Glossary of Terms used in Heat Treatment of Iron and Steel
Glossary of Terms used in Heat Treatment of Iron and Steel
Steel is normally defined as an alloy of iron and carbon with the carbon content ranging between a few hundred of a percent upto around 2 %. Other alloying elements can amount in total to around 5 % in low-alloy steels and higher in more highly alloyed steels such as tool steels and stainless steels. Steels can show a wide variety of properties depending on composition as well as the phases and micro constituents present, which in turn depend on the heat treatment. Heat treatment of steel is governed by iron-carbon phase diagram (Fig 1).
Fig 1 Iron- carbon phase diagram
A large number of terms are used in the heat treatment of steels. They are described below.
Acicular ferrite – It is a highly sub-structured non-equiaxed ferrite which forms upon continuous cooling by a mixed diffusion and shear mode of transformation which begins at a temperature slightly higher than the temperature transformation range for upper bainite. It is distinguished from bainite in that it has a limited amount of carbon available and hence, there is only a small amount of carbide present.
Aerated bath nitriding – It is a type of liquid nitriding in which air is pumped through the liquid bath creating agitation and increased chemical activity.
Age hardening – It is the hardening by aging, normally after rapid cooling or cold working.
Age softening – It is the spontaneous decrease of strength and hardness which takes place at room temperature in certain strain hardened alloys, especially those of aluminum.
Aging – It is a change in the properties of certain metals and alloys which occurs at ambient or moderately elevated temperatures after hot working or a heat treatment (quench aging in ferrous alloys, natural or artificial aging in ferrous and non-ferrous alloys) or after a cold working operation (strain aging). The change in properties is often, but not always, due to a phase change (precipitation), but never involves a change in chemical composition of the metal or alloy.
Air-hardening steel – It is steel containing sufficient carbon and other alloying elements to harden fully during cooling in air or other gaseous mediums from a temperature above its transformation range. The term is generally restricted to steels which are capable of being hardened by cooling in air in fairly large sections, around 50 mm or more in diameter.
Allotropy – It is a near synonym for polymorphism. Allotropy is generally restricted to describing polymorphic behaviour in elements, terminal phases, and alloys whose behaviour closely parallels that of the predominant constituent element.
Alpha iron – It is the body-centered cubic form of pure iron, stable below 910 deg C.
Annealing – It is a generic term denoting a treatment, consisting of heating to and holding at a suitable temperature followed by cooling at a suitable rate, used primarily to soften metallic materials, but also to simultaneously produce desired changes in other properties or in microstructure. The purpose of such changes can be, but is not confined to (i) inducing softness, (ii) improvement of machinability, (iii) facilitation of cold work, (iv) improving cold working properties, (v) improvement of mechanical or electrical properties, (vi) obtaining a desired structure, (vii) removing stresses, and / or (viii) increase in stability of dimensions. When the term is used without qualification, full annealing is implied. When applied only for the relief of stress, the process is properly called stress relieving or stress relief annealing. The time temperature cycle used vary widely both in maximum temperature attained and in cooling rate employed, depending on the composition of the steel, its condition, and the result desired.
In ferrous alloys, annealing normally is done above the upper critical temperature, but the time-temperature cycles vary widely in both maximum temperatures attained and in cooling rate employed, depending on composition, material condition, and results desired. When applicable, the commercial process names are used. These are black annealing, blue annealing, box annealing, bright annealing, cycle annealing, flame annealing, full annealing, graphitizing, inter-critical annealing, isothermal annealing, malleablizing, order hardening, process annealing, quench annealing, spheroidizing, and sub-critical annealing.
Annealing carbon – It is the fine, apparently amorphous carbon particles formed in white cast iron and certain steels during prolonged annealing. It is also called temper carbon.
Annealing twin – It is a twin form in a crystal during recrystallization.
Anneal to temper – It is a final partial anneal which softens a cold worked alloy to a specified level of hardness or tensile strength.
Artificial aging – It is aging above room temperature.
Athermal transformation – It is a reaction which proceeds without benefit of thermal fluctuations. Hence, thermal activation is not required. In contrast, a reaction which occurs at constant temperature is an isothermal transformation. Hence, thermal activation is necessary in this case and the reaction proceeds as a function of time.
Ausforming – It is thermo-mechanical treatment of steel in the metastable austenitic condition below the recrystallization temperature followed by quenching to obtain martensite and / or bainite.
Austempering – It is a heat treatment for ferrous alloys in which a part is quenched from the austenitizing temperature at a rate fast enough to avoid formation of ferrite or pearlite and then held at a temperature just above Ms temperature until transformation to bainite is complete. It is carried out in a medium having a rate of heat abstraction high enough to prevent the formation of high temperature transformation products. Although designated as bainite in both austempered steel and austempered ductile iron (ADI), austempered steel consists of two phase mixtures containing ferrite and carbide, while ADI consists of two phase mixtures containing ferrite and austenite.
Austenite – It is a solid solution of one or more elements in face-centered cubic iron. Unless otherwise designated (such as nickel austenite), the solute is generally assumed to be carbon.
Austenitic grain size – It is the size attained by the grains of steel when heated to the austenitic region. It can be revealed by appropriate etching of cross sections after cooling to room temperature.
Austenitizing – It is 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.
Bainite – It is a metastable aggregate consisting of dispersed carbide in ferrite resulting from the transformation of austenite at temperatures below the pearlite range but above Ms temperature. Its appearance is in the form of relatively coarse ferrite laths between which carbides are precipitated as platelets if formed in the upper part of the bainite transformation range and acicular, resembling tempered martensite, if formed in the lower part.
Bainitic hardening – It is quench-hardening treatment resulting principally in the formation of bainite.
Batch furnace – It is a furnace which is 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.
Belt furnace – It is a continuous-type furnace which uses a mesh-type or cast-link belt to carry parts through the furnace.
Beta annealing – It consists of 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.
Black annealing – It is box annealing or pot annealing ferrous alloy sheet, strip, and wire to impart a black colour to the oxidized surface.
Black oxide – It is a black finish on a metal produced by immersing it in hot oxidizing salts or salt solutions.
Blank carburizing – It consists of simulating the carburizing operation without introducing carbon. This is usually accomplished by using an inert material in place of the carburizing agent, or by applying a suitable protective coating to the ferrous alloy.
Blank nitriding – It consists of simulating the nitriding operation without introducing nitrogen. This is normally accomplished by using an inert material in place of the nitriding agent or by applying a suitable protective coating to the ferrous alloy.
Blue annealing – It consists of heating hot-rolled ferrous sheet in an open furnace to a temperature within the transformation range and then cooling in air, in order to soften the metal. The formation of a bluish oxide on the surface is incidental.
Blue brittleness – It is the brittleness shown by some steels after being heated to some temperature within the range of around 205 deg C to 370 deg C, particularly if the steel is worked at the elevated temperature. Killed steels are virtually free of this kind of brittleness.
Bluing – It consists of subjecting the scale-free surface of a ferrous alloy to the action of air, steam, or other agents at a suitable temperature, thus forming a thin blue film of oxide and improving the appearance and resistance to corrosion. This term is ordinarily applied to sheet, strip, or finished parts. It is used also to denote the heating of springs after fabrication to improve their properties.
Boriding or boronizing – It is the thermo-chemical treatment involving the enrichment of the surface layer of an object with borides. This surface-hardening process is performed below the Ac1 temperature.
Box annealing – It is annealing a metal or alloy in a sealed container under conditions which minimize oxidation. In box annealing a ferrous alloy, the charge is usually heated slowly to a temperature below the transformation range, but sometimes above or within it, and is then cooled slowly. This process is also called close annealing or pot annealing.
Breaks – Breaks are creases or ridges normally in ‘untempered’ or in aged material where the yield point has been exceeded. Depending on the origin of the break, it can be termed a cross break, a coil break, an edge break, or a sticker break.
Bright annealing – It consists of annealing in a protective medium to prevent discoloration of the bright surface.
Bright nitriding – It consists of nitriding in a protective medium to prevent discoloration of the bright surface.
Brine quenching – It is a quench in which brine (salt water-chlorides, carbonates, and cyanides) is the quenching medium. The salt addition improves the efficiency of water at the vapour phase or hot stage of the quenching process.
