Through hardening

Steels having a carbon content between 0.3% and 0.8% are capable of being through hardened. As the carbon content increases, so does the degree of hardness achievable. The depth to which a steel grade will fully harden depends upon the rate of quenching, with faster quenches in brine or water producing a deeper hardening effect than with oil, air or inert gas. The addition of alloying elements, such as manganese, nickel, chromium and molybdenum, increase the achievable hardening depth, i.e. the hardenability of the steel is thereby increased.

For every steel composition there is a limiting ruling section at which the specified combination of properties can be achieved. In parallel with hardening, the brittleness of the steel increases. This is the reason for the secondary treatment which follows hardening, which is termed tempering. The instability of steel in the as-hardened condition, due to the high level of internal stresses present, is prone to induce cracking. The cracking tendency increases with increasing hardenability and with the severity of the quenchant used in hardening. In order to relieve the internal stress produced in the microstructural change causing hardening (the formation of martensite), it is necessary to re-heat the quenched steel to a temperature below the martensite transformation finish temperature, suitable for the particular steel in question.

The cracking propensity increases with increasing hardness i.e. with increasing carbon and alloy content. Thus, tempering has to be carried out with as short a time delay as possible following hardening, particularly for tool steels. During tempering, in addition to stress relief, many steels undergo a further sub-microscopic structural change consisting of the precipitation of carbide particles from the martensite. Tempering produces a reduction in hardness and a corresponding improvement in ductility. The effect is both time and temperature dependant, with higher temperatures and longer soak times producing maximum reduction in hardness and increase in ductility. Ultimately with some steels, over-tempering can produce a breakdown of the martensite structure and the formation of a spheroidised carbide structure.

Low alloy steels are usually tempered in the range 450-650°C for the most useful combination of mechanical properties. Some high alloy tool steels exhibit secondary hardening during the tempering treatment, due to the precipitation of hard alloy carbides.