an arrest or critical point is shown on heating or cooling curves.

Crystalline Fracture

A type of fracture that appears bright and glittering, it having formed along the cleavage planes of the individual crystals. Normally an indication that brittle fracture has occurred.


nbsp;(1) The separation, usually from a liquid phase on cooling, of a solid crystalline phase. (2) Sometimes erroneously used to explain fracturing that actually has occurred by fatigue.


Chemical symbol for Copper.

Cyanide Hardening

A process of introducing carbon and nitrogen into the surface of steel by heating it to a suitable temperature in a molten bath of sodium cyanide, or a mixture of sodium and potassium cyanide, diluted with sodium carbonate and quenching in oil or water. This process is used where a thin case and high hardness are required.

Cycle Annealing

An annealing process employing a predetermined and closely controlled time-temperature cycle to produce specific properties or microstructures.

Dead Soft

A temper of nonferrous alloys and some ferrous alloys corresponding to the condition of minimum hardness and tensile strength produced by full annealing.


A term used in reference to the absorption of heat without a corresponding increase in temperature, when steel is heated through the critical points (phase changes).


The loss of carbon from the surface of steel as a result of heating in a carbon weak atmosphere. During the rolling of steel hot surfaces are exposed to the decarburizing effects of oxygen in the atmosphere and as a result the surface is depleted of carbon. In steels where the components are to be subsequently heat treated it is necessary to remove the decarburized surface by machining.

Delta Iron

When pure or practically carbon-free iron is cooled from above its melting point it solidifies at about 1535ºC as delta iron having a body-centered cubic lattice structure, which persists down to about 1400ºC. On further cooling it undergoes an allotropic change to gamma iron which has a face-centered cubic lattice and is non-magnetic.


A crystal that has a treelike branching pattern most evident in cast metals slowly cooled through the solidification range.


Elements such as silicon and aluminum when added to molten steel react to form stable oxides and reduce the amount of dissolved oxygen. The solubility of oxygen in steel is reduced as temperature is lowered during solidification and the excess oxygen combines to form carbon monoxide. If the molten metal is not deoxidized the effervescence produced by the evolution of carbon monoxide during solidification would result in blow holes and porosity. Steel treated in this way is termed, "Killed Steel".


It is necessary to remove the scale from hot rolled bars or coil before bright drawing. This is normally carried out by shot blasting or pickling in acid. Other methods of descaling steel products include sand blasting, flame descaling and tumbling.


A process of burning out defective areas on the surface of ingots, blooms or billets. The condition of the surface is such that it can then be rolled or forged into a satisfactory product.

Dew Point

The temperature and pressure at which a gas begins to condense to a liquid.

Dew Point Analyzer

An atmosphere monitoring device that measures the partial pressure of water vapor in an atmosphere.

Diamond Pyramid Hardness Test

This test, more commonly known as the Vickers test, finds greater use in the laboratory than the workshop. It employs a pyramid shaped diamond with an included angle of 136° which is impressed into the specimen using loads of 5 to 120 kg making a small square impression. This test is used for finished or polished components because the impression can be very small. The diamond pyramid hardness number is obtained from a calculation based on measuring the diagonals of the impressions in the steel.


The term die is most commonly used in tooling, i.e. press tools "punch and die" but there are many other types of die, e.g. thread cutting dies, forming dies, forging dies, die-casting dies, etc. The term when applied to steel often refers to drawing dies through which hot rolled wire and bar are drawn to produce the finish and dimensional accuracy that is required for bright steel.

Direct Quenching

nbsp;(1) Quenching carburized parts directly from the carburizing operation. (2) Also used for quenching pearlitic malleable parts directly from the malleablizing operation.

Direct-Fired Tunnel-Type Furnace

A continuous-type furnace where the work is conveyed through a tunnel-type heating zone, and the parts are hung on hooks or fixtures to minimize distortion.


A discontinuity in the crystal lattice of a metal. The movement of dislocations under stress may be used to explain slip, creep, plastic yielding, etc.


The chemical breakdown of a compound into simpler compounds or elements. One of the most common examples is the dissociation of ammonia (NH3) into nitrogen and hydrogen.


A natural carbonate of calcium and magnesium generally used as a flux in blast furnaces.

Double Aging

Employment of two different aging treatments to control the type of precipitate formed from a supersaturated matrix in order to obtain the desired properties. The first aging treatment, sometimes referred to as intermediate of stabilizing, is usually carried out at higher temperature than the second.

Double Tempering

A treatment in which a quench-hardened ferrous metal is subjected to two complete tempering cycles, usually at substantially the same temperature, for the purpose of ensuring completion of the tempering reaction and promoting stability of the resulting microstructure.


The process of pulling metal wire, rods, or bars through a die with the effect of altering the size, finish and mechanical properties. In the USA, it is a term used for tempering.

Drop Forging

An operation in which a metal shape is formed by forcing hot metal into impressions formed in solid blocks of hardened alloy steel, the forging dies. The dies are made in halves, one attached to the rising and falling block of the drop forge and the other to the stationary anvil. Drop forgings are widely used in the automotive industry for crankshafts, stub-axles, gears, etc.

Ductile Cast Iron

A cast iron that has been treated while molten with an element such as magnesium or cerium to induce the formation of free graphite nodules or spherulites, which imparts a measurable degree of ductility to the cast metal. Also known as nodular cast iron, spherulitic graphite cast iron and SG iron.

Ductile Fracture

Fracture characterized by tearing of metal accompanied by appreciable gross plastic deformation and expenditure of considerable energy. Contrast with brittle fracture.


The property of metal which permits it to be reduced in cross sectional area without fracture. In a tensile test, ductile metals show considerable elongation eventually failing by necking, with consequent rapid increase in local stresses. Measured by elongation or reduction of area in a tensile test, by height of cupping in an Erichsen test or by other means.

Dye Penetrant Inspection

A method for detecting surface porosity or cracks in metal. The part to be inspected is cleaned and coated with a dye which penetrates any flaws that may be present. The surface is wiped clean and coated with a white powder. The powder absorbs the dye held in the defects indicating their location.

Elastic Limit

The maximum stress that can be applied to a metal without producing permanent deformation. When external forces act upon a material they tend to form internal stresses within it which cause deformation. If the stresses are not too great the material will return to its original shape and dimension when the external stress is removed.


The property which enables a material to return to its original shape and dimension.

Electrical Steels

Steels which are characterized by their magnetic properties and are intended for the manufacture of electrical circuits. They are supplied in the form of cold rolled sheet or strip, generally less than 2mm thick and up to 1500mm wide. Grain orientated steels have preferential magnetic properties in the direction of rolling and non- grain orientated steels have similar magnetic properties both transversely and in the direction of rolling.

Electroslag Refining

A specialized steel making process in which a rolled or a cast ingot in the form of an electrode is remelted in a water cooled copper mould. The melting is activated by resistive heat generated in a conductive slag. The resulting product has a similar basic chemical composition to the original ingot, but is characterized by high purity and low inclusion content. Typical applications include high integrity components for the aerospace industry.

Elevated Temperature Drawing

A process of drawing steel bars at elevated temperatures (normally 250-300°C) which under optimum conditions produce steels that have higher tensile and yield strengths than those cold drawn with the same degree of reduction. The process is little used in the United Kingdom.


A test to measure the ductility of steel. When a material is tested for tensile strength it elongates a certain amount before fracture takes place. The two pieces are placed together and the amount of extension is measured against marks made before starting the test and is expressed as a percentage of the original gauge length.


Reduction in ductility or toughness or both due to a chemical or physical change.

End-Quench Hardenability Test

A laboratory procedure for determining the hardenability of a steel or other ferrous alloy; widely referred to as the Jominy test. Hardenability is determined by heating a standard specimen above the upper critical temperature, placing the hot specimen in a fixture so that a stream of cold water impinges on one end, and, after cooling to room temperature is completed, measuring the hardness near the surface of the specimen at regularly spaced intervals along its length. The data are normally plotted as hardness versus distance form the quenched end.

Endurance Limit

The maximum stress below which a material can presumably endure an infinite number of stress cycles. If the stress is not completely reversed, the value of the mean stress, the minimum stress or the stress ratio also should be stated. Compare with fatigue limit.

Equiaxed Crystals

Crystals, each of which has axes approximately equal in length. These are normally present in the centre of a steel ingot.

Equilibrium Diagram

A graphical representation of the temperature, pressure and composition limits of phase fields in an alloy system as they exist under conditions of complete equilibrium. In metal systems, pressure is usually considered constant.


Treatment of a prepared metal surface with acid or other chemical reagent which, by differential attack, reveals the structure.


1) An isothermal reversible reaction in which a liquid solution is converted into two or more intimately mixed solids on cooling, the number of solids formed being the same as the number of components in the system. (2) An alloy having the composition indicated by the eutectic point on an equilibrium diagram. (3) An alloy structure of intermixed solid constituents formed by eutectic reaction.


A mixture of two or more constituents which forms on cooling from a solid solution and transforms on heating at a constant minimum temperature. A eutectoid steel contains approximately 0.83% carbon.

Expendable Thermocouples

Those thermocouples made of fabric or plastic covered wire. The wire is provided in coils or on spools. Insulation usually consists of glass braid, asbestos, or ceramic fiber cloth on each conductor plus glass braid overall.


The production of a section by forcing a billet to flow through a die. Often used for producing complex sections, the process is used with both hot and cold metal. Seamless tubes are produced by forcing a hot billet to flow through a die over a mandrel positioned centrally in the die.


Chemical symbol for Fluorine.

Face Centered Cubic Lattice

An arrangement of atoms in crystals in which the atomic centers are disposed in space in such a way that one atom is located at each of the corners of the cube and one at the centre of each face. Steel in the face-centered cubic arrangement is termed austenite.


The phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the ultimate tensile strength of the material. Fatigue failure generally occurs at loads which applied statically would produce little perceptible effect. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress. There is no obvious warning as such a crack forms without appreciable surface deformation, making it difficult to detect. Fractures often start from small nicks or scratches or fillets that cause a localized concentration of stress. Failure can be influenced by a number of factors including size, shape and design of the component, condition of the surface or operating environment.