Brittle fracture – It consists of separation of a solid accompanied by little or no macroscopic plastic deformation. Typically, brittle fracture occurs by rapid crack propagation with less expenditure of energy than for ductile fracture.
Burning – It consists of (i) permanently damaging a metal or alloy by heating to cause either incipient melting or inter-granular oxidation or (ii) in grinding, getting the work hot enough to cause discoloration or to change the microstructure by tempering or hardening.
Calorizing – It consists of imparting resistance to oxidation to an iron or steel surface by heating in aluminum powder at 800 deg C to 1000 deg C.
Carbo-nitriding – It is a case hardening process in which a suitable ferrous material is heated above the lower transformation temperature in a gaseous atmosphere of such composition as to cause simultaneous absorption of carbon and nitrogen by the surface and, by diffusion, create a concentration gradient. The process is completed by cooling at a rate which produces the desired properties in the work piece.
Carbonization – It consists of conversion of an organic substance into elemental carbon.
Carbon potential – It is a measure of the ability of an environment containing active carbon to alter or maintain, under prescribed conditions, the carbon level of the steel. In any particular environment, the carbon level attained depends on such factors as temperature, time, and steel composition.
Carbon restoration – It consists of replacing the carbon lost in the surface layer from previous processing by carburizing this layer substantially to the original carbon level. It is sometimes called recarburizing.
Carburizing – It consists of absorption and diffusion of carbon into solid ferrous alloys by heating, to a temperature usually above Ac3, in contact with a suitable carbonaceous material. It is a form of case hardening which produces a carbon gradient extending inward from the surface, enabling the surface layer to be hardened either by quenching directly from the carburizing temperature or by cooling to room temperature, then reaustenitizing and quenching.
Carburizing flame – It consists of a gas flame which introduces carbon into some heated metals, as during a gas welding operation. A carburizing flame is a reducing flame, but a reducing flame is not necessarily a carburizing flame.
Car furnace – It is 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.
Case – It is that portion of a ferrous alloy, extending inward from the surface, whose composition has been altered so that it can be case hardened. Typically considered to be the portion of the alloy (i) whose composition has been measurably altered from the original composition, (ii) which appears dark on an etched cross section, or (iii) which has hardness, after hardening, equal to or greater than a specified value. It differs with the core.
Case hardening – It is a generic term covering several processes applicable to steel which change the chemical composition of the surface layer by absorption of carbon, nitrogen, or a mixture of the two and, by diffusion, create a concentration gradient. Normally case hardening is followed by suitable heat treatment. It is also used to designate the hardened surface layer of a piece of steel which is large enough to have a distinctly softer core or centre. The processes normally used are carburizing and quench hardening, cyaniding, nitriding, and carbo-nitriding. The use of the applicable specific process name is preferred.
Caustic quenching – It consists of quenching with aqueous solutions of 5 % to 10 % sodium hydroxide (NaOH).
Cementation – It is the introduction of one or more elements into the outer portion of a metal object by means of diffusion at high temperature.
Cementite – It is a compound of iron and carbon, known chemically as iron carbide and having the approximate chemical formula Fe3C. It is characterized by an orthorhombic crystal structure. When it occurs as a phase in steel, the chemical composition is altered by the presence of manganese and other carbide-forming elements.
Checks – They are numerous, very fine cracks in a coating or at the surface of a metal part. Checks can appear during processing or during service and are most often associated with thermal treatment or thermal cycling. They are also called check marks, checking, or heat checks.
Coalescence – It is the growth of grains at the expense of the remainder by absorption or the growth of a phase or particle at the expense of the remainder by absorption or re-precipitation.
Coarsening – It is an increase in the grain size, normally, but not necessarily, by grain growth.
Coherent precipitate – It is a crystalline precipitate which forms from solid solution with an orientation that maintains continuity between the crystal lattice of the precipitate and the lattice of the matrix, normally accompanied by some strain in both lattices. Because the lattices fit at the interface between precipitate and matrix, there is no discernible phase boundary.
Cold die quenching – It consists of a quench utilizing cold, flat, or shaped dies to extract heat from a part. Cold die quenching is slow, expensive, and is limited to smaller parts with large surface areas. It is also called cold dry die quenching.
Cold treatment – It is exposing the steel to suitable sub-zero temperature for the purpose of obtaining desired conditions or properties, such as dimensional or micro-structural stability. When the treatment involves the transformation of retained austenite, it is normally followed by a tempering treatment.
Columnar structure – It is a coarse structure of parallel elongated grains formed by unidirectional growth, most often observed in castings, but sometimes in structures resulting from diffusional growth accompanied by a solid-state transformation.
Combined carbon – It is that part of the total carbon in steel or cast iron which is present as other than free carbon.
Conditioning heat treatment – It is a preliminary heat treatment used to prepare a material for desired reaction to a subsequent heat treatment. For the term to be meaningful, the exact heat treatment is to be specified.
Congruent transformation – It is an isothermal or isobaric phase change in which both of the phases concerned have the same composition throughout the process.
Continuous cooling transformation (CCT) diagram – It is a set of curves drawn using logarithmic time and linear temperature as coordinates, which defines for each cooling curve the beginning and end of the transformation of the initial phase.
Continuous precipitation – It consists of precipitation from a supersaturated solid solution in which the precipitate particles grow by long range diffusion without recrystallization of the matrix. Continuous precipitates grow from nuclei distributed more or less uniformly throughout the matrix. The precipitates are normally randomly oriented, but can form a Widmanstatten structure. It is also called general precipitation.
Continuous-type furnace – It is a furnace used for heat treating materials which progress continuously through the furnace, entering one door and being discharged from another. Examples are belt furnace, direct-fired tunnel-type furnace, rotary retort furnace, and shaker-hearth furnace.
Controlled cooling – It consists of cooling from an elevated temperature in a predetermined manner, to avoid hardening, cracking, or internal damage, or to produce desired micro-structure or mechanical properties.
Cooling curve – It consists of a curve showing the relation between time and temperature during the cooling of a material.
Cooling stresses – These are residual stresses resulting from non uniform distribution of temperature during cooling.
Core – It is the interior portion of steel which is substantially softer than the surface layer (case) after case hardening. The term core is also used to designate the relatively soft central portion of certain hardened tool steels. Typically considered to be the portion which (i) appears light on an etched cross section, (ii) has an essentially unaltered chemical composition, or (iii) has a hardness, after hardening, less than a specified value.
Critical cooling rate – It is the rate of continuous cooling required to prevent undesirable transformation. For steel, it is the minimum rate at which austenite is to be continuously cooled to suppress transformations above the Ms temperature.
Critical diameter – It is the diameter of the bar which can be fully hardened with 50 % martensite at its centre.
Critical point – It is the transformation temperature at which (i) the temperature or pressure at which a change in crystal structure, phase or physical properties occurs, or (ii) in an equilibrium diagram, that specific value of composition, temperature and pressure, or combinations thereof, at which the phases of a heterogeneous system are in equilibrium.
Critical strain – It is the strain just sufficient to cause recrystallization. Since the strain is small, normally only a few percent, recrystallization takes place from only a few nuclei, which produces a recrystallized structure consisting of very large grains.
Critical temperature – It consists of (i) synonymous with critical point if the pressure is constant, and (ii) the temperature above which the vapour phase cannot be condensed to liquid by an increase in pressure.
Critical temperature ranges – These are synonymous with transformation ranges, which is the preferred term.
Cryogenic treatment – A cryogenic treatment is the process of treating work pieces to cryogenic temperatures (i.e. below −190 deg C) in order to remove residual stresses and improve wear resistance on steels and even composites. In addition to seeking enhanced stress relief and stabilization, or wear resistance, cryogenic treatment is also sought for its ability to improve corrosion resistance by precipitating micro-fine eta carbides, which can be measured before and after in a part.
Curie temperature – It is the temperature of magnetic transformation below which a metal or alloy is ferromagnetic and above which it is paramagnetic.
Cyaniding – It is a case-hardening process in which a ferrous material is heated above the lower transformation range in a molten salt containing cyanide to cause simultaneous absorption of carbon and nitrogen at the surface and, by diffusion, create a concentration gradient. Quench hardening completes the process.
Cycle annealing – It is an annealing process employing a predetermined and closely controlled time-temperature cycle to produce specific properties or micro-structures.
