Soft spots will result when plain water is used as a
quenching medium, if the vapor stage of the quench is
Cracking during heat treatment may be caused by
not broken up. In this stage, air bubbles or air pockets
heating the material unevenly, by heating it to too high
form on the surface of the metal and retard the cooling
a temperature, by soaking it for too long a time, or by
rate wherever they touch the metal.
quenching it so it cools unevenly. Some steels are given
extra preheats to minimize the danger of cracking from
uneven heating. Steel that has been overheated or
soaked should be allowed to cool in air to room
Some permanent change in dimensions may occur
temperature, and then it should be heated to the correct
during heat treatment. In some cases this change of size
temperature. However, metals and alloys that have been
is unavoidable. In others it is merely the result of
severely overheated cannot be salvaged; they are
incorrect heat treatment.
actually burned, and no amount of subsequent heat
Oil-hardened and air-hardened steels tend to shrink
treatment can restore them to their original condition.
during hardening. This size change is normal and
Uneven cooling is a major cause of cracking,
cannot be prevented. However, it must be allowed for
particularly in some steels. Factors that contribute to
in the design of any part that must be precisely
uneven cooling are poor design, the presence of scale
dimensioned. These steels tend to shrink excessively,
or other material on the surface of the metal, and the
more than the normal amount, if they are not heated
presence of gas pockets in various recesses of the part.
sufficiently for hardening. They tend to increase in size
Scale should usually be removed before the material is
if they are overheated. Very close control of
quenched. Gas pockets in recesses of the part can be
temperature is necessary for successful heat treatment
avoided by circulation or agitation of the quenching
of these steels.
When metal scales, some of the surface metal is
Tool steels that have been deformed or worked
lost. Thus, scaling causes a decrease in size. Scaling can
while cold tend to crack during hardening unless they
usually be prevented by controlling the furnace
are fully annealed before the hardening treatment is
atmosphere. If furnaces with controlled atmospheres
started. These steels must also be tempered immediately
are not available, a fuels source such as a small block
after they are hardened.
of wood or a small amount of charcoal can be placed in
the furnace chamber. This will reduce the oxygen in an
electric furnace to about 3%.
Nitrided steels increase in dimension during the
Any change of shape in the form of a twist or a bend
nitriding process. This increase in size cannot be
is known as warping. Poor design, uneven heating
prevented, but parts to be nitrited should be machined
through the lower temperature range, and uneven
slightly undersized to allow for the increase.
cooling are common causes of warping. Preheating
tends to minimize the danger of warping from uneven
Steel that has been cold-drawn may undergo a
permanent increase in size when it is heated, because
sometimes prevent warping. This is particularly true of
cold-drawing leaves the metal highly stressed. The size
parts that are rough machined on one side and smooth
increase in cold-drawn steel can be avoided by
ground on the other. Air-hardening steels tends to warp
annealing the steel before machining the part to size.
them if they are not protected from drafts while being
Excessive shrinkage occurs whenever there is a
great difference in the cooling rates of the outer and the
inner portions of the metal being heat-treated. The flush
quenching method should be used to prevent shrinkage
of the metal being heat-treated.
Soft spots in a hardened piece can usually be traced
to the use of the wrong quenching medium, the use of
incorrect quenching procedures, the presence of scale
on some parts of the surface, or the use of the wrong
kind of tongs for handling the material. Soft spots in
Spalling is the surface cracking or flaking of steel.
case-hardened steels are usually caused by packing the
The cracks are usually very shallow, but in severe cases
pieces so they touch each other.
fairly large sections of the surface may peel away.