steel to a temperature below the critical range, holding
FORMS OF HEAT TREATMENT
this temperature for a sufficient period, and then
The various heat-treating processes are similar in
cooling in water, oil, or air. In this process, the degrees
that they involve the heating and cooling of metals.
of strength hardness and ductility obtained depend
They differ, however, in the temperatures to which the
directly upon the temperature to which the steel is
metals are heated, the rates at which they are cooled,
heated. High tempering temperatures improve ductility
and, of course, in the final result. The most common
at the sacrifice of tensile, yield strength, and hardness.
forms of heat treatment for ferrous metals are
annealing, normalizing, hardening, tempering, and case
hardening. Most nonferrous metals can be annealed but
The objective in case hardening is to produce a hard
never tempered, normalized, or case hardened.
case over a tough core. Case hardening is ideal for parts
Successful heat-treating requires close control over all
that require a wear-resistant surface and, at the same
factors affecting the heating and cooling of metals.
time, must be tough enough internally to withstand the
Such control is possible only when the proper
applied loads. The steels best suited to case hardening
equipment is available, and the equipment is selected to
are the low-carbon and low-alloy steels. If high-carbon
fit the particular job.
steel is case-hardened, the hardness penetrates the core
and causes brittleness. In case hardening, the surface of
the metal is changed chemically by inducing a high
Annealing is used to reduce residual stresses,
carbide or nitride content. The core is unaffected
induce softness, alter ductility, or refine the grain
chemically. When heat-treated, the surface responds to
structure. Maximum softness in metal is accomplished
hardening while the core toughens. The common
by heating it to a point above the critical temperature,
methods of case hardening are carburizing, nitriding,
holding at this temperature until the grain structure has
been refined, followed by slow cooling.
CARBURIZING.--Carburizing consists of hold-
ing the metal at an elevated temperature while it is in
contact with a solid or gaseous material rich in carbon.
The process requires several hours, as time must be
Normalizing is a process whereby iron base alloys
allowed for the surface metal to absorb enough carbon
are heated to approximately 100°F (56°C) above the
to become high-carbon steel. The material is then
upper critical temperature, followed by cooling to room
quenched and tempered to the desired hardness.
temperature in still air. Normalizing is used to establish
materials of the same nature with respect to grain size,
NITRIDING.--Nitriding consists of holding
special alloy steel, at temperatures below the critical
point, in anhydrous ammonia. Absorption of nitrogen
as iron nitride into the surface of the steel produces a
greater hardness than carburizing, but the hardened area
Hardening is accomplished by heating the metal
extends to a lesser depth.
slightly in excess of the critical temperature, and then
CYANIDING.--Cyaniding is a rapid method of
rapidly cooling by quenching in oil, water, or brine.
producing surface hardness on an iron base alloy of
This treatment produces a fine grain structure, extreme
low-carbon content. It may be accomplished by
hardness, maximum tensile strength, and minimum
immersion of the steel in a molten bath of cyanide salt,
ductility. Generally, material in this condition is too
or by applying powdered cyanide to the surface of the
brittle for most practical uses, although this treatment is
heated steel. The temperature of the steel during this
the first step in the production of high-strength steel.
process should range from 760° to 899°C (1,400° to
1,650°F), depending upon the type of steel, depth of
case desired, type of cyanide compound, and time
Tempering (drawing) is a process generally applied
exposed to the cyanide. The material is dumped directly
to steel to relieve the strains induced during the
from the cyanide pot into the quenching bath.
hardening process. It consists of heating the hardened