I'm not sure it has anything to do with flux core welding. If you are sure the pipe is bulging out, I was able to place a bottle jack on the huh spot on mine,and wrap a chain around it. It pushed back in a essay enough but I have some pretty stubborn thick walls on my pipe, so it springs back. I'm going to need more firepower on mine, but might work on yours. Keep playing with it before you put flanges on just to make sure you don't need to make any door adjustment. As a last ditch attempt I may have to form my door do the pipe...