Dead soft – It is a temper of nonferrous alloys and some ferrous alloys corresponding to the condition of minimum hardness and tensile strength produced by full annealing.
Decalescence – It is a phenomenon, associated with the transformation of alpha iron to gamma iron on the heating (super-heating) of iron or steel, revealed by the darkening of the metal surface owing to the sudden decrease in temperature caused by the fast absorption of the latent heat of transformation. It differs with recalescence.
Decarburization – It consists of loss of carbon from the surface layer of a carbon-containing alloy due to reaction with one or more chemical substances in a medium which contacts the surface.
Degrees of freedom – It is the number of independent variables (such as temperature, pressure, or concentration within the phases present) which can be altered at will without causing a phase change in an alloy system at equilibrium, or it is the number of such variables which are to be fixed arbitrarily to define the system completely.
Dew point – It is the temperature and pressure at which a gas begins to condense to a liquid.
Dew point analyzer – It is an atmosphere monitoring device which measures the partial pressure of water vapour in an atmosphere.
Differential heating – It consists of any method of heating so controlled as to produce a desired non uniform temperature distribution in a steel object. It is a heating process by which the temperature is made to vary throughout the steel object being heated so that on cooling different portions, a desired stress distribution or variation in properties is present within the object.
Differential quenching – It is the selective quenching of the different parts of the same steel object.
Diffusion – It consists of (i) spreading of a constituent in a gas, liquid, or solid, tending to make the composition of all parts uniform, or (ii) the spontaneous movement of atoms or molecules to new sites within a material.
Diffusion coefficient – It is a factor of proportionality representing the amount of substance diffusing across a unit area through a unit concentration gradient in unit time.
Dilatometer – It is an instrument for measuring the linear expansion or contraction in a metal resulting from changes in such factors as temperature and allotropy.
Direct-fired tunnel-type furnace – It is 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.
Direct quenching – It consists of quenching carburized parts directly from the carburizing operation. It is also used for quenching pearlitic malleable parts directly from the malleablizing operation.
Discontinuous precipitation – It is the precipitation from a supersaturated solid solution in which the precipitate particles grow by short range diffusion, accompanied by recrystallization of the matrix in the region of precipitation. Discontinuous precipitates grow into the matrix from nuclei near grain boundaries, forming cells of alternate lamellae of precipitate and depleted (and recrystallized) matrix. It is often referred to as cellular or nodular precipitation.
Dissociation – It is as applied to heterogeneous equilibria, the transformation of one phase into two or more new phases of different composition.
Double aging – It consists of employment of two different aging treatments to control the type of precipitate formed from a super-saturated matrix in order to obtain the desired properties. The first aging treatment, sometimes referred to as intermediate or stabilizing, is normally carried out at higher temperature than the second.
Double tempering – it is a treatment in which a quench-hardened ferrous metal is subjected to two complete tempering cycles, normally at substantially the same temperature, for the purpose of ensuring completion of the tempering reaction and promoting stability of the resulting micro-structure.
Drawing – It is drawing the temper and is synonymous with tempering.
Ductile cast iron – It is a cast iron which has been treated while molten with an element such as magnesium or cerium to induce the formation of free graphite as nodules or spherulites, which imparts a measurable degree of ductility to the cast metal. It is also known as nodular cast iron, spherulitic graphite cast iron, and SG iron.
Ductile fracture – It is the fracture characterized by tearing of metal accompanied by appreciable gross plastic deformation and expenditure of considerable energy.
Ductility – It is the ability of a material to deform plastically without fracturing, measured by elongation or reduction of area in a tensile test, by height of cupping in an Erichsen test, or by other means.
475 deg C embrittlement – It is the embrittlement of stainless steels upon extended exposure to temperatures between 400 deg C and 510 deg C. This type of embrittlement is caused by fine, chromium-rich precipitates which segregate at grain boundaries with the time at temperature directly influencing the amount of segregation. Grain boundary segregation of the chromium-rich precipitates increases strength and hardness, decreases ductility and toughness, and changes corrosion resistance. This type of embrittlement can be reversed by heating above the precipitation range.
Elastic limit – It is the maximum stress which a material is capable of sustaining without any permanent strain (deformation) remaining upon complete release of the stress.
Electron-beam heat treating – It is a selective surface hardening process which rapidly heats a surface by direct bombardment with an accelerated stream of electrons.
Embrittlement – It is the severe loss of ductility or toughness or both, of a material, normally a metal or alloy. Many forms of embrittlement can lead to brittle fracture. Many forms can occur during thermal treatment or elevated-temperature service (thermally induced embrittlement). Some of these forms of embrittlement, which affect steels, include blue brittleness , 475 deg C embrittlement, quench-age embrittlement, sigma-phase embrittlement, strain-age embrittlement, temper embrittlement, tempered martensite embrittlement, and thermal embrittlement. In addition, steels and other metals and alloys can be embrittled by environmental conditions (environmentally assisted embrittlement). The forms of environmental embrittlement include acid embrittlement, caustic embrittlement, corrosion embrittlement, creep-rupture embrittlement, hydrogen embrittlement, liquid metal embrittlement, neutron embrittlement, solder embrittlement, solid metal embrittlement, and stress-corrosion cracking.
Enantiotropy – It is the relation of crystal forms of the same substance in which one form is stable above a certain temperature and the other form stable below that temperature. Ferrite and austenite are enantiotropic in ferrous alloys.
End-quench hardenability test – It is a laboratory procedure for determining the hardenability of a steel or other ferrous alloy. It is 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 is normally plotted as hardness versus distance from the quenched end.
Equilibrium diagram – It is 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 normally considered constant.
Eutectic – It consists of (i) 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, (ii) an alloy having the composition indicated by the eutectic point on an equilibrium diagram, and (iii) an alloy structure of intermixed solid constituents formed by a eutectic reaction.
Eutectic alloy – It is the alloy composition which freezes at constant temperature similar to a pure metal. It is the lowest melting (or freezing) combination of two or more metals. The alloy structure (homogeneous) of two or more solid phases formed from the liquid eutectically.
Eutectic carbide – It is carbide formed during freezing as one of the mutually insoluble phases participating in the eutectic reaction of ferrous alloys.
Eutectic melting – It is melting of localized microscopic areas whose composition corresponds to that of the eutectic in the system.
Eutectoid – It consists of (i) an isothermal reversible reaction in which a solid 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, (ii) an alloy having the composition indicated by the eutectoid point on an equilibrium diagram, or (iii) an alloy structure of intermixed solid constituents formed by a eutectoid reaction.
Extra hard – It is a temper of non-ferrous alloys and some ferrous alloys characterized by tensile strength and hardness about one-third of the way from full hard to extra spring temper.
Extra spring – It is a temper of non-ferrous alloys and some ferrous alloys corresponding approximately to a cold worked state above full hard beyond which further cold work is not measurably increase the strength and hardness.
Ferrite – It is a solid solution of one or more elements in body-centered cubic iron. Unless otherwise designated (for instance, as chromium ferrite), the solute is generally assumed to be carbon. On the iron-carbon equilibrium diagrams, there are two ferrite regions separated by an austenite area. The lower area is alpha ferrite while the upper, delta ferrite. If there is no designation, alpha ferrite is assumed.
Ferritizing anneal – It is a treatment given as-cast gray or ductile (nodular) iron to produce an essentially ferritic matrix. For the term to be meaningful the final micro-structure desired or the time-temperature cycle used is to be specified.
Final annealing – It is an imprecise term used to denote the last anneal given to a non-ferrous alloy prior to shipment.
Finish annealing – It is a sub-critical annealing treatment applied to cold-worked low- carbon 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.
Finishing temperature – It is the temperature at which hot working is completed.
Fixturing – It is the placing of parts to be heat treated in a constraining or semi constraining apparatus to avoid heat-related distortions.
Flame annealing – It consists of annealing in which the heat is applied directly by a flame.
Flame hardening – It is a process for hardening the surfaces of hardenable ferrous alloys in which an intense flame is used to heat the surface layers above the upper transformation temperature, whereupon the work piece is immediately quenched.
Flame straightening – It is the correcting distortion in metal structures by localized heating with a gas flame.
Fluidized-bed heating – It is heating carried out in a medium of solid particles suspended in a flow of gas.
Fog quenching – It is quenching in a fine vapour or mist.
