My friend had to modify his oil pan to clear the oil pump, so I fabricated an extension on the bottom of his pan using 1/16-inch steel plate, and then I TIG-welded it all together. Next, I filled the oil pan with water, and from the outside I blasted compressed air against the weld to see if there were any pinholes. Unfortunately, there were many. I rewelded the bad spots and blew air against the welds again, only to find many pinholes were still there. It became a game of weld and test and chase, until one spot on the pan got so bad I cut it out and replaced it with new metal. I noticed when holding the cut-out portion up to the light, you could see lots of tiny holes going through the metal. This isn't the first time this has happened to me when trying to make liquid-tight welds on steel.
What am I doing wrong? I cleaned the metal very well before welding, and tried different argon settings. I'm puzzled about what goes on after the bead is laid down that can create pinholes that often can't be seen.
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Welding porosity is caused by gas (generally oxygen) that forms in the weld puddle before it solidifies. Many, many things can cause this. The top of the list is always contamination. If the base metal or the filler rod has any sort of contamination-oil, paint, rust, corrosion, or heat scale-some porosity is almost guaranteed. Hot-rolled steel is lousy for TIG welding, because it comes with a thick scale on the surface that's difficult to remove completely. I'm suspicious thatsome imported steel may have contaminants in it, too.
The argon shielding gas is crucial to making non-porous welds, too. Normally, a flow of 12 to 14 CFH is sufficient for welding steel sheetmetal, but there are several variables here, such as the cup diameter, how far the tungsten extends past the cup, and the tightness and integrity of the gas hose and connections (even tiny cracks or worn spots in the hose can draw in air, like a carburetor draws in gas).
There is a special collet body available, called a "gas lens," that focuses the gas flow and allows you to stick the tungsten out farther than normal and still have good weld coverage, which is helpful when you're getting into a tight area. Too much argon flow can cause turbulence, which might draw air into the shielding gas, causing contamination.
When you weld, the air around you must be absolutely still. The slightest breeze or air movement can upset the shielding effect of the argon gas coming through the cup. Last, although this is rarely a problem, the purity of the gas in the bottle can be an issue. I got a contaminated bottle once, and I couldn't weld anything worth beans until I replaced it with a good bottle.
Torch technique is a factor, too. The torch should be held very close to the work, and ideally no more than 20 degrees off of vertical (you do have some leeway here), but you must never let the tungsten electrode touch the base metal, the rod, or the puddle-or it will become contaminated. Once the tungsten is contaminated, even slightly, you'll see a brown smudge next to the weld, and you'll be creating porosity as you go. When you get even the slightest contamination, you should stop immediately and resharpen the tungsten to a fine point. For years, I used the old standby 2 percent thoriated tungsten. More recently, I've switched to 2 percent ceriated tungsten, and one of the benefits is that it doesn't get contaminated nearly as often as the thoriated tungsten, for reasons I don't understand, but this property is very much appreciated!
The next thing to look at is the kind of filler rod you're using. The difference between gas welding rod and TIG-specific rod is the amount of deoxidizing they receive. The best TIG rod is ER 70 S-6. Most welding distributors sell S-2, S-3, or S-4 rod, which isn't bad, but the S-6 rod has special processing, as well as extra deoxidizers and cleaning agents to help it work better in difficult situations. You should definitely seek this out for demanding jobs. Most steel MIG wire is ER 70 S-6, so you can unroll MIG wire, straighten it, and use this as filler rod for TIG welding.
Your technique for applying the welding rod is important, too. You shouldn't swizzle the end of the rod in the puddle, never withdrawing it-the proper technique is to apply the rod with a series of dabs into the puddle. But be careful-when you withdraw the rod from the puddle, keep it close to the cup so it doesn't go outside the shield of argon gas streaming from the cup. If the glowing red-hot end of the rod contacts air, it oxidizes rapidly, and this could be part of the problem you're having with porosity.
Last, if you do have a little porosity in a weld, even one bubble, it generally gets worse when you try to touch it up with regular rod. For some reason, stainless steel rod (I'd suggest 308L) or silicon bronze rod is much better for filling individual pinholes. Another option is to drill through each pinhole to remove the porous metal, and fill the hole with fresh rod (I'd suggest a quarter-inch-diameter drill bit). The last resort is to cut the porous areas away and replace them with new metal, as you did on the oil pan. (Yes, I've done this, too, on occasion!) Again, carefully clean all the metal that became discolored when being welded the first time. Even the light heat scale left on the metal after welding can cause porosity, too
I have a DVD that covers many aspects of TIG welding, including eliminating contamination and porosity. Check it out on our website listed below.
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