Glossary

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

alloy steel. Steel containing specified quantities of alloying elements (other than carbon and the commonly accepted amounts of manganese, copper, silicon, sulfur, and phosphorus), added to effect changes in mechanical or physical properties.

alpha iron. The "body-centered cubic" form of pure iron, stable below 912 °C.

annealing. A generic term denoting a treatment, consisting of heating to and holding at a suitable temperature followed by cooling at a suitable rate, used primarily to soften metallic materials, but also to simultaneously produce desired changes in other properties or in microstructure.

anodizing. Forming a conversion coating on a metal surface by anodic oxidation; most frequently applied to aluminum.

arc welding. A group of welding processes that fuse metals together by heating them with an arc, with or without the use of filler metal.

austenite. A solid solution of carbon in close-packed cubic iron.

bainite. A metastable aggregate of ferrite and cementite resulting from the transformation of austenite at temperatures below the pearlite range but above Ms. Bainite formed in the upper part of the bainite transformation range has a feathery appearance; bainite formed in the lower part of the range has an acicular appearance resembling that of tempered martensite.

binary alloy. An alloy containing only two component elements.

brass. An alloy consisting mainly of copper (over 50%) and zinc, to which smaller amounts of other elements may be added.

brazing. A group of welding processes that join solid materials together by heating them to a suitable temperature and using a filler metal having a liquidus above 450 °C (840 °F) and below the solidus of the base materials. The filler metal is distributed between the closely-fitted surfaces of the joint by capillary attraction.

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 millimeters.

brittle fracture. Separation of a solid accompanied by little or no macroscopic plastic deformation. Typically, brittle fracture occurs by rapid crack propagation with less expenditure of energy than for ductile fracture.

brittleness. The quality of a material that leads to crack propagation without appreciable plastic deformation.

bronze. A copper-rich, copper-tin alloy with or without small proportions of other elements such as zinc and phosphorus. By extension, certain copper-base alloys containing considerably less tin than other alloying elements, such as manganese bronze (copper-zinc plus manganese, tin and iron) and leaded tin bronze (copper-lead plus tin and sometimes zinc). Also, certain other essentially binary copper-base alloys containing no tin, such as aluminum bronze (copper-aluminum), silicon bronze (copper-silicon) and beryllium bronze (copper-beryllium). Also, trade designations for certain specific copper-base alloys that are actually brasses, such as architectural bronze (57% Cu, 40% Zn, 3% Pb) and commercial bronze (90% Cu, 10% Zn).

carbon steel. Steel containing carbon up to about 2% and only residual quantities of other elements except those added for deoxidation, with silicon usually limited to 0.60% and manganese to about 1.65%. Also termed "plain carbon steel," "ordinary steel" and "straight carbon steel."

case hardening. A generic term covering several processes applicable to steel that change the chemical composition of the surface layer by absorption of carbon, nitrogen, or a mixture of the two and, by diffusion, create a concentration gradient. The processes commonly used are carburizing and quench hardening; cyaniding; nitriding; and carbonitriding. Also includes induction hardening.

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.

cementite. A compound of iron and carbon, known as iron carbide and having the approximate chemical formula Fe3C. When it occurs as a phase in steel, the chemical composition will be altered by the presence of manganese and other carbide-forming elements.

Charpy test. A pendulum-type, single-blow impact test in which the specimen, usually notched, is supported at both ends as a simple 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.

cold extrusion. Conversion of an ingot or billet into lengths of uniform cross section by forcing metal to flow plastically through a die orifice. Impact extrusion is the process (or resultant product) in which a punch strikes a slug (usually unheated) in a confining die. The metal flow may be either between punch and die or through another opening. Impact extrusion of unheated slugs is often called cold extrusion.

cold work. permanent strain produced by an external force in a metal below its recrystallization temperature.

constitution diagram. A graphical representation of the temperature and composition limits of phase fields in an alloy system as they actually exist under the specific conditions of heating or cooling.

coring. A condition of variable composition between the center and surface of a unit of microstructure (such as a dendrite, grain, carbide particle) resulting from non-equilibrium solidification, which occurs over a range of temperatures. Also called segregation.

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

creep. Time-dependent strain occurring under stress. The creep strain occurring at a diminishing rate is called primary creep; that occurring at a minimum and almost constant rate, secondary creep; and that occurring at an accelerating rate, tertiary creep.

delta ferrite. See ferrite.

delta iron. Solid phase of pure iron which is stable at temperatures between 1390 °C and 1539 °C and possesses the "body-centered cubic metal structure".

diamond pyramid hardness. An indentation hardness number determined from a hardness test employing a diamond pyramid indenter and variable loads, enabling the use of one hardness scale for all ranges of hardness.

