Quench & Temper
specific mechanical properties
When a part is expected to provide specific levels of strength and
toughness, it is usually put through a quench-and-temper process, which
by industry convention is usually simply referred to as heat treating.
The first step in the quench-and-temper is to heat the parts to a
uniform temperature above the transformation range, usually about 1650º F.
The hot parts are then quenched in a liquid, usually oil (for medium
carbon plate or alloy plate like 1045, 4140, or 4340) or water (for
low carbon alloy grades like A514). The rapid cooling imparts a very
high degree of hardness, often 600 Bhn or more. Some grades, such as
4140, are often hardened by quenching in rapidly moving air; this process
is also known as normalize-and-temper.
As-quenched, the parts are too brittle to be used, and are returned
to the furnace for another cycle, this time at a lower temperature
that will reduce or temper the hardness and restore some ductility.
The final surface hardness that can be achieved in this process is
dependent on the carbon content of the steel. The alloy content dictates
the depth of that hardness and the ultimate mechanical properties that
can be expected. A 1045 part can have the same surface hardness as
one cut from 4340, but the core hardness and tensile strength of the
two parts will be quite different.