Most stainless piping failures in welded systems start at the weld - not because the weld metal is weak, but because the rules of stainless welding were not followed. Austenitic stainless does not need the preheat and post-weld heat treatment of carbon steel; instead it needs four quieter disciplines: the right filler, controlled heat input, a low interpass temperature, and shielding on both sides of the joint.
Break any one and the result is sensitization, hot cracking, or a weld zone that corrodes faster than the pipe around it. This guide sets out the rules, the filler selection logic, and the defects to look for.
Product Forms
Bar & Rod
Plate & Sheet
Coil & Strip
Pipe & Tube
Fitting: Flange, Tee, Elbow, Reducer etc.
Forging: Ring, Shaft, Circle, Block etc.
Why stainless welding is different
Carbon steel welding is about avoiding hydrogen cracking and restoring toughness; stainless welding is about preserving the corrosion resistance the heat destroys. When austenitic stainless is held in the 425-870°C band - exactly what happens in the heat-affected zone of a weld - chromium carbides precipitate at grain boundaries and the metal beside them becomes chromium-depleted and un-protectable.
The weld therefore has to be made so that the time in that band is minimized, and the filler has to be chosen so that the weld metal itself stays corrosion-resistant. Those two objectives drive every rule below.
Rule 1: match the filler to the parent metal - and prefer the low-carbon version
| Base metal | Preferred filler | Why |
| 304 / 304L | 308L / 308L-type | Low carbon prevents weld sensitization |
| 316 / 316L | 316L-type | Molybdenum in filler matches pitting resistance |
| 321 / 347 | 347-type | Niobium stabilizes the weld, matches the stabilized base |
| Duplex 2205 / 2507 | Over-alloyed duplex filler (22/25 Cr type) | Ensures austenite-ferrite balance in the weld |
| Nickel alloys (625, C276) | Matching nickel filler (ERNiCrMo-3, ERNiCrMo-4) | Weld metal meets base corrosion resistance |
The low-carbon filler rule is the simplest and most violated one: welding a 316 base with a 316L filler keeps the weld metal's carbon low even though the base is 316, and the weld is then more sensitization-resistant than a 316 weld metal would be. It costs nothing and buys a margin.
Rule 2: control heat input
High heat input means a wide heat-affected zone and more time in the sensitization band. The discipline is moderate travel speed, stringer beads rather than wide weaving, and the lowest heat input that still gives a sound weld. This matters most in thin sections and in the first passes, where the temperature exposure is proportionally larger.
Rule 3: keep interpass temperature low
For austenitic stainless, the interpass temperature is typically limited to about 150°C - the multi-pass weld must be allowed to cool between passes so the accumulated heat does not push the joint through the sensitization band for too long.
Duplex grades are stricter still, often limited below 150°C and sometimes near 100°C, because over-heating drives the phase balance wrong and can form sigma phase. A fitter who treats stainless like carbon steel and welds pass after pass while the joint stays hot is building the corrosion failure into the pipe.
Rule 4: shield both sides of the joint
The root pass of an austenitic weld oxidizes on the inside if it is not shielded, and the oxidized root becomes a corrosion and contamination site that is nearly impossible to fix from outside.
GTAW root passes in corrosion-critical piping get a back-purge with argon on the inside of the joint, and the purge is held until the root is protected. The evidence of a skipped purge is the dark, scaled root - which is why internal heat-tint removal is part of the surface story from the pickling guide.
The defects the inspector looks for
| Defect | Cause | Result if ignored |
| Hot cracking in the weld metal | High heat input, restrained joint, impurities | Weld failure under load |
| Sensitized heat-affected zone | Over-heating, high interpass, wrong filler | Intergranular corrosion in service |
| Oxidized root | No back-purge | Pitting and contamination at the root |
| Sigma phase (duplex) | Excessive heat, slow cooling | Brittle, low-toughness weld zone |
| Iron contamination | Carbon-steel brushes and tools | Local rust and pitting on stainless surfaces |
What to specify on the purchase order
- Welding standard and procedure: ASME IX or equivalent qualified WPS
- Filler: the grade called out (e.g. 316L filler for 316 pipe)
- Interpass temperature limit and back-purge requirement for corrosion service
- Post-weld examination: VT, and RT or UT where the service demands it
- Surface: heat-tint removal and pickling of the weld zone for chloride service
FAQ
Q: Does stainless need preheat and post-weld heat treatment like carbon steel?
A: No. Austenitic stainless needs no preheat and, for most services, no PWHT. The treatment that matters is the opposite: keeping heat low and cooling between passes to avoid the sensitization band.
Q: Why use 316L filler on 316 pipe?
A: The lower carbon of the filler keeps the weld metal sensitization-resistant. It is a standard practice that costs little and protects the weld zone in chloride service.
Q: What happens if the root pass is not back-purged?
A: The inside of the root oxidizes. The scaled root is a contamination site that can pit in service and is difficult to clean on installed pipe. Corrosion-critical lines specify back-purge for this reason.
Q: Is duplex stainless harder to weld than austenitic?
A: Yes - the phase balance must be protected. Heat input and interpass temperature are stricter, and the filler is over-alloyed to maintain the austenite-ferrite ratio. Qualified duplex procedures are mandatory for good results.