Fatigue Limit

The maximum value of the applied alternating stress which a test piece can stand indefinitely.

Fatigue Testing

Fatigue tests are made with the object of determining the relationship between the stress range and the number of times it can be applied before causing failure. Testing machines are used for applying cyclically varying stresses and cover tension, compression, torsion and bending or a combination of these stresses.


Chemical symbol for Iron.


Ferrite is the name given any solid solution in which alpha iron is the solvent. Ferrite is strictly a structure name and means nothing as to composition.

Ferritic Steel

A term usually applied to a group of stainless steels with a chromium content in the range of 12- 18o and whose structure consists largely of ferrite. Such steels possess good ductility and are easily worked but do not respond to any hardening or tempering processes. Types of applications include automotive trim and architectural cladding.

Ferritizing Anneal

A treatment given as-cast gray or ductile (nodular) iron to produce an essentially ferritic matrix. For the term to be meaningful, the final microstructure desired or the time-temperature cycle used must be specified.

Ferro Alloys

Alloys of iron with chromium, manganese, silicon, tungsten, molybdenum or vanadium. Used in steelmaking as a means of introducing these alloying elements into the cast or as deoxidizers.


The removal of sand adhering to castings by hammering, tumbling or shot blasting.


In rolling mill practice a fin is a projection extending from the side of rolled sections. It causes considerable trouble and is the result of overfill. The fin, formed when the bar or shape is fed through one pass, is likely to be rolled back into the bar at the next pass. It is rarely encountered in modern rolling mills.

Final Annealing

An imprecise term used to denote the last anneal given to a nonferrous alloy prior to shipment.

Finish Annealing

A sub critical annealing treatment applied to cold-worked low-or medium-carbon steel. Finish annealing, which is a compromise treatment, lowers residual stresses, thereby minimizing the risk of distortion in machining while retaining most of the benefits to machinability contributed by cold working. Compare with final annealing.


The placing of parts to be heat-treated in a constraining or semi-constraining apparatus to avoid heat-related distortions. See Racking.

Flame Annealing

Annealing in which the heat is applied directly by a flame.

Flame Hardening

A surface hardening process in which heat is applied by a high temperature flame followed by quenching jets of water. It is usually applied to medium to large size components such as large gears, sprockets, slide ways of machine tools, bearing surfaces of shafts and axles, etc. Steels most suited have a carbon content within the range 0.40-0.55%.


A fin that arises from metal in excess of that required to fill the final impression in a forging die and is exuded from the parting line between the dies; similarly it can arise at the mould joint in a casting.

Forced-Air Quench

A quench utilizing blasts of compressed air against relatively small parts such as a gear.


A process of working metal to a finished shape by hammering or pressing and is primarily a "hot" operation. It is applied to the production of shapes either impossible or too costly to make by other methods or needing properties not obtainable by casting. Categories of forgings include Hammer, Press, Drop or Stamping.


The relative ease with which a metal can be shaped through plastic deformation.


The separation of material into two or more parts. Fractures are often described by the appearance of the surface of the break in a piece of steel. Crystalline is bright and glittering, failure having developed along the cleavage planes of individual crystals and can be typical of brittle material. A silky fracture has a smooth dull grain indicative of ductile material such as a mild steel. In tensile testing fractures are described by shape, e.g. cup and cone.

Fracture Stress

1) The maximum principal true stress at fracture. Usually refers to unnotched tensile specimens. (2) The (hypothetical) true stress that will cause fracture without further deformation at any given strain.

Free Carbon

The part of the total carbon in steel or cast iron that is present in elemental form as graphite or temper carbon.

Free Ferrite

Ferrite that is formed directly from the decomposition of hypoeutectoid austenite during cooling, without the simultaneous formation of cementite. Also called proeutectoid ferrite.

Freecutting Steels

Steels which have had additions made to improve machinability. The most common additives are sulfur and lead, other elements used include tellurium, selenium and bismuth.

Freezing Range

That temperature range between liquidus and solidus temperatures in which molten and solid constituents coexist.

Full Annealing

Annealing a ferrous alloy by austenitizing and then cooling slowly through the transformation range. The term is meant to denote an annealing cycle that produces minimum strength and hardness, but for the actual value to be known requires the composition and starting condition of the material as well as the time-temperature cycle.

Full Hard

A temper corresponding approximately to a cold-worked state beyond which the material can no longer be formed by bending. In specifications, a full hard temper is commonly defined in terms of minimum hardness or minimum tensile strength (or alternatively, a range of hardness or strength) corresponding to a specific percentage of cold reduction following full annealing. For aluminum, a full hard temper is equivalent to a reduction of 75% from dead soft; for austenitic stainless steels, a reduction of about 50% to 55%.


Chemical symbol for Gallium.

Galvanic Action

When iron and steel are subject to conditions of aqueous corrosion the incidence and rate at which the corrosion takes place will alter if the steel is coupled with other metals or alloys that are also exposed to the electrolyte. Copper, brass, bronze, lead and nickel are more "noble" and act as auxiliary cathodes to the steel and accelerate its anodic dissolution, that is, its corrosion. Magnesium, zinc and zinc-base alloy are nearly always less noble and tend to divert the attack from the steel to themselves. The galvanic relationship of various metals is an important factor affecting corrosion.

Gamma Iron

The allotropic form of iron existing between the temperature 910°C and 1400°C is known as Gamma Iron. It has a face centered cubic lattice and is non-magnetic. Gamma iron containing carbon or other elements in solution is known as austenite.

Gas Carburising

A heat treatment method used in the case- hardening of steel. Carbon is absorbed into the outer layers of the components by heating in a current of gas, rich in carbon compounds. The process is more versatile than some other methods as the depth of the case and the limiting carbon content of the case can be controlled by the composition of the atmosphere, the dew point and the temperature.

Gauge Length

Used in the mechanical testing of steel, it is the length marked on the parallel portion of a tensile test piece from which the elongation is measured.

Gauge Plate

An alloy tool steel supplied in flat and square section with the surfaces ground to close limits. It is also known as Ground Flat Stock and is used for the manufacturing of gauges, punches, dies, jigs, templates etc.


Chemical symbol for Germanium.


An individual crystal in a polycrystalline metal or alloy; it may or may not contain twinned regions and sub grains.

Grain Flow

Fiber like lines appearing on polished and etched sections of forgings, which are caused by orientation of the constituents of the metal in the direction of workings during forging. Grain flow produced by proper die design can improve required mechanical properties of forgings.

Grain Growth

Growth of some grains at the expense of others, resulting in an overall increase in average grain size.

Grain Size Control

When a steel is austenitised by heating to above the critical range, time is required for the production of a homogeneous structure during which there is a tendency towards grain growth. Although subsequent hot and cold working affect the grain size, it is originally controlled at the steel making stage by the addition of aluminium.

Grain Size Measurement

Grain size is normally quantified by a numbering system. Coarse 1-5 and fine 5-8. The number is derived from the formula N=2n-1 where n is the number of grains per square inch at a magnification of 100 diameters. Grain size has an important effect on physical properties. For service at ordinary temperatures it is generally considered that fine grained steels give a better combination of strength and toughness, whereas coarse grained steels have better machinability.

Granular Fracture

A type of irregular surface produced when metal is broken, characterized by a rough, grain like appearance as differentiated from a smooth, silky or fibrous type. It can be sub classified into transgranular and intergranular forms. This type of fracture is frequently called crystalline fracture, but the inference that the metal broke because it "crystallized" is not justified, because all metals are crystalline when in the solid state. Contrast with fibrous fracture, silky fracture.


An annealing process applied to cast iron and steels with a high carbon and high silicon content by which the combined carbon is wholly or in part transformed to graphitic or free carbon.

Grey Iron

Also known as flake iron on account of all or part of the carbon content being in the form of graphite distributed through the metal as flakes.


A machining process:- (a) to shape components that are too hard to be machined by conventional methods such as hardened tool steels and case or induction hardened components. (b) to obtain a high degree of dimensional accuracy and surface finish on a component.

Grinding Cracks

Cracks can arise from incorrect grinding and appear in the form of a network. They are caused by the generation of high heat and rapid cooling in the area of contact and they mostly occur when grinding fully hardened material such as tool steel.


Chemical symbol for Hydrogen.

Hard Metal Facing

A method of increasing the wear resistance of a metal by the deposition of a hard protective coating. Alloys such as Stellite or a metallic carbide are most often used for the coating.

Hard Metals

A group of materials more commonly known as cemented carbides. They consist of mixtures of one or more of the finely divided carbides of tungsten, titanium, tantalum and vanadium embedded in a matrix of cobalt or nickel by sintering. Widely used for cutting tools where for many applications they have replaced conventional high speed steels.


The property that determines the depth and distribution of hardness when steel is heated to a given temperature and then quenched (more precisely it may be defined as an inverse measure of the severity of cooling conditions necessary to produce on continuous cooling a martensitic structure in a previously austenitized steel i.e. to avoid transformations in the pearlitic and bainitic ranges). The lower the cooling rate to avoid these transformations, the greater the hardenability. The critical cooling rate is largely a function of the composition of the steel. In general the higher the carbon content, the greater the hardenability, whilst alloying elements such as nickel, chromium, manganese and molybdenum increase the depth of hardening for a given ruling section.


Increasing the hardness of steel by heat treatment. The steel is heated to a suitable austenitizing temperature, holding at that temperature for a sufficient time to affect the desired solution of carbon and other alloying elements, then quenching in a suitable medium, such as water, oil, air, polymer or molten salts to form martensite.


The hardness of steel is generally determined by testing its resistance to deformation. A number of methods are employed including Brinell, Vickers and Rockwell. The steel to be tested is indented by a hardened steel ball or diamond under a given load and the size of the impression is then measured. For steel there is an empirical relationship between hardness and tensile strength and the hardness number is often used as a guide to the tensile strength, e.g. 229 Brinell = 772N/mm2 (50 tons/


In steel making terms this is often used to define the batch or cast produced from a single melting operation.

