[Ed Caffrey]

Hi Dan!

Generally those bubbles occur when there is a low spot between the layers. It usually occurs after the first re-stack/folding. When you grind the forge scale off before re-stacking/folding, make sure you grind a scarf into the pieces (the surface of the pieces are slightly convex, with the highest spot being along the centerline of the pieces. Another trick is to make sure the grind lines are running across the pieces (from one side to the other). This creates a capillary action for the flux, allowing it to be squeezed outward from the centerline of the billet. If your grind lines are running lengthwise on the pieces, there's a good chance that something will get trapped in there, creating a bubble.
Something else to think about are your dies (hammer or press). About a year ago I went for a couple of weeks where every billet I welded had a bubble in it.....it wound up being the press dies. There was a dent in the upper die, and it was actually causing the bubble. I replaced the dies and the problem was solved. And finally, one other possibility... sometimes bubbles are caused by trying to weld with drawing dies. The drawing dies do not draw all the layers equally...especially in a hammer. The drawing dies will draw the outer layers more so than the inner layers, and it will allow a gap/bubble to form, especially if it's an inital weld.

Hopefully you can sift through this and figure out what applies to your situation.

Let us know if you solve the problem!