GLOSSARY OF METALLURGICAL TERMS

[ Back ] [ Home ] [ Table of Contents ] [ Next ]

A

A₁ temperature.   The eutectoid temperature of a steel.

A₂ temperature.   Curie temperature, where bcc iron upon reaching this temperature, 1420⁰F (770⁰C),  becomes nonmagnetic.

A₃ temperature.   The temperature at which proeutectoid ferrite begins to separate from austenite under conditions of slow cooling.

A_cm temperature.   The temperature at which proeutectoid cementite begins to separate from austenite under conditions of slow cooling.

abrasion.   The process of grinding or wearing away using abrasive.

abrasive.   A substance capable of grinding away another material.

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

aging.  A change in properties that occurs at ambient or moderately elevated temperatures after hot working or a heat treating operation (quench aging in 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 does not involve a change in chemical composition.

allotriomorph.   A particle of a phase that has no regular external shape.

allotropy.   The property whereby certain elements may exist in more than one crystal structure.   See polymorphism.

alloy.    A substanc having metallic properties and composed of two or more chemical elements of which at least one is a metal.

alloy steel.   Steel containing significant quantities of alloying elements (other than carbon and the commonly accepted amounts of managanese, silicon, sulphur and phosphorus) added to effect changes in mechanical or physical properties.   Those containing less than 5% total metallic alloying elements tend to be termed low-alloy steels, and those containing more than 5% tend to be termed high-alloy steels.

alloying element.   An element added to a metal, and remaining in the metal, that effects changes in structure and properties.

angstrom unit (A).   A unit of linear measure equal to 10^-10 m, or 0.1 nm; not an accepted Si unit, but still sometimes used for small distances such as interatomic distances and some wavelengths.

annealing.   A general term denoting treatment of metal that involves heating to a suitable temperature followed by cooling at a suitable rate to produce discrete changes in microstructure and properties.   See full annealing, homogenizing annealing, spheroidizing annealing and subcritical annealing.   Full annealing is implied when the term "annealing" is used without qualification.

annealing twin.   A twin formed in a metal during an annealing heat treatment.

arc welding.   A group of welding processes wherein the metal or metals being joined are coalesced by heating with an arc, with or without the application of pressure and with or without the use of filler metal.

artifact.   In microcopy, a false structure introduced during preparation of a specimen.

austempering.   Cooling an austenitized steel at a rate high enough to suppress formation of high-temperature transformation products, then holding the steel at a temperature below that for pearlite formation and above that for martensite formation until transformation to an essentially bainitic structure is complete.

austenite.   Generally, a solid solution of one or more alloying elements in the fcc polymorph of iron (g-Fe).   Specifically, in carbon steels, the interstitial solid solution of carbon in g-Fe.

austenitic grain size.   The size of the grains in steel heated to the austenitic region.

austenitizing.   Forming austenite by heating a steel to between A₁ and A₃ or between A₁and A_cm (partial austenitizing), or above A₁ or  A_cm (complete austenitizing).   When used without qualification, the term implies complete austenitizing.

autoradiograph.   A radiograph recorded photographically by radiation spontaneously emitted by radioisotopes that are produced in, or added to, the material.   This technique identifies the locations of the radioisotopes.

autotempering.   Tempering occuring immediately after martensite has formed, either as the martensite cools from the M_s temperature to room temperature or at room temperature.   Also known as self tempering.

B

bainite.   A eutectoid transformation product of ferrite and a fine dispersion of carbide, generally formed at temperatures below 840 to 930⁰F (450 to 500⁰C0; upper bainite is an aggregate containing parallel lath-shape units of ferrite, produces the so-called "featherly" appearance in optical microscopy, and is formed at temperature above about 660⁰F (350⁰C); lower bainite consists of individual plate-shape units and is formed at temperature below about  660⁰F (350⁰C).

bamboo grain structure.   A structure in wire or sheet in which the boundaries of the grains tend to be aligned normal to the long axis and to extend completely through the thickness.

banding.   Inhomogeneous distribution of alloying elements or phases aligned in filaments or plates parallel to the direction of working.  See ferrite-pearlite banding a segregation banding.

billet.   A solid, semifinished steel round or square product that has been hot worked by forging, rolling or extrusion; usually smaaler than a bloom.

blister steel.   High-carbon steel produced by carburizing wrought iron.   The bar, originally smooth, is covered with small blisters when removed from the cementation (carburizing) furnace.

bloom.   (1) Ancient: iron produced in a solid condition directly by the reduction of ore in a primitive furnace.   The carbon content is variable but usually low.   Also known as bloomery iron.   The earliest ironmaking process, but still used in underdeveloped countries.   (2) Modern: a semifinished hot rolled product, rectangular in section, usually produced on a blooming mill but sometimes made by forging.

bloomery.   A primitive furnace used for direct reduction of ore to iron.

box annealing.   Annealing a metal or alloy in a sealed container under conditions that minimize oxidation.

braze welding.   A group of welding processes in which metals are joined by a filler metal that has a liquidus temperature below the solidus of the parent metal, but above 840⁰F (450⁰C).   The filler metal is not distributed by capillary action.   Compare brazing.

brazing.   A welding process in which a third metal or alloy of comparatively low melting point is melted and caused, sometimes with the assistance of a flux, to wet the surfaces to be joined.

brittle fracture.   fracture preceded by little or no plastic deformation.

brittleness.   The tendency of a material to fracture without first undergoing significant plastic deformation.

buffer.   A substance added to aqueous solutions to maintain a constant hydrogen-ion concentration, even in the presence of acids or alkalis.

burning.   (1) During austenitizing, permanent damage of a metal or alloy by heating to cause incipient melting; see overheating.  (2) During subcritical annealing, particularly in continuous annealing, production of a severely decarburized and grain-coarsened surface layer that results from heating to an excessively high temperature for an excessively long time.

