What are the best techniques for welding stainless steel? That’s like asking what is the best way to go fishing? It depends on a lot of things, doesn’t it? Lots of things like what type of stainless, what thickness, what is the application, and for what industry?
Let’s focus on three industries: Food service, Aerospace, and Nuclear.
1. Food Service
Most stainless steel in the food service industry is 300 series stainless. Type 304 .063″ thick stainless sheet metal to be exact. If you go to any Fast food counter and check out all the counters, shelving, cookers and such, you will notice it is all made from welded stainless steel sheet metal. Food service codes require 304 stainless steel to be utilized in food preparation areas because it does not rust easily. All welds are supposed to be performed in such a way to not trap bacteria and other crud. Back sides of stainless steel welds should be shielded with argon so that they are not sugared and full of pits that could trap bacteria like salmonella. All wire brushing should be done with a stainless steel brush and welds that are not perfectly smooth should be blended smooth with some type of abrasive wheel and then cleaned with alcohol.
Tig welding is almost always the best process since spatter and slag are absent. Tig welding rods should be 308L for welding 304 stainless. L is for low carbon because
Carbon is bad when it comes to corrosion resistance in stainless.
Tips for welding SS sheet metal : 1 amp per 1 thousands of thickness, keep the hot tip of the filler rod shielded and snip it if it gets oxidized, use chill bars made of aluminum, copper, or bronze whenever possible. Filler rod should generally not be bigger than thickness of metal welded. Keep bead width to around 4 times the thickness of the sheet. Use a gas lens style cup, a #7 or bigger. A 1/16 thoriated or lanthanated electrode will easily weld 16 ga .063″ sheet. Keep electrodes clean and sharp.
Stainless steel alloys used in the aerospace and aviation industries are a bit more varied. There are lots of them. Austenitic stainless steels like 321, 347, 316, and 304 are common, but so are martensitic stainless grades like 410 steel , Greek Ascoloy, and Jethete M190. Another family of stainless used on commercial aircraft is the Precipitation Hardening variety. A PH at the end like 15-5ph, 17-4ph, 15-7ph, 17-7ph indicates that the steel is precipitation hardening. That means holding it at high temperature for extended time allows the steel to harden. PH grades are sometimes much more difficult to weld than the straight 300 series because of alloy elements and complex metallurgical reactions to heat while welding.
Tig Welding and automated plasma welding are the most commonly utilized processes.
Tips for Aerospace tig welding SS : Clean, Clean, Clean. Use as big a tig cup and possible 3/4 ” –1″. Use minimal heat and use any means to prevent distortion (skip welding, fixturing, small beads) shield the back side of everything you weld with argon using tooling, or home made purge boxes.
Nuclear piping systems use a lot of stainless steel and most of it is TIG welded. Or at least the root pass is tig welded. Most of the stainless piping is basic 304L stainless but other grades like 316 are used also. Tig root passes with stick fill is pretty common with heavy wall pipe but anything under schedule 80 is usually just Tig welded all the way out because x ray results are better that way. Stick welding is kind of hard with stainless.
Instrumentation lines that are 1/2″ and less in diameter and are often done with orbital tig welding because it is so repeatable. Welding small diameter tubing manually is tough.
There are all kinds of other applications for welding corrosion resistant alloys like boat propellers, headers and exhaust, and marine hardware like sailboat parts. One thing to remember is that they distort easily so precautions like heat sinks, chill blocks, and minimizing heat input are all important techniques to use.