Preheating
Preheating means heating the part in a controlled way before welding. It slows down the cooling after welding and prevents hardening, hydrogen cracking, and stress cracking – especially important for hardenable steels.
The heat has to go into the plate, not just into the weld. Thick or cold material draws the heat away immediately, making the weld glass-hard. If you preheat the part, it cools down slower, and the structure stays calm. The hydrogen from moisture or rust also gets time to diffuse out of the weld metal instead of causing cold cracks later. The more carbon and alloying elements in the steel, the more critical this becomes. You can tell by the carbon equivalent. When preheating, do it over the whole area and from both sides, not just work a point with the flame. Check the temperature with thermocrayons or a contact thermometer; don't trust your hand. Maintain the interpass temperature until the last pass, and then let the part cool down slowly and evenly, if necessary with a blanket. Cooling with water is taboo.