Forced-air quench – It is a quench utilizing blasts of compressed air against relatively small parts such as a gear.
Free carbon – It is the part of the total carbon in steel or cast iron which is present in elemental form as graphite or temper carbon.
Free ferrite – It is the ferrite which is formed directly from the decomposition of hypo-eutectoid austenite during cooling, without the simultaneous formation of cementite.
Freezing range – It is that temperature range between liquidus and solidus temperatures in which molten and solid constituents coexist.
Full annealing – It is heating to and holding at some temperature above the transformation range, followed by cooling slowly at a rate through the transformation range such that the hardness of the steel approaches a minimum. It is an imprecise term which denotes an annealing cycle to produce minimum strength and hardness. For the term to be meaningful, the composition and starting condition of the material and the time-temperature cycle used are to be stated.
Full hard – It is a temper of non-ferrous alloys and some ferrous alloys 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 normally 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 a full anneal. For austenitic stainless steels, it is a reduction of around 50 % to 55 %.
Gamma iron – It is the face-centered cubic form of pure iron, stable from 910 deg C to 1400 deg C.
Gas cyaniding – It is a misnomer for carbo-nitriding.
Grain-boundary liquation – It is an advanced stage of overheating in which material in the region of austenitic grain boundaries melts. It is also termed as burning.
Grain coarsening – It is a heat treatment which produces excessively large austenitic grains.
Grain growth – It is an increase in the average size of the grains in poly-crystalline metal, normally as a result of heating at elevated temperature.
Grain refiner – It is a material added to a molten metal to induce a finer-than-normal grain size in the final structure.
Grain refining – It is heating from some temperature below the transformation range to a suitable temperature above that range followed by cooling at a suitable rate.
Grain size – It is for metals, a measure of the areas or volumes of grains in a poly-crystalline material, normally expressed as an average when the individual sizes are fairly uniform. In metals containing two or more phases, the grain size refers to that of the matrix unless otherwise specified. Grain sizes are reported in terms of number of grains per unit area or volume, average diameter, or as a grain size number derived from area measurements.
Graphitic carbon – It is the free carbon in steel or cast iron.
Graphitization – It is the formation of graphite in iron or steel. Where graphite is formed during solidification, the phenomenon is called primary graphitization; where formed later by heat treatment, secondary graphitization.
Graphitizing – It is annealing of a ferrous alloy in such a way that some or all of the carbon is precipitated as graphite.
Grossmann chart – It is a chart describing the ability of a quenching medium to extract heat from a hot steel work-piece in comparison to still water.
Guinier-Preston (G-P) zone – It is a small precipitation domain in a super-saturated metallic solid solution. A G-P zone has no well defined crystalline structure of its own and contains an abnormally high concentration of solute atoms. The formation of G-P zones constitutes the first stage of precipitation and is normally accompanied by a change in properties of the solid solution in which they occur.
Half hard – It is a temper of non-ferrous alloys and some ferrous alloys characterized by tensile strength about mid-way between that of dead soft and full hard tempers.
Hardenability – It is the relative ability of a ferrous alloy to form martensite when quenched from a temperature above the upper critical temperature. Hardenability is commonly measured as the distance below a quenched surface where the metal shows a specific hardness (for example, 50 HRC) or a specific percentage of martensite in the micro-structure.
Hardening – It is increasing of the hardness by suitable treatment, normally involving heating and cooling. When applicable, then more specific terms are to be used such as age hardening, flame hardening, induction hardening, laser hardening, precipitation hardening, and quench hardening.
Hardness profile – It is the hardness as a function of distance from a fixed reference point (normally from the surface).
Hard temper – It is same as full hard temper.
Heat tinting – It is the coloration of a metal surface through oxidation by heating to reveal details of the micro-structure.
Heat-treatable alloy – It is an alloy which can be hardened by heat treatment.
Heat-treating film – It is a thin coating or film, usually an oxide, formed on the surface of metals during heat treatment.
Heat treatment – It is heating and cooling a solid metal or alloy in such a way as to obtain desired conditions or properties. Heating for the sole purpose of hot working is excluded from the meaning of this definition.
Heat treatment solution – It is a treatment in which an alloy is heated to a suitable temperature and held at this temperature for a sufficient length of time to allow a desired constituent to enter into solid solution, followed by rapid cooling to hold the constituent in solution. The material is then in a super-saturated unstable state which may subsequently exhibit age hardening.
Holding – It is the portion of the thermal cycle during which the temperature of the object is maintained constant.
Holding temperature – It is the constant temperature at which the object is maintained.
Holding time – It is the time for which the temperature of the object is maintained constant.
Homogeneous carburizing – It is the use of a carburizing process to convert a low-carbon ferrous alloy to one of uniform and higher carbon content throughout the section.
Homogenizing – It is holding at high temperature to eliminate or decrease chemical segregation by diffusion.
Horizontal batch furnace – It is a versatile batch-type furnace which 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 – It is an imprecise term used to cover a variety of quenching procedures in which a quenching medium is maintained at a prescribed temperature above 70 deg C.
Hot-wire analyzer – It is an electrical atmosphere analysis device which is based on the fact that the electrical resistivity of steel is a linear function of carbon content over a range from 0.05 % C to saturation. The device measures the carbon potential of furnace atmospheres (typically). This term is not to be confused with the hot-wire test which measures heat extraction rates.
Hot-wire test – It is the method used to test heat extraction rates of various quenchants. Faster heat-extracting quenchants permit more electric current to pass through a standard wire because it is cooled more quickly.
Hyper-eutectic alloy – It is in an alloy system showing a eutectic, any alloy whose composition has an excess of alloying element compared with the eutectic composition, and whose equilibrium micro-structure contains some eutectic structure.
Hyper-eutectoid alloy – It is in an alloy system showing a eutectoid, any alloy whose composition has an excess of alloying element compared with the eutectoid composition, and whose equilibrium micro-structure contains some eutectoid structure.
Hypo-eutectic alloy – It is in an alloy system showing a eutectic, any alloy whose composition has an excess of base metal compared with the eutectic composition, and whose equilibrium micro-structure contains some eutectic structure.
Hypo-eutectoid alloy – It is in an alloy system showing a eutectoid, any alloy whose composition has an excess of base metal compared with the eutectoid composition, and whose equilibrium micro-structure contains some eutectoid structure.
Ideal critical diameter – Under an ideal quench condition, it is the bar diameter which has 50 % martensite at the centre of the bar when the surface is cooled at an infinitely rapid rate (that is, when H = infinity, where H is the quench severity factor).
Immersed-electrode furnace – It is a furnace used for liquid carburizing of parts by heating molten salt baths with the use of electrodes immersed in the liquid.
Impact tube – It is also known as Pitot tube.
Induction hardening – It is a surface-hardening process in which only the surface layer of a suitable ferrous work piece is heated by electro-magnetic induction to above the upper critical temperature and immediately quenched.
Induction heating – It is 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 – It is the tempering of steel using low-frequency electrical induction heating.
Infrared analyzer – It is an atmosphere-monitoring device which measures a gas (usually carbon monoxide, carbon dioxide, and methane) presence based on specific wave-length absorption of infrared energy.
Intense quenching – It is the quenching in which the quenching medium is cooling the part at a rate at least two and a half times faster than still water.
Inter-critical annealing – It is any annealing treatment which involves heating to, and holding at, a temperature between the upper and lower critical temperatures to obtain partial austenitization, followed by either slow cooling or holding at a temperature below the lower critical temperature.
Inter-granular – It is between crystals or grains. It is also called inter-crystalline. It differs with transgranular.
Inter-granular cracking – It is the cracking or fracturing which occurs between the grains or crystals in a poly-crystalline aggregate. It is also called inter-crystalline cracking. It differs with transgranular cracking.
Inter-granular fracture – It is the brittle fracture of a metal in which the fracture is between the grains, or crystals, which form the metal. It is also called inter-crystalline fracture. It differs with transgranular fracture.
Intermediate annealing – It is the annealing of wrought metals at one or more stages during manufacture and before final treatment.
Interrupted aging – It consists of aging at two or more temperatures, by steps, and cooling to room temperature after each step.
Interrupted quenching – It is a quenching procedure in which the work piece is removed from the first quench at a temperature substantially higher than that of the quenchant and is then subjected to a second quenching system having a different cooling rate than the first.
