Distortion is caused by stresses in a part that are relieved during thermal treatment. These stresses include induced machining stresses from variations in the machining procedure and stresses caused by non-metallic inclusions due to differential rates of expansion.

Distortion can also result from stresses associated with the volumetric growth caused by surface treatments. When you change the composition of a metallic surface by diffusion techniques – carburizing and nitriding – there will be growth due to a surface volume change. There will also be a size change in any metal due to phase changes that occur as a result of the application of heat. In steel, this includes changes from ferrite to austenite and austenite to martensite.

Whenever steel is rolled, hammered, forged or machined (including turning, milling, hobbing, broaching, etc.), induced residual stresses will accumulate in the steel surface. The stresses can only be effectively released when heat is applied. This means that during preheat the steel is stress relieving itself – hence distortion. Distortion cannot be stopped. It can be reduced, however, by prior stress relieving, careful preheat to the hardening temperature and choosing the most appropriate quench medium when cooling rapidly to form martensite.