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

ductility. The ability of a material to deform plastically without fracturing; measured by elongation or reduction of area in a tensile test.

elastic strain. A change in dimensions directly proportional to and in phase with an increase or decrease in applied force.

elastic modulus. Same as modulus of elasticity.

elasticity. Ability of a solid to deform in direct proportion to and in phase with increases or decreases in applied force.

electrode. In arc welding, a current-carrying rod that supports the arc between the rod and work, or between two rods, as in twin carbon-arc welding. It may or may not furnish filler metal.

electrolyte. A liquid, most often a solution, that will conduct an electric current.

electroplating. Electrodepositing a metal or alloy in an adherent form on an object serving as a cathode.

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

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.

equilibrium. A dynamic condition of physical, chemical, mechanical, or atomic balance which appears to be a condition of rest rather than one of change.

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) A microstructure of intermixed solid constituents formed by a eutectic reaction.

eutectoid. An isothermal reversible reaction in which a solid solution is converted into two or more intimately mixed solids on cooling, the number of solids formed being the same as the number of components in the system.

extrusion. Conversion of an ingot or billet into lengths of uniform cross section by forcing the plastic metal through a die orifice of the desired cross-sectional outline. In "direct extrusion," the die and ram are at opposite ends of the extrusion stock, and the product and ram travel in the same direction. "Hot extrusion" is similar to cold extrusion except that preheated stock is used and the pressure application is slower.

fatigue. The phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the tensile strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress.

fatigue strength. The maximum stress that can be sustained for a specified number of cycles without failure, the stress being completely reversed within each cycle unless otherwise stated.

ferrite. A solid solution of carbon in "body-centered cubic" iron. In the Fe-C diagram, there are two ferrite regions separated by an austenite area. The lower area is alpha ferrite, the upper, delta ferrite.

filler metal. Metal added in making a brazed, soldered, or welded joint.

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

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

forming. Making a change in the shape or contour of a metal part.

fracture. The separation of a material into two or more parts.

furnace brazing. A mass-production brazing process in which the filler metal is preplaced on the joint, then the entire assembly is heated to brazing temperature in a furnace. Usually, a protective furnace atmosphere is required, and welding of the joint surfaces is accomplished without using a brazing flux.

gamma iron. The close packed cubic form of pure iron, stable from 912 to 1390 °C .

hardenability. The ability of a steel to form martensite.

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, and quench hardening.

hardness. Resistance of metal to plastic deformation, usually by indentation. However, the term may also refer to stiffness or temper, or to resistance to scratching, abrasion or cutting. Indentation hardness may be measured by various hardness tests, such as Brinell, Rockwell and Vickers.

heat-affected zone. That portion of the base metal that was not melted during brazing, cutting, or welding, but whose microstructure and mechanical properties were altered by the heat.

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.

homogenizing. Heating at high temperature to eliminate or decrease chemical diffusion and segregation.

hot shortness. A tendency for some alloys to separate along grain boundaries when stressed or deformed at temperatures near the melting point. Hot shortness is caused by a low-melting constituent, often present only in minute amounts, that is segregated at grain boundaries.

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

hypoeutectoid alloy. In an alloy system exhibiting a eutectoid, any alloy whose composition has an excess of the base metal compared to the eutectoid composition, and whose equilibrium microstructure contains some eutectoid structure.

impact test. A test to determine the behavior of materials when subjected to high rates of loading, usually in bending, tension or torsion. The quantity measured is the energy absorbed in breaking the specimen by a single blow, as in Charpy and Izod tests.

impurities. Elements or compounds whose presence in a material is undesirable.

inclusions. Nonmetallic materials in a solid metal matrix.

indentation hardness. The resistance of a material to indentation. This is the usual type of hardness test, in which a pointed or rounded indenter is pressed into a surface under a substantially static load.

intergranular. Between grains or along grain boundaries.

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.

liquidus. In a constitution or equilibrium diagram, the locus of points representing the temperatures at which the various compositions in the system begin to freeze on cooling or finish melting on heating. See also solidus.

machinability. The relative ease of machining a metal.

malleability. The characteristic of metals that permits plastic deformation in compression without rupture.

martensite. A generic term for microstructures formed by diffusionless phase transformation. Martensite is characterized by an acicular microstructure in both ferrous and nonferrous alloys. In steels, the structure is hard and highly strained. The amount of the high-temperature phase that transforms to martensite on cooling depends to a large extent on the lowest temperature attained, there being a rather distinct beginning temperature (Ms) and a temperature at which the transformation is essentially complete (Mf).

