Needle peening on welding

You peen the weld bead with a needle scaling gun after welding to reduce the stresses set up by the heat of the weld. Particularly apposite to welding of cast iron where the bead contraction will tend to pull chunks out of the base material.

(You can achieve the same effect with a small toffee hammer and lots of patience)

AWEM

Peening is done to the actual weld bead using an air chisel or a needle gun. The idea is to stress relieve the weld. The problem is that the effect is not easily predictable. One welder may peen longer than another. A needle gun is simply an air chisel with a bundle of steel rods on its end. the steel rods chip and remove welding flux. Google around on mechanical methods of stress relieving. Randy

Thank you Raja

using a needle gun wouldn't be very effective in peening steels. What's more important is that while peening one should be very careful to keep the peening operation on just the weld bead and not on the adjacent base material. Additionally, the weld bead in question should have a convex contour that will lend it to being expanded by the blows from the peening hammer. If there aren't any marks on the metal, then the peening isn't happening.

The point in peening is to stretch the metal that's just shrunk from it's transition from being just solidified to the shape it takes at room (or nearly) room temperature.

And there is such a thing as over peening. The use of accurate measuring tools with punch marks on either side of the weld can tell you if you're effective in using peening or not.

For heavy section and high strength steels, we've used chipping or riveting guns, air chisles are a bit light.

On thin stock, you might need to "back up" the peening tool too. Like putting some type of anvil on the other side of the work from the side the peening is happening. Heavy sledgehammers can be effective.