2.3 Tempering

When you heat and quench a plain carbon steel as described previously, you not only harden the steel, you also make it very brittle. In this condition it is unsuitable for immediate use. For instance, a chisel would shatter if you hit it with a hammer. After hardening we have to carry out another process known as tempering. This greatly reduces the brittleness and increases the toughness. However, the tempering process also reduces the hardness to some extent.

Tempering consists of reheating the hardened steel workpiece to a suitable temperature and again quenching it in oil or water. The tempering temperature to which the workpiece is reheated depends only upon the use to which the workpiece is going to be put. In a workshop, the tempering temperature is usually judged by the ‘temper colour’ of the oxide film that forms on the surface of the workpiece.

2.4 Annealing

Annealing processes are used to soften steels that are already hard. This hardness may be imparted in two ways.

• Quench hardening. This has previously been described.

• Work hardening. This occurs when the metal has been cold worked. It becomes hard and brittle at the point where cold working occurs as this causes the grain structure to deform. For example, if a strip of metal is held in a vice, bending the metal back and forth causes it to work harden at the point of bending. It will eventually become sufficiently hard and brittle to break off at that point.

Full annealing

The temperatures for full annealing are the same as for hardening. To anneal (soften the workpiece), you allow the hot metal to cool down as slowly as possible. Small components can be buried in crushed limestone or in ashes. Larger components and batches of smaller components will have been heated in furnaces. When the correct temperature has been reached, the component is ‘soaked’ at this temperature so that the temperature becomes uniform throughout its mass. The furnace is then shut down, the flue dampers are closed and the furnace is sealed so that it cools down as slowly as possible with the work inside it.

Although such slow cooling results in some grain growth and weakening of the metal, it will impart maximum ductility. This results in the metal being in the correct condition for cold forming. However, because of its extreme softness and grain growth the metal will tend to tear and leave a poor surface finish if it is machined.

Stress-relief annealing

This process is reserved for steels with a carbon content below 0.4%. Such steels will not satisfactorily quench harden but, as they are relatively ductile, they will be frequently cold worked and become work hardened. Since the grain structure will have become severely distorted by the cold working, the crystals will begin to reform and the metal will begin to soften (theoretically) at 500◦C. In practice, the metal is rarely so severely stressed as to trigger recrystallization at such a low temperature. Stress relief annealing is usually carried out between 630◦C and 700◦C to speed up the process and prevent excessive grain growth. Stress-relief annealing is also known as:

• Process annealing since the work hardening of the metal results from cold-working (forming) processes.

• Inter-stage annealing since the process is often carried out between the stages of a process when extensive cold working is required.

The degree of stress-relief annealing and the rate of cooling will depend not only upon the previous processing the steel received before annealing, but also upon the processing it is to receive after annealing. If further cold working is to take place, the maximum softness and ductility is required. This is achieved by prolonged heating and very slow cooling to encourage grain growth. However, if grain refinement, strength and toughness are more important, then heating and cooling should be more rapid.