Tempered martensite

Tempering is the targeted relaxation of hardened martensite. The carbon from the supersaturated lattice precipitates as fine carbide, which lowers hardness and increases toughness. Without tempering, the weld remains hard and brittle, and the part will tear under load – especially if hydrogen is involved. When welding quenched and tempered steels, you have to be careful not to put in too much heat. Otherwise, you over-temper the weld, the carbides coarsen, and strength drops. You can take advantage of the natural tempering effect from the subsequent bead if the interpass temperature is right. When tempering deliberately, heat the part slowly and uniformly, hold it at temperature long enough, and then let it cool slowly in still air. Rapid cooling after tempering only creates new stresses, which you don't want.