Steels are prepared by further purification (reduction) of pig iron.
Alloys can also be prepared by mixing finely powdered portions of the
constitutent metals, compacting the mixture under high pressure, and
removing the impurities.
HEAT TREATMENT
Heat treatment can make a metal harder, stronger, and more resistant
to impact.
This process consists, in general, of a series of
operations involving controlled heating and cooling of metals in a
solid state.
The purpose is to change the metal's property or
combination of properties so that the metal is more useful,
serviceable, or safe for a particular application or design.
Heat
treatment can also make a metal softer and more ductile.
No one
heat-treating operation can produce all of these characteristics.
Some properties are often improved at the expense of others.
In
being hardened, for example, a metal can become brittle. The various
heat-treating processes are similar in that they all involve heating
and cooling the metals.
However, they differ in three important
ways. The first two ways are the temperatures to which the metal is
heated and the rate at which it is cooled. The third difference is
the finished metal.
The most common forms of heat treatment for ferrous metals are
hardening, tempering, annealing, normalizing, case hardening, and
hot-working.
The paragraphs that follow discuss each of these
treatments. Most nonferrous metals can be annealed and many of them
can be hardened by heat treatment.
However, only one nonferrous
metal, titanium, can be case-hardened, and none can be normalized or
tempered.
Hardening.
For most steels the hardening treatment consists of
heating the steel to the correct temperature and then cooling it
rapidly by plunging the hot steel into oil or brine.
Although most
steels must be cooled rapidly for hardening, a few can be cooled from
the hardening (specified) temperature by air.
Hardening increases
the durability and strength of the steel, but it makes it less
ductile.
Many nonferrous metals can also be hardened and
strengthened by the same method.
Tempering. After the hardening treatment, steel is often
harder than necessary and it is too brittle for most practical
uses.
In addition, rapid cooling from the hardening temperature
causes internal stresses that can cause flaws in the metal.
To
relieve the internal stresses and reduce brittleness, steel
is tempered after hardening. Tempering consists of heating the
steel to a temperature below that at which it was hardened,
holding the metal at that temperature for a predetermined time,
and then cooling it, usually in still air. The resultant
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