Work hardening and annealing
Most of the common metals cannot be hardened by
heat treatment, but nearly all metals will harden to
some extent as a result of hammering, rolling or
bending. Annealing is a form of heat treatment for
softening a metal which has become work hardened
so that further cold working can be carried
out. The common metals differ quite a lot in their
degree and rate of work hardening. Copper hardens
rather quickly under the hammer and, as this also
reduces the malleability and ductility of the metal,
it needs frequent annealing in order that it may be
further processed without risk of fracture. Lead, on
the other hand, may be beaten into almost any
shape without annealing and without undue risk of
fracture. It possesses a degree of softness which
allows quite a lot of plastic deformation with very
little work hardening. However copper, though less
164Repair of Vehicle Bodies
soft than lead, is more malleable. Aluminium will
withstand a fair amount of deformation by beating,
rolling and wheeling before it becomes necessary
to anneal it. The pure metal work hardens much
less rapidly than copper, though some of the sheet
aluminium alloys are too hard or brittle to allow
very much cold working. Commercially pure iron
may be cold worked to a fair extent before the
metal becomes too hard for further working.
Impurities in iron or steel impair the cold working
properties to the extent that most steels cannot be
worked cold (apart from very special low-carbon
mild steel sheets used in the car industry), although
nearly all steels may be worked at the red-heat
condition.
The exact nature of the annealing process used
depends to a large extent on the purpose for which
the annealed metal is to be used. There is a vast
difference in technique between annealing in a
steel works where enormous quantities of sheet
steel are produced, and annealing in a workshop
where single articles may require treatment.
Briefly, cold working causes deformation by crushing
or distorting the grain structure within the
metal. In annealing, a metal or alloy is heated to a
temperature at which recrystallization occurs, and
then allowed to cool at a predetermined rate. In
other words, crystals or grains within the metal
which have been displaced and deformed during
cold working are allowed to rearrange themselves
into their natural formation during the process of
annealing. Iron and low-carbon steels should be
heated to about 900 °C and allowed to cool very
slowly to ensure maximum softness, as far as possible
out of contact with air to prevent oxidation of
the surface; this can be done by cooling the metal
in warm sand. High-carbon steels require similar
treatment except that the temperature to which the
steel needs to be heated is somewhat lower and is
in the region of 800 °C. Copper should be heated
to a temperature of about 550 °C or dull red, and
either quenched in water or allowed to cool out
slowly. The rate of cooling does not affect the
resulting softness of this metal. The advantage of
quenching is that the surface of the metal is
cleaned of dirt and scale. Aluminium may be
annealed by heating to a temperature of 350 °C.
This may be done in a suitable oven or salt bath. In
the workshop aluminium is annealed by the use of
a blowpipe, and a stick or splinter of dry wood is
rubbed on the heated metal; when the wood leaves
a charred black mark the metal is annealed.
Sometimes a piece of soap is used instead of the
wood; when the soap leaves a brown mark the
heating should be stopped. The metal may then be
quenched in water or allowed to cool out slowly
in air. Zinc becomes malleable between 100 and
150 °C, and so may be annealed by immersing it in
boiling water. Zinc should be worked while still
hot, as it loses much of its malleability when cold.