Microalloying element

Keep the heat input low and don't let the interpass temperature get too high. If the zone gets too hot or cools too slowly, the fine carbides and nitrides coarsen and the grain grows – then you lose strength and toughness in the heat-affected zone. If it cools too quickly, on the other hand, the alloying elements remain forcibly dissolved and can't develop their effect. So work with a steady hand and controlled cooling, then the structure stays fine and the weld holds.