Glossary of Terms

GLOSSARY OF SELECT TERMS RELATED TO HEAT TREATING
A
Ac1, Etc.
SEE “TRANSFORMATION TEMPERATURE.”
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.
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
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.
ALUMINIZING
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.
ANNEALING
A very general term describing the heating of a metal to a suitable temperature, holding for a suitable time, and cooling at a suitable rate to accomplish the objective of the treatment. Annealing may be done to:
  1. Relieve stresses
  2. Induce softness structure
  3. Improve physical, electrical, or magnetic properties
  4. Improve machinability microstructure
  5. Refine the crystallinestructure
  6. Remove gases
  7. Produce a specifictemper
ATMOSPHERE
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, sulphur, etc.
AUSTEMPERING
A heat treating operation in which austenite is quenched to and held at a constant temperature (usually between 450 degrees (F) and 800 degrees (F) until transformation to bainite is complete. In some steels at certain hardness levels, bainite is tougher than quenched and tempered structures.
AUSTENITE
Austenite is the name given any solid solution in which gamma iron is the solvent. Austenite is a structure name and means nothing as to composition. Austenite 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.
AUSTENITIZING
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.
B
BAINITE
The product formed when austenite transforms between 450°F and 900°F. Bainite is an acicular aggregate of ferrite and carbide and varies in hardness between Rc 30 and Rc 55.
BANDED STRUCTURE
A layering effect that is sometimes developed during the hot rolling of steel.
BARK
An older term used to describe the decarburized skin that develops on steel bars heated in a non-protective atmosphere.
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.
BELT FURNACE
A continuous-type furnace that uses a mesh-type or cast-link belt to carry parts through the furnace.
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.
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.
BREAKS
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
Annealing work in a protective atmosphere so that there is no discoloration as the result of heating. In some atmospheres oxides may be reduced.
BRINELL HARDNESS TEST
A test for determining the hardness of a material by forcing a hard steel or carbide ball of specified diameter into it under a specified load. The result is expressed as the Brinell hardness number, which is the value obtained by dividing the applied load in kilograms by the surface area of the resulting impression in square millimetres.
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.
C
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.
CARBONITRIDING
A heat treatment for steel which adds carbon and nitrogen from an atmosphere rich in such elements.
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
Steel which is essentially iron plus carbon with no intentionally added alloy. Also known as ordinary steel, straight carbon steel, or plain carbon steel.
CARBURIZING
Adding carbon to the surface of steel by heating it in contact with carbon-rich solids, liquids or gases.
CASE
The surface layer of a steel whose composition has been changed by the addition of carbon, nitrogen, chromium, or other material at high temperature.
CASE HARDENING
A heat treatment in which the surface layer of a steel is made substantially harder than the interior by altering its composition.
CAST IRON
A generic term for a large family of cast ferrous alloys in which the carbon content exceeds solubility of carbon in austenite at the eutectic temperature. Most cast irons contain at least 2% carbon, plus silicon and sulfur, and may or may not contain other alloying elements. For the various forms gray cast iron, white cast iron, malleable cast iron and ductile cast iron, the word “cast” is often left out, resulting in “gray iron,” “white iron,” “malleable iron,” and “ductile iron,” respectively.
CEMENTITE
The common name for iron carbide, Fe3C, the chemical combination of iron and carbon.
CHARPY TEST
A pendulum-type single-blow impact test in which the specimen, usually notched, is supported at both ends as a simpe beam and broken by a falling pendulum. The energy absorbed, as determined by the subsequent rise of the pendulum, is a measure of impact strength or notch toughness. Contrast with Izod Test.
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
Plastic deformation of a metal at a temperature low enough so that re-crystallization does not occur during cooling.
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 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 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 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.
CORE
The interior part of a steel whose composition has not been changed in a case hardening operation.
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.
CREEP STRENGTH
(1) The constant nominal stress that will cause a specified quantity of creep in a given time at constant temperature.
(2) The constant nominal stress that will cause a specified rate of secondary creep at constant temperature.
CRITICAL COOLING RATE
The rate of continuous cooling required to prevent undesirable transformation. For steel, it is the minimum rate at which austenite must be continuously cooled to suppress transformations above the Ms temperature.
CRITICAL DIAMETER
(D) Diameter of the bar that can be fully hardened with 50% martensite at its center.
