Welding Stainless Steel 304 and 316: Filler, Heat Input and Passivation

Filler selection

The default rule is to weld one grade up:

• 304/304L is welded with 308L filler.
• 316/316L is welded with 316L filler, which carries the molybdenum needed to preserve pitting resistance.
• Dissimilar joints (304 to carbon steel) use an over-alloyed filler such as 309L.

The "L" matters: filler carbon is kept at or below 0.03% so the deposit itself resists sensitization. Where a base metal is plain 304 or 316 (not L), using L-grade filler and controlling heat input is the main defence for the weld metal.

Heat input and sensitization

Between roughly 450 °C and 850 °C, chromium combines with carbon to form carbides at grain boundaries — sensitization — leaving chromium-depleted zones that corrode preferentially (weld decay, knife-line attack). Stainless practice therefore works the opposite way to carbon steel:

• Keep heat input low to moderate (commonly in the region of 0.5–1.5 kJ/mm, process dependent).
• Limit interpass temperature, typically to about 150 °C.
• Use stringer beads rather than wide weaves, and let the joint cool between passes.

Austenitic stainless also conducts heat poorly and expands roughly half again as much as carbon steel, so distortion control (balanced sequences, backstep welding, adequate fixturing) is part of the procedure, not an afterthought.

Technique points

• Shielding: the root side of pipe and sheet welds needs an inert backing gas (argon purge) until the root is sealed by several millimetres of metal; an oxidized, sugared root has poor corrosion resistance and fatigue performance.
• Cleanliness: dedicated stainless-only brushes and abrasives; iron contamination from shared carbon-steel tools embeds and rusts. Degrease before welding.
• Never use the stainless piece as a workbench for grinding carbon steel nearby — grinding dust contamination is a common cause of mystery rust spots.

After welding: cleaning and passivation

Welding leaves heat tint — a chromium-depleted oxide film whose straw-to-blue colours signal reduced corrosion resistance underneath. For corrosion service the tint and the depleted layer must go:

1. Mechanical cleaning (stainless brushing, fine abrasives) for light duty.
2. Pickling with nitric/hydrofluoric paste or bath for full restoration of the surface.
3. Passivation in nitric or citric acid (per ASTM A967 practice) rebuilds the chromium oxide film on an already clean surface. Passivation is not a cleaner — it does not remove heavy tint or scale.

A correctly welded, cleaned and passivated 316 joint should be indistinguishable in corrosion performance from the parent metal; skipping the postweld steps is the usual reason it is not.

Common defects and their causes

• Porosity: almost always contamination — moisture, grease, paint or zinc on the joint faces, or a shielding problem (lost gas coverage, draughts). Stainless shows contamination porosity sooner than carbon steel because the pool is less forgiving of dirty surfaces.
• Sugared root: insufficient backing purge; the root oxidizes into a rough, blackened, corrosion-prone surface that cannot be repaired from outside.
• Cracking: rare in 304/316 itself (the austenitic structure with a small ferrite content in the deposit resists solidification cracking), but a real risk when the wrong filler is used on dissimilar joints or when carbon contamination enters the pool — one more reason for dedicated tools and clean wire.