I was assembling some parts today and I needed to weld some bolts to a plate. I countersunk the holes, dropped in flat head socket head cap screws that I ground to a smaller diameter head, but still larger than the hole. The bolts were cleaned by bead blasting so they were bare steel. I held them in place with nuts and TIG welded them. The welds looked great. Then I ground down the welds to make them smooth and some of them were full of bubbles. So, I cleaned and welded them again with more rod and it kept getting worse the more I tried to fill the bubbles. I even tried to grind out the weld and weld again, and it just kept getting worse. I even tried MIG to no avail.
I assume this has something to do with the carbon in the socket head screw, but I've done this before with no issues though I usually have some small pits after I grind the welds. What's going on? I know bolts are welded on all the time. All I need to do is keep the bolts from rotating but they need to be smoothed down too.