Martensitic Needle Structure

Martensite forms when you cool the steel too quickly after welding. The structure becomes glass-hard and brittle, which can quickly lead to cracks, especially in higher carbon or alloyed steels. To avoid this, you have to slow down the cooling, for example by preheating or letting it cool slowly. If you can't treat the parts properly after welding, at least don't let them cool in air; insulate them or bury them in warm sand. This prevents stress cracks and keeps the part stable.