Heat Treatable Alloy

An alloy that can be hardened by heat treatment.

Heat Treatment

A process where solid steel or components manufactured from steel are subject to treatment by heating to obtain required properties, e.g. softening, normalizing, stress relieving, hardening. Heating for the purpose of hot-working as in the case of rolling or forging is excluded from this definition.

High Speed Steel

The term `high speed steel was derived from the fact that it is capable of cutting metal at a much higher rate than carbon tool steel and continues to cut and retain its hardness even when the point of the tool is heated to a low red temperature. Tungsten is the major alloying element but it is also combined with molybdenum, vanadium and cobalt in varying amounts. Although replaced by cemented carbides for many applications it is still widely used for the manufacture of taps, dies, twist drills, reamers, saw blades and other cutting tools.


The portion of the thermal cycle during which the temperature of the object is maintained constant.

Holding Temperature

The constant temperature at which the object is maintained.

Holding Time

Time for which the temperature of the object is maintained constant.


An annealing treatment at fairly high temperature designed to eliminate or reduce chemical segregation.

Hooke's Law

This states that, “within the limits of elasticity the strain produced by a stress of any one kind is proportional to the stress.” The stress at which a material ceases to obey Hooke's Law is known as the Limit of Proportionality.

Horizontal Batch Furnace

A versatile batch-type furnace that can give light or deep case depths, and because the parts are not exposed to air, horizontal batch furnaces can give surfaces almost entirely free of oxides.

Hot Quenching

Cooling in a medium, the temperature of which is substantially higher than room temperature.

Hot Work

The rolling, forging or extruding of a metal at a temperature above its recrystallisation point.


An undesirable impurity if present in steel and a cause of fine hairline cracks especially in alloy steels. Modern vacuum treatment eliminates this problem.

Hydrogen Embrittlement

The brittleness induced in steel by the absorption of atomic hydrogen, most commonly from a pickling or plating operation.

Hyper-Eutectoid Steel

A steel that contains more than 0.83% carbon which with appropriate heat treatment consists of pearlite and cementite.

Hypereutectic Alloy

In an alloy system exhibiting a eutectic, any alloy whose composition has an excess of alloying element compared with the eutectic composition, and whose equilibrium microstructure contains some eutectic structure.


Chemical symbol for Iodine.

Impact Energy

The amount of energy required to fracture a material, usually measured by means of an Izod test or Charpy test. The type of specimen and test conditions affect the values and therefore should be specified.

Impact Test

A test designed to give information on how a specimen of a known material will respond to a suddenly applied stress, e.g. shock. The test ascertains whether the material is tough or brittle. A notched test piece is normally employed and the two methods in general use are either the Izod or Charpy test. The result is usually reported as the energy in ft.lbs. or KJ. required to fracture the test piece.


Chemical symbol for Indium.

Inclusion Count

A method of assessing the number and size of non-metallic inclusions present in metal.


Particles of foreign material in a metallic matrix. The particles are usually compounds (such as oxides, sulfides or silicates), but may be of any substance that is foreign to (and essentially insoluble in) the matrix. In general they are detrimental to mechanical properties but much depends on the number, their size, shape and distribution.

Induction Hardening

A widely used process for the surface hardening of steel. The components are heated by means of an alternating magnetic field to a temperature within or above the transformation range followed by immediate quenching. The core of the component remains unaffected by the treatment and its physical properties are those of the bar from which it was machined, whilst the hardness of the case can be within the range 37/58 Rc. Carbon and alloy steels with a carbon content in the range 0.40/0.45% are most suitable for this process.

Induction Heating

Heating by combined electrical resistance and hysteresis losses induced by subjecting a metal to the varying magnetic field surrounding a coil carrying alternating current.

Induction Tempering

Tempering of steel using low-frequency electrical induction heating.


The mass of metal that results from casting molten steel into a mould. An ingot is usually rectangular in shape and is subsequently rolled into blooms and billets for rods, bars and sections and slabs for plates, sheet and strip. With the increasing use of the continuous casting process the ingot route is less used as the molten steel is now directly cast into a bloom or billet.

Ingot Mould

The receptacle into which molten steel is poured to form an ingot. After solidification the steel is suitable for subsequent working, i.e. rolling or forging.

Intercrystalline Corrosion

Chromium-nickel austenitic stainless steels are prone to this form of corrosion when they are welded and subsequently in contact with certain types of corrosive media. When heated within a temperature range of 450-800oC precipitation of the chromium carbides takes place at the grain boundaries in the area of the weld and these areas no longer have the protection of the chromium on the peripheries of the grains. This type of corrosion is also known as Weld Decay and Intergranular Corrosion. The most common way to avoid the problem is to select a grade of steel that is very low in carbon i.e. 0.03% or less, or one that is stabilised with niobium or titanium.


Between crystals or grains. Also called intercrystalline.

Intergranular Cracking

Cracking or fracturing that occurs between the grains or crystals in a polycrystalline aggregate. Also called intercrystalline cracking.

Intergranular Fracture

Brittle fracture of a metal in which the fracture is between the grains, or crystals, that form the metal. Also called intercrystalline fracture.

Intermediate Annealing

Annealing wrought metals at one or more stages during manufacture and before final treatment.

Interrupted Aging

Aging at two or more temperatures, by steps, and cooling to room temperature after each step. See aging, and compare with progressive aging and step aging.

Interrupted Quenching

Rapid cooling to a selected temperature by quenching in a suitable medium, usually molten salt, holding at the temperature for an appropriate time and then cooling to room temperature. This process is used to minimize the risk of distortion.


The term iron, as used in the chemical or scientific sense of the word, refers to the chemical element iron or pure iron and is the chief constituent of all commercial iron and steel.

Isothermal Annealing

Heating to and holding at a temperature above the transformation range, then cooling to and holding at a suitable temperature until the austenite to pearlite change is complete.

Isothermal Transformation

A change in phase that takes place at a constant temperature. The time required for transformation to be completed, and in some instances the time delay before transformation begins, depends on the amount of supercooling below (or superheating above) the equilibrium temperature for the same transformation.

Isothermal Transformation Curve

Also known as the Time Temperature Transformation Curve. If a small piece of steel is heated sufficiently slowly for it to become austenitic and then plunged into a salt bath and held at a constant temperature below the upper critical point for a definite time followed by rapid quenching, it is possible by examination to determine the extent to which the transformation of the austenite has occurred. By taking a number of specimens of the same steel and treating them in the same way, but varying the holding temperature and time the behavior of the steel with time and temperature can be studied. The information obtained can be plotted as time- temperature transformation curves which is useful in heat treatment practice, particularly for martempering and austempering.

Isothermal Treatment

A type of treatment in which a part is quenched rapidly down to a given temperature, then held at that temperature until all transformation is complete.

Izod Impact Test

A test specimen, usually of square crossed section is notched and held between a pair of jaws, to be broken by a swinging or falling weight. When the pendulum of the Izod testing machine is released it swings with a downward movement and when it reaches the vertical the hammer makes contact with the specimen which is broken by the force of the blow. The hammer continues its upward motion but the energy absorbed in breaking the test piece reduces its momentum. A graduated scale enables a reading to be taken of the energy used to fracture the test piece. To obtain a representative result the average of three tests is used and to ensure that the results conform to those of the steel specification the test specimens should meet the standard dimensions laid down in BS 131. Contrast with Charpy test.

Jominy Test

A method for determining the hardenability of steel. The Jominy test is covered by BS 4437:1987. A standard test piece 25mm x 100mm is heated to a pre- determined temperature and quenched by a jet of water sprayed onto one end. When the specimen is cold, hardness measurements are made at intervals along the test piece from the quenched end and the results are plotted on a standard chart from which is derived the hardenability curve. BS 970 contains hardenability curves for many of the steels in the Standard. Properly carried out, this test will illustrate the effect of mass upon a chosen steel when heat treated and indicate if the steel is of a shallow, medium or deep hardening type. See end-quench hardenability test.


A unit of energy. One joule is equal to the energy expended in one second by one ampere against the resistance of one ohm. In the mechanical testing of steel it is the unit used in the Charpy V notch impact test.


Chemical symbol for potassium.

Kaldo Process

A method of producing steel from molten iron, using an inclined rotating converter and a water cooled oxygen lance inserted through the converter mouth. Originating in Sweden, this process is no longer in use in the UK.

Killed Steel

The term indicates that the steel has been completely deoxidized by the addition of an agent such as silicon or aluminium, before casting, so that there is practically no evolution of gas during solidification. Killed steels are characterized by a high degree of chemical homogeneity and freedom from porosity.

Knoop Hardness

Microhardness determined from the resistance of metal to indentation by a pyramidal diamond indenter, having edge angles of 172°, 30°, and 130°, making a rhombohedral impression with one long and one short diagonal.

L-D Process

An oxygen steel making process named after the towns in Austria, Linz and Donawitz, where it was first developed. It is a modified Bessemer process, steel is produced in a solid bottom converter by injection of oxygen into the molten iron bath from a water cooled lance inserted through the converter mouth. Present day BOS (basic oxygen steelmaking) plants are developments of the L- D Process.


Chemical symbol for Lanthanum.


A defect appearing as a seam on a rolled bar. Laps are rolled over pieces of material that arise when a bar is given a pass through the rolls after a sharp overfill or fin has been formed, causing the protrusion to be rolled into the surface of the product. The presence of oxides usually prevents the lap welding to the original bar surface, so that in subsequent cold working it is carried through as a longitudinal crack.

Latent Heat

Thermal energy absorbed or released when a substance undergoes a phase change.

Leaded Steels

When added to steel, lead does not go into solution but exists in a very finely divided state along the grain boundaries. It greatly assists machinability as it acts as a lubricant between the steel and the tool face. Lead is normally added in amounts between 0.15-0.35% and when combined with similar amounts of sulfur, optimum machinability is attained as in such steel as BS 970 230M07 Pb.


