Lath martensite

Martensite forms when steel cools quickly. Depending on carbon content and cooling rate, the structure becomes hard and brittle or tougher. When welding, you have to control the cooling: Too fast cooling makes the weld and heat-affected zone hard and crack-prone, too slow cooling makes the material soft. If you preheat the parts and let them cool slowly after welding, you usually get a usable structure.