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.

melting range. The range of temperature over which an alloy changes from solid to liquid; the range of temperature from solidus to liquidus at any given composition on a phase diagram.

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metastable. Refers to a state of pseudoequilibrium that has a higher free energy than the true equilibrium state.

microstructure. The structure of a metal as revealed by microscopic examination of the etched surface of a polished specimen.

modulus 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

necking. (1) Reducing the cross-sectional area of metal in a localized area by stretching. (2) .

nitriding. Introducing nitrogen into a solid ferrous alloy by holding at a suitable temperature (below Ac1 for ferritic steels) in contact with a nitrogenous material, usually ammonia or molten cyanide of appropriate composition. Quenching is not required to produce a hard case.

pearlite. A lamellar aggregate of ferrite and cementite resulting from the transformation of austenite at temperatures above the bainite range.

phase diagrams. See constitution diagram.

plastic strain. Strain or deformation which remains permanent after the removal of the load or stress which caused the strain.

powder metallurgy. The process of producing metal powders and pressing and sintering them for the production of massive materials and shaped objects.

proeutectic phase. A phase which forms before the eutectic temperature is reached.

proeutectoid phase. A phase which forms before the eutectoid temperature is reached.

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

reduction of area. (1) Commonly, the difference, expressed as a percentage of original area, between the original 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.

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.

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

shot peening. Cold working the surface of a metal by metal-shot impingement.

sinter. To heat a mass of fine particles for a prolonged time below the melting point, usually to cause agglomeration.

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

soldering. A group of processes that join metals by heating them to a suitable temperature below the solidus of the base metals and applying a filler metal having a liquidus not exceeding 450 °C. Molten filler metal is distributed between the closely fitted surfaces of the joint by capillary action. Soft solders are usually lead-tin alloys.

solidification. The change in state from liquid to solid on cooling through the melting temperature or melting range.

solidus. In a phase diagram, the locus of points representing the temperatures at which various compositions finish freezing on cooling or begin to melt on heating. See also liquidus.

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

spheroidite. An aggregate of iron or alloy carbides of essentially spherical shape dispersed throughout a matrix of ferrite.

stainless steel. Any of several steels containing 12 to 30% chromium as the principal alloying element.

steel. An iron-base alloy containing manganese, usually carbon, and often other alloying elements. In carbon steel and low-alloy steel, the maximum carbon 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 being at about 5% metallic alloying elements.

stiffness. The ability of a metal or shape to resist elastic deflection. For identical shapes, the stiffness is proportional to the modulus of elasticity. For a given material, the stiffness increases with increasing moment of inertia, which is computed from cross-sectional dimensions.

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.

strength-to-weight ratio. The ability of a material to support a load divided by its density, an important parameter in applications where weight is important.

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 is force divided by the original area.

temper. In heat treatment, reheating hardened steel or hardened cast iron to some temperature below the eutectoid temperature for the purpose of decreasing hardness and increasing toughness.

tensile strength. In tensile testing, the ratio of maximum load to original cross-sectional area. Also called ultimate strength.

tool steel. Any of a class of carbon and alloy steels commonly used to make tools. Tool steels are characterized by high hardness and resistance to abrasion, often accompanied by high toughness and resistance to softening at elevated temperatures. These attributes are generally attained with high carbon and alloy contents.

toughness. Ability of a metal to absorb energy and deform plastically before fracturing. It is usually measured by the energy absorbed in a notch impact test, but the area under the stress-strain curve in tensile testing is also a measure of toughness.

ultimate strength. The maximum conventional stress (tensile, compressive or shear) that a material can withstand.

uniform strain. The strain occurring prior to the beginning of localization of strain (necking); the strain to maximum load in the tension test.

Vickers hardness test. An indentation hardness test employing a 136° diamond pyramid indenter (Vickers) and variable loads enabling the use of one hardness scale for all ranges of hardness from very soft lead to tungsten carbide.

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

weld metal. That portion of a weld that has been melted during welding.

weldability. A measure of the ability of a material to be welded under a given set of conditions. Implicit in this definition is the ability of the completed weldment to fulfill all functions for which the part was designed.

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

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. Associate with the onset of plastic flow.

Young's modulus. 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.