Chemical symbol for Lithium.

Limiting Range of Stress

The greatest range of stress that a metal can withstand for an indefinite number of cycles without failure. If exceeded, the metal fractures after a certain number of cycles, which decrease as the range of stress increases.

Limiting Ruling Section

The maximum diameter of cross section of a bar or component in which certain specified mechanical properties are achieved after heat treatment.


A term used to determine a minimum and maximum. In a mechanism, it should denote the minimum and maximum sizes for each part, between which the parts will function properly in conjunction with each other and outside of which they will not. The words "limits" and "tolerances" are often interchanged, "tolerance" represents the difference between the minimum and maximum limits.

Limits of Proportionality

The stress (load divided by original area of cross section of the test piece) at which the strain (elongation per unit of gauge length) ceases to be proportional to the corresponding stress. It is usually determined from a load-elongation diagram, obtained by plotting extensometer readings and is the stress at which the load-elongation line ceases to be straight.

Liquid Carburising

A widely used method of case-hardening steel that eliminates scaling and the tendency to decarburization and results in clean components. Sodium cyanide is the common media for this process, usually heated within the range of 900-930°C. It is advisable to pre-heat the components in neutral salts to avoid a temperature drop resulting from immersing cold components into the cyanide. After carburizing, either single quench hardening or refining and hardening and tempering is carried out.

Liquid Penetrant Inspection

A type of nondestructive inspection that locates discontinuities that are open to the surface of a metal by first allowing a penetrating dye or fluorescent liquid to infiltrate the discontinuity, removing the excess penetrant, and then applying a developing agent that causes the penetrant to seep back out of the discontinuity and register as an indication. Liquid penetrant inspection is suitable for both ferrous and nonferrous materials, but is limited to the detection of open surface discontinuities in nonporous solids.


Simply defined as a measure of the ease with which a metal can be machined satisfactorily.

Macroscopic Stresses

Residual stresses that vary from tension to compression in a distance (presumably many times the grain size) that is comparable to the gage length in ordinary strain measurements, hence, detectable x-ray or dissection methods.


The general crystalline structure of a metal and the distribution of impurities seen on a polished or etched surface by either the naked eye or under low magnification of less than x10.

Magnetic Crack Detection

The bar or component to be tested is magnetized by passing a heavy current through it or by making it the core of a coil through which a heavy current is passed. Cracks or inclusions cause the magnetic flux to break the surface forming free magnetic poles. When the component is sprayed with a suspension of finely divided magnetic particles they collect at the free poles to visibly show the presence of defects.


It can be defined as the property of a metal to be deformed by compression without cracking or rupturing. The load may be applied slowly or suddenly and will determine whether the material will be suitable for forging or rolling into thin sheet.

Malleable Cast Iron

A cast iron made by prolonged annealing of white cast iron in which decarburization or graphitization, or both, take place to eliminate some or all of the cementite. The graphite is in the form of temper carbon. If decarburization is the predominant reaction, the product will exhibit a light fracture surface, hence, "whiteheart malleable;" otherwise, the fracture surface will be dark, hence, "blackheart malleable." Ferritic malleable has a predominantly ferritic matrix; pearlitic malleable may contain pearlite, spheroidite or tempered martensite depending on heat treatment and desired hardness.


One of the most important constituents of steel in which it fulfils a number of functions. It acts as a mild de-oxidizing agent. It combines with the sulfur present to form globular inclusions of Manganese Sulfide which are beneficial to machining. It increases tensile strength and the hardenability of steel.


A precipitation-hardening treatment applied to a special group of iron-base alloys to precipitate one or more intermetallic compounds in a matrix of essentially carbon-free martensite.


A heat treatment involving austenitisation followed by step quenching, at a rate fast enough to avoid the formation of ferrite, pearlite or bainite to a temperature slightly above the Ms point. Soaking must be long enough to avoid the formation of bainite. The advantage of martempering is the reduction of thermal stresses compared to normal quenching. This prevents cracking and minimizes distortion. Also called Marquenching.


The hard constituent produced when steel is cooled from the hardening temperature at a speed greater than its critical cooling rate. Martensite is an acicular phase when seen in the microstructure of steel.

Martensitic Transformation

A reaction that takes place in some metals on cooling, with the formation of an acicular structure called martensite.

Mass Effect

A term used to signify the effect of size and shape during heat treatment, since it is the rate of cooling of a piece of steel which determines the properties resulting from the hardening and quenching process.


The mass or principal constituent (e.g. iron in the case of steel) in which other constituents are embedded.

Maximum Stress

In the testing of the strength of steel a sample is machined into a standard test piece and is stretched in a tensile testing machine until it breaks. The results are expressed in N/mm2 and is the value of the maximum load reached in the test divided by the original cross sectional area of the specimen.

McQuaid EHN Grain Size Test

A method of assessing grain size. It consists of a test piece at 927°C for 8 hours by slow cooling and subsequent microscopical examination. The grain size is measured at x100 magnification and compared to standard charts, the figures range from No. 1 - very coarse, to No. 8 - very fine.

Mechanical Properties

The properties of a material that reveal its elastic and inelastic behavior when force is applied, thereby indicating its suitability for mechanical applications; for example, modulus of elasticity, tensile strength, elongation, hardness, and fatigue limit. Compare with physical properties.


A trade name applied to a certain type of cast iron.

Melting Point

The temperature at which a solid begins to liquefy.


nbsp;(1) Forming a metallic coating by atomized spraying with molten metal or by vacuum deposition. Also called spray metallizing. (2) Applying an electrically conductive metallic layer to the surface of a nonconductor.


Chemical symbol for Magnesium.

Micro Segregation

Segregation within a grain, crystal or small particle.


The hardness of a material as determined by forcing an indenter such as a Vickers or Knoop indenter into the surface of a material under very light load; usually, the indentations are so small that they must be measured with a microscope. Capable of determining harnesses of different microconstituents within a structure or of measuring steep hardness gradients such as those encountered in case hardening.


A unit of length equal to one millionth of a meter (0.001mm).


The structure that is observed when a polished and etched specimen of metal is viewed in an optical microscope at magnifications in range of approximately x25 to x1500.

Mill Scale

The heavy oxide layer formed during hot fabrication or heat treatment of metals.


Chemical symbol for Manganese.


Chemical symbol for Molybdenum.

Modulus of Elasticity

This is a measure of rigidity based on the ratio of stress to corresponding strain in an elastic material. When a material is subjected to an external load it becomes distorted or strained. With metals, provided the loading is not too great, they return to their original dimensions when the load is removed, i.e. they are elastic. Within the limits of elasticity, the ratio of the linear stress to the linear strain is termed the modulus of elasticity or more commonly known as Young’s Modulus.

Modulus of Rupture

Nominal stress at fracture in a bend test or torsion test. In bending, the modulus of rupture is the bending moment at fracture divided by the section modulus. In torsion, modulus of rupture is the torque at fracture divided by the polar section modulus.


Element, chemical symbol Mo. A grayish metal that does not easily tarnish in air and has a high melting point of 2620°C (4748°F). Its use as an alloying element in steel increases hardenability and in low alloy steels reduces the risk of temper brittleness. When added to stainless steels it increases their resistance to corrosion. It is also used in high-speed steels.


Chemical symbol for Nitrogen.


Chemical symbol for Sodium.

Natural Aging

Spontaneous aging of a supersaturated solid solution at room temperature.


Chemical symbol for Niobium.

Neutral Flame

A gas flame in which there is no excess of either fuel or oxygen in the inner flame. Oxygen from ambient air is used to complete the combustion of CO2 and H2 produced in the inner flame.


Chemical symbol for Nickel.


A metallic element used in some steels. A silvery-white metal of medium hardness, highly ductile and resistant to chemical and atmospheric corrosion. Widely used as an alloying agent in iron and copper base alloys. As an alloying element in steel, it imparts a finer and more homogeneous structure. The most suitable composition for cold working is said to be 0.2 to 0.5 percent carbon and 2 to 3.5 percent nickel. Also used for plating and coating (see nickel coating). Nickel increases hardenability, thus permitting steel to be oil-hardened instead of water-quenched. In larger quantities, 8.00 percent and upwards, nickel is the constituent, together with chromium, of many corrosion resistant and stainless austenitic steels.


Also known as columbium. Niobium is a strong carbide forming element which is added to certain 18/8% chromium-nickel stainless steels as a stabilizer to prevent inter-granular corrosion arising from welding.


A case-hardening process in which nitrogen is introduced into the surface of a ferrous alloy, usually of special composition. All machining, stress relieving, as well as hardening and tempering are normally carried out before nitriding. The parts are heated in a special container through which ammonia gas is allowed to pass. The ammonia splits into hydrogen and nitrogen and the nitrogen reacts with the steel penetrating the surface to form nitrides. This takes place in an ammonia atmosphere of about 500° to 540°C (932° to 1004°F) or in contact with nitrogenous material to produce surface hardening by the absorption of nitrogen, without quenching. Nitriding steels offer many advantages: a much higher surface hardness is obtainable when compared with case-hardening steels; they are extremely resistant to abrasion and have a high fatigue strength.


Any of several processes in which both nitrogen and carbon are absorbed into the surface layers of a ferrous material at temperatures below the lower critical temperature and, by diffusion, create a concentration gradient. Nitrocarburizing is done mainly to provide an antiscuffing surface layer and to improve fatigue resistance. Compare with carbonitriding.


Nitrogen is a gas that forms approximately 79% by volume or 77% by weight of the atmosphere. It can combine with many metals to form nitrides and is thus applied to the case-hardening of steel, the usual source for this purpose being ammonia.

Noble Metals

Metals such as gold, silver and platinum which are resistant to corrosion by all but the most powerful acids.

Non Destructive Testing

Those forms of testing that do not result in permanent damage or deformation to the part being tested. Typical examples are magnetic crack detection, ultrasonic inspection, X-Ray inspection and gamma radiography.

