2.1 Through-hardening

Basically the process by which we ‘through-harden’ (also referred to as quench hardening) consists of heating a suitable steel to a critical temperature that is dependent upon the carbon content of the steel and cooling it quickly (quenching it) in water or quenching oil. The hardness attained will depend upon:

• The carbon content of the steel (the higher the carbon content the harder the steel).

• The rate of cooling (the faster the cooling the harder the steel).

For example, to harden a chisel made from 8% carbon steel, the temperature to which the steel is heated should be between 800◦C and 830◦C.

That is, the steel should glow ‘cherry red’ in colour. It is then quenched by cooling it in cold water. Any increase in temperature will make no difference to the hardness of the chisel. It will only result in ‘grain growth’ and weakening of the steel. Grain growth means that the hotter the steel becomes and the longer it is kept at excessively high temperatures the bigger the grains in the metal will grow by merging together.

Therefore the workpiece should never be cooled more quickly than is required to give the desired degree of hardness (critical cooling rate). The most common substances used for quenching (quenching media) are:

• Brine (a solution of sodium chloride and water) is the most rapid quenching bath – it will give the greatest hardness and is the most likely to cause cracking.

• Water is less severe and is the most widely used quenching bath for plain carbon steels.

• Oil is the least severe of the liquid quenching media. Only plain carbon steels of the highest carbon content will harden in oil, and then only in relatively small sections. Oil quenching is mostly used with alloy die steels and tool steels.

• Air blast is the least severe of any of the quenching media used. It can be applied only to heavily alloyed steels of small section such as high-speed steel tool bits.

2.2 Quenching, distortion and cracking

Quenching and distortion

When quenching hot metal, some thought must be given to the way the work is lowered into the bath to avoid distortion and to get the most effective quenching. For example, when hardening a chisel, the shank of the chisel is held with tongs and the chisel is dipped vertically into the quenching bath.

• This results in the cutting end of the steel entering the bath first and attaining maximum hardness whilst the quenching medium is at its minimum temperature.

• The shank is masked to some extent by the tongs and this result in reduced hardness. This does not matter in this example as we want the shank to be tough rather than hard so that it does not shatter when struck with a hammer.

• The chisel should be stirred around in the bath so that it is constantly coming into contact with fresh and cold water. It also prevents steam pockets being generated round the chisel that would slow up the cooling rate and prevent maximum hardness being achieved.

• Dipping a long slender workpiece like a chisel vertically into the bath prevents distortion. This is shown in Fig. 1. This figure also shows what happens if the component is quenched flat and also how the shape of the component itself can cause uneven cooling and distortion.

Figure 1 Causes of distortion: (a) distortion caused by an unbalanced shape being hardened; (b) how to quench long, slender components to avoid distortion

Cracking

Figure 2 shows some typical causes of cracking occurring during and as a result of heat treatment. Careful design and the correct selection of materials can result in fewer problems in the hardening shop.

Figure 2 Causes of cracking: (a) incorrect engineering that promotes cracking; (b) correct engineering to reduce cracking

• Avoid sharp corners and sudden changes of section.

• Do not position holes, slots and other features near the edge of the workpiece.

• Do not include screw threads in a hardened component. Apart from the chance of cracking occurring, once hardened you cannot run a die down the thread to ease it if it has become distorted during the hardening process.

• For complex shapes, which are always liable to cracking and distortion during hardening, always use an alloy steel that has been formulated so that minimum distortion and shrinkage (movement) will occur during heat treatment. Such steels are oil or air hardened and this also reduces the chance of cracking.