C

capped steel.   Semikilled steel cast in a bottle-top mold and covered with a cap fitting into the top of the mold to cause the top metal to solidify.   Pressure increases in the sealed-in mlten metal, resulting in a surface condition in the ingot similar to that of the rimmed steel.

carbide.   A compound of carbon with one or more metallic elements.

carbon equivalent (for rating of weldability).   A value that takes into account the equivalent additive effects of carbon and other alloying elements on a particular characteristic of a steel.   For rating weldability, a formula commonly used is:

            CE = C + (Mn/6) + [(Cr + Mo + V)/5] + [(Ni + Cu)/15].

carbon potential.   A measure of the capacity of an environment containing active carbon to alter or maintain, under prescribed conditions, the carbon concentration in a steel.

carbon restoration.   Replacing the carbon lost in the surface layer during previous processing by carburizing this layer to substantially the original carbon level.

carbon steel.   A steel containing only residual quantities of elements other than carbon, except those added for deoxidation or to counter the deleterious effects of residual sulphur.   Silicon is usually limited to about 0.60%, and manganese to about 1.65%.   Also termed plain carbon steel, ordinary steel, straight carbon steel.

carbonitriding.   A case hardening process in which a suitable ferrous material is heated above the lower transformation temperature is a gaseous atmosphere having a composition that results in simultaneous absorption of carbon and nitrogen by the surface and, by diffusion, creates a concentration gradient.   The process is completed by cooling at a rate that produces the desired properties in the workpiece.

carburizing.   A process in which an autenitized ferrous material is brought into contact with carbonaceous atmosphere having sufficient carbon potential to cause absorption of the carbon at the surface and, by diffision, create a concentration gradient.

case.   In a ferrous alloy, the outer portion that has been made harder than the inner portion, or core (see case hardening).

case hardening.   A generic term covering several processes applicable to steel that change the chemical composition of the surface layer by absorption of carbon or nitrogen, or a mixture of the two, and, by diffusion, create a concentration gradient.

cast iron.   Iron containing more carbon than the solubility limit in austenite (about 2%).

cast steel.   Steel in the form of casting, usually containing less than 2% carbon.

casting.   (1) An object at or near finished shape obtained by solidification of a substance in a mold.   (2) Pouring molten metal into a mold to produce an object of desired shape.

cementation.   (1) Introduction of one or more elements into the outer layer of a metal object by means of diffusion at high temperature.   (2) An obsolete process used to convert wrought iron to blister steel by carburizing.   Wrought iron bars were packed in sealed chests with charcoal and heated at about 2000⁰F (1100⁰C) for 6 to 8 days.   Cementation was the predominant method of manufacturing steels, particularly high-carbon tool steels, prior to the introduction of the bessermer and open-hearth methods.

cementite.   A metastable carbide, with composition Fe₃C and orthorhombic crystal structure, having limited substitutional solubility for carbide-forming elements, notably manganese.

chafery.   A charcoal-fired furnace used in early ironmaking processes to reheat a bloom of wrought iron for forging to consolidate the iron and expel entrapped slag.

chemical polishing.   Improving the specular reflectivity of a metal surface by chemical treatment.

cleavage.   Fracture of a crystal by crack propagation across a crystallographic plane of low index.

cleavage fracture.   Fracture of a grain, or most of the grains, in a polycrystalline metal by cleavage, resulting in bright reflecting facets.

cleavage plane.   A characteristic crystallographic plane or set of planes in a crystal on which cleavage fracture occurs easily.

columnar structure.   A structure consisting of elongated grains whose long axes are parallel.

conatituent.   A phase, or combination of phases, that occurs in a characteristic configuration in a microstructure.

constitutional diagram.    A graphical represrentation of the temperature and composition limits of phase fields in an alloy system as they actually exist under specific conditions of heating and cooling (synonymous with phase diagram).   A constitutional diagram may be, or may approximate, an equilibrium diagram, or may represent metastable conditions or phases.   Compare equilibrium diagram.  

continuous phase.   In an alloy or portion of an alloy containing more than one phase, the phase that forms the background or matrix in which the other phase or phases are present as isolated volumes.

controlled rolling.   A hot rolling process in which the temperature of the steel is closely controlled, particularly during the final rolling passes, to produce a fine grain microstructure.

coring.   A variation of composition between the center and surface of a unit of structure (such as a dendrite, a grain or a carbide particle) resulting from nonequilibrium growth over a range of temperature.

corrosion.   Deterioration of a metal by chemical or electrochemical reaction with its environment.

creep.   Time dependent strain occurring under stress.

critical cooling rate.   The limiting rate at which austenite must be cooled to ensure that a particular type of transformation product is formed.

critical point.   (1) The temperature or pressure at which a change in crystal structure, phase or physical properties occurs, same as transformation temperature.   (2)  In an equilibrium diagram, that specific combination of composition, temperature and pressure at which the phases of an inhomogeneous system are in equilibrium.

critical strain.   That strain which results in the formation of very large grains during crystallization.

critical temperature.   Synonymous with critical point if pressure is constant.

cross rolling.   A (hot) rolling process in which rolling reduction is carried out in a direction perpendicular to, as well as a direction parallel to, the length of the original slab.

crucible steel.   High-carbon steel produced by melting blister steel in a covered crucible.   Crucible steel was developed by Benjamin Huntsman in about 1760 and remained in use until the late 1940's.

crystalline fracture.   A fracture of a polycrystalline metal characerized by a grainy appearance.   Compare fibrous fracture.