Non Magnetic Steels

Austenitic steels such as the 14% manganese steels and the 303 type 18/8% chromium-nickel stainless steels.

Non-Expendable Thermocouples

Those thermocouples that are not covered with fabric or plastic insulation. One type consists of ceramic insulators over bare thermocouple wire, sometimes inserted in a tube for stability and protection. A second type consists of a combination of thermocouple wires, mineral insulation, and a protecting metal sheath compacted into a small diameter.


A heat treatment process that has the object of relieving internal stresses, refining the grain size and improving the mechanical properties. The steel is heated to 800-900°C according to analysis, held at temperature to allow a full soak and cooled in still air.

Notched Bar Test

A test to determine the resistance of a material to a suddenly applied stress, i.e. shock. A notched test piece is employed in an Izod or Charpy machine and the results are recorded in ft.lbs. or Joules.


Chemical symbol for Oxygen.


A term applied, in the case of metals, to the absorption or entrapment of gases.

Oil Hardening

Quench-hardening treatment involving cooling in oil.

Oil Hardening Steel

Used to describe tool or alloy steels where oil is used as the quenching medium in the hardening process.

Oil Quenching

Hardening of carbon steel through cooling in an oil bath at a desired rate. Oils are categorized as conventional, fast, martempering, or hot quenching.

Open Hearth Furnace

Developed in the middle of the last century, the open hearth or Siemens-Martins process, as it is known, accounted for a major proportion of UK steel production until the early 1970's. For economic and quality reasons it has been replaced by the Electric Arc Furnace and the Basic Oxygen Steelmaking process. There are no open hearth furnaces in use in Britain today but they are still in use in Russia and Eastern Europe.

Orange Peel Effect

An effect that arises on the surface of steel sheets when they are stretched beyond their elastic limit.


An ore is a material that contains a metal in such quantities that it can be mined and worked commercially to extract that metal. The metal is usually contained in chemical combination with some other element in addition to various impurities.


Chemical symbol for Osmium.


Aging under conditions of time and temperature greater than those required to obtain maximum change in a certain property, so that the property is altered in the direction of the initial value. See aging.


Failure of tools and components in heat treatment can arise through overheating. This may be caused due to quenching from a temperature too high for the type of steel involved. Overheating is evidenced by cracking, grain-coarseness, erratic surface hardness and pitting.


A common form of chemical reaction which is the combining of oxygen with various elements and compounds. The corrosion of metals is a form of oxidation, rust on iron for example is iron oxide.

Oxy-Acetylene Welding

A process for joining two pieces of metal in which the required high temperature is obtained by the combustion of acetylene gas and oxygen. The gases are thoroughly mixed in the nozzle or tip of the welding torch to ensure perfect combustion. The weld may be formed directly between two adjoining surfaces, but usually metal from a welding rod is fused in between the surfaces of the joint.


Oxygen is one of the chief constituents of the atmosphere of which it forms approximately one fifth. It is odorless and invisible. Although oxygen itself does not burn it is extremely efficient in supporting combustion, nearly all other chemical elements combine with it under evolution of heat. It has many uses in industry and is essential to the BOS (Basic Oxygen Steelmaking Process).

Oxygen Probe

An atmosphere-monitoring device that electronically measures the difference between the partial pressure of oxygen in a furnace or furnace supply atmosphere and the external air.


Chemical symbol for Phosphorus.


A chemical treatment applied to ferrous metals to improve their corrosion resistance. The process is based on a manganese phosphate solution which produces a fairly thick coating. This can subsequently be painted or impregnated with oil.

Partial Annealing

An imprecise term used to denote a treatment given cold worked material to reduce its strength to a controlled level or to affect stress relief. To be meaningful, the type of material, the degree of cold work, and the time-temperature schedule must be stated.


A heat treatment process often applied to high carbon wire. The steel is heated to a suitable temperature well above the transformation range, followed by cooling in air or a bath of molten lead or salt. A structure is produced suitable for subsequent cold drawing and which will give the desired mechanical properties in the finished state.


Chemical symbol for Palladium.


A distinctive two-phase lamellar structure in steel consisting of thin platelets of iron carbide (Fe3C) in a ferrite (essentially, Fe) matrix. The fine pearlitic structure (small grain size) allows maximum drawability and strengthening in steel wire, and is responsible for the mechanical properties of unhardened steel.


Chemical symbol for Lead.

pH Value

A method of expressing differences in the acidity or alkalinity of a solution. A figure of 7 is regarded as neutral, figures below this indicate the decree of acidity and above alkalinity.


An element that forms 0.12% of the earth's crust, chiefly in the form of phosphates. Its presence in steel is usually regarded as an undesirable impurity due to its embrittling effect, for this reason its content in most steels is limited to a maximum of 0.050%.

Physical Properties

Properties of a metal or alloy that are relatively insensitive to structure and can be measured without the application of force; for example, density, electrical conductivity, coefficient of thermal expansion, magnetic permeability and lattice parameter. Does not include chemical reactivity. Compare with mechanical properties.


A process to chemically remove scale or oxide from steel to obtain a clean surface. When applied to bars or coils prior to bright drawing, the steel is immersed in a bath of dilute sulfuric acid heated to a temperature of around 80oC. An inhibitor is added to prevent attack and pitting of the cleaned metal. After pickling, a washing process takes place followed by immersion in a lime-water bath to neutralize any remaining acid. For environmental reasons shot blasting has largely replaced pickling.

Pig Iron

The product of the blast furnace. The term was derived from the method of casting the bars of the pig iron in depressions or moulds formed in the sand floor adjacent to the furnace. These were connected to a runner (known as a sow) and when filled with metal the runner and the numerous smaller moulds were supposed to resemble a litter of suckling pigs, hence the term pig iron.

Pinch Pass

A term applied when, after annealing, sheet or strip is lightly rolled with the object of preventing stretcher lines or kinks on subsequent cold working.


A defect that arises during the solidification of steel in the ingot mould. As steel contracts on solidification a central cavity forms in the upper portion of the ingot, if this is not completely removed before rolling into bars a central defect known as "pipe" results. The risk of piping is considerably reduced on continuously cast steel due to molten steel being available to fill any shrinkage cavity.

Plasma Spraying

A thermal spraying process in which the coating material is melted with heat from a plasma torch that generates a non-transferred arc (defined in plasma-arc welding); molten coating material is propelled against the basis metal by the hot, ionized gas issuing from the torch.

Poisson's Ratio

If a square bar is stressed in a testing machine in the direction of its length so that the length increases, there is a contraction in each opposite direction, which produces a decrease in the thickness of the bar. The ratio between the contraction at right angles to a stress and the direct extension is called the Poisson's ratio. Its value in steel is in the order of 0.28.

Pot Quenching

Quenching carburized parts directly from the carburizing pot or box.

Powder Metallurgy

A method of producing components by pressing or moulding metal powders which may be simultaneously or subsequently heated to produce a coherent mass.


Used in the hardening process. Heating before some further thermal or mechanical treatment. For tool steel, heating to an intermediate temperature immediately before final austenitizing. For some nonferrous alloys, heating to a high temperature for a long time, to homogenize the structure before working. In welding and related processes, heating to an intermediate temperature for a short time immediately before welding, brazing, soldering, cutting, or thermal spraying. Pre-heating reduces the time of exposure to the hardening temperature and helps to minimize scaling and decarburization.

Precipitation Hardening

Hardening caused by the precipitation of a constituent from a supersaturated solid solution. See also age hardening and aging.

Precipitation Heat Treatment

Artificial Aging in which a constituent precipitates from a supersaturated solid solution.

Projection Welding

A welding process that uses small projections on one or both components of the weld to localize the heat and pressure, the projections collapse when the weld is made.

Proof Stress

The stress that will cause a specified small, permanent extension of a tensile test piece. Commonly the stress to produce 0.2% extension is quoted in N/mm2 for steel. This value approximates to the yield stress in materials not exhibiting a definite yield point.

Proportional Limit

The stress (load divided by original area of cross section of the test piece) at which the strain (elongation per unit of gauge length) ceases to be proportional to the corresponding stress. If the load is removed for any stress up to this point, the material will spring back, or assume its original dimensions. The limit is usually determined from a load-elongation diagram, obtained by plotting extensometer readings and is the stress at which the load-elongation line ceases to be straight.

Pusher Furnace

A type of continuous furnace in which parts to be heated are periodically charged into the furnace in containers, which are pushed along the hearth against a line of previously charged containers thus advancing the containers toward the discharge end of the furnace, where they are used.

Quench Aging

Aging induced by rapid cooling after solution heat treatment.

Quench Annealing

Annealing an austenitic ferrous alloy by solution heat treatment followed by rapid quenching.

Quench Cracking

Fracture of a metal during quenching from elevated temperature. Most frequently observed in hardened carbon steel, alloy steel, or tool steel parts of high hardness and low toughness. Cracks often emanate from fillets, holes, corners, or other stress raisers and result from high stresses due to the volume changes accompanying transformation to martensite.


Rapid cooling from a high temperature by immersion in a liquid bath of oil or water. Molten salts may also be used.


Chemical symbol for Radium.


A term used to describe the placing of parts to be heat treated on a rack or tray. This is done to keep parts in a proper position to avoid heat-related distortions and to keep the parts separated. See fixturing.


A method of non-destructive testing. Internal examination of a metallic structure or component is carried out by exposing it to a beam of X-Ray or gamma radiation. Internal defects can be seen on a screen or recorded on film.


Chemical symbol for Rubidium.


Chemical symbol for Rhenium.


The re-arrangement of crystals in cold worked metal brought about by heating so that the deformed crystals are absorbed by newly-formed crystals and the effects of work hardening are removed. Also occurs when steel is heated through the transformation range and when steel is hot worked.


nbsp;(1) To increase the carbon content of molten cast iron or steel by adding carbonaceous material, high-carbon pig iron, or a high-carbon alloy. (2) To carburize a metal part to return surface carbon lost in processing; also known as carbon restoration.


Equipment for transferring heat from gaseous products of combustion to incoming air or fuel. The incoming material passes through pipes surrounded by a chamber through which the outgoing gasses pass.