CRITICAL POINT
A temperature point at which a structure change either starts, is completed, or both when a material is being heated or cooled.
CRITICAL RANGE
The temperature range between an upper and lower critical point for a given material.
CRYSTALLIZATION
(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.
CYCLE ANNEALING
An annealing process employing a predetermined and closely controlled time-temperature cycle to produce specific properties or microstructures.
D
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.
DECARBURIZING
The process (usually unintentional) of removing carbon from the surface of a steel, usually at high temperature, when in contact with certain types of atmosphere.
DENDRITE
A crystal that has a treelike branching pattern most evident in cast metals slowly cooled through the solidification range.
DEOXIDIZING
(1) The removal of oxygen from molten metals by use of suitable deoxidizers.
(2) Sometimes refers to the removal of undesirable elements other than oxygen by the introduction of elements or compounds that readily react with them.
(3) In metal finishing, the removal of oxide films from metal surfaces by chemical or electrochemical reaction.
DEW POINT
The temperature and pressure at which a gas begins to condense to a liquid.
DEW POINT ANALYZER
An atmosphere monitoring device that measure the partial pressure of water vapor in an atmosphere.
DIRECT QUENCHING
(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.
DEOXIDIZING
(1) The removal of oxygen from molten metals by use of suitable deoxidizers.
(2) Sometimes refers to the removal of undesirable elements other than oxygen by the introduction of elements or compounds that readily react with them.
(3) In metal finishing, the removal of oxide films from metal surfaces by chemical or electrochemical reaction.
DISSOCIATION
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.
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.
DRAW
The common term used interchangeably with Tempering.
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.
DUCTILITY
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.
E
ELASTIC LIMIT
The maximum stress to which a material may be subjected without any permanent strain remaining upon complete release of stress.
EMBRITTLEMENT
The servere loss of ductility or toughness or both, of a material, usually a metal or alloy.
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.
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.
EUTECTIC
(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.
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.
F
FATIGUE
Failure by progressive fracture caused by repeated applications or reversals of stress.
FERRITE
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.
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.
FINAL ANNEALING
An imprecise term used to denote the last anneal given to a nonferrous alloy prior to shipment.
FINISH ANNEALING
A subcritical 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.
FIXTURING
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.
FORCED-AIR QUENCH
A quench utilizing blasts of compressed air against relatively small parts such as a gear.
FLAME HARDENING
A process consisting of heating a desired area, usually localized, with an oxyacetylene torch or other type of high temperature flame and then quenching to produce a desired hardness.
FORMABILITY
The relative ease with which a metal can be shaped through plastic deformation. See drawability.
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. Contrast with combined alloy.
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.
FREEZING RANGE
That temperature range between liquidus and solidus temperatures in which molten and solid constituents coexist.
FULL ANNEALING
An imprecise term that denotes an annealing cycle designed 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 must be stated.
FULL HARD
A temper of nonferrous 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 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%.
G
GRAIN
An individual crystal in a polycrystalline metal or alloy; it may or may not contain twinned regions and subgrains.
GRAIN FLOW
Fiberlike 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.
GRANULAR FRACTURE
A type of irregular surface produced when metal is broken, characterized by a rough, grainlike appearance as differentiated from a smooth, silky or fibrous type. It can be subclassified 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.
H
HARDENABILITY
The fundamental characteristic of a steel which determines the ease of preventing the transformation of austenite to anything else but martensite during the quench.
HARDENING
Increasing hardness by suitable treatment, usually involving heating and cooling.
HEAT TREATMENT
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 TREATABLE ALLOY
An alloy that can be hardened by heat treatment.
HOLDING
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.
HOMOGENIZING
An annealing treatment at fairly high temperature designed to eliminate or reduce chemical segregation.
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.
HYDROGEN EMBRITTLEMENT
The brittleness induced in steel by the absorption of atomic hydrogen, most commonly from a pickling or plating operation.
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.
I
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.
INCLUSIONS
Particles of impurities (usually oxides, sulphides, silicates and such) which separate from the liquid steel and are mechanically held during solidification.In some grades of steel, inclusions are made intentionally high to aid machinability.
INDUCTION HARDENING
A form of hardening in which the heating is done by induced electrical current.
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.