D

decarburization.   Loss of carbon from the surface of a ferrous alloy as a result of heating in a medium that reacts with carbon.

decoration (of dislocations).   Segregation of solute atoms to the line of a dislocation in a crystal.   In ferrite, the dislocations may be decorated with carbon or nitrogen atoms.

deformation bands.   Generally, bands in which deformation has been concentrated inhomogeneously.   See kink band, microbands and shear bands for specific types.

degenerate structure.   Usually refers to pearlite that does not have an ideally lamellar structure.   The degree of degenaracy may vary from slight perturbations in the lamellar arrangement to structures that are not recognizably lamellar.

dendrite.   A crystal that has grown in treelike branching mode.

dendritic segragation.   Inhomogeneous distribution of alloying elements through the same dendrites.

deoxidation.   (1) Removal of oxygen from molten metals by use of suitable chemical agents.   (2) Sometimes refers to removal of undesirable elements other than oxygen by the introduction of elements or compounds that readily react with them.

diffusion.   (1) Spreading of a constituent in a gas, liquid or solid, tending to make the composition of all parts uniform.   (2) The spontaneous movement of atoms or molecules to new sites within a material.

dilatometer.   An instrument for measuring the expansion or contraction of a solid metal resulting from heating, cooling, polymorphic changes, etc.

dislocation.   A linear defect in the structure of a crystal. 

ductility.   the capacity of a material to deform plastically without fracturing.

E

elastic limit.   The maximum stress to which a material may be subjected without any permanent strain remaining upon complete release of the stress.

elastic strain.   Dimensional changes accompanying stress where the original dimensions are restored upon release of the stress.

electron beam microprobe analyzer.   An instrument for selective chemical analysis of a small volume of material.   An electron beam bombards the area of interest and x-radiation thereby emitted is analyzed in a spectrometer.

electropolishing.   Improving the specular reflectivity of  a metal surface by electrochemical dissolution.

elongation after fracture.   In tensile testing, the increase in that gauge length measured after fracture of the specimen within the gauge length and usually expressed as a percentage of the original gauge length.

epitaxy.   Induced orientation of the lattice of a crystal of a surface deposit by the lattice of the substrate crystal.

equiaxed structure.   A structure in which the grains have approximately the same dimensions in all directions.

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 thermodynamical equilibrium.   In condensed system, pressure is usually considered constant.

etchant.   A chemical solution used to etch a metal to reveal structural details.

etching.   Subjecting the surface of a metal to preferential chemical or electrolytic attack to reveal structural details.

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

F

fatigue.   The phenomenon leading to fracture under repeated or fluctuating stresses (having a maximum value less than the tensile strength of the material).

ferrite.   Generally, a solid solution of one or more alloying elements in the bcc polymorph of iron (a-iron).   Specifically, in carbon steels, the interstitial solid solution of carbon in a-iron.

ferrite-pearlite banding.   Inhomogeneous distribution of ferrite and pearlite aligned in filaments or plates parallel to the direction of working.

ferritic grain size.   The grain size of the ferritic matrix of a steel.

fiber.   (1) The characteristic of wrought metal that indicates directional properties.   It is revealed by etching a longitudinal section or manifested by the fibrous appearance of a fracture.   It is caused chiefly by extension of the constituents of the metal, both metallic and nonmetallic, in the direction of working.   (2) The pattern of preferred orientation of metal crystals after a given deformation process.   See preferred orientation.

fibrous fracture.   A fracture whose surface is characterized by a dull gray or silky appearance.   Compare crystalline fracture.

filler metal.   A third material that is melted concurrently with the parent metals during fusion or braze welding.   It is usually, but not necessarily, of different composition from the parent metals.

finery.   A charcoal-fueled hearth furnace used in early processes for converting cast iron to wrought iron by melting and oxidizing it in an air blast, then repeatedly oxidizing the product in the presence of a slag.   The carbon oxidizes more rapidly than the iron so that a wrought iron of low carbon content is produced.

flash.   (1) In forging, the excess metal forced between the upper and lower dies.   (2) In resistance butt welding, a fin formed perpendicular to the direction of applied pressure.

flow lines.   (1) Texture showing the direction of metal flow during hot or cold working.   Flow lines often can be revealed by etching the surface or a section of a metal part.   (2) In mechanical metallurgy, paths followed by volume elements of metal during deformation.

flow stress.   The uniaxial true stress required to cause plastic deformation at a specified value of strain.

flux.   (1) In refining, a material used to remove undesirable substances as a molten mixtures.   It may also be used as a protection covering for molten metal.   (2) In welding, a material used to prevent the formation of, or to dissolve and facilitate the removal of, oxides and other undesirable substances.

forge welding.   Welding hot metal by applying pressure or blows.

forging.   Plastically deforming metal, usually hot,into desired shapes with compressive force, with or without dies.

fractography.   Descriptive treatment of fracture, especially in metals, with specific reference to photography of the fracture surface.

fragmentation.   The subdivision of a grain into small discrete crystallites outlined by a heavily deformed network of intersecting slip bands as a result of cold working.   These small crystals or fragments differ from one another in orientation and tend to rotate to a stable orientation determined by the slip systems.

free machining.   Pertains to the machining characteristics of an alloy to which one or more ingredients have been introduced to produce small broken chips, low power consumption, better surface finish or longer tool life.

full annealing (ferrous materials).   An annealing treatment in which a steel is austenitized by heating to a temperature above the upper critical temperature (A₃ or A_cm) and then cooled slowly to room temperature.   A typical cooling would be 210⁰F/h (100⁰C/h).   Compare normalizing.   Use of the term "annealing" without qualification implies full annealing.

fusion welding.   Any welding process in which fusion is employed to complete the weld. 