Red Hardness

A term sometimes associated with high speed steel because it has the property of retaining sufficient hardness for cutting metals even when heated to a temperature high enough to cause a dull redness. The tungsten content has a significant influence on this property.

Reduction of Area

1) Commonly, the difference, expressed as a percentage of original area, between the cross-sectional area of a tensile test specimen and the minimum cross-sectional area measured after complete separation. (2) The difference, expressed as a percentage of original area, between original cross-sectional area and that after straining of the specimen.


a) The removal of impurities and metallic oxides from the molten bath by the reaction of the slag and other additions. (b) A heat treatment process with the object of refining or making the grain size of the steel uniform.


1) A material of very high melting point with properties that make it suitable for such uses as furnace linings and kiln construction. 2) The quality of resisting heat.

Refractory Metal

The refractory metals include niobium (formerly known as columbium), tantalum, molybdenum, tungsten and rhenium. With the exception of two of the platinum-group metals, osmium and iridium, the refractory metals have the highest melting temperatures (greater than 2000 °C, or 3630 °F) and the lowest vapor pressures of all metals. They are readily degraded by oxidizing environments at moderately low temperatures, a property that has restricted their use as high-temperature materials. Protective coating systems have been developed, mostly for niobium alloys, to permit their use in high-temperature oxidizing aerospace applications.

Residual Stress

The stress which exists in an elastic solid body in the absence of, or in addition to, the stresses caused by an external load. Such stresses can arise from deformation during cold working such as cold drawing or stamping, in welding from weld metal shrinkage, and in changes in volume due to thermal expansion.


A vessel used for distillation of volatile materials, as in separation of some metals and in destructive distillation of coal.


Chemical symbol for Rhodium.

Rockwell Hardness Testing

A method for testing the hardness of metals by determining the depth of penetration of a steel ball or a diamond sphero-conical indentor. The value is read from a dial and is an arbitrary number related to the depth of penetration. For testing hard steels, a sphero-conical diamond is used with a 150 kg load, the result is read from the black scale on the dial and is prefixed with the letter C. A hardened tool steel would typically give a reading of 62Rc. For softer metals Scale B is used with a 1/16" diameter steel ball and a standard load of 100 kgs.


The process of shaping metal by passing it between rolls revolving at the same peripheral speed and in opposite directions. In steel there are a number of different types of rolling mill for processing the ingot to its finished shape. These are variously known as Cogging mills, Slabbing mills, Billet mills, Bar mills and Strip mills, which produce plate, sections, bars, sheet and strip. Cold rolling of previously hot rolled strip is carried out to produce strip that is accurate to size and with a smooth bright polished surface.

Rolling Lap

A fault arising from the overfilling or mis-alignment of rolls, the result is a bulge on the bar which is rolled into the metal and is lapped over. It remains throughout subsequent working and appears as a longitudinal crack.

Rotary Retort Furnace

A continuous-type furnace in which the work advances by means of an internal spiral, which gives good control of the retention time within the heated chamber.


Chemical symbol for Ruthenium.

Ruling Section

More accurately termed limiting ruling section. One of the most important factors associated with the choice of steel for a given purpose is to ensure that the desired mechanical properties are obtained throughout the section when the material has been heat treated. The limiting ruling section determines the maximum diameter or cross-section of a bar or component in which the specified properties can be achieved by a given heat treatment. The analysis of the steel also has an important bearing on this.


Chemical symbol for Sulfur.

Salt Bath

A method of heating steel using a bath of molten salts. Salt baths give uniform heating and prevent oxidation, they are used for hardening, tempering or quenching. The type of salt used depends on the temperature range required. For hardening, sodium cyanide, sodium carbonate and sodium chloride are in common use.


Chemical symbol for Antimony.


The oxidized surface of steel produced during hot working, as in rolling, and by exposure to air or steam at elevated temperature.


Also termed deseaming. It is a process for burning out defective areas on the surface of ingots or semi-finished products such as billets so that the product is suitable for subsequent rolling or forging.

Scleroscope Test

A hardness test in which the loss in kinetic energy of a falling metal "tup," absorbed by indentation upon impact of the tup on the metal being tested, is indicated by the height of rebound.


It forms the basic raw material for making steel by the electric arc process. Steel offers ecological advantages as it can be recycled enabling the discarded car of today to appear as part of a new model tomorrow. Scrap is sorted and graded before use and the necessary elements are added during the steel making process to achieve the desired specifications.


Chemical symbol for Selenium. Seams A surface defect caused during the steel making process. Seams are generally formed from blow holes in the ingot, non metallic inclusions, or stresses arising during the solidification stage. They appear as longitudinal discontinuities in the bar.

Secondary Hardness

An increase in hardness which sometimes occurs when hardened steel is re-heated. It can be caused by the transformation of retained austenite to martensite or by the precipitation of alloy carbides.


A term applied to the concentration and partial separation of one or more elements from solution during solidification of liquid steel in an ingot mould. Sulfur and phosphorus tend to segregate to a greater extent than other elements which can have a particular adverse effect on machinability in high sulfur free-cutting steels. Modern steel making and continuous casting have largely overcome this problem.

Selective Heating

Intentionally heating only certain portions of a work piece.

Selective Quenching

Quenching only certain portions of an object.


An element that closely resembles sulfur in its properties. The main use in steel is as a free cutting additive but due to high cost its use is limited to stainless steel. One of the benefits being the ability to obtain a very good surface finish on machined components.

Severity of Quench

Ability of quenching medium to extract heat from a hot steel work piece; expressed in terms of the H value.

SG Iron

An abbreviation for Spheroidal Graphite Cast Iron. As the name implies, graphite is present in spheroidal form instead of flakes and compared with Grey Cast Iron it has higher mechanical strength, ductility and increased shock resistance.

Shaker-Hearth Furnace

A continuous type furnace that uses a reciprocating shaker motion to move the parts along the hearth.


nbsp;(1) That type of force that causes or tends to cause two contiguous parts of the same body to slide relative to each other in a direction parallel to their plane of contact. (2) A type of cutting tool with which a material in the form of wire, sheet, plate or rod is cut between two opposing blades. (3) The type of cutting action produced by rake so that the direction of chip flow is other than at right angles to the cutting edge.

Shearing Test

The test applied to metal to determine the stress required to fracture it across its section.


A process developed in Britain in 1904 by Sherard Cowper-Coles. It is a method of producing a protective zinc coating on iron and steel products.


A thin piece of material placed between two surfaces to obtain a proper fit, adjustment, or alignment. The piece can also be analyzed to measure furnace carbon potential (that is, because while in the furnace it will quickly carburize to a level equal to the furnace carbon potential).

Shore Scleroscope

An instrument that measures the hardness of a sample in arbitrary terms of elasticity. A diamond tipped hammer is allowed to fall freely down a graduated glass tube on to the sample under test. The hardness is measured by the height of the rebound. In another form the rebounding hammer actuates the pointer of a scale so that the height of the rebound is recorded.

Slack Quenching

The incomplete hardening of steel due to quenching from the austenitizing temperature at a rate slower than the critical cooling rate for the particular steel, resulting in the formation of one or more transformation products in addition to martensite.

Slot Furnace

A common batch furnace where stock is charged and removed through a slot or opening.

Snap Temper

A precautionary interim stress-relieving treatment applied to high-hardenability steels immediately after quenching to prevent cracking because of delay in tempering them at the prescribed higher temperature.


Prolonged holding at a selected temperature to effect homogenization of structure or composition.

Solid Solution

A single, solid, homogenous crystalline phase containing two or more chemical species.

Solution Heat 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.


A chipping or flaking of a surface due to any kind of improper heat treatment or material dissociation.


Heating and cooling to produce a spheroidal or globular form of carbide in steel. Spheroidizing methods frequently used are: 1. Prolonged holding at a temperature just below Ae1; 2. Heating and cooling alternately between temperatures that are just above and just below Ae1; 3. Heating to a temperature above Ae1 or Ae3 and then cooling very slowly in the furnace or holding at a temperature just below Ae1; 4. Cooling at a suitable rate from the minimum temperature at which all carbide is dissolved, to prevent re-formation of a carbide network, and then reheating in accordance with method 1 or 2 above. (Applicable to hypereutectoid steel containing a carbide network.)


The formation of sheet metal blanks into hollow circular shapes. This is carried out on a lathe with forming tools which service to press and shape the metal. Annealing may be needed during and/or after the operation to remove the effects of work hardening.

Spot Welding

A process for joining steel sheets. The two parts are held between electrodes and the heat generated at the interface between the sheets causes local welding when pressure is applied.

Spray Quenching

A quenching process using spray nozzles to spray water or other liquids on a part. The quench rate is controlled by the velocity and volume of liquid per unit area per unit of time of impingement.

Spring Steel

The steels used for spring making depend on the application and type of spring. They range from plain carbon grades in the range 0.5% to 1.00% C. to Chromium, Chromium-Vanadium, Nickel-Chromium-Molybdenum, Silico- Manganese and Silicon-Manganese-Chromium-Molybdenum types. Full details can be found in BS5770.

Spring Temper

A temper of nonferrous alloys and some ferrous alloys characterized by tensile strength and hardness about two-thirds of the way from full hard to extra spring temper.


A term applied to a number of processes: a) A type of heat treatment to relieve internal stresses: b) The retarding or prevention of a particular reaction by the addition of a stabilizing element; c) A thermal and/or mechanical treatment given to magnetic material in order to increase the permanency of its magnetic properties or condition.

Stabilizing Treatment

1) Before finishing to final dimensions, repeatedly heating a ferrous or nonferrous part to or slightly above its normal operating temperature and then cooling to room temperature to ensure dimensional stability service. (2) Transforming retained austenite in quenched hardenable steels, usually cold treatment. (3) Heating a solution-treated stabilized grade of austenitic stainless steel to 879( to 900(C (1600( to 1650(F) to precipitate all carbon as TiC, NbC or TaC so that sensitization is avoided on subsequent exposure to elevated temperature.

