Tempering process

When tempering, the steel is heated below its transformation temperature. This relieves the stresses from welding or hardening without the structure fundamentally transforming. If you skip it or do it sloppily, the steel stays hard and brittle and can crack or distort later. But you must not go too hot either, otherwise you unnecessarily lose hardness and the grain becomes coarse. It is important to let the part cool slowly and evenly after heating to prevent new stresses from building up.