5.13.2 Normalizing
Normalizing is similar to annealing except that the rate of cooling is increased by allowing steel to cool
in the air instead of in a furnace. Normalizing is used to control grain size and lessen residual
stresses. Normalized steels are harder and have higher strengths than steels that have been
annealed.
5.13.3 Quenching
Quenching is probably the most common and well-known heat treatment. Quenching can be described
as an operation that provides for rapid cooling of steel from the austenitic temperature (FCC) to lower
temperatures such as room temperature. If cooling is rapid enough, steel will become much harder
and stronger. Different rates of cooling can be obtained by immersing the piece in air, water, oil, brine
and molten salts or molten metals.
Quenching is particularly useful for tools that must be hard and that must maintain their sharpness
under severe conditions. Note that maximum hardness is generally accompanied by brittleness. To
optimize mechanical properties, applying a subsequent heat treatment is often necessary. The
treatment is called tempering.
5.13.4 Tempering
Quenched steels exhibit a wide range of mechanical properties. Hardness, tensile and yield strength,
and brittleness will be very high. On the other hand, toughness and ductility will be much lower.
Tempering is an operation designed to modify steel properties resulting from quenching. Tempering is
essentially a reheating process and is always done at temperatures where no structure change occurs.
Its usual purpose is to increase toughness, reduce brittleness and alleviate high internal stresses.