Platlet martensite

Needle martensite forms during very rapid cooling, especially in higher-carbon steels. This makes the structure extremely hard and brittle, which can quickly lead to cracks during welding. The problem is that these cracks often only appear after cooling or under load. When welding such steels, you absolutely must slow down the cooling, for example by preheating or allowing slow cooling. Never quench with water, as that promotes martensite formation. Better to cover the weld so it cools evenly.