Martensite Aging

Martensite is a glass-hard but also brittle structure. It forms when you cool the steel too quickly after welding. Then the weld becomes hard and prone to cracking, which you don't want. To avoid this, you have to slow down the cooling. You do this by preheating the part or not drawing the heat away immediately after welding. The higher the carbon content of the steel, the more careful you have to be with cooling, otherwise the weld will crack.