Q.Hey Ron, I am having a problem TIG welding an area on a steel gas tank I'm building. I was getting small pinholes that would be left behind once the welds were made. When I go back to fill the pinhole, it forms a bubbling crater that would get thicker and deeper as I added rod to it. As I apply more heat, the crater will bubble, spit, and hiss like it is releasing trapped gasses inside the weld. At first I thought it might have had something in the actual weld that was trying to float to the top of the weld pool, but for whatever reason wouldn't. So from there I cut out the entire area that was affected and made a replacement patch piece to weld back in. I made certain that I cut back far enough to be on the original unwelded steel. When I welded the new piece back in, the same thing started again. Now I am wondering if I was given a piece of hot-rolled steel and it could be causing this type of problem. Is there a problem with welding hot-rolled steel? As far as I know, I have never tried to weld hot-rolled before as I always order cold-rolled. One other thing that I've noticed with the bubbling is when I fill the spot with stainless rod it seems to stabilize the weld pool and fill the porosity. Do you think there would be any reason for the stainless helping out like it did? I'd like your input before I finish welding the entire piece back in. Please let me know what you think. Thanks once again for all the help you offer,Allan ReevesVia E-mail
A.There are many factors that can cause the sort of "bubbling" or "out-gassing" that you describe. First of all, I do hope that the tank was well vented as you were welding, since welding on a sealed container will surely cause problems. Also, I want to remind our readers that we are talking here about welding on a tank that has NEVER HAD GASOLINE IN IT. Welding on used tanks requires taking some very special precautions, or you can cause an explosion!
Most often, porosity in a weld is caused by contamination of some sort. This could be in the base metal, the filler metal, or even in the shielding gas (or incomplete coverage by the gas). Most commonly the contamination is on the base metal. Any trace of paint, grease, or oil can cause this problem. Yes, the scale on the surface of hot-rolled steel can cause porosity, but even hot-rolled steel can be successfully TIG welded if the scale is scrupulously cleaned off. I've found that even a tiny bit of rust or oxidation on the metal can cause this outgassing. The metal to be welded has to be absolutely clean, both inside and out. If you cut out the "bad" section with a torch, plasma cutter, or die grinder, any discoloration (bluing) on the metal must be carefully cleaned off, or it can cause further bubbling.
The welding rod most commonly used for TIG welding-ER 70-S2-is deoxidized. Using non-deoxidized rod (like that designed for gas welding) can also cause this outgassing, so be sure you are using the correct rod. A lot of welding rod these days is made offshore, and the quality can vary widely. You might try getting rod from a different source if you are still having problems. It really helps to clean your welding rod, too. The wire is drawn through dies when being manufactured, and the lubricants used for drawing can cause weld contamination if not completely cleaned off the rod.
I have also found that using stainless filler rod is sometimes a cure for minor porosity, although I don't understand why.