Stainless Steel

Can be defined as a group of corrosion resisting steels containing a minimum 10% chromium and in which varying amounts of nickel, molybdenum, titanium, niobium as well as other elements may be present. An Englishman, Harry Brearley, is generally acknowledged to be the pioneer who developed stainless steels for commercial use.

Statistical Process Control

The application of statistical techniques for measuring and analyzing the variation in processes.

Statistical Quality Control

The application of statistical techniques for measuring and improving the quality of processes and products (includes statistical process control, diagnostic tools, sampling plans, and other statistical techniques).


Generally defined as a metallic product whose principal element is iron and where the carbon content is not more than 2%. (The presence of large quantities of carbide forming elements may modify the upper limit of the carbon content.)


Strain. A measure of the relative change in the size or shape of a body. Linear strain is the change per unit length of a linear dimension. True strain (or natural strain) is the natural logarithm of the ratio of the length at the moment of observation to the original gage length. Conventional strain is the linear strain over the original gage length. Shearing strain (or shear strain) is the change in angle (expressed in radians) between two lines originally in right angles. Deformation produced by a stress is expressed as the change per unit of original dimension or as angular displacement in the case of shear. When used alone, the term usually refers to the linear strain in the direction of the applied stress.

Strain Aging

The gradual changes in physical and mechanical properties, in particular hardness and tensile strength, which takes place following cold rolling or deformation. At atmospheric temperatures, this may take place over a number of weeks but can be accelerated by heating.

Strain Hardening

The loss of ductility and gain in hardness resulting from strain aging. An increase in hardness and strength caused by plastic deformation at temperatures below the recrystallization range.


Force or load per unit area, often thought of as force acting through a small area within a plane. Can be divided into normal, perpendicular to the surface, and shear, parallel to the surface, components. True stress denotes the stress where force and area are measured at the same time. Conventional stress, as applied to tension and compression tests, is force divided by original area. Nominal stress, ignoring stress raisers and disregarding plastic flow, in a notch bend test, for example, it is bending moment divided by minimum section modulus.

Stress Equalizing

A low-temperature heat treatment used to balance stresses in cold-worked material without an appreciable decrease in the mechanical strength produced by cold working.

Stress Relieving

A heat treatment including heating and soaking at a suitable temperature (e.g. 600-650°C) followed by cooling at an appropriate rate in order to reduce internal stresses without substantially modifying the steel's structure. This treatment may be used to relieve stresses induced by machining, quenching, welding or cold working.

Stress Strain Curve

A graph in which stress (load divided by the original cross sectional area of the test piece) is plotted against strain (the extension divided by the length over which it is measured).

Sub-Critical Annealing

Heating to, and holding at, some point below the critical temperature. Subsequent cooling may be in air. This form of heat treatment has a variety of uses depending on the temperature and specification of the steel, its purpose is often to soften the material.

Sub-zero Treatment

A low temperature treatment carried out after quenching on hardened steel to transform the retained austenite into martensite. It involves immersing the component in a bath of solid carbon dioxide at a temperature of minus 70-80°C.


Generally regarded as an impurity in steel as it can have detrimental effects on strength, ductility and weldability as well as producing hot and cold shortness. Its content in most steels is limited to a maximum of 0.050%. Sulfur is beneficial to machining and is added to freecutting steels in amounts up to 0.35% with the manganese content increased to overcome any detrimental effects.


Cooling below the temperature at which an equilibrium phase transformation can take place, without actually obtaining the transformation.


Heating above the temperature at which an equilibrium phase transformation should occur without actually obtaining the transformation.

Surface Hardening

A generic term covering several processes applicable to a suitable ferrous alloy that produces, by quench hardening only, a surface layer that is harder or more wear resistant than the core. There is no significant alteration of the chemical composition of the surface layer. The processes commonly used are carbonitriding, carburizing, induction hardening, flame hardening, nitriding, and nitrocarburizing. Use of the applicable specific process name is preferred.


A method of forming or reducing steel or other metals to a desired shape by a series of blows rapidly applied by dies or hammers. The process is applied to wires, rods and tubes and can be used for a variety of pointing, tapering, sizing and reducing operations.


The particles of metal arising from machining or grinding operations, much of it finds its way to the steel maker for remelting.


Chemical symbol for Tantalum.


A rare metal of silver white color having excellent corrosion resistance and a high melting point. It is widely used for chemical process equipment and specialized aero-space and nuclear applications.


Chemical symbol for Tellurium.


Its main use in the steel industry is as an additive in leadbearing freecutting steels to further improve their machinability. Its presence in the steel is either within the manganese sulfide particles, where it is partially soluble, or as particles combined with lead or manganese. For certain applications it offers significant improvements in machinability but the added cost is a factor that should be taken into account.


A term to which a number of definitions can be applied. These include: a) The operation of tempering; b) The degree of hardness left in a steel bar after quenching and tempering; c) The grading of the hardness of low carbon cold rolled strip, e.g. Hard, Half Hard, Quarter Hard, Skin Passed, Soft; d) An indication of the amount of carbon present in a tool steel, e.g. razor temper, file temper, die temper, etc.

Temper Brittleness

The loss in impact resistance that is present in some low and medium carbon alloy steels when tempered in the range of 350ºC - 600ºC. It is revealed by the notched bar impact test but not the tensile test.

Temper Color

A thin, tightly adhering oxide skin that forms when steel is tempered at a low temperature, or for a short time, in air or a mildly oxidizing atmosphere. Before the use of instruments such as pyrometers, colors were used to judge temperatures when hardening and tempering. For example, on carbon tool steel where the tempering range may typically be from 200ºC to 350ºC, the colors change with the rise in temperature giving Light Straw at around 210ºC, Purple at 275ºC, and Grey at 330ºC. The practice still continues in workshops where controlled heat treatment facilities are not available.

Temper Rolling

A light pass given to annealed cold rolled strip to prevent the formation of kinks and stretcher strain markings on subsequent cold working. Also termed Pinch pass and Skin pass.

Tempered Martensite Embrittlement

Embrittlement of ultrahigh-strength steels caused by tempering in the temperature range of 205 to 400°C (400 to 750°F); also called 350°C or 500°F embrittlement. Tempered martensite embrittlement is thought to result from the combined effects of cementite precipitation on prior-austenite grain boundaries or interlath boundaries and the segregation of impurities at prior-austenite grain boundaries.


A heat treatment applied to ferrous products after hardening. It consists of heating the steel to some temperature below the transformation range and holding for a suitable time at the temperature, followed by cooling at a suitable rate. The object of tempering is to decrease hardness and increase toughness to produce the desired combination of mechanical properties.

Tensile Strength

The maximum load applied in breaking a tensile test piece divided by the original cross-sectional area of the test piece. Originally quoted as tons/ it is now measured as Newtons/ Also termed Maximum Stress and Ultimate Tensile Stress.

Tensile Test

A standard test piece is gripped at either end by suitable apparatus in a testing machine which slowly exerts an axial pull so that the steel is stretched until it breaks. The test provides information on proof stress, yield point, tensile strength, elongation and reduction of area.

Thermal Fatigue

Fracture resulting from the presence of temperature gradients that vary with time in such a manner as to produce cyclic stresses in a structure.

Thermal Shock

The development of a steep temperature gradient and accompanying high stresses within a structure.

Thermal Stress

Stresses in metal resulting from non-uniform temperature distribution.

Thermocouple, Expendable

Such a thermocouple is made of fabric- or plastic-covered wire. The wire is provided in coils or on spools. Insulation usually consists of glass braid, asbestos, or ceramic fiber cloth on each conductor plus glass braid overall.

Thermocouple, Non-Expendable

Those thermocouples that are not covered with fabric or plastic insulation. One type consists of ceramic insulators over bare thermocouple wire, sometimes inserted in a tube for stability and protection. A second type consists of a combination of thermocouple wires, mineral insulation, and a protecting metal sheath compacted into a small diameter.

Thomas Process

The Continental name for the basic Bessemer steel making process, now superseded by modern day BOS plants.


Chemical symbol for Titanium.

Time Temperature Transformation Curve

An isothermal transformation diagram showing the relationship between temperature and the time taken for the decomposition of austenite when the transformation occurs at constant temperature.


When present in steel it is an undesirable impurity which gives rise to temper brittleness. When used as a coating on steel, it has a good resistance to corrosion for many applications.


Small amounts added to steel contribute to its soundness and give a finer grain size. In austenitic stainless steels it acts as a carbide stabilizer and is used to prevent intercrystalline corrosion, commonly termed "weld decay". Titanium carbide is also used with tungsten carbide in the manufacture of hard metal tools.


The amount of variation permitted on dimensions or surfaces. The tolerance is equal to the difference between the maximum and minimum limits of any specified dimension.

Tool Steel

A generic term applied to a wide range of steels, both plain carbon and alloy. It includes steels suitable for various types of cutting tools, press tools, hot and cold heading dies, moulds for plastics and die- casting, extrusion tools, hand tools, etc.

Torsional Strength

The resistance of a bar to twisting. Closely related to its shear strength.

Total Carbon

The sum of the free and combined carbon (including carbon in solution) in a ferrous alloy.


The ability of a metal to rapidly distribute within itself both the stress and strain caused by a suddenly applied load, or more simply expressed, the ability of a material to withstand shock loading. It is the exact opposite of "brittleness" which carries the implication of sudden failure. A brittle material has little resistance to failure once the elastic limit has been reached.

Transformation Hardening

Heat treatment comprising austenitization followed by cooling under conditions such that the austenite transforms more or less completely into martensite and possibly into bainite.

Transformation Range

The temperature range in which a constitutional change occurs on heating or cooling a metal in the solid state. It is the range where austenite forms and ferrite or carbide progressively dissolves while ferrous alloys are being heated. Also, the temperature range within which austenite decomposes to form ferrite and carbide on cooling.

