Don't forget to Ping that Weld

Paul, Got a question for you. I work in the mines, when we are welding something thick our boss tells us to "ping" our welds (excessive chipping /with air-chipper). I've never heard of that before. What’s your input?
Matt

Matt, Your Boss is offering some good advice.  He's actually asking you to peen your weld (I'm sure he says "ping" but this is what he means).
Peening a weld helps reduce what's called "residual stress". When you heat and cool metal it wants to move, but typically the weldment doesn't allow it to move. This builds residual stress in the part.  When residual stress exceeds "Yield Strength" you'll get distortion.  When residual stress exceeds "Tensile Strength" you'll get weldment failure.
Also, welding screws with the materials grain structure (at an atomic level). There are several ways to deal with this. You can Post Weld Heat Treat the weldment: raising it to what's called the transition temperature (Around 1600f), holding it there for some predetermined time and then allowing it to cool in some controlled manner. Or you could apply vibration (vibratory stress relief), vibrating the weldment during welding and shortly thereafter. Or you can peen, hitting the weldment with a peening hammer (or a scaler) to help reduce this stress.
So, what your Boss has you doing is stress relieving each pass to reduce distortion and /or the risk of cracking.  Listen to him.

Good question.
PWC