Steel is an important engineering material because,
although cheap, it can be given a wide range of
mechanical properties by heat treatment. Heat
treatment can both change the size and shape of
the grains, and alter the microconstituents. The
shape of the grains can be altered by heating the
steel to a temperature above that of recrystallization.
The size of the grains can be controlled by the
temperature and the duration of the heating, and
the speed at which the steel is cooled after the
heating; the microconstituents can be altered by
heating the steel to a temperature that is sufficiently
high to produce the solid solution austenite
so that the carbon is dispersed, and then cooling it
at a rate which will produce the desired structure.
The micrograin structure of carbon steel has the
following constituents:
Ferrite Pure iron.
Cementite Carbon and iron mixed.
Pearlite A sandwich layer structure of ferrite and
cementite.
All low-carbon steels of less than 0.83 per cent
carbon content consist of a combination of ferrite
and pearlite. Carbon steel containing 0.83 per cent
carbon is called eutectoid and consists of pure
pearlite structure. Steels over 0.83 per cent carbon
up to 1.2 per cent carbon are a mixture of cementite
and pearlite structures. If a piece of carbon
steel is heated steadily its temperature will rise at a
uniform rate until it reaches 700 °C. At this point,
even though the heating is continued, the temperature
of the steel will first remain constant for a
short period and then continue to rise at a slower
rate until it reaches 775 °C. The pause in the temperature
rise and the slowing down of the rate indicate
that energy is being absorbed to bring about a
Metal forming processes and machines 165
chemical and structural change in the steel. The
carbon in the steel is changing into a solid solution
with the iron and forming what is known as
austenite. The temperature at which this change in
the structure of the steel starts is 700 °C, which is
known as the lower critical point; the temperature
at which the change ends is known as the upper
critical point. The difference between these points
is termed the critical range. The lower critical
point is the same for all steels, but the upper critical
point varies with the carbon content as shown
in Figure 5.1. Briefly, steels undergo a chemical
and structural change, forming austenite, when
heated to a temperature above the upper critical
point; if allowed to cool naturally they return to
their normal composition.
Steel can be heat treated by normalizing, hardening,
tempering and case hardening as well as by
annealing, which has already been described in
Section 5.2.1.
Normalizing
Normalizing is a process used to refine the grain
structure of steel after it has been subjected to prolonged
heating above the critical range (as in the
case of forging) and to remove internal stresses
caused by cold working. The process may appear
to differ little from annealing, but as its name suggests
the effect of normalizing is to bring the steel
back to its normal condition and no attempt is
made to soften the steel for further working.
Normalizing is effected by slowly heating the steel
to just above its upper critical range for just sufficient
time to ensure that it is uniformly heated, and
then allowing it to cool in still air.
Hardening
It has already been said that if a piece of steel is
allowed to cool naturally after heating to above its
upper critical point, it will change from austenite
back to its original composition. If, however, the
temperature of the heated steel is suddenly lowered
by quenching it in clean cold water or oil, this
change back from austenite does not take place,
and instead of pearlite, a new, extremely hard and
brittle constituent is formed, called martensite.
This process makes steels containing 0.3 per cent
or more carbon extremely hard, but steels having a
carbon content of less than 0.3 per cent cannot be
hardened in this way because the small amount of
carbon produces too little martensite to have any
noticeable hardening effect. The steel to be hardened
should be quenched immediately it is uniformly
heated to a temperature just above the
upper critical point. It is also important not to overheat
the steel and to allow it to cool to the quenching
temperature. Whether water or oil is used for
quenching depends upon the use to which the steel
is to be put. Water quenching produces an
extremely hard steel but is liable to cause cracks
and distortion. Oil quenching is less liable to cause
these defects but produces a slightly softer steel.
A more rapid and more even rate of cooling can be
obtained if the steel is moved about in the cooling
liquid, but only that part of the steel which is to
be hardened should be moved up and down in the
liquid in order to avoid a sharp boundary between
the soft and hard portions.
A workshop method of hardening carbon tool
steel is to heat the steel, using the forge or oxyacetylene
blowtorch, to a dull red colour (see Table 5.3)
and then quench it in water or oil. This would
harden the article ready for tempering.
Tempering
Hardened steel is too brittle for most purposes, and
the process of tempering is carried out to allow the
steel to regain some of its normal toughness and
ductility. This is done by heating the steel to a temperature
below the lower critical point, usually
Figure 5.1 Changes in structure of carbon steel
with temperature and carbon content
166 Repair of Vehicle Bodies
between 200 and 300 °C, thereby changing some
of the martensite back to pearlite. The exact temperature
will depend on the purpose for which the
steel is intended; the higher the temperature, the
softer and less brittle the tempered steel becomes.
Methods used for controlling the tempering
process depend upon the size and class of the article
to be tempered. One method is to heat the hardened
steel in a bath of molten lead and tin, the
melting points of various combinations of these
two metals being used as an indication of the temperature.
Another method of tempering small articles
is to polish one face or edge and heat it with a
flame. This polished surface will be seen to change
colour as the heat is absorbed. The colour changes
are caused by the formation at different temperatures
of thin films of oxide, called tempering
colours (Table 5.4). After tempering, the steel is
either quenched or allowed to cool naturally.
Case hardening
Although mild steels having a carbon content of
less than 0.3 per cent cannot be hardened, the surface
of the mild steel can be changed to a highcarbon
steel. Case hardening of mild steel can be
divided into three main processes:
1 Carburizing
2 Refining and toughening the core
3 Hardening and tempering the outer case.
A method called pack carburizing is often used in
small workshops. After thoroughly cleaning them,
the steel parts to be carburized are packed in an
iron box so that each part is entirely surrounded by
2.5 cm of carburizing compound. The box is sealed
with fireclay, heated in a furnace to 950 °C and
kept at that temperature for two to twelve hours,
during which time the carbon in the compound is
absorbed by the surface of the steel parts. The steel
parts are then allowed to cool slowly in the box. At
this stage their cores will have a coarse grain structure
due to prolonged heating, and the grain is
refined by heating the steel parts to 900 °C and
quenching them in oil. Next the casing is hardened
by reheating the steel to just under 800 °C and then
quenching it in water or oil. Tempering of the casing
may then be carried out in the normal way.
Small mild steel parts can be given a very thin
casing by heating them in a forge to a bright red
heat and coating them with carburizing powder.
The parts are then returned to the forge and kept
at a bright red heat for a short period to allow the
carbon in the compound to penetrate the surface of
the metal. Finally the parts are quenched and ready
for use.
