Metallurgical Terms
A
Abnormal Steels
(a) Carbon steels showing relatively poor low deformation creep behaviour
usually as indicated by abnormally high creep rates. This usually occurs when
high aluminum additions are made and is thought to be associated with removal of
nitrogen from solid solution as AIN.
(b) A name given by McQuaid and Ehn to carburizing steels which tended to show
soft spots on quenching after carburizing. The cause is low hardenability
associated with fine grain size.
Accm Ac1 Ac3 Ac4
(See Transformation Temperatures)
Ac2
(See Magnetic Changes)
Acid Brittleness (Pickling Brittleness)
Lack of ductility induced in steel, especially wire or sheet when it is
pickled in dilute acid to remove scale, or during electro-plating. Attributed to
absorption of hydrogen.
Acid Open Hearth
(See Open Hearth Furnace)
Acid Slag
(See Slag)
Acid Steel
Steel melted in a furnace with an acid lining, i.e., consisting of a
siliceous refractory such as ganister or Sand, and under a siliceous sing.
Neither sulphur nor phosphorus is removed to any appreciable extent during the
process and for this reason a higher grade of raw material is required than in
the basic process. Acid steel may be produced either by the open hearth,
Bessemer or electric processes.
Ag
(Chemical Symbol for Silver)
Ageing (Age Hardening)
A process causing structural change which may occur gradually in certain
metals and alloys at atmospheric temperature (natural ageing), or more rapidly
at higher temperatures (artificial ageing). As a result of ageing, the proof
stress, maximum stress and hardness values are increased, with some reduction in
ductility. These effects are caused by precipitation from a supersaturated solid
solution so that the ageing treatment is usually preceded by a solution
treatment at a much higher temperature. The precipitate may be submicroscopic.
There is a tendency to apply the term "ageing" to steels; "age hardening" to
non-ferrous alloys. Where the ageing is produced by heating at elevated
temperatures, i.e., artificial ageing, the effect is often referred to as
precipitation hardening.
Air Hardening Steel (Self Hardening)
Strictly the term refers to a steel which becomes martensitic, i.e., fully
hardened, on cooling in air from above its critical point, and does not require
rapid quenching in oil or water, but it may also be applied to varying degrees
of non-martensitic hardening, e.g., where the steel, although not wholly
martensitic, attains adequate hardness on cooling in air. Such steels are
produced by the addition of certain alloying elements which lower their critical
range on cooling; a typical example contains 0•30 % carbon, 1•3% chromium and
4•5% nickel. It should be noted that sufficiently rapid air cooling can be
obtained only if the mass of the steel does not exceed a certain section which
varies according to the composition.
Al
(Chemical Symbol for Aluminum)
Allotropy
The property possessed by some elements of existing in two or more states
(allotropes) differing widely in properties and each stable within certain
limiting conditions of temperature and pressure, e.g., carbon has three
allotropic varieties, diamond, graphite and amorphous carbon. The allotropy of
iron modifies the solubility of carbon, and it is because of this that steel can
be hardened. Pure alpha iron (ferrite) exists up to 910˚C and pure gamma iron
(austenite) from 900˚C-1405˚C. Above 1405˚C and up to the melting point of
l539˚C it exists as delta iron. These temperatures are modified by alloy
additions so that in certain steels, e.g., 18/8 and 14% manganese steel, the
austenitic condition is stable at room temperature. The lattice of alpha and
delta iron is body centered cubic whilst that of gamma iron is face centered
cubic.
Alloy Cast Iron
(See Cast Iron)
Alloy Steel
A steel to which one or more alloying elements other than carbon ha\•e been
deliberately added with the object of conferring particular properties upon it.
(Cf. Carbon steel)
Alpha Iron
The allotropic form of iron, which in pure iron is stable below 910˚C, the
atoms being arranged in a body centered cubic space lattice. It is magnetic
below the magnetic change point, which, in pure iron, occurs at 770˚C. Above
this point, it was formerly known as beta iron.
Alumina (A12O3)
The oxidation product of aluminum; as such may be a constituent of
non-metallic inclusions. Alumina is also used as a refractory (alumino-silicates
are the principal constituents of fireclay refractories) and as an abrasive
(corundum and emery).
Annealing
Heating steel and holding it at a suitable temperature followed by cooling
at a suitable rate, with the object of improving softness, machinability, and
cold-working properties or of removing stresses and obtaining a desired
structure. Usually (full annealing) the steel is heated to a temperature at
which the carbide is wholly or partly taken into solution; subsequently the
steel is slowly cooled, generally in the furnace. Sub-critical annealing is done
at a temperature just below that at which carbide commences to be taken into
solution.