G

gas welding.   Welding with a gas flame.

grain.   An individual crystal in a polycrystalline metal or alloy, including twinned regions or subgrains if present.

grain-boundary liquation.   An advanced stage of overheating in which material in the region of austenitic grain boundaries melts.   Also known as burning.

grain-boundary sulfide precipitation.   An intermediate stage of overheating in which sulfide inclusions are redistributed to the austenitic grain boundaries by partial solution at the overheating temperature and reprecipitation during subsequent cooling.

grain-coarsening.   A heat treatment that produces excessively large austenitic grains.

grain flow.   Fiberlike lines appearing on polished and ectched sections of forgings, caused by orientation of the constituents of the metal in the direction of working during forging.

grain growth.    An increase in the average size of the grains in polycrystalline metal, usually a result of heating at elevated temperature.

grain size.   A measure of the areas or volumes of grains in a polycrystalline metal or alloy, usually 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 size is reported in terms of number of grains per unit area or volume, average diameter, or as a number derived from area measurements.

granular fracture.   Atype of irregular surface produced when metal fractures, 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 implication that the metal has crystallized is completely misleading.

graphite.   The polymorph of carbon with a hexagonal crystal structure.

graphitization.   Formation of graphite in iron or steel.   Primary graphitization refers to formation of graphite during solidification.   Secondary graphitization refers to later formation during heat treatment.

grinding.   Removing material from a workpiece with a grinding wheel or abrasive belt.

H

hardenability.   In ferrous alloys, the property that determines the depth and distribution of hardness induced by quenching.

hardening.   Increasing hardness by suitable treatment, usually involving heating and cooling.   When applicable, the following more specific terms should be used: age hardening, case hardening, flame hardening, induction hardening, precipitation hardening, quench hardening.  

hardness (indentation).   Resistance of a metal to plastic deformation by indentation.   Various hardness tests such as Brinell, Rockwell and Vickers may be used.   In the Vickers test, a diamond pyramid with an included face angle of 136⁰ is used as the indenter.

heat-affected zone.   That portion of the base metal which was not melted during brazing, cutting or welding, but within which microstructure and physical properties were altered by the treatment.

heat tinting.   Colouration of a metal surface through oxidation by heating to reveal details of structure.

heat treatment.   Heating and cooling a solid metal or alloy in such a way that desired structures, conditions or properties are attained.   Heating for sole purpose of hot working is excluded from the meaning of this term.

hematite.   The oxide of iron of highest valency which has a composition close to the stoichiometric composition Fe₂O₃.

homogenizing annealing.   An annealing treatment carried out at a high temperature, approaching the solidus temperature, for a sufficiently long time that inhomogeneous distributions of alloying elements are reduced by diffusional processes.

hot working.   Deformation under conditions that result in recrystallization.

hypereutectoid alloy.   In a eutectoid system, any alloy containing more than the eutectoid concentration of solute.

hypoeutectic alloy.   In a eutectic system, any alloy containing less than the eutectic concentration of solute.

I

idiomorph.   A particle of a phase that has a regular external shape.

impact test.   A test  for determining the bahaviour of materials when subjected to high rates of loading under conditions designed to promote fracture, usually in bending, tension or torsion.   The quantity measured is the energy absorbed when the specimen is broken by a single blow.

impurities.   Elements or components whose presence in a material is undesired.

inclusion.   A nonmetallic material in a solid metallic matrix.

ingot.   A casting suitable for hot working or remelting.

ingot iron.   Commercially pure iiron.

.intercrystalline.   Between crystals, or between grains.   Same as intergranular.

internal oxidation.    Formation of oxides beneath the surface of a metal.

interstitial solid solution.   A solid solution in which the solute atoms occupy (interstitial) positions between the atoms in the structure of the solvent.   See also substitutional solid solution.

intracrystalline.   Within and across crystals or grains.    Same as transcrystalline and transgranular.

iron.   An element that has an average atomic number of 55.85 and that always, in engineering practice, contains small but signaficant amounts of carbon.   Thus iron-carbon alloys containing less than about 0.1% C may be referred to as irons.   Alloys with higher carbon contents are always termed steels.

isothermal transformation (IT) diagram.   A diagram that shows the isothermal time required for transformation of austenite to commence and to finish as a function of temperature.   Same as time-temperature-transformation (TTT) diagram or S-curve.

K

killed steel.   Steel deoxidized with a strong deoxidizing agent, such as silicon or aluminum, to reduce the oxygen content to such a level that no reaction occurs between carbon and oxygen during solidification.

kink band (deformation).   In polycrystalline materials, a volume of crystal that has rotated physically to accommodate differential deformation between adjourning parts of a grain while the band itself has deformed homogeneously.   This occurs by regular bending of the slip lamellae along the boundaries of the band.