Interval test – It is the method used to test heat extraction rates of various quenchants. This test measures the increase in temperature of a quenchant when a standard bar of metal is quenched for five seconds. Faster quenchants show greater temperature increases.
Ion carburizing – It is a method of surface hardening in which carbon ions are diffused into a work piece in a vacuum through the use of high-voltage electrical energy. It is synonymous with plasma carburizing or glow discharge carburizing.
Ion nitriding – It is a method of surface hardening in which nitrogen ions are diffused into a work piece in a vacuum through the use of high-voltage electrical energy. It is synonymous with plasma nitriding or glow discharge nitriding.
Isothermal annealing – It is austenitizing a ferrous alloy and then cooling to and holding at a temperature at which austenite transforms to a relatively soft ferrite carbide aggregate.
Isothermal transformation – It is a change in phase which 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 super cooling below (or superheating above) the equilibrium temperature for the same transformation.
Isothermal transformation (IT) diagram – It is the set of curves drawn using logarithmic time and linear temperature as coordinates, which define for each level of temperature the beginning and end of the transformation of the initial phase under isothermal conditions.
Jominy test – It is the end-quench hardenability test.
Kish – It is the free graphite which forms in molten hyper-eutectic cast iron as it cools. In castings, the kish can segregate toward the surface, where it lodges at or immediately beneath the casting surface.
Laser hardening – It is a surface-hardening process which uses a laser to quickly heat a surface. Heat conduction into the interior of the part quickly cools the surface, leaving a shallow martensitic layer.
Latent heat – It is the thermal energy absorbed or released when a substance undergoes a phase change.
Ledeburite – It is the eutectic of the iron-carbon system, the constituents being austenite and cementite. The austenite decomposes into ferrite and cementite on cooling below the Ar1.
Leidenfrost phenomenon – It is slow cooling rates associated with a hot vapour blanket which surrounds a part being quenched in a liquid medium such as water. The gaseous vapour envelope acts as an insulator, thus slowing the cooling rate.
Liquation temperature – It is the lowest temperature at which partial melting can occur in an alloy which shows the greatest possible degree of segregation.
Liquid carburizing – It is surface hardening of steel by immersion into a molten bath consisting of cyanides and other salts.
Liquid nitriding – It is a method of surface hardening in which molten nitrogen-bearing, fused-salt baths containing both cyanides and cyanates are exposed to parts at sub-critical temperatures.
Liquid nitro-carburizing –It is a nitro-carburizing process (where both carbon and nitrogen are absorbed into the surface) utilizing molten liquid salt baths below the lower critical temperature.
Localized precipitation – It is the precipitation from a super-saturated solid solution similar to continuous precipitation, except that the precipitate particles form at preferred locations, such as along slip planes, grain boundaries, or incoherent twin boundaries.
Magnetic test – It is the method used to test heat extraction rates of various quenchants. The test works by utilizing the change in magnetic properties of metals at their Curie point (the temperature above which metals lose their magnetism).
Malleable cast iron – It is 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 shows a light fracture surface, hence, ‘white-heart malleable’. Otherwise, the fracture surface is dark, hence, ‘black-heart malleable’. Ferritic malleable has a predominantly ferritic matrix while the pearlitic malleable can contain pearlite, spheroidite or tempered martensite depending on heat treatment and desired hardness.
Malleablizing – It is a process of annealing white cast iron in which the combined carbon is wholly or partly transformed to graphitic or free carbon and, in some cases, part of the carbon is removed completely.
Maraging – It is a precipitation-hardening treatment applied to a special group of iron-base alloys to precipitate one or more inter-metallic compounds in a matrix of essentially carbon-free martensite.
Marquenching – It is same as martempering.
Martempering – It is quenching from a temperature above the transformation range to some temperature above the upper limit of martensite formation, holding at that temperature long enough to permit equalisation of temperature without transformation of the austenite followed by cooling in air. This results into the formation of martensite which can be tempered as desired. When the process is applied to carburized material then the controlling Ms temperature is that of the case. This variation of the process is frequently called marquenching.
Martempering is a hardening procedure in which an austenitized steel object is quenched into an appropriate medium whose temperature is maintained substantially at the Ms temperature of the steel object, held in the medium until its temperature is uniform throughout but not long enough to permit bainite to form, and then cooled in air. The treatment is followed by tempering.
Martensite – It is a generic term for micro-structures formed by diffusion less phase transformation in which the parent and product phases have a specific crystallographic relationship. Martensite is characterized by an acicular pattern in the micro-structure in both ferrous and non-ferrous alloys. In alloys where the solute atoms occupy interstitial positions in the martensitic lattice (such as carbon in iron), the structure is hard and highly strained. But where the solute atoms occupy substitutional positions (such as nickel in iron), the martensite is soft and ductile. The amount of high-temperature phase which transforms to martensite on cooling depends to a large extent on the lowest temperature attained, there being a rather distinct beginning temperature (Ms) and a temperature at which the transformation is essentially complete (Mf).
Martensite range – It is the temperature interval between Ms and Mf.
Martensitic transformation – It is a reaction which takes place in some metals on cooling, with the formation of an acicular structure called martensite
McQuaid-Ehn test – It is a test to reveal grain size after heating into the austenitic temperature range. Eight standard McQuaid-Ehn grain sizes rate the structure, No. 8 is the finest while the No. 1 is the coarsest.
Mf temperature – For any alloy system, it is the temperature at which martensite formation on cooling is essentially finished.
Micro-hardness – It is 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. Normally, the indentations are so small that they are to be measured with a microscope. It is capable of determining hardness of different micro-constituents within a structure, or of measuring steep hardness gradients such as those encountered in case hardening.
Microscopic stress – It is the residual stress which varies from tension to compression in a distance (presumably approximating the grain size) that is small compared with the gauge length in ordinary strain measurements. It is not detectable by dissection methods, but can sometimes be measured from line shift or line broadening in an x-ray diffraction pattern.
Micro segregation – It is the segregation within a grain, crystal, or small particle.
Mill scale – It is the heavy oxide layer formed during hot fabrication or heat treatment of metals.
Monotropism – It is the ability of a solid to exist in two or more forms (crystal structures), but in which one form is the stable modification at all temperatures and pressures. Ferrite and martensite is a monotropic pair below Ac1 in steels, for example. It can also be spelled monotrophism.
Ms temperature – For any alloy system, it is the temperature at which martensite starts to form on cooling.
Natural aging – It is spontaneous aging of a super-saturated solid solution at room temperature.
Neutral flame – It is 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.
Neutralization number – It is an ASTM (American Society for Testing and Materials) number given to quenching oils which reflects the oil’s tendency towards oxidation and sludging. It is also known as saponification number.
Nitriding – It consists of introducing nitrogen into the surface layer of a solid ferrous alloy by holding at a suitable temperature (below Ac1 for ferritic steels) in contact with a nitrogenous material, normally ammonia or molten cyanide of appropriate composition. Quenching is not required to produce a hard case.
Nitro-carburizing – It consists of 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. Nitro-carburizing is done mainly to provide an anti-scuffing surface layer and to improve fatigue resistance.
Normalizing – It consists of heating a ferrous alloy to a suitable temperature above the transformation range and then cooling in air to a temperature substantially below the transformation range.
Nucleation – It is the initiation of a phase transformation at discrete sites, the new phase growing on nuclei.
Nucleus – It is the first structurally stable particle capable of initiating recrystallization of a phase or the growth of a new phase, and possessing an interface with the parent matrix. The term is also applied to a foreign particle which initiates such action.
Oil hardening – It is quench-hardening treatment involving cooling in oil.
Oil quenching – It is hardening of carbon steel in an oil bath. Oils are categorized as conventional, fast, martempering, or hot quenching.
Optical pyrometer – It is an instrument for measuring the temperature of heated material by comparing the intensity of light emitted with a known intensity of an incandescent lamp filament.
Order-disorder transformation – It is a phase change among two solid solutions having the same crystal structure, but in which the atoms of one phase (disordered) are randomly distributed, and in the other, the different kinds of atoms occur in a regular sequence upon the crystal lattice, that is, in an ordered arrangement.
Order hardening – It is a low-temperature annealing treatment that permits short-range ordering of solute atoms within a matrix, which greatly impedes dislocation motion.