Anodizing
A process of coating aluminum or aluminum alloys with a layer consisting
essentially of aluminum oxide. The aluminum is made the anode in an electrolytic
cell containing dilute chromic, sulphuric, or oxalic acid. The cathode may
consist of lead, iron or carbon according to the electrolyte used. Oxygen is
generated at the anode and attacks the aluminum, giving rise to a tenacious
corrosion-resistant film. The film is somewhat porous, and it is usually
"sealed" by means of lanoline dissolved in spirit. If desired, the anodized
surface can be coloured with various dyes before sealing.
A.O.D. (Argon-Oxygen Decarburizing)
A process used in the production of stainless steel whereby the carbon
content can be reduced to low levels with little loss of chromium. Stainless
steel of the required alloy content is transferred from the are melting furnace
to a refining vessel into which controlled amounts of argon and oxygen are
simultaneously injected. The injection of argon has the effect of lowering the
partial pressure of carbon monoxide in the melt thus helping to promote the
carbon-oxygen reaction without excessive oxidation of chromium.
Arc Furnace
A Steel melting furnace that normally has three electrodes, one phase of a
three-phase current being brought to each electrode. In plan, the electrodes are
situated at the apex of an equilateral triangle. The current travels from
electrode to electrode through the medium of arcs made with the bath. The
electrodes used in this furnace may be either graphite or amorphous carbon.
Graphite electrodes have approximately four times the conductivity of amorphous
carbon and thus an electrode only half the diameter will carry the same current.
The basic process, in which the hearth is of rammed magnesite or dolomite, may
be used for the production of either ingots or castings, while the acid process,
in which the furnace lining consists of silica sand or ganister over silica
brick, is employed principally for the production of castings.
Arc Process
(See Electric Steel)
Argon Arc Welding
(See Welding)
Armco Iron
A nearly pure commercial iron, manufactured by the American Rolling Mill
Co., containing less than 0•1% impurities, e.g., carbon 0•012 %, manganese 0•017
%, phosphorus 0•005 %, sulphur 0•025%.
Arrest Points
(See critical points)
As
Chemical symbol for Arsenic
ASEA-SKF
A vacuum degassing process with provision for stirring and re-heating to
compensate for the temperature drop which occurs during degassing. Steel melted
in an electric are furnace in the normal way is tapped into a special ladle
where it can be stirred, vacuum degassed, reheated and given alloy additions to
complete the refining process.
Ausforming
A hardening process in which the steel is first austenitized, cooled to a
temperature in the region of 400˚C, where it is heavily worked and then quenched
to martensite. The result is very high strength combined with good ductility.
Austempering
An interrupted quenching process which consists essentially of heating steel
to an appropriate temperature above the critical range to render it austenitic
and then, instead of cooling to room temperature in one of the conventional
cooling media, transferring the steel to a hot quenching bath maintained at a
predetermined, constant temperature below the critical range, but above the
martensitic change point (Ms point) usually between 260˚C and 370˚C; the steel
is held at this temperature for a certain time to ensure the complete direct
transformation of the austenite in the final products (e.g. pearlite and/or
bainite), after which the material may be cooled to atmospheric temperature in
an convenient manner. (See also Critical Cooling Rate.)
Austenite
The allotropic form of iron (gamma iron) which has a face centered cubic
lattice, the parameter of which increases with increasing carbon content.
Austenite, containing only carbide or iron in solution, is not stable at
ordinary temperatures, nor can it be completely retained in solution by
quenching, but its stability is greatly increased by the addition of certain
alloying elements. (See also Allotropy, Austenitic Steels and Gamma Iron.)
Austenitic Steels
Steels consisting of austenite, which, owing to the presence of high
percentages of certain alloying elements such as manganese and nickel, are
stable, for most practical purposes, at normal temperatures. Typical examples of
austenitic steels include 13% manganese steel, and the corrosion-resistant type
containing about 18% chromium and 8% nickel.
Austenitizing
Heating steel so that it becomes completely austenitic. This is usually, the
first stage in a heat treatment operation such as hardening normalizing, full
annealing, etc.
Auto-Frettage
A cold-working process, chiefly applied to cylinders and tubes of heavy wall
thickness, e.g., guns. Auto- frettage may be affected by expanding the bore by
hydraulic pressure until practically all the metal has been stressed beyond its
elastic limit. On removing the applied pressure the cylinders are left in a stat
of internal stress with compressive stress at the bore and tensile stress at the
outside. Thus, before failure can occur, any bursting force acting from the bore
must overcome this internal compressive stress before the steel is subject to
tension.