Two-stage tempering
A targeted heat treatment process for quenched martensite involving two sequential temperature stages for optimized carbide precipitation. This relieves residual stresses and precisely adjusts toughness.
Martensite is hard and brittle, you get it by rapid cooling. In two-stage tempering, you let the part cool to an intermediate temperature first to relieve stresses and let the structure settle. Then you cool slowly further, e.g., in the furnace, to make the material tougher. If you mess this up, you'll have residual stresses and the part can crack or distort. Cooling too fast just makes the martensite harder and more susceptible, too slow makes the structure too soft. So: first cool down cleanly to an intermediate temperature, then cool slowly and controlled, then it'll be fine.