Lower shelf energy region

Tieflage means: The material breaks without warning, smooth and brittle. This happens when the workpiece gets too cold during welding or cools too quickly. Then the weld and heat-affected zone become hard and prone to cracking. So you have to preheat the part properly and maintain the interpass temperature so the structure can recrystallize. After welding, let it cool slowly, best to cover it. If you don't do that, you'll quickly get a cold crack in the weld.