Orsat analyzer – It is an atmosphere analysis device in which gases are absorbed selectively (volumetric basis) by passing them through a series of pre-selected solvents.
Over-aging – It is the 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.
Overheating or overheated – Steel is said to have been overheated if, after exposure to an unduly high temperature, it develops an undesirably coarse grain structure but is not permanently damaged. The structure damaged by overheating can be corrected by suitable heat treatment or by mechanical work or by a combination of the two. When the original properties cannot be restored by further heat treating, by mechanical working, or by a combination of working and heat treating, the overheating is known as burning and the structure is known as the burnt structure.
Oxidation – It is a reaction in which there is an increase in valence resulting from a loss of electrons. It is also a corrosion reaction in which the corroded metal forms an oxide. It is normally applied to reaction with a gas containing elemental oxygen, such as air.
Oxidized surface (on steel) – It is the surface having a thin, tightly adhering, oxidized skin (from straw to blue in colour), extending in from the edge of a coil or sheet. It is sometimes called annealing border.
Oxidizing agent – It is a compound which causes oxidation, thereby itself becoming reduced.
Oxidizing flame – It is a gas flame produced with excess oxygen in the inner flame.
Oxygen probe – It is an atmosphere-monitoring device which electronically measures the difference between the partial pressure of oxygen in a furnace or furnace supply atmosphere and the external air.
Pack carburizing – It is a method of surface hardening of steel in which parts are packed in a steel box with the carburizing compound and heated to the elevated temperatures.
Pack nitriding – It is method of surface hardening of steel in which parts are packed in a steel box with the nitriding compound and heated to the elevated temperatures.
Partial annealing – It is an imprecise term used to denote a treatment given cold-worked material to reduce the strength to a controlled level or to effect stress relief. To be meaningful, the type of material, the degree of cold work, and the time-temperature schedule need to be stated.
Patenting – It is, in wire making, a heat treatment applied to medium-carbon or high-carbon steel before the drawing of wire or between drafts. This process consists of heating to a temperature above the transformation range and then cooling to a temperature below Ae1 in air or in a bath of molten lead or salt to produce a structure which facilitates subsequent cold working and gives the desired mechanical properties in the finished state.
Pearlite – It is a metastable lamellar aggregate of ferrite and cementite resulting from the transformation of austenite at temperatures above the bainite range.
Phase diagram – It is a graphical representation of the temperature and composition limits of phase fields in an alloy system as they actually exist under the specific conditions of heating or cooling (synonymous with constitution diagram). A phase diagram can be an equilibrium diagram, an approximation to an equilibrium diagram, or a representation of metastable conditions or phases.
Pirani gauge – It is an instrument used to measure the pressure inside a vacuum chamber. The gauge measures electrical resistance in a wire filament which changes in temperature depending on atmospheric pressure.
Pitot tube – It is an instrument which measures the stagnation pressure of a flowing fluid, consisting of an open tube pointing into the fluid and connected to a pressure-indicating device. It is also known as impact tube.
Plasma carburizing – It is same as ion carburizing.
Plasma nitriding – It is same as ion nitriding.
Plastic deformation – It is the permanent (inelastic) distortion of metals under applied stresses which strain the material beyond its elastic limit.
Polymorphism – It is the property of a chemical substance crystallizing into two or more forms having different structures, such as diamond and graphite.
Post-heating – It consists of heating the weldments immediately after welding, for tempering, for stress relieving, or for providing a controlled rate of cooling to prevent formation of a hard or brittle structure.
Pot annealing – It is the same as box annealing.
Precipitation hardening – It results in hardening due to the precipitation of a constituent from a super-saturated solid solution which occurs in certain suitably quenched steel on heating to and holding at some temperature below the transformation range.
Precipitation heat treatment – It is the artificial aging in which a constituent precipitates from a super-saturated solid solution.
Preheating – It is heating before some further thermal or mechanical treatment. For tool steel, it is heating to an intermediate temperature immediately before final austenitizing. For some non-ferrous alloys, it is 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.
Press quenching – It is a quench in which hot dies are pressed and aligned with a part before the quenching process begins. Then the part is placed in contact with a quenching medium in a controlled manner. This process avoids part distortion.
Process annealing – It is an imprecise term denoting various treatments used to improve workability. For the term to be meaningful, the condition of the material and the time-temperature cycle used is to be stated.
Progressive aging – It is aging by increasing the temperature in steps or continuously during the aging cycle.
Pseudo-carburizing – It is same as blank carburizing.
Pseudo-nitriding – it is same as blank nitriding.
Pusher furnace – It is 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 removed.
Pyrometer – It is a device for measuring temperatures above the range of liquid thermometers.
Quarter hard – It is a temper of non-ferrous alloys and some ferrous alloys characterized by tensile strength about midway between that of dead soft and half hard tempers.
Quench-age embrittlement – It is the embrittlement of low-carbon steels resulting from precipitation of solute carbon at existing dislocations and from precipitation hardening of the steel caused by differences in ferrite at different temperatures. Quench-age embrittlement is normally by rapid cooling of the steel from temperatures slightly below Ac1 (the temperature at which austenite begins to form), and can be minimized by quenching from lower temperatures.
Quench aging – It is aging induced by rapid cooling after solution heat treatment. It is a change in properties which can occur gradually at atmospheric temperature and more rapidly at higher temperature following rapid cooling (precipitation hardening).
Quench annealing – It is annealing an austenitic ferrous alloy by solution heat treatment followed by rapid quenching.
Quench cracking – It is the fracture of a metal during quenching from elevated temperature. It is 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.
Quench hardening – It is hardening suitable alpha-beta alloys (most often certain copper to titanium alloys) by solution treating and quenching to develop a martensitic-like structure. In case of ferrous alloys, it is hardening by austenitizing and then cooling at a rate such that a substantial amount of austenite transforms to martensite.
Quenching – It is rapid cooling. When applicable, the more specific terms are to be used such as brine quenching, caustic quenching, cold die quenching , forced-air quenching, intense quenching, oil quenching, press quenching, spray quenching, direct quenching, fog quenching, hot quenching, interrupted quenching, selective quenching, time quenching, and water quenching.
Racking – It is a term used to describe the placing of parts to be heat treated on a rack or tray. This is done to keep pans in a proper position to avoid heat-related distortions and to keep the parts separated.
Recalescence – It is the phenomenon, associated with the transformation of gamma iron to alpha iron on the cooling (super cooling) of iron or steel, revealed by the brightening (reglowing) of the metal surface owing to the sudden increase in temperature caused by the fast liberation of the latent heat of transformation.
Recarburize – It is used to increase the carbon content of molten cast iron or steel by adding carbonaceous material, high-carbon pig iron, or a high-carbon alloy. It is also used to carburize a metal part to return surface carbon lost in processing. It is also known as carbon restoration.
Recovery – It is the reduction or removal of work-hardening effects, without motion of large-angle grain boundaries.
Recrystallization – It is the formation of a new, strain-free grain structure from that existing in cold-worked metal, normally accomplished by heating. It consists of the change from one crystal structure to another, as being occurred on heating or cooling through a critical temperature.
Recrystallization annealing – It is annealing cold-worked metal to produce a new grain structure without phase change.
Recrystallization temperature – It is the approximate minimum temperature at which complete recrystallization of a cold-worked metal occurs within a specified time.
Recuperator – It is the 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 gases pass.
Reducing flame – It is a gas flame produced with excess fuel in the inner flame.
Refractory – It is a material of very high melting point with properties which make it suitable for such uses as furnace linings and kiln construction. It has the quality of resisting heat.
Regenerator – It is the same as recuperator except the gaseous products of combustion heat brick checker work in a chamber connected to the exhaust side of the furnace while the incoming air and fuel are being heated by the brick checker-work in a second chamber, connected to the entrance side. At intervals, the gas flow is reversed so that incoming air and fuel contact hot checker-work while that in the second chamber is being reheated by exhaust gases.
Residual stress – It is an internal stress not depending on external forces resulting from such factors as cold working, phase changes, or temperature gradients.
Retort – It is a vessel used for distillation of volatile materials, as in separation of some metals and in destructive distillation of coal.
Reverberatory furnace – It is a furnace with a shallow hearth, usually non-regenerative, having a roof which deflects the flame and radiates heat toward the hearth or the surface of the charge.
