Iron is the major constituent in the steel used in tooling,
to which carbon is added in order that the steel may harden. Alloys are put
into steel to enable it to develop properties not possessed by plain carbon
steel, such as ability to harden in oil or air, increased wear resistance,
higher toughness, and greater safety in hardening.
Heat treatment of ferrous materials involves several
important operations which are customarily to under various headings, such as
normalizing, spheroidizing, stress relieving, annealing, hardening, tempering,
and case hardening.
Normalizing
This is the operation of heating to temperature about 100F
to 200F above the critical range a
cooling in still air. This is about 100F over the regular hardening
temperature. The purpose of normalizing is usually to refine grain structure
that have been coarsened in forging. With most of the medium carbon forging
steels, alloyed and unalloyed, normalizing is highly recommended after forging
and before machining to produce more homogeneous structures and in most cases
improved machinability.
High alloy air hardening steels are never normalized, since
to do so would them to harden and defeat the primary purpose.
Spheroidizing
This is a form of annealing which, in the process of heating
and cooling steel, produces a rounded or globular form of carbide the hard
constituent in steel.
Tool steel are normally spherodized for best machinability.
This is accomplished by heating to a temperature of 1380-1400F for carbon
steels and higher for many alloy tool steels, holding at heat one to four
hours, and cooling slowly in the furnace.
Stress Relieving
This is a method of relieving the internal stresses set up
in steel during forming, cold working, and cooling after welding or machining.
It is the most simple heat treatment and is accomplished by merely heating to 1200-1350F followed by air or
furnace cooling.
Large dies are usually roughed out, then stress relieved and
finish machined. This will minimize change of shape not only during machining
but during subsequent heat treating as well. Welded sections will also have
locked in stress owing to a combination of differential heating and cooling
cycle as well as to changes in cross section. Such stresses will cause considerable
movement in machining operations.
Annealing
The process of annealing consist of heating the stell to an elevated
temperature for a definite period of time and, usually, cooling it slowly.
Annealing is done to produce homogenization and to establish normal equilibrium
conditions, with corresponding characteristic properties.
Tool steel as purchased is generally in the annealed
condition. Sometimes it is necessary to rework a tool that has been hardened,
and the tool must then be annealed. For this type of anneal, the steel is heated slightly above
its critical range and than cooled very slowly.
Finished parts may be annealed without surface deterioration
by placing them in a closed pot and covering with compounds that will combine
with the air present to form a reducing atmosphere. Partially spent carburizing
compound is widely used, as well as cast iron chips, charcoal, and commercial
neutral compounds.
Hardening
This is the process of heating to a temperature above the
critical range, and cooling rapidly enough through the critical range to
appreciably harden the steel.
A simplified theory of hardening steel is that iron has two
distinct and different atomic
arrangements, one existing at room temperature or gain near the melting point,
and one above the critical temperature. Without this phenomenon it would be
impossible to harden iron base alloys by heatreatment.
Tempering
This is the process of heating quenched and hardened steel
and alloy to some temperature below the lower critical temperature to reduce
internal stresses set up in hardening. Thus the hard martensite resulting from
the quenching operation is changed in tempering in the direction of the equilibrium
properties, the degree being dependent on the tempering temperature and rate of
cooling.
Case Hardening
The addition of carbon to the surface of steel parts and
subsequent hardening operation are important phase in heat treating. The
process may involve the use of molten sodium cyanide mixtures, pack carburizing
with activated solid material, such as charcoal, or coke, gas, or oil
carburizing, and dry cyaniding.
Whether a solid carbonaceous packing material is used, or a
liquid gas, the objective is to produce a hard, wear resistant surface with a
core of such hardness or toughness as is best suited for the purpose. The
carbon content of the surface is raised to 0.80-1.20% and the case depth can be
closely controlled by the time, the temperature, and the carburizing medium
used. Pack carburizing is generally done at 1700F for eight hours to produce a
case depth of 1/16 in, Light cases up to 0.005in. can be obtained in liquid
cyanide baths and case depths to 1/32in. are economically practical in liquid
carburizing baths.
Usually low carbon and low carbon alloy steel are
carburized. The usual carbon range is 0.10 to 0.30% carbon, though higher
carbon content steel may be carburized as well.
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