strength, hardness, and ductility depend upon the temperature
to
which the steel is heated during the tempering process.
Annealing.
In general, annealing is the opposite of hardening.
Metals are annealed to relieve internal stresses, soften them, make
them more ductile, and refine their grain structure.
Annealing
consists of heating the metal to the proper temperature, holding it
at that temperature for the required time, and then cooling the metal
to room temperature.
The big difference between hardening and
annealing is the cooling rate. To produce maximum softness in steel,
the metal must be cooled very slowly.
This can be done by burying
the hot metal in sand, ashes, or some other substance that does not
conduct heat readily.
Another method is to shut off the heat and
allow the furnace and metal to cool together.
The first method is
called packing and the latter is called furnace cooling.
Normalizing.
Only ferrous metals can be normalized.
In the
process, the metal is heated to the required temperature, held at
that temperature until it is uniformly heated, and then removed to
cool in still air.
Steel parts are normalized to relieve the
internal stress set up by machining, forging, bending, or welding.
Case Hardening. When a low-carbon steel is used where a hardened
surface is desirable, the character of its surface can be altered to
form a very hard case. The hard surface is obtained by adding carbon
to the steel.
This is done by keeping the steel at a sufficiently
high temperature and in contact with a suitable material containing
carbon. The steel is heated to 1,700 to 1,800F for several hours.
The prolonged heating at a high temperature develops a coarse grain
in the core.
To refine the structure, the metal must be reheated
slightly above the critical temperature of the core and then
quenched.
It must then be reheated slightly above the critical
temperature of the case, and again quenched.
The double heat
treatment produces a hardened case with a fine structure and a
ductile core with a full measure of toughness.
Hot Working. Almost all steel is hot-worked from the ingot
into some form from which it is either hot- or cold-worked to
the finished shape. When an ingot is stripped from its mold,
its surface is solid, but the core is still molten. The ingot
is then placed in a soaking pit to retard heat loss so the
core solidifies gradually. After soaking, the temperature
is equalized throughout the ingot. Then, to make it easier
to handle, the ingot is reduced to intermediate size by rolling.
The rolled shape is called a bloom when its sectional dimensions
are 6 x 6 inches or larger, and it is approximately square.
The section is called a billet when it is almost square and its
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