Transformation Temperature

The temperature at which a change in phase occurs or the limiting temperature of a transformation range. These critical points are denoted by symbols, e.g. Ac1; the temperature at which austenite begins to form on heating. There are 12 principal temperatures to which symbols are applied. The following symbols are used for irons and steels: Accm. In hypereutectoid steel, the temperature at which solution of cementite in austenite is completed during heating; Ac1. The temperature at which austenite begins to form during heating; Ac3. The temperature at which transformation of ferrite to austenite is completed during heating; Ac4. The temperature at which austenite transforms to delta ferrite during heating; Aecm, Ae1, Ae3, Ae4. The temperatures of phase changes at equilibrium; Arcm. In hypereutectoid steel, the temperature at which precipitation of cementite starts during cooling; Ar1. The temperature at which transformation of austenite to ferrite or to ferrite plus cementite is completed during cooling; Ar3. The temperature at which austenite begins to transform to ferrite during cooling; Ar4. The temperature at which delta ferrite transforms to austenite during cooling; Ar°. The temperature at which transformation of austenite to pearlite starts during cooling; M1. The temperature at which transformation of austenite to martensite is completed during cooling; M1 (or Ar°). The temperature at which transformation of austenite to martensite starts during cooling; NOTE: All these changes, except formation of martensite, occur at lower temperatures during cooling than during heating, and depend on the rate of change temperature.

Transgranular Cracking

Cracking or fracturing that occurs through or across a crystal or grains. Also called transcrystalline cracking.

Transgranular Fracture

Fracture through or across the crystals or grains of a metal. Also called transcrystalline fracture or intracrystalline fracture.

Transition Temperature

The temperature at which a transition from ductile to brittle fracture takes place in steel. It is usually determined by making a series of Charpy impact tests at various temperatures, the transition temperature is usually taken as the point where 50% of the fracture is brittle.

Transverse Strength

A measurement of strength when the load is applied across the longitudinal flow of the grain of a metal. Certain impurities such as sulfur have a detrimental effect on the transverse strength. This can be minimized by the inclusion modification process.

Transverse Test

A test taken at right angles to the principal direction of rolling or forging.

TTT Curve

An abbreviation of Time Temperature Transformation Curve.


A form of surface hardening, the process involves nitrogen but does not achieve the hardness of conventional nitriding.


When used as an alloying element it increases the strength of steel at normal and elevated temperatures. Its "red hardness" value makes it suitable for cutting tools as it enables the tool edge to be maintained at high temperatures. In conjunction with other alloying elements it finds applications in heat resisting and other severe service conditions.


Chemical symbol for Uranium.

Ultimate Analysis

In chemistry, this is a quantitative analysis in which percentages of all elements in the substance are determined.

Ultimate Tensile Strength

The highest load applied in breaking a tensile test piece divided by the original cross- sectional area of the test piece.

Ultrasonic Inspection

A means of locating defects in steel. When acoustic energy in the ultrasonic range is passed through steel, the sound waves tend to travel in straight lines, rather than diffusing in all directions as they do in the audible range. If there is a defect in the path of the beam it will cause a reflection of some of the energy, depleting the energy transmitted. This casts an acoustic shadow which can be monitored by a detector placed opposite the transducer or energy source. If the acoustic energy is introduced as a very short burst, then the reflected energy coming back to the originating transducer can also be used to show the size and depth of the defect. Ultrasonic techniques can be used to detect deeply located defects or those contained in the surface layer. Skill and experience are required in interpreting the results portrayed on the cathode ray tube.

Unkilled Steel

Steel which has been insufficiently deoxidized and evolves gas during solidification with the formation of blow-holes.


Working a piece of steel so that its length is shortened and its cross-sectional area is increased. Its effect is to increase ductility in the radial and tangential directions.


A white malleable metal which is softer than steel. Its specific gravity is 18.7, it melts at a temperature of 2400ºC.


Chemical symbol for Vanadium.

Vacuum Annealing

Annealing carried out at sub-atmospheric pressure.

Vacuum Arc Remelting

A process used for producing advanced steels to the most demanding and critical specifications, particularly in such areas as aerospace applications. The steel is first produced to a very close analysis and the resulting ingot is slowly remelted in a Vacuum Arc Remelting furnace for up to 14 hours. Such steels are, by necessity, expensive to manufacture.

Vacuum Degassing

A ladle of molten metal is placed within a chamber which is then evacuated. This reduces the gas content, particularly hydrogen, as well as reducing non- metallic inclusions. Modern secondary steel making processes using Vacuum Arc Degassing units that include automated stirring and control of temperature and chemical analysis, ensure a consistent and high quality product.

Vacuum Deposition

Condensation of thin metal coatings on the cool surface of work in a vacuum.

Vacuum Furnace

A furnace using low atmospheric pressures instead of a protective gas atmosphere like most heat-treating furnaces. Vacuum furnaces are categorized at hot wall or cold wall, depending on the location of the heating and insulating components.


Steels containing vanadium have a much finer grain structure than steels of similar composition without vanadium. It raises the temperature at which grain coarsening sets in and increases hardenability where it is in solution in the austenite prior to quenching. It also lessens softening on tempering and confers secondary hardness on high speed steels. Vanadium is used in nitriding, heat resisting, tool and spring steels in conjunction with other alloying elements.

Vapor Plating

Deposition of a metal or compound on a heated surface by reduction or decomposition of a volatile compound at a temperature below the melting points of the deposit and the base material. The reduction is usually accomplished by a gaseous reducing agent such as hydrogen. The decomposition process may involve thermal dissociation or reaction with the base material. Occasionally used to designate deposition on cold surfaces by vacuum evaporation.

Vickers Hardness Test

A method of determining the hardness of steel whereby a diamond pyramid is pressed into the polished surface of the specimen and the diagonals of the impression are measured with a microscope fitted with a micrometer eye piece. The rate of application and duration are automatically controlled and the load can be varied.


Chemical symbol for Tungsten, from wolfram.

Water Quenching

A quench in which water is the quenching medium. The major disadvantage of water quenching is its poor efficiency at the beginning or hot stage of the quenching process.


The process of joining together two pieces of metal so that bonding accompanied by appreciable interatomic penetration takes place at their original boundary surfaces. The boundaries more or less disappear at the weld, and integrating crystals develop across them. Welding is carried out by the use of heat or pressure or both and with or without added metal. There are many types of welding including Metal Arc, Atomic Hydrogen, Submerged Arc, Resistance Butt, Flash, Spot, Stitch, Stud and Projection.


Thin hair-like growths on metal that are barely visible to the naked eye, they are stronger than the metals from which they are formed, probably because they are free from defects.

White Annealing

A heat treatment process carried out on pickled steel with the objective of eliminating the hydrogen that has entered the steel during the pickling operation and thus removing any tendency to hydrogen embrittlement.

White Layer

Compound layer that forms as a result of the nitriding process.

Widmanstatten Structure

A microstructure resulting when steels are cooled at a critical rate from extremely high temperatures. It consists of ferrite and pearlite and has a cross-hatched appearance due to the ferrite having formed along certain crystallographic planes.


The alternative name for tungsten.

Woody Fracture

A fracture that is fibrous or woody in appearance due to the elongation of the individual grains. This may be accentuated by the presence of slag or by a banded structure. It is grey and dull and is characteristic of ductile but non-homogeneous material such as wrought iron.

Work Hardening

The increase in hardness and strength produced by cold plastic deformation or mechanical working.

Working Zone

That portion of the enclosed volume of a piece of thermal processing equipment occupied by parts or raw material during the soaking portion of a thermal treatment. It is usually, but not always, a high percentage of the total enclosed volume. It may include more than one control zone.

Wrought Iron

A commercial iron that has little use today and has been replaced by mild steel. It was commonly produced by the puddling process. The temperatures employed in its production are too low to render it fluid, it is heated until it forms a pasty mass then it is squeezed or forged. The process does not lend itself to removal of impurities so it contains an appreciable quantity of slag. It will not respond to any heat treatment designed to increase the hardness or strength.

X-Ray Crystallography

X-ray photographs of metals are a means of providing information which in many cases cannot be obtained by microscopic methods. The lines produced by each element, or phase are characteristic, and their general pattern enables the crystalline structure to be identified. The scale of the pattern can be used to determine accurately the size of the unit cell and, therefore, the distance apart of the individual atoms. From the relative intensity of the lines it is possible to deduce the distribution throughout the unit cell, the various types of atoms in an alloy or the degree of preferred orientation in the material.

Yield Point

Can be defined as the point where a tensile test piece begins to extend permanently. If the load is reduced to zero, the test piece will not return to its original length.

Yield Strength

The stress at which general plastic elongation of the test piece takes place. This point is well defined in hardened and tempered or annealed structures but can be ill defined in "as drawn" structures. It is the stress at which a material exhibits a specified deviation from proportionality of stress and strain. An offset of 0.2% is used for many metals. Compare with tensile strength.

Young's Modulus

A measure of rigidity based on the ratio of stress to corresponding strain in an elastic material. Within the limits of elasticity, the ratio of the linear stress to the linear strain is termed the modulus of elasticity or Young’s Modulus and may be written as E =(Stress/Strain). It is this property that determines how much a bar will sag under its own weight or under a loading when used as a beam within its limit of proportionality. For steel, Young’s Modulus is of the order of 205000 N/mm2.


Zinc is a metallic chemical element, it has a white color with a bluish tinge. It has a high resistance to atmospheric corrosion and a major use is as a protective coating for iron and steel sheet and wire. Galvanized sheets are a prime example. The melting point of zinc is 419ºC.


Acts as a deoxidizing element in steel and combines with sulfur.


Chemical symbol for Zinc.


Chemical symbol for Zirconium. was derived from the fact that it is capable of cutting metal at a much higher rate than carbon tool steel and continues to cut and retain its hardness even when the point of the tool is heated to a low red temperature. Tungsten is the major alloying element but it is also combined with molybdenum, vanadium and cobalt in varying amounts. Although replaced by cemented carbides for many applications it is still widely used for the manufacture of taps, dies, twist drills, reamers, saw blades and other cutting tools.