Tempered martensite microstructure
The microstructure resulting from the thermal tempering of freshly quenched martensite, consisting of a ferritic matrix with extremely fine, homogeneously distributed transition carbides. It minimizes internal stresses while maintaining high toughness.
Tempered martensite structure forms when you reheat quenched martensite in a controlled way. The hard, brittle structure partially transforms and becomes tougher without losing hardness completely. When welding, you must be careful not to unintentionally create such structural changes in the base material due to heat input. This often happens in the heat-affected zone when the material cools quickly and then is reheated. If you're dealing with such steels, control the cooling and interpass temperature. Cooling too fast makes the structure hard and crack-sensitive; cooling too slow can make it overly soft. So don't cut it close—keep cooling uniform and let the part cool down slowly after welding.