INTERGRANULAR
Between crystals or grains. Also called intercrystalline.
INTERGRANULAR CORROSION
Corrosion occurring preferentially at grain boundaries, usually with slight or negligible attack on the adjacent grains.
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 QUENCH
Stopping the cooling cycle at a predetermined temperature and holding at this temperature for a specific time before cooling to room temperature. Usually done to minimize the likelihood of cracking, or to produce a particular structure in the part.
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 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 TEST
A pendulum-type single-blow impact test in which the specimen, usually notched, is fixed at one end and broken by a falling pendulum. The energy absorbed, as measured by the subsequent rise of the pendulum, is a measure of impact strength or notch toughness. Contrast with Charpy test.
J
JOMINY TEST
See end-quench hardenability test.
K
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
LATENT HEAT
Thermal energy absorbed or released when a substance undergoes a phase change.
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.
M
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.
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.
MARAGING
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.
MARTEMPERING OR MARQUENCHING
Martempering is a form of interrupted quenching in which the steel is quenched rapidly from its hardening temperature to about 450(F, held at 450(F until the temperature is uniform, then cooled in air to room temperature. Actual hardening does not occur until the air cooling starts and is accomplished with a minimum temperature differential. Martempering is indicated for low to medium alloy steels when distortion may be a problem.
MARTENSITE
The very hard transformation product which forms austenite when a steel is quenched and cooled below about 450(F. Technically, martensite can be considered to be a supersaturated solution of carbon in tetragonal (distorted cubic) iron. Under the microscope it appears as an acicular or needle like structure. Hardness of martensite will vary from Rc 30 to Rc 68 depending on the carbon content.
MARTENSITIC TRANSFORMATION
A reaction that takes place in some metals on cooling, with the formation of an acicular structure called martensite.
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.
METALLIZING
(1) Forming a metallic coating by atomized spraying with molten metal or by vacuum deposition. Also alled spray metallizing.
(2) Applying an electrically conductive metallic layer to the surface of a nonconductor.
MICROHARDNESS
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.
MICROSEGREGATION
Segregation within a grain, crystal or small particle.
MICROSTRUCTURE
The structure of a metal as revealed at high magnification, usually at 100x and higher.
MILL SCALE
The heavy oxide layer formed during hot fabrication or heat treatment of metals.
MODULE OF ELASTICITY
A measure of the rigidity of metal. Ratio of stress, below the proportional limit, to corresponding strain. Specifically, the modulus obtained in tension or compression is Young’s modulus, stretch modulus or modulus of extensibility; the modulus obtained in torsion or shear is modulus of rigidity, shear modulus or modulus of torsion; the modulus covering the ratio of the mean normal stress to the change in volume per unit is the bulk modulus. The tangent modulus and secant modulus are not restricted within the proportional limit; the former is the slope of the stress-strain curve at a specified point; the latter is the slope of a line from the origin to a specified point on the stress-strain curve. Also called elastic modulus and coefficient of elasticity.
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.
N
NATURAL AGING
Spontaneous aging of a supersaturated solid solution at room temperature.
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.
NITRIDING
The process of adding nitrogen to the surface of a steel, usually from dissociated ammonia as the source. Nitriding develops a very hard case after a long time at comparatively low temperature, without quenching.
NITROCARBURIZING
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.
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.
NORMALIZING
The process of heating steel to a temperature above its transformation range, followed by air cooling. The purpose of normalizing may be to refine grain structure prior to hardening the steel, to harden the steel slightly, or to reduce segregation in castings or forgings.
O
OIL HARDENING
Quench-hardening treatment involving cooling in oil.
OIL QUENCHING
Hardening of carbon steel in an oil bath. Oils are categorized as conventional, fast, martempering, or hot quenching.
OVERAGING
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 again.
OVERHEATING
Heating a metal or alloy to such a high temperature that its properties are impaired. 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.
OXIDATION
(1) A reaction in which there is an increase in valence resulting from a loss of electrons.
(2) A corrosion reaction in which the corroded metal forms an oxide; usually applied to reaction with a gas containing elemental oxygen, such as air.
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.
P
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.
PEARLITE
A metastable lamellar aggregate of ferrite and cementite resulting from the transformation of austenite at temperatures above the bainite range.
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.
