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.