Peening
Peening means working the still-warm weld immediately after welding with a ball or needle hammer. Hammering stretches the weld and relieves shrinkage stresses that could otherwise cause cracking—the most important protection for crack-prone materials like cold-work steel and cast iron.
What Peening Does
During welding, metal melts and shrinks as it solidifies. This shrinkage creates tensile stresses in the weld and heat-affected zone. If you peen the still-hot weld immediately, it gets pressed down and yields outward—exactly in the direction of the shrinkage forces. This greatly reduces the stresses and significantly reduces the room for cracking.
Why It's Crucial for Cold Work Steel
Cold work steels harden glass-hard in the heat-affected zone. Hard zones plus shrinkage stresses mean cold cracking risk. Peening relieves exactly these stresses before they trigger cracks. That's why it's a must when welding tool steels, not optional.
The Two-Person Technique
The trick from the shop floor: best to have a second person swinging the hammer. You lay the weld bead and lift the torch, your colleague peens the still-glowing weld immediately. Repeat this process bead by bead until the end. This way, every layer gets peened while it's still hot and plastic.
Limits
Peening doesn't replace preheating or controlled cooling—it's an additional measure. With high-alloy martensitic steels, too much peening can actually create new stresses, so work with moderation.