Martensite Start Temperature

The martensite start temperature is the point where the structure transforms from austenite to martensite during cooling. If you cool too fast during welding and go below this temperature, you get a hard, brittle structure that cracks quickly. You don't want that. So you have to slow down and control the cooling to prevent martensite formation. How you do that depends on the material – sometimes by preheating, sometimes by letting it cool slower. The exact temperature where it gets critical varies by steel grade.