Rockwell hardness test – It is an indentation hardness test based on the depth of penetration of a specified penetrator into the specimen under certain arbitrarily fixed conditions.
Rotary retort furnace – It is 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.
Salt bath heat treatment – It is the heat treatment carried out in a bath of molten salt.
Saponification number – It is a number given to quenching oils which reflects the oils amount of compounding with fatty materials, which thereby helps evaluate the condition of these oils in service.
Secondary hardening – It is a hardening effect which occurs on cooling certain previously hardened steel from a particular range of tempering temperature.
Selective heating It is the intentional heating of only certain portions of the work piece.
Selective quenching – It is quenching only certain portions of an object.
Self-hardening steel – It is the preferred term is the air-hardening steel.
Sensitization – In austenitic stainless steels, the precipitation of chromium carbides, normally at grain boundaries, on exposure to temperatures of around 540 deg C to 845 deg C, leaving the grain boundaries depleted of chromium and therefore susceptible to preferential attack by a corroding (oxidizing) medium.
Severity of quench – It is the ability of quenching medium to extract heat from a hot steel work piece. It is expressed in terms of the ‘H’ value.
Shaker-hearth furnace – It is a continuous type furnace which uses a reciprocating shaker motion to move the parts along the hearth.
Shell hardening – It is a surface-hardening process in which a suitable steel work piece, when heated through and quench hardened, develops a martensitic layer or shell which closely follows the contour of the piece and surrounds a core of essentially pearlitic transformation product. This result is accomplished by a proper balance among section size, steel hardenability, and severity of quench.
Shim – It is 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 quickly carburizes to a level equal to the furnace carbon potential).
Sigma phase – It is a hard, brittle, non-magnetic intermediate phase with a tetragonal crystal structure, containing 30 atoms per unit cell, space group (P4)2/mnm, occurring in many binary and ternary alloys of the transition elements. The composition of this phase in the various systems is not the same, and the phase normally shows a wide range in homogeneity. Alloying with a third transition element normally enlarges the field of homogeneity and extends it deep into the ternary section.
Sigma-phase embrittlement – It is embrittlement of iron-chromium alloys (most notably austenitic stainless steels) caused by precipitation at grain boundaries of the hard, brittle inter-metallic sigma phase during long periods of exposure to temperatures between around 565 deg C and 980 deg C. Sigma phase embrittlement results in severe loss in toughness and ductility and can make the embrittled material structure susceptible to inter-granular corrosion.
Signal-to-noise ratio – it is the ratio of the average response to the root-mean-square variation about the average response. Ratio of variances associated with the two parts of the performance measurement.
Siliconizing – It is diffusing silicon into solid metal, normally steel, at an elevated temperature.
Sintering – It is the bonding of adjacent surfaces in a mass of particles by molecular or atomic attraction on heating at high temperatures below the melting temperature of any constituent in the material. Sintering strengthens a powder mass and normally produces densification and, in powdered metals, recrystallization.
Slack quenching – It is 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 – It is a common batch furnace where stock is charged and removed through a slot or opening.
Snap temper – It is 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.
Soaking – It is the prolonged holding at a selected temperature to effect homogenization of structure or composition.
Soft temper – It is the same as dead soft temper.
Solution heat treatment – It is heating an alloy to a suitable temperature, holding at that temperature long enough to cause one or more constituents to enter into solid solution, and then cooling rapidly enough to hold these constituents in solution.
Sorbite – It is now an obsolete term. It is a fine mixture of ferrite and cementite produced either by regulating the rate of cooling of steel or by tempering steel after hardening. The first type is very fine pearlite difficult to resolve under the microscope and the second type is tempered martensite.
Spalling – It is a chipping or flaking of a surface due to any kind of improper heat treatment or material dissociation.
Spinodal hardening – It is same as aging.
Spheroidite – It is an aggregate of iron or alloy carbides of essentially spherical shape dispersed throughout a matrix of ferrite.
Spheroidizing – It is subjecting steel to a selected temperature cycle usually within or near the transformation range in order to produce a suitable globular form of carbide for such purpose as (i) improving machinability, (ii) facilitating subsequent cold working, and (iii) obtaining a desired structure for subsequent heat treatment. Spheroidizing methods frequently used are (i) prolonged holding at a temperature just below Ae1, (ii) heating and cooling alternately between temperatures which are just above and just below Ae1, (iii) heating to a temperature above Ae1 or Ae3 and then cooling very slowly in the furnace or holding at a temperature just below Ae1, and (iv) cooling at a suitable rate from the minimum temperature at which all carbide is dissolved, to prevent the reformation of a carbide network, and then reheating in accordance with method 1 or 2 above. It is applicable to hyper-eutectoid steel containing a carbide network.
Spinodal structure – It is a fine homogeneous mixture of two phases which form by the growth of composition waves in a solid solution during suitable heat treatment. The phases of a spinodal structure differ in composition from each other and from the parent phase but have the same crystal structure as the parent phase.
Spray quenching – It is 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 temper – It is a temper of non-ferrous alloys and some ferrous alloys characterized by tensile strength and hardness about two-thirds of the way from full hard to extra spring temper.
Stabilizing treatment – It consists of (i) before finishing to final dimensions, repeatedly heating a ferrous or non-ferrous part to or slightly above its normal operating temperature and then cooling to room temperature to ensure dimensional stability in service, (ii) transforming retained austenite in quenched hardenable steels, normally by cold treatment, and (iii) heating a solution treated austenitic stainless steel which contains controlled amounts of titanium or niobium plus tantalum to a temperature below the solution heat treating temperature to cause precipitation of finely divided, uniformly distributed carbides of those elements, thereby substantially reducing the amount of carbon available for the formation of chromium carbides in the grain boundaries on subsequent exposure to temperature in the sensitizing range so that sensitization is avoided on subsequent exposure to elevated temperature.
Statistical process control – It is the application of statistical techniques for measuring and analyzing the variation in processes.
Statistical quality control – It is the application of statistical techniques for measuring and improving the quality of the processes and the products (includes statistical process control, diagnostic tools, sampling plans, and other statistical techniques).
Stead’s brittleness – It is a condition of brittleness which causes trans-crystalline fracture in the coarse grain structure which results from prolonged annealing of thin sheets of low-carbon steel previously rolled at a temperature below around 705 deg C. The fracture usually occurs at about 45 degrees to the direction of rolling.
Step aging – It is aging at two or more temperatures, by steps, without cooling to room temperature after each step.
Strain-age embrittlement – It is a loss in ductility accompanied by an increase in hardness and strength which occurs when low carbon steel (especially rimmed or capped steel) is aged following plastic deformation. The degree of embrittlement is a function of aging time and temperature, occurring in a matter of minutes at around 200 deg C but requiring a few hours to a year at room temperature.
Strain aging – It is aging following plastic deformation. It is a change in properties which can occur gradually at atmospheric temperature and more rapidly at higher temperature following plastic straining.
Stress equalizing – It is 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 – It is a process to reduce internal residual stresses in steel by heating the steel to a suitable temperature and holding for a proper time at that temperature and then cooling slowly. This treatment is applied to relieve internal residual stresses induced during casting, quenching, normalizing, machining, cold working, or welding.
Sub-critical annealing – It is a process anneal performed on ferrous alloys at a temperature below Ac1.
Sub-zero treatment – It is cooling hardened steel to a temperature sufficiently below zero deg C to promote the transformation of any retained austenite to martensite.
Submerged-electrode furnace – It is a furnace used for liquid carburizing of parts by heating molten salt baths with the use of electrodes submerged in the ceramic lining.
Super-cooling – It consists of cooling below the temperature at which an equilibrium phase transformation can take place, without actually obtaining the transformation.
Super-heating – It consists of heating above the temperature at which an equilibrium phase transformation occurs without actually obtaining the transformation.
Surface hardening – It is a generic term covering several processes applicable to a suitable ferrous alloy which 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 normally used are carbo-nitriding, carburizing, induction hardening, flame hardening, nitriding, and nitro-carburizing.
