Surface hardness

Case hardness is caused by the transformation of the structure at the edge of the component when the material is quickly heated and quenched. This makes the surface wear-resistant but also more brittle. In welding, this is a problem: if the heat-affected zone cools too quickly, it becomes hard and crack-sensitive. So you have to slow down the cooling, for example by preheating or welding more slowly. With high-carbon steels, the risk is particularly high because hard, brittle martensite forms quickly.