Risk of hardening and cracking At the welding point, the metal is liquefied at about 1,700 degrees Celsius. The massive workpiece is significantly cooler, however, so stresses in the material are inevitable. If they are too great, cracks can form. Moreover, the weld seam and the base material might not bond properly if weld edges in the cold workpiece do not fully melt. If the material cools too rapidly, on the other hand, unwanted hardening can occur. High-strength steels in particular demand special care during welding, in order not to impair their tensile strength. So, in order to minimize the temperature differences, the workpieces are preheated to 100 to 300 degrees Celsius before welding. Though still significantly cooler than the extreme temperatures at the weld point, that’s still warm enough to reduce the gradient significantly. The preheating also delays cooling and prevents bonding failures. In addition, it helps prevent hydrogen-assisted cracking: During welding, moisture can penetrate the weld point. In the high heat, hydrogen atoms are then released and absorbed by the liquid metal. After cooling, the steel loses its hydrogen solubility. Under stress, the hydrogen atoms begin to migrate in the material. When they encounter each other, the molecules bond – H becomes H2 – with a volume that is a thousand times greater. This can cause hydrogen cracking. Preheating gives the gas the time it needs to escape from the metal.