 L

lamellar tear.   A system of cracks or discontinuities aligned generally parallel to the worked surface of a plate.   Usually associated with a fusion weld in thick plate.

lamination.   An abnormal structure resulting in a separation or weakness aligned generally parallel to the worked surface of the metal.

lath martensite.   Martensite formed,tly in steels containing less than about 1.0% C and solely in steels containing less than about 0.5% C, as parallel arrays or packets of lath-shape units about 0.1 to 0.3 mm thick, and having a habit plane that is close to {111}.

liquation.   Partial melting of an alloy.

liquidus.   In a constitutional diagram, the locus of points representing the temperatures at which various components commence freezing on cooling or finish melting on heating.

luders lines or bands.   Elongated surface markings or depressions caused by localized plastic deformation that results from discontinuous (inhomogeneous) yielding.

M

M_f  temperature   The temperature at which martensitic transformation is essentially complete during cooling after austenitization.

M_s  temperature   The temperature at which a martensitic transforamtion starts during cooling after austenization.   

machinability.   The capacity of a material to be machined easily.

macroetching.   Etching of a metal surface with the objective of accelerating gross structural details, for observing by the unaided eye or at magnifications not exceeding ten diameters.

macrograph.   A graphic reproduction of a prepared surface of a specimen at amagnification not exceeding ten diameters.  When photographed, the reproduction is known as a photomacrograph (not a macrophotograph).

macrostructure.   The structure of a metal as revealed by examination of the etched surfae at a magnification not exceeding ten diameters.

magnetite.   The oxide of iron of intermediate valence which has a composition close to the stoichiometric composition Fe₃O₄.

manual welding.   Welding wherein the entire welding operation is performed and controlled by hand.

martempering.   (1)   A hardening procedure in which an austenitized ferrous material is quenched into an appropriate medium at a temperature just above the M_s temperature of the material, held in the medium until the temperature is uniform through-out - but not long enough for bainite to form - and then cooled in air.   The treatment is frequently followed by tempering.   (2)   When the process is applied to carburized material, the controlling M_s temperature is that of the case.  This variation of the process is frequently called marquenching.

martensite.   In steel, a metastable transition phase with a body-centred-tetragonal crystal structure formed by diffusionless transformation of austenite generally during cooling between the M_s and  M_f  temperatures.

martensite range.   The interval between the  M_s and M_f  temperatures.

matrix.   The principap phase or aggregate in which another constituent is embedded.

mechanical twin.   A twin formed in a metal during plastic deformation by simple shear of the structure.

mechanical polishing.   A method of producing a specularly reflecting surface by use of abrasives.

melting point.   The temperature at which a pure metal, compound or eutectic changes from solid to liquid; the temperature at which the liquid and the solid are in equilibrium.

metal.   An opaque, lustrous, elemental substance that is a good conductor of heat and electricity and, when polished, a good reflector of light.   Most metals are malleable and ductile and are, in general, denser than other substances.

metallograph.   An optical instrument designed for both visual observation and photomicrography of prepared surfaces of opaque materials at magnifications ranging from 25 to about 1500 diameters.

metastable.   Possessing a state of pseudo-equilibrium that has a free energy higher than that of the true equilibrium state but from which a system does not change spontaneously.

microbands (deformation).   Thin sheetlike volumes of constant thickness in which cooperative slip occurs on a fine scale.   They are an instability which carry exclusively the deformation at medium strains when normal homogeneous slip is precluded.   The sheets are aligned at +/-55⁰ to the compression direction and are confined to individual grains, which usually contain two sets of bands.  Compare shear bands.

microcrack.   A crack of microscopic size.

micrograph.   A graphic reproduction of the prepared surface of a specimen at a mignification greater than ten diameters.   When photographed, the reproduction is known as a photomicrograph (not a microphotograph).

microsegregation.   Segregation within a grain, crystal or small particle.   See coring.

microstructure.   The structure of a prepared surface of a metal as revealed by a microscope at a magnification greater than ten diameters.

mild steel.   Carbon steel containing a maximum of about 0.25% C.

N

neutral aging.   Spontaneous aging of a supersaturated solid solution at room temperature.   See aging.

necking.   Local reduction of the cross-sectional area of metal by stretching.

Neumann band.   A mechanical (deformation) twin in ferrite.

nodular pearlite.   Pearlite that has grown as a colony with an approximately spherical morphology.

normalizing.   Heating a ferrous alloy to a suitable temperature above A₃ or A_cm and then cooling in still air to a temperaturez substially below A₁.   The cooling rate usually is in the range 900 to 1800⁰F/h (500 to 1000⁰C/h).

notch brittleness.   A measure of the susceptibility of a material to brittle fracture at locations of stress concentration.   For example, in a notch tensile test amaterial is said to be "notch brittle" if its notch strength is less than its tensile strength; otherwise, it is said to be "notch ductile".

notch ductility.   See notch brittleness.

notch sensitivity.   A measure of the reduction in strength of a metal caused by the presence of stress concentration.   Values can be obtained from static,impact or fatigue tests.

nucleation.   Initiation of a phase transformation at discrete sites, the new phase growing from nuclei.   See nucleus (1)

nucleus.   (1)   The first structurally stable particle capable of initiating recrystallization of a phase or the growth of a new phase, and separated from the matrix by an interface.   (2)   The heavy central core of an atom, in which most of the mass and the total positive electrical charge are concentrated.

O

orientation (crystal).   Direction in space of the axes of the lattice of a crystal with respect to a chosen reference or coordinate system.   See also preferred orientation.

overaging.  Aging under conditions of time and temperature greater than those required to obtain maximum change in a certain property.   See aging.

