TEMPERING

Tempering is essential after most hardening operations to restore some toughness to the structure. It is frequently performed as an integral part of the cycle in a seal quench furnace, with the parts fully protected against oxidation and decarburisation throughout the process. Generally tempering is conducted in the temperature range 150 to 700°C, depending on the type of steel and is time dependent as the microstructural changes occur relatively slowly.

Caution: Tempering can,in some circumstances, m a k e the steel brittle which is the opposite of what it is intended to achieve.

There are two forms of this brittleness Temper Brittleness which affects both carbon and low alloy steels when either, they are cooled too slowly from above 575°C, or are held for excessive times in the range 375 to 575°C. The embrittlement can be reversed by heating to above 575°C and rapidly cooling.

Blue Brittleness affects carbon and some alloy steels after tempering in the range 230 to 370°C The effect is not reversible and susceptible steels should not be employed in applications in which they sustain shock loads. If there is any doubt consult with the heat treater or in house metallurgical department about the suitability of the steel type and the necessary heat treatment for any application.