Critical Cooling Time for Martensite Formation

The cooling time between 800 and 500 degrees determines whether martensite forms in the heat-affected zone or not. If the part cools too slowly, you get a coarse, soft structure that has no toughness left and cracks quickly. If you cool too fast, it gets hard and brittle. So you have to control the cooling rate, especially with higher-strength steels. Adjust your heat input and preheating so you stay within a reasonable window, then the weld will hold.