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)   Chemical combination with oxygen to form an oxide.

oxidized surface.   A surface having a thin, tightly adhering oxidized skin.

P

pack rolling.   Hot rolling a pack of two or more sheets of metal; scale prevents  the sheets from being welded together.

pancake grain structure.   A structure  in which the lengths and widths of individual grains are large compared to their thicknesses.

pass.   (1)   A single transfer of metal through a stand of rolls.   (2)   The open space between two grooved rolls through which metal is processed.   (3)   The weld deposited in one run along the axis of a weld.

patenting.   A heat treatment applied to medium and high-carbon steel prior to cold drawing to wire.  The treatment involves austenitization followed by isothermal transformation at a temperature that produces a microstructure of very fine pearlite.

pattern welding.   A process in which strips or other small sections of iron or steel are twisted together and then forge welded.   Homogeneity and toughness are thereby improved.  A regular decorative pattern can be developed in the final product.   Commonly used for making swords as early as the 3rd century A.D.

pearlite.   A eutectoid transformation product of ferrite and cementite that ideally has a lamellar structure but that is always degenrate to some extent.

peritectic.   An isothermal reversible reaction in which a liquid phase reacts with a solid phase to produce another solid phase.

phase.   A physical homogeneous and distinct portion of a material system.

phase diagram.   Synonymous with constitutional diagram.

photomacrograph.   See macrograph.

photomicrograph.   See micrograph.

physical properties.  Properties, other than mechanical properties, that pertain to the physical nature of a material; e.g., density, electrical conductivity, thermal expansion, reflectivity, magnetic susceptibility, etc.

pig iron.   (1)   High-carbon iron made by reduction of iron ore in the blast furnace.   (2)   Cast iron in the form of pigs.

piling.   A process in which several bars are stacked and hot rolled together, with the objective of improving the homogeneity of the final product.   Used in primitive ironmaking.

plastic deformation.   Deformation that remains, or will remain, permanent after release of the stress that caused it.

plasticity.   The capacity of a metal to deform nonelastically without rupturing.

plate.   A flat rolled metal product of some minimum thichness and width arbitrarily dependent on the type of metal.

plate martensite.   Nartensite formed, partly in steels containing more than 0.5% C and solely in steels containing more than 1.0% C, as lenticular-shape plates on irrational habit planes.

polishing.   Producing a specularly reflecting surface.

polycrystalline.   Comprising an aggregate of more than one crystal, and usually a large number of crystals.

polymorphism.   The property whereby certain substances may exist in more than one crystalline form, the particular form depending on the conditions of crystallization - e.g., temperature and pressure. Among elements,this phenomenon is also called allotropy.

preferred orientation.   A condition of a polycrystalline aggregate in which the crystal orientations are not random.

primary crystal.  The first type of crystal that separates from a melt during solidification.

proeutectoid.   (phase).. Particles of a phase that precipitate during cooling after austinitizing but before the eutectoid transformation takes place.

proof stress.   See yield strength.

puddling process.   A process for making wrought iron in which cast iron is melted in a hearth furnace and rabbled with slag and oxide until a pasty mass is obtained. This process was developed by Henry Cort about 1784 and remained in use until 1957, although on a very small scale during the present century.

Q

quench aging.   Aging that occurs after quenching following solution heat treatment.

quench hardening.   Hardening by austenitizing and then cooling at a rate such that a substantial amount of austenitie is transformed to martensite.

quanching.   Rapid cooling.

R

recarburizing.  (1)   Increasing the carbon content of molten cast iron or steel by adding carbonaceous material, high-carbon pig iron or high-carbon alloy.  (2)   Carburizing a metal part to return surface carbon lost in processing.

recovery.   Reduction or removal of work-hardening effects, without motion of large-angle grain boundaries.

recrystallization.  (1)   A change from one crystal structure to another, such as that occurring on heating or cooling through a critical temperature.   (2)   Formation of a new, strain-free grain, structure from the structure existing in cold worked metal.

recrystallization annealing.   Annealing cold worked metal to produce a new grain structure without a phase change.

recrystallization temperture.   The approximate minimum temperature at which complete recrystallization of a cold worked metal occurs within a specified time.

residual elements.   Small quantities of elements unintentionally present in an alloy.

residual stress.   Stress present in a body that is free of external forces or thermal gradients.

resistance welding.   Welding with electrical resistance haeting and pressure, the work being part of an electrical circuit.

resolution.   The capacity of an optical or radiation system to separate closely spaced forms or entities; also, thedegree to which such forms or entities can be discriminated.

resulfurized steel.   Steel to which sulfur has been added in controlled amounts after refining.   The sulfur is added to improve machinability.

rimmed steel.   Low-carbon steel containing sufficient iron oxide to produce continuous evolution of carbon monoxide during ingot solidificstion, resulting in a case or rim of metal virtually free of voids.

rolling.   Reducing the cross-sectional area of metal stock, or otherwise shaping metal products, through the use of rotating rolls.

