Normalizing
Normalizing is a process whereby iron base alloys
are heated to approximately 100°F (56°C) above the
upper critical temperature, followed by cooling to room
temperature in still air. Normalizing is used to establish
materials of the same nature with respect to grain size,
composition, structure, and stress.
Hardening
Hardening is accomplished by heating the metal
slightly in excess of the critical temperature, and then
rapidly cooling by quenching in oil, water, or brine. This
treatment produces a fine grain structure, extreme
hardness, maximum tensile strength, and minimum
ductility. Generally, material in this condition is too
brittle for most practical uses, although this treatment is
the first step in the production of high-strength steel.
Tempering
Tempering (drawing) is a process generally applied
to steel to relieve the strains induced during the
hardening process. It consists of heating the hardened
steel to a temperature below the critical range, holding
this temperature for a sufficient period, and then cooling
in water, oil, or air. In this process, the degrees of
strength hardness and ductility obtained depend directly
upon the temperature to which the steel is heated. High
tempering temperatures improve ductility at the
sacrifice of tensile, yield strength, and hardness.
Case Hardening
The objective in casehardening is to produce a hard
case over a tough core. Casehardening is ideal for parts
that require a wear-resistant surface and, at the same
time, must be tough enough internally to withstand the
applied loads. The steels best suited to case hardening
are the low-carbon and low-alloy steels. If high-carbon
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
carbide or nitride content. The core is unaffected
chemically. When heat treated, the surface responds to
hardening while the core toughens. The common
methods of case hardening are carburizing, nitriding,
and cyaniding.
CARBURIZING.Carburizing consists of holding
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 allowed
for the surface metal to absorb enough carbon to become
high-carbon steel. The material is then quenched and
tempered to the desired hardness.
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 extends
to a lesser depth.
CYANIDING.Cyaniding is a rapid method of
producing surface hardness on an iron base alloy of
low-carbon content. It may be accomplished by
immersion of the steel in a molten bath of cyanide salt,
or by applying powdered cyanide to the surface of the
heated steel. The temperature of the steel during this
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 exposed
to the cyanide. The material is dumped directly from the
cyanide pot into the quenching bath.
HEAT TREATMENT OF FERROUS
METALS (STEEL)
The first important consideration in the heat
treatment of a steel part is to know its chemical
composition. This, in turn, determines its upper critical
point. When the upper critical point is known, the next
consideration is the rate of heating and cooling to be
used. Uniform-heating furnaces, proper temperature
controls, and suit able quenching mediums are used in
carrying out these operations.
Principles of Heat Treatment of Steel
Changing the internal structure of a ferrous metal is
accomplished by heating it to a temperature above its
upper critical point, holding it at that temperature for a
time sufficient to permit certain internal changes to
occur, and then cooling to atmospheric temperature
under predetermined, controlled conditions.
At ordinary temperatures, the carbon in steel exists
in the form of particles of iron carbide scattered
throughout the iron mixture known as ferrite. The
number, size, and distribution of these particles
determine the hardness of the steel. At elevated
temperatures, the carbon is dissolved in the mixture in
the form of a solid solution called austenite, and the
carbide particles appear only after the steel has been
cooled. If the cooling is slow, the carbide particles are
relatively coarse and few. In this condition the steel is
soft. If cooling is rapid, as by quenching in oil or water,
the carbon precipitates as a cloud of very fine carbide
15-39