Metallurgical Terms

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

A

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

ALUMINIZING
A method for forming an aluminum or aluminum alloy coating on a metal employing diffusion, hot spraying or hot dipping.

ANNEALING
A heat treatment that alters the micro structure of a material causing changes in properties such as strength and hardness

Annealing may be done to:
A. Relieve stresses
B. Induce softness structure
C. Improve physical, electrical, or magnetic properties
D. Improve machinability microstructure
E. Refine the crystalline
F. Remove gases
G. Produce a specific

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

AUSTEMPERING
The austemper process offers benefits over the more conventional oil quench and temper method of heat treating springs and stampings that requires the uppermost in distortion control. The process consists of quenching the part from the proper austentizing temperature directly into a liquid salt bath at a temperature between 590 to 710 degrees Farenheit.

AUSTENITE
A nonmagnetic solid solution of ferric carbide or carbon in iron, used in making corrosion-resistant steel.

AUSTENITIZING TEMPERATURE
The temperature at which a steel is substantially all austenite.

B

BAINITE
Steel formed by austempering, having an acicular structure of ferrite and carbides, exhibiting considerable.

BANDED STRUCTURE
Appearance of a metal showing parallel bands in the direction of rolling or working.

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

BRIGHT ANNEALING
Heating and cooling a metal in an inert atmosphere to inhibit oxidation; surface remains relatively bright.

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

CARBONITRIDING
Carbonitriding is a modified form of gas carburizing, rather than a form of nitriding. The modification consists of introducing ammonia into the gas carburizing atmosphere to add nitrogen to the carburized case as it is being produced. Nascent nitrogen forms at the work surface by the dissociation of ammonia in the furnace atmosphere; the nitrogen diffuses into the steel simultaneously with carbon. Typically, carbonitriding is carried out at a lower temperature and for a shorter time than is gas carburizing, producing a shallower case than is usual in production carburizing.

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

CARBURIZING
To treat, combine, or impregnate with carbon, as when casehardening steel.

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

CEMENTITE
A compound of iron and carbon, known chemically as iron carbide and having the approximate chemical formula Fe3C. It is characterized by an orthorhombic crystal structure. When it occurs as a phase in steel, the chemical composition will be altered by the presence of manganese and other carbide-forming elements.

COLD WORKING
Any work, grinding, surfacing, and drilling that is done on the glass that has been finished after the annealing process is completed. Working without heat.

CORE
The interior part of a steel whose composition has not been changed in a case hardening operation.

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.

D

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.

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.

DRAW
The common term used interchangeably with Tempering.

F

FATIGUE
Failure by progressive fracture caused by repeated applications or reversals of stress.

FERRITE
Any of a group of nonmetallic, ceramiclike, usually ferromagnetic compounds of ferric oxide with other oxides, especially such a compound characterized by extremely high electrical resistivity and used in computer memory elements, permanent magnets, and various solid-state devices.

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

G

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

H

HARDENABILITY
The hardenability of a metal alloy is its capability to be hardened by heat treatment. It should not be confused with hardness, which is a measure of the material's resistance to indentation or scratching. It is an important property for welding, since it is inversely proportional to the ease of welding a material.

HOMOGENIZING
A process of heat treatment at high temperature intended to eliminate or decrease chemical segregation by diffusion.

HYDROGEN EMBRITTLEMENT
In oxygen-bearing copper, a condition of low ductility resulting from absorption of hydrogen at high temperature. Internal reduction of cuprous oxide and creation of intergranular holes or cracks by the accompanying generation of steam.

I

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.

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 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.

M

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.

MICROSTRUCTURE
The structure of a metal as revealed at high magnification, usually at 100x and higher.

N

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.

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.

Q

QUENCHING
Cooling from high temperature, usually at a fast rate.

S

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.

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 solid solution, and then cooling rapidly enough to hold these constituents in solution.

SPHEROIDIZING
Heating to a high temperature (austenitic) then cooling at an extremely slow rate through active temperature control. Results in spherically diffused carbon areas with mostly iron rich compositions, also known as spheroidite, as opposed to elongated bands of pearlite. Results in extreme softness and ductility, often only necessary when a high degree of forming is necessary.

T

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

W

WORK HARDNESS
Hardness developed in metal resulting from cold working.