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Engineering Metals and Materials Table of Contents
Heat Treating Terms and Definitions #2
AIR HARDENING STEEL -An alloy steel which does not
require quenching from a high temperature to harden but which is
hardened by simply cooling in air from above its critical temperature
ANNEALING - Applies normally to softening by changing the
microstructure and is a term used to describe the heating and cooling
cycle of metals in the solid state. The term annealing usually implies
relatively slow cooling in carbon and alloy steels. The more important
purposes for which steel is annealed are as follows: To remove stresses;
to induce softness; to alter ductility, toughness, or electric, magnetic
or other physical and mechanical properties; to change the crystalline
structure; and to produce a definite microstructure.
AUSTEMPERING - This is a method of hardening steel by quenching from
the austenitizing temperature into a heat extracting medium (usually
salt) which is maintained at some constant temperature level between
400 and 800 and holding the steel in this medium until austenite is
transformed to bainite. The austempering process is limited to sections
less than `/2 diameter. The advantages of this method of interrupted
quenching are increased ductility and toughness at the resulting
hardness of RC 45-55.
AUSTENITE - The solid solution of iron and carbon which is attained
by heating to high temperatures above the upper critical temperature.
This temperature or temperature range is called the austenitizing
temperature and must be attained to obtain the proper microstructure and
full hardness of steel in heat treating. The austenitizing temperature
varies for the different grades of carbon, alloy and tool steels.
BAINITE - A decomposition or transformation product of austenite
which is a type of microconstituent or structure in steel. This term is
used by metallurgists to describe a particular structure of steel when
the steel is polished, etched and examined with a microscope.
BRINELL HARDNESS - A hardness number determined by applying a 3000
kilogram load to the surface of the material to be tested through a
hardened steel ball of 10mm. The diameter of the depression is measured
and the hardness is the ratio of load to spherical area of the
impression. Tables of numbers have been prepared, and the hardness is
read from the table from the diameter of the depression.
CARBURIZING - Adding carbon to the surface of steel by heating the
metal below its melting point in contact with carbonaceous solids,
liquids, or gases.
CASE HARDENING - A heat treatment or a combination of heat treatments
of surface hardening involving a change in the composition of the outer
layer of an iron-base alloy in which the surface is made substantially
harder by inward diffusion of a gas or liquid followed by appropriate
thermal treatment. Typical hardening processes are carburizing,
cyaniding, carbo-nitriding and nitriding.
CYANIDING - Surface hardening by carbon and nitrogen absorption of a
steel article or a portion of it by heating at a suitable temperature in
contact with cyanide salt, followed by quenching.
DECARBURIZATION - When steel is subjected to high temperatures, such
as are used in hot rolling, forging, and heat treating in a media
containing air, oxygen, or hydrogen there is a loss of carbon at the
surface which is known as decarburization. This resultant loss of carbon
or chemistry change at the surface of the steel part reduces the
strength of the part by reducing the size of the section and produces a
softer surface hardness than the core of the part.
FLAME HARDENING - A heat treat method used to harden the surface of
some parts where only a small portion of the surface is hardened and
where the part might distort in a regular carburizing or heat treating
operation. The operation consists of heating the surface to be hardened
by an acetylene torch to the proper quenching temperature followed
immediately by a water quench and proper tempering. Generally wrought or
cast steels with carbon contents of .30 to .40%, low alloy steels, and
ductile and malleable cast irons are suitable for flame hardening.
HARDENABILITY- This relates to the ability of steel to harden deeply
upon quenching, and takes into consideration the size of the part and
the method of quenching. The test used to determine the hardenability of
any grade of steel is the Jominy Test.
HARDENING - The heating and quenching of certain iron-base alloys
from a temperature above the critical temperature range for the purpose
of producing a hardness superior to that obtained when the alloy is not
quenched. This term is usually restricted to the formation of
Heat Treating Terms and Definitions #2
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