S

scale.   A layer of oxidation products formed on a metal at high temperature.

scarf joint.   A butt joint in which the plane of the joint is inclined with respect to the main axes of the members.

segregation.   Nonuniform distribution of alloying elements, impurities or phases.

segregation banding.   Inhomogeneous distribution of alloying elements aligned in filaments or plates parallel to the direction of working.

self diffusion.   The spontaneous movement of an atom to a new site in a crystal of its own species.

semikilled steel.   Steel that is incompletely deoxidized and contains sufficient dissolved oxygen to react with the carbon to form carbon monoxide and thus offset solidification shrinkage.

shear. 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.

shear bands (deformation).   Bands in which deformation has been concentrated inhomogeneously in sheets that extend across regional groups of grains.   Usually only one system is present in each regional groups of grains, different systems being present in adjoining groups.   The bands are noncrystallographic and form on planes of maximum shear stress (55⁰  to the compression direction).   They carry most of the deformation at large strains.   Compare microbands.

shear steel.   Steel produced by forge welding together several bars of blister steel, providing a more homogeneous product.

sheet.  A flat-rolled metal product of some maximum thickness and minimum width arbitrarily dependent on the type of metal.   Sheet is thinner than plates.

shortness.   A form of brittleness in metal.   It is designated as "cold", "hot", and "red", to indicate the temperature range in which the brittleness occurs.

sintering.   Bonding of adjacent surfaces of particles in a mass of metal powders, or in a compact, by heating.

slab.   A piece of metal, intermediate between ingot and plate, at least twice as wide as it is thick.

slag.   A nonmetallic product resulting from mutual dissolution of flux and nonmetallic impurities in smelting and refining operations.

slip.   Plastic deformation by irreversible shear displacement of one part of a crystal relative to another in a definite crystallographic direction and on a definite crystallographic plane.

slip direction.   The crystallographic direction in which translation of slip takes place.

slip line.   Trace of a slip plane on a viewing surface.

slip plane.   The crytallographic plane on which slip occurs in a crystal.

solid solution.   A solid crystalline phase containing two or more chemical species in concentrations that may vary between limits imposed by phase equilibrium.

solidus.   In a constitutional diagram, the locus of points representing the temperatures at which various components finish freezing on cooling or begin to melt on heating.

solute.   The component of either a liquid or solid solution that is present to the lesser or minor extent; the component that is dissolved in the solvent.

solution heat treatment.   A heat treatment in which an alloy is heated to a suitable temperature, held at that temperature long enough to cause one or more constituents to enter into solid solution, and then cooled rapidly enough to hold these constituents in solution.

solvent.   The component of either a liquid or solid solution that is present to the greater or major extent; the component that dissolves the solute.

solvus.   In a phase or equilibrium diagram, the locus of points representing the temperature at which solid phases with various compositions coexist with other solid phases; that is, the limits of solid solubility.

spheroidized structure.   A microstructure consisting of a matrix containing spheroidal particles of another constituent.

spheroidizing.   Heating and cooling to produce a spheroidal  or globular form of carbide in steel.

spheroidizing annealing.   A subcritical annealing treatment intended to produce spheroidization of cementite or other carbide phases.

steel.   An iron-base alloy usually containing carbon and other alloying elements.   In carbon steel and low-alloy steel, the maximum carbon content is about 2.0%; in high-alloy steel, about 2.5%.   The dividing line between low-alloy and high-alloy steels is generally regarded as the 5% level of total metallic alloying elements.   Steel is differentiated from two general classes of "iron" - namely, cast irons, which have high carbon concentrations, and relatively pure irons, which have low carbon concentrations.

strain.   A measure of the relative change in the size of a body. Linear strain is the change per unit length of a linear dimension.   True (or natural) strain is the natural logarithm of the ratio of the length at the moment of observation to the original gauge length.   Shearing strain is the change in angle (expressed in radians) between two reference lines originally at right angles.   When the term is used alone, it usually refers to linear strain in the direction of the applied stress.

strain aging.   Aging induced by cold worked.   See aging.

strain hardening.   an increase in hardness and strength caused by plastic deformation at temperature below the recrystallization range.

stress.   Force per unit area.   True stress denotes stress determined by measuring force and area at the same time.   Conventional stress, as applied to tension and compression tests, is force divided by original area.   Nominal stress is stress computed by single elasticity formulae.

stress-corrosion cracking.   Failure by cracking under the combined action of corrosion and stress, either external (applied) or internal (residual).   Cracking may be either intergranular or transgranular, depending on the metal and the corrosive medium.

stress relieving.   Heating to a suitable temperature, holding long enough to reduce residual stresses and then cooling slowly enough to minimize the development of new residual stresses.

stretcher strains.   Elongated markings that appear on the surfaces of some materials when they are deformed just past the yield point.   These markings lie approximately parallel to the direction of maximum shear stress and are the result of the localized yielding.   See also Luders lines.

strip.   A sheet of metal whose length is many times its width.

sub-boundary structure (subgrain structure).   A network of low-angle boundaries (usually with misorientations of less than one degree) within the main grains of a microstructure.

subcritical annealing.   An annealing treatment in which a steel is heated to a temperature below the A₁ temperature and then cooled slowly to room temperature.

subgrain.   A portion of a crystal or grain slightly different in orientation from neighbouring portions of the same crystal.   Generally, neighbouring subgrains are separated by low-angle boundaries.

substitutional solid solution.   A solid solution in which the solvent and solute atoms are located randomly at the atom sites in the crystal structure of the solution.

substrate.   The layer of metal underlying a coating, regardless of whether the layer is base metal.

sulfide spheroidization.   A stage of overheating in which sulfide inclusions are partly or completely spheroidized.

sulfur print.   A macrographic method of examining distribution of sulfide inclusions.

supercooling.   Cooling to a temperature below that of an equilibrium phase transformation without the transformation taking place.

superheating.   (1)   Heating a phase to a temperature above that of a phase transformation without the transformation taking place.   (2)   Heating molten metal to a temperature above the normal casting temperature to obtain more complete refining or greater fluidity.

surface hardening.   A generic term covering several processes applicable to a suitable ferrous alloy that produce, by quench hardening only, a surface 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 induction hardening, flame hardening and shell hardening.   Use of the applicable specific process name is preferred.

