Stainless Steel Welding: Filler Selection, Prep, and Heat Control
Stainless steel welding is fundamentally different from carbon steel welding. The same techniques and habits that work on mild steel will ruin stainless. Heat control is stricter, cleanliness is critical, and filler metal selection directly affects whether the finished weld resists corrosion or fails in service. This guide covers the core principles that apply regardless of which welding process you use.
Filler Metal Selection
The filler must match the base metal grade. This is not optional.
304 stainless gets ER308L (wire) or E308L-16 (stick). The L means low carbon, which prevents sensitization.
316 stainless gets ER316L or E316L-16. The molybdenum in these fillers matches the base metal and maintains chloride corrosion resistance.
Stainless to carbon steel joints get ER309L or E309L-16. The higher alloy content compensates for dilution from the carbon steel side.
Unknown stainless gets E312-16. Its duplex microstructure resists hot cracking even with heavy dilution from unknown base metals.
Using the wrong filler creates a weld with different corrosion properties than the base metal. The weld becomes the weak point.
Heat Control
Stainless steel retains heat much longer than carbon steel and conducts it more slowly. This means the heat-affected zone stays hot longer, which causes two problems.
Sensitization occurs when the temperature stays between 800-1500F (430-815C) for too long. Carbon combines with chromium to form carbide precipitates at grain boundaries, depleting the surrounding area of chromium and destroying corrosion resistance. Using low-carbon (L grade) filler helps but does not eliminate the risk.
Distortion is worse on stainless than carbon steel because of higher thermal expansion. Tack frequently, use backstep techniques, and minimize heat input.
Keep interpass temperature below 350F (175C). If the part is too hot to hold your hand near, let it cool before welding the next pass.
Cleaning and Contamination
Carbon steel contamination on stainless causes rust staining and pitting. Use dedicated stainless tools: wire brushes, grinding discs, flap discs, and clamps that have never touched carbon steel. Even a stainless wire brush that was previously used on carbon steel can embed iron particles in the stainless surface.
Clean the joint and surrounding area with acetone or a stainless-safe solvent before welding. Remove all oil, grease, marker, and fingerprints.
After welding, passivate the weld area with a pickling paste or passivation solution to restore the chromium oxide layer that provides corrosion resistance.
Back Purging for TIG
When TIG welding stainless pipe or tubing, the root side of the weld must be protected from oxidation. Without protection, the root oxidizes into a rough, discolored surface called sugaring. Sugared roots have drastically reduced corrosion resistance and may be rejected by inspection.
Purge the inside of the pipe with argon before and during welding. Use aluminum tape, silicone plugs, or water-soluble purge dams to seal the pipe. Flow argon through the pipe at 10-15 CFH until the oxygen level drops below 100 ppm (use an oxygen analyzer for critical work).
For non-critical work where purging is impractical, Solar Flux paste applied to the back side of the joint provides limited protection as an alternative.
Classification system defined by AWS A5.4, AWS A5.9, Lincoln Electric stainless welding guide.