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
The absolute value of the ratio of the transverse strain to the corresponding axial strain, in a body subjected to uniaxial stress; usually applied to elastic conditions.
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.
PREHEATING
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.
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.
Q
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.
QUENCHING
Cooling from high temperature, usually at a fast rate.
R
RACKING
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.
RECARBURIZE
(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.
RECRYSTALLIZATION
(1) Formation of a new, stain-free grain structure from the existing in cold worked metal, usually accomplished by heating.
(2) The change from one crystal structure to another, as occurs on heating or cooling through a critical temperature.
RECUPERATOR
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.
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.
REFRACTORY
(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.
RESIDUAL STRESS
An internal stress not depending on external forces resulting from such factors as cold working, phase changes, or temperature gradients.
RETORT
A vessel used for distillation of volatile materials, as in separation of some metals and in destructive distillation of coal.
ROCKWELL HARDNESS TEST
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
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.
S
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.
SECONDARY HARDNESS
The higher hardness developed by certain alloy steels when they are cooled from a tempering operation. This should always be followed by a second tempering operation.
SELECTIVE HEATING
Intentionally heating only certain portions of a workpiece.
SELECTIVE QUENCHING
Quenching only certain portions of an object.
SEVERITY OF QUENCH
Ability of quenching medium to extract heat from a hot steel workpiece; expressed in terms of the H value.
SHAKER-HEARTH FURNACE
A continuous type furnace that uses a reciprocating shaker motion to move the parts along the hearth.
SHEAR
(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.
SHIM
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).
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.
SOAKING
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.
SPALLING
A chipping or flaking of a surface due to any kind of improper heat treatment or material dissociation.
SPHEROIDIZING
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.)
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 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.
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°C to 900°C (1600°F to 1650°F) to precipitate all carbon as TiC, NbC or TaC so that sensitization is avoided on subsequent exposure to elevated temperature.
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).
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. When the term “strain” is used alone it usually refers to the linear strain in the direction of applied stress.
STRAIN HARDENING
An increase in hardness and strength caused by plastic deformation at temperatures below the recrystallization range.
STRESS
Force per unit area, often thought of as force acting through a small area within a plane. It can be divided into components, normal and parallel to the plane, called normal stress and shear stress, respectively. 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
Heating to a suitable temperature, holding long enough to reduce residual stresses and then cooling slowly to minimize the development of new residual stresses.
SUBCRITICAL ANNEALING
A process anneal performed on ferrous alloys at a temperature below Ac1.
SUPERCOOLING
Cooling below the temperature at which an equilibrium phase transformation can take place, without actually obtaining the transformation.
SUPERHEATING
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.
T
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. The color, which ranges from straw to blue depending on the thickness of the oxide skin, varies with both tempering time and temperature.
TEMPERED MARTENSITE EMBRITTLEMENT
Embrittlement of ultrahigh-strength steels caused by tempering in the temperature range of 205 to 400 degrees C (400 to 750 degrees F); also called 350 degrees C or 500 degrees 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.
TEMPERING
Reheating quenched steel to a temperature below the critical range, followed by any desired rate of cooling. Tempering is done to relieve quenching stresses, or to develop desired strength characteristics.
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
A device for measuring temperatures, consisting of lengths of two dissimilar metals or alloys that 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 electromotive force is produced that is roughly proportional to the difference in temperature between the hot and cold junctions.
TOTAL CARBON
The sum of the free and combined carbon (including carbon in solution) in a ferrous alloy.
TOUGHNESS
The ability of a metal to absorb energy and deform plastically before fracturing.
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 TEMPERATURE
The temperature at which a change in phase occurs. This term is sometimes used to denote the limiting temperature of a transformation range. 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.
U
ULTIMATE STRENGTH
The maximum conventional stress (tensile, compressive or shear) that a material can withstand.
ULTRA-SONIC TESTING
A nondestructive test applied to sound-conductive materials having elastic properties for the purpose of locating inhomogeneities of structural discontinuities within a material by means of an ultrasonic beam.
V
VACUUM ANNEALING
Annealing carried out at sub-atmospheric pressure.
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.
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.
W
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.
WHITE LAYER
Compound layer that forms as a result of the nitriding process.
WORK HARDNESS
Hardness developed in metal resulting from cold 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.
X
Y
YIELD STRENGTH
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.
Z