T

taper section.   A section  made at an acute angle to a surface of interest, thereby achieving a geometrical magnification of depth.   A sectioning angle of 5⁰ 43' achieves a depth magnification of 10:1.

teeming.   Pouring molten metal from a ladle into ingot molds.   The term applies particularly to the specific operation of pouring either iron or steel into ingot molds.

temper brittleness.A reversible increase in the ductile-brittle transition temperature in steels heated in, or slowly cooled through, the temperature range from about 700 to 1100 ⁰F (375 to 575 ⁰C).

temper rolling.   Light cold rolling of sheet steel.   The operation is performed to improve flatness, to minimize the formation of stretch strains, and to obtain a specified hardness or temper.

tempering.    In heat treatment, reheating hardened steel to some temperature below the A₁ temperature for the purpose of decreasing hardness and/or increasing toughness.   The process also is sometimes applied to normalized steel.

tensile strength.   In tensile testing, the ratio of the maximum force sustained to the original cross-sectional area.

texture.   In a polycrystalline aggregate, the state of distribution of crystal orientation.   In the usual sense, it is synonymous with preferred orientation, in which the distribution is not random.

thermal analysis.   A method of studying transformations in metal by measuring the temperatures at which thermal arrests occur.

time-temperature-transformation   (TT) diagram.   See isothermal transformation (IT) diagram.

toughness.   Capacity of a metal to absorb energy and deform plastically before fracturing.

transcrystalline.    Same as intracrystalline.

transformation ranges (transformation temperature ranges).   Those ranges of temperature within which austenite forms during heating and transforms during cooling.  The two ranges are distinct, sometimes overlapping but never coinciding.   The limiting temperatures of these ranges depend on the composition of the alloy and on the rate of change of temperature, particularly during cooling.   See transformation temperature.

transformation temperature.   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:

 A₁ - The temperature of the eutectoid transformation.

 A₃ - The temperature at which proeutectoid ferrite begins to separate from austenite under conditions of slow cooling.

 A_cm - The temperature at which proeutectoid cementite begins to separate from austenite under conditions of slow cooling.

 M_f  - The temperature at which transformation of austenite to martensite finishes during cooling.

 M_s - The temperature  at which transformation of austenite to martensite starts during cooling.

transgranular.   Same as intracrystalline.        

transition temperature (ductile-brittle transition temperature).   An arbitrarily defined temperature that 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 cleavage.

triple point.   The intersection of the boundaries of three adjoining grains, as observed in a section.

twin.   Two portions of a crystal having a definite orientation relationship; one may be regarded as the parent, the other as the twin.   The orientation of the twin is either a mirror image of the orientation of the parent across a "twinning plane" or an orientation that can be derived by rotating the twin portion about a "twinning axis".

twin, annealing.   A twin produced as the result of heat treatment.

twin, crystal.   A twinned region produced by a shearlike distortion of the parent crystal structure during deformation.   In ferrite, deformation twins form on {211} planes.

U

upper yield stress.   See yield point.

upset.   (1) Localized increase in cross-sectional area resulting from the application of pressure during mechanical fabrication or welding.   (2)   That portion of welding cycle during which the cross-sectional area is increased by the application of pressure.

V

vacancy.   A type of structural imperfection in which an individual atom site is temporarily unoccupied.

veining.   A type of sub-boundary structure that can be delineated because of the presence of a greater-than-average concentration of precipitate or solute atoms.

W

Waloon process.   An early two-hearth process for making wrought iron by refining cast iron.   The conversion proper was carried out in a hearth known as a finery; reheating for forging was carried out in a second hearth furnace known as a chafery.

weld.   A union made by welding.

weld bead.   A deposit of filler metal from a single welding pass.

weldability.   Suitability of a metal for welding under specific conditions.

welding.   Joining two or more pieces of material by applying heat or pressure, or both, with or without filler metal, to produce a localized union through fusion or recrystallization across the interface.

wetting agent.   A surface-active agent that produces wetting by decreasing the cohesion within the liquid.

Widmanstatten structure.   A structure characterized by a geometric pattern resulting from the formation of a new phase on certain crystallographic planes in the parent phase.   The orientation of the lattice in the new phase is related crystallographically to the orientation of the lattice in the parent phase.

wootz.   A carbon steel containing 1 to 1.6% C produced by melting a bloomery iron or an inhomogeneous steel with charcoal in a crucible.   The process originated in India as early as the 3rd century A.D.

work hardening.   same as strain hardening.

wrought iron.   An iron produced by direct reduction of ore or by refining molten cast iron under conditions where a pasty mass of solid iron with included slag is produced.   The iron has a low carbon content.

wustite.   The oxide of iron of lowest valence which exists over a wide range of compositions that do not quite include the stoiciometric composition FeO.

Y

yield point.   The first stress in a material less than the maximum obtainable stress at which an increase in strain occurs without an increase in stress.   Also known as upper yield stress.

yield strength.   The stress at which a material exhibits a specified limiting deviation from the proportionality of stress to strain.   The deviation is expressed in terms of strain.   Also known as proof stress.

 The Ferrous Metals Data Book

 The Ferrous Metals Data Book