Temper – It consists of (i) in heat treatment, reheating hardened steel, or hardened cast iron to some temperature below the eutectoid temperature for the purpose of decreasing hardness and increasing toughness. The process also is sometimes applied to normalized steel, (ii) in tool steels, temper is sometimes used, but inadvisedly, to denote the carbon content, and (iii) in nonferrous alloys and in some ferrous alloys (steels which cannot be hardened by heat treatment), the hardness and strength produced by mechanical or thermal treatment, or both, and characterized by a certain structure, mechanical properties, or reduction in area during cold working.
Temper carbon – It is the free or graphitic carbon which comes out of solution normally in the form of rounded nodules in the structure during graphitizing or malleablizing. It is same as annealing carbon.
Temper colour – It is a thin, tightly adhering oxide skin which forms when steel is tempered at a low temperature, or for a short time, in air or a mildly oxidizing atmosphere. The colour, which ranges from straw to blue depending on the thickness of the oxide skin, varies with both tempering time and temperature.
Tempered martensite embrittlement – It is the embrittlement of ultrahigh-strength steels caused by tempering in the temperature range of 205 deg C to 400 deg C. It is also called the 350 deg C 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.
Temper embrittlement – It is embrittlement of alloy steels caused by holding within or cooling slowly through a temperature range just below the transformation range. Embrittlement is the result of the segregation at grain boundaries of impurities such as arsenic, antimony, phosphorus, and tin. It is nomally manifested as an upward shift in ductile-to-brittle transition temperature. Temper embrittlement can be reversed by re-tempering above the critical temperature range, then cooling rapidly.
Tempering – It is heating hardened, normalized or mechanical worked steel to some temperature below the transformation range and holding for a suitable time at that temperature, followed by cooling at a suitable rate. The process is normally applied for the purpose of producing a desired combination of mechanical properties.
Thermal analysis – It is a method for determining transformations in a metal by noting the temperatures at which thermal arrests occur. These arrests are manifested by changes in slope of the plotted or mechanically traced heating and cooling curves. When such data are secured under nearly equilibrium conditions of heating and cooling, the method is commonly used for determining certain critical temperatures required for the construction of equilibrium diagrams.
Thermal electro-motive force – It is the electromotive force generated in a circuit containing two dissimilar metals when one junction is at a temperature different from that of the other.
Thermal fatigue – It is the fracture resulting from the presence of temperature gradients which vary with time in such a manner as to produce cyclic stresses in a structure.
Thermal shock – It is the development of a steep temperature gradient and accompanying high stresses within a structure.
Thermal stress – It is the stress in metal resulting from non uniform temperature distribution.
Thermo-chemical treatment – It consists of heat treatment carried out in a medium suitably chosen to produce a change in the chemical composition of the object by exchange with the medium.
Thermocouple – It is a device for measuring temperatures, consisting of lengths of two dissimilar metals or alloys which are electrically joined at one end and connected to a voltage-measuring instrument at the other end. When one junction is hotter than the other, a thermal electro-motive force is produced which is roughly proportional to the difference in temperature between the hot and cold junctions.
Thermo-mechanical working – It is a general term covering a variety of processes combining controlled thermal and deformation treatments to obtain specific properties.
Three-quarters hard – It is a temper of non-ferrous alloys and some ferrous alloys characterized by tensile strength and hardness around midway between those of half hard and full hard tempers.
Time quenching – It is a term used to describe a quench in which the cooling rate of the part being quenched is to be changed abruptly at some time during the cooling cycle.
Time-temperature-transformation (TTT) diagrams – It is same as isothermal transformation (IT) diagram.
Total carbon – It is the sum of the free and combined carbon (including carbon in solution) in a ferrous alloy.
Total indicator reading – It is the same as the total indicator variation.
Total indicator variation – It is the difference between the maximum and minimum indicator readings during a checking cycle.
Toughness – It is the ability of a metal to absorb energy and deform plastically before fracturing.
Trans-crystalline – It is the same as trans-granular.
Transformation hardening – It is the 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-induced plasticity – It is a phenomenon, occurring mainly in certain highly alloyed steels which have been heat treated to produce metastable austenite or metastable austenite plus martensite, whereby, on subsequent deformation, part of the austenite undergoes strain-induced transformation to martensite. Steels capable of transforming in this manner, normally referred to as TRIP steels, are highly plastic after heat treatment, but show a very high rate of strain hardening and thus have high tensile and yield strengths after plastic deformation at temperatures between around 20 deg C and 500 deg C. Cooling to -195 deg C can or cannot be required to complete the transformation to martensite. Tempering normally is done following transformation.
Transformation ranges – These consist of those ranges of temperature within which a phase forms during heating and transforms during cooling. The two ranges are distinct, sometimes overlapping but never coinciding. The limiting temperatures of the ranges depend on the composition of the alloy and on the rate of change of temperature, particularly during cooling.
Transformation temperature – It is the temperature at which a change in phase occurs. The term is sometimes used to denote the limiting temperature of a transformation range. The following symbols are used for iron and steels.
Accm – In hyper-eutectoid steel, it is the temperature at which the solution of cementite in austenite is completed during heating.
Ac1 – It is the temperature at which austenite begins to form during heating.
Ac3 – It is the temperature at which transformation of ferrite to austenite is completed during heating.
Ac4 – It is the temperature at which austenite transforms to delta ferrite during heating.
Aecm, Ae1, Ae3, and Ae4 – These are the temperatures of phase changes at equilibrium.
Arcm – In hyper-eutectoid steel, it is the temperature at which precipitation of cementite starts during cooling.
Ar1 – It is the temperature at which transformation of austenite to ferrite or to ferrite plus cementite is completed during cooling.
Ar3 – It is the temperature at which austenite begins to transform to ferrite during cooling.
Ar4 – It is the temperature at which delta ferrite transforms to austenite during cooling.
Ar’ – It is the temperature at which transformation of austenite to pearlite starts during cooling.
Mf – It is the temperature at which transformation of austenite to martensite finishes during cooling.
Ms, (or Ar”) = It is the temperature at which transformation of austenite to martensite starts during cooling.
It is to be noted that all the changes except the formation of martensite occur at lower temperatures during cooling than during heating, and depend on the rate of change of temperature.
Trans-granular – It is through or across crystals or grains. It is also called intra-crystalline or trans-crystalline.
Trans-granular cracking – It consists of cracking or fracturing that occurs through or across a crystal or grain. It is also called trans-crystalline cracking and differs with inter-granular cracking.
Trans-granular fracture – It is the fracture through or across the crystals or grains of a metal. It is also called trans-crystalline fracture or intra-crystalline fracture and differs with inter-granular fracture.
Transition temperature – It consists of (i) an arbitrarily defined temperature which lies within the temperature range in which metal fracture characteristics (as usually determined by tests of notched specimens) change rapidly, such as from primarily fibrous (shear) to primarily crystalline (cleavage) fracture. It is normally used definitions are ‘transition temperature for 50 % cleavage fracture’, and ’transition temperature for half maximum energy’, and (ii) it is sometimes used to denote an arbitrarily defined temperature within a range in which the ductility changes rapidly with temperature.
TRIP steel – It is a commercial steel product showing the transformation-induced plasticity.
Troostite – It is now an obsolete term. It is a previously un-resolvable rapidly etching fine aggregate of carbide and ferrite produced either by tempering martensite at low temperature or by quenching a steel at a rate slower than the critical cooling rate. Now the preferred terminology for the first product is tempered martensite and for the latter, fine pearlite.
Under-cooling – It is same as super-cooling.
Vacuum annealing – It is annealing carried out at sub-atmospheric pressure.
Vacuum carburizing – It is a high-temperature gas carburizing process using furnace pressures between 0.07 kg/sq cm and 0.56 kg/sq cm during the carburizing portion of the cycle.
Vacuum furnace – It is a furnace using low atmospheric pressures instead of a protective gas atmosphere like most heat treating furnaces. Vacuum furnaces are categorized as hot wall or cold wall, depending on the location of the heating and insulating components.
Vacuum nitro-carburizing – It is a sub-atmospheric nitro-carburizing process using a basic atmosphere of 50 % ammonia / 50 % methane, containing controlled oxygen additions of upto 2 %.
Walking-beam furnace – It is a continuous-type furnace consisting of two sets of rails, one stationary and the other movable. Only the work being processed has to be heated because trays or fixtures are not needed.
Water quenching – It is 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.
White layer – It is the compound layer which forms as a result of the nitriding process.