316 Filler Wire Versus 309 Wire?

by Chaz

When tig welding mild steel, how much weaker is the weld if using 316 filler wire instead of 309 wire?

Answer: By Steve Bleile.

I am not sure I can answer your question, but 309 and 316 are stainless. You can weld mild carbon steel with stainless rods, but I would not recommend it. The stainless rods are developed to provide specific chemical properties and I'm not sure whether or not that effects strength of the weld. The problem is that the stainless weld will have different characteristics than the mild steel base metal.

The best bet for welding mild steel to mild steel is good metal preparation, a perfect fit, and ER70S-2 tig rod. If the metals being joined are dissimilar, the 309 would be appropriate.

There are two ways to look at the strength of a welded joint. One is to make the properties of the joint equal to the base metal. This typically requires a full penetration weld and in some cases a heat treatment to relieve stress in the heat affected zone.

Another way to look at it is the amount of strength required for a specific situation. If a welded joint only needs to support a couple of hundred pounds, you don't need much weld. A good quarter inch fillet weld with 7018 should hold a little more than 2000 lbs per inch of weld. Economics plays a roll here. In the welding industry, it's not cost effective to put down more weld than you need and also, it's not good to put a whole lot more heat on the metal than necessary.

Getting back to the original question about stainless rod on mild steel. First, the properties of the stainless weld are going to be completely different than the carbon base metal. Second, stainless disperses heat slower than carbon steel, so the base metal and weld will be cooling at different rates. Third, and to me this is the most important, you will be creating a new alloy where the carbon steel base metal and stainless filler intermix, and you have no control of what that alloy will be. All three of those things will tend to weaken, not strengthen, the metal at the tie in area next to the weld and If a welded joint breaks, it usually breaks right aside of the weld.

Stainless does run nice on carbon steel, and if it's a cosmetic, non structural weld it doesn't matter. Maybe a little waste of money, but if your not doing a whole lot it may not be important. For structural welds there are no secrets, just basic (boring) fundamentals. Good design, metal preparation, fit up and weld technique.

Learn more from Steve.