Why tap?

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I am new to welding, and while i see plenty of people do it and a few things i've read say to do it, i'm curious as to why you tap a fresh weld with a hammer right away after completing?
Reply:just to show who's boss.
Reply:^ When a weld cools, it contracts.  The thought behind peening is to relieve some of that tensile stress / pulling that results from the contraction.  I've heard it's most useful when doing cast iron, which likes to crack if you've got too much stress around the weld.  Other than that, I haven't heard of it being done much.
Reply:"Peening" seems to be one of those things that's kind of widely misunderstood (and mis-used).   Just tapping a weld with a hammer does not constitute "peening" it.   You have to cause a fairly serious level of plastic deformation all down the surface of a weld for it to really be considered as having been "peened".   It usually requires a serious power tool to accomplish and not just whacking it with a manual hammer.
Reply:are you sure he is not thinking of a chipping hammer to remove slag?Miller Multimatic 255
Reply:I'm thinking he is watching the welder peen slag also.
Reply:I wondered the same thing, and every time i asked no one gave me a good explanation. A couple of weeks ago I was welding a 20ga SS butt joint, blew a hole and put way too much heat in trying to fill it. When flipped my hood up I seen the weld area rising right before eyes. That's when i had the ah-ha moment. The next time it happened I had my body hammer ready and smacked it a couple of time and stopped it in its tracks.
Reply:Here's a link to Mr Shrink. Someone here posted it before. Peening is covered (very little) at about 15:00From what I understand is that the weld wants to contract while cooling. So peening it pushes it down and forces the weld to spread and flaten.
Reply:Originally Posted by Louie1961are you sure he is not thinking of a chipping hammer to remove slag?
Reply:From the video's i've been watching, i'm going to guess it's the slag reason... they are not even close to doing what they are doing in that video!.Thanks guys