Why Stainless Welding Is Not "Just Like Carbon, Only Shinier"

Carbon steel forgives a lot in the weld shop. A wide heat-affected zone, some oxidation, a slightly hot interpass temperature — most of it comes out in the PWHT or in service. Austenitic stainless steel does not forgive. The same heat input that produces a tough carbon weld produces sensitization, distortion and heavy oxide scale on a 304 or 316 joint, and all three shorten the life of the line.

Three rules cover most of the difference. First, control the heat input — keep interpass temperature low and stringer beads tight. Second, purge the back side of every weld with clean, low-dew-point argon or argon/helium; do not let oxygen touch the root. Third, match the filler to the base metal on chemistry, not on diameter. A stainless steel pipe welded correctly will outlast a carbon steel pipe in the same service. Welded poorly, it will fail in months.

Pick the Filler Metal Before You Pick the Torch

Filler metal selection is a chemistry decision, not a preference decision. AWS A5.4 (covered electrodes) and A5.9 (bare wire) give the standard answers, and the standard answer for the four grades that cover 95% of industrial stainless work is short:

The L-grade rule is the one most often broken. If the base pipe is supplied as 304 (not 304L) and the spool is going to be field-welded in chloride-bearing service, the right fix is either to upgrade the pipe to 304L or to weld with the E308L filler — the low-carbon weld metal is what prevents sensitization at the HAZ. Sticking with E308 on a non-L base pipe is the single most common stainless weld defect found in service, and it shows up as intergranular corrosion along the HAZ within a few years.

Purging: The Step That Saves More Joints Than Any Other

Austenitic stainless does not tolerate oxygen at the root. When the back side of a TIG root pass sees air, the chromium in the weld metal oxidizes preferentially, the root surface turns grey or straw-coloured, and the chromium-depleted layer becomes the first place to fail under corrosion or thermal cycling. The fix is mechanical: displace the air with an inert gas before the arc strikes, and keep it displaced until the root has cooled below about 200 °C.

The two purge options on a real site

For shop fabrication, a full-pipe purge with inflatable dam-style purge dams at each end of the joint is the cleanest answer. The dams sit just outside the gap, the cavity is purged through a small port, and the welder strikes the arc into a known-low-oxygen environment. For field welds, a water-soluble paper purge dam or a foam dam with a trailing gas hose is more common, especially on long runs where the spool cannot be rotated. Both work; both fail if the inlet gas is wet, the dam leaks, or the welder is in a hurry.

The cheapest way to find a bad purge is to cut a representative spool and look at the root. A correct purge is bright silver, sometimes with a faint pale-gold tint. Anything straw, blue, or grey means oxygen was present and the joint will pay for it later.

Heat Input and Interpass: The Hidden Variable

Heat input is measured in kJ/mm and is the product of voltage, current and travel speed. For 304/316 austenitic pipe, the standard band is 0.5–1.5 kJ/mm. Going above 1.5 kJ/mm does not weld faster — it sensitization-heats the HAZ, grows the grain, and in extreme cases pushes the ferrite balance in the weld towards brittle phases. Going below 0.5 kJ/mm produces lack of fusion, which is the other classic stainless defect.

Interpass temperature is the second control. Keep it below 150 °C for 304/316 service and below 100 °C for thin-wall sanitary tube. The standard field discipline is a temperature-indicating crayon on the adjacent unheated pipe, marked before the welder strikes the next bead. Without that crayon, the second bead is started on a weld that is already too hot, and the HAZ on the first bead sensitizes under the second pass.

• Use stringer beads, not weave beads, on austenitic stainless. Weave beads build a wider HAZ and concentrate heat in one spot.
• Grind, not brush, between passes. A stainless wire brush is often contaminated with carbon steel from previous work; that contamination is what becomes rust spots on a hygienic line.
• Grinding discs must be dedicated to stainless. A disc that has been used on carbon steel leaves iron embedment in the root face — a known root cause of rust staining and pitting in chloride service.
• Back purge must stay on until the root has cooled. Pulling the purge as soon as the arc stops produces a heavy oxide tint on the inside of the root.

WPS and PQR: How the Procedure Is Actually Qualified

A WPS (Weld Procedure Specification) is the document that the welder works to. The PQR (Procedure Qualification Record) is the test record that proves the WPS can make an acceptable joint. For stainless work covered by ASME Section IX, both have to exist before a production weld is struck, and the welder has to be qualified on the WPS within the last six or twelve months depending on the project specification.

In practice, the WPS for a 316L spool on a chloride service tends to be tighter than the WPS for a generic 304 line. Specify the essential variable limits in advance — joint geometry, filler metal classification, shielding gas composition, purge gas flow, heat input range, interpass cap, and preheat (usually 10–15 °C minimum for shop work in cold climates). If the WPS does not include the purge gas as an essential variable, the welder can — and will — vary it from spool to spool.

Welding the Joints Around the Pipe

A stainless steel pipe line is not just pipe. It is pipe, pipe fittings, pipe flanges, and the bolted joints that connect to valves and equipment. Every one of those transitions is a weld that has to be made with the same discipline as the line pipe itself.

Butt-weld fittings (long-radius elbows, equal tees, concentric and eccentric reducers) are the most common transition. They are delivered with B16.25 bevels matched to the pipe schedule, and the WPS that welds the pipe is the same WPS that welds the fitting — chemistry matches, thickness matches, purge procedure matches. The inspection regime is the same: visual + RT or UT, then pickling and passivation of the entire spool once the welds are complete.

Flanged joints are where the discipline tends to break down. A stainless weld-neck flange (ASTM A182 F316L) is welded into the line with the same filler and purge as the pipe, but the flange face itself must be protected from arc strikes and from grinding sparks. A single carbon-steel grind mark on a 316L flange face becomes a rust spot at the next inspection, and in hygienic or pharmaceutical service that rust spot is a deviation report. Keep a stainless-only grinding disc on the flange side, mask the face with a plastic cover during fit-up, and remove the cover only when the bolts go in.

Post-Weld Treatment: Pickling, Passivation, and What the Inspector Checks

Stainless welds leave the heat-tinted layer (oxidation of chromium) that the previous sections of this guide have tried to prevent. When prevention is incomplete — and it usually is on a real site — the post-weld treatment fixes it. Pickling paste or a pickling bath removes the chromium-depleted layer and the underlying scale. Passivation rebuilds the chromium-oxide passive layer that gives the alloy its corrosion resistance.

The standard references are ASTM A380 (cleaning, descaling and passivation) and ASTM A967 (chemical passivation treatments). On a sanitary line, A270 with a Ra ≤ 0.8 µm internal finish, the post-weld passivation is followed by a mechanical or electropolish step. The buyer should require:

• Pickling of all field welds and any shop welds where heat tint is visible. Pickling paste, dwell time per supplier data sheet, then thorough water rinse.
• Passivation certificate citing the chemistry used (typically nitric acid per A967 or citric acid for greener passivation), the temperature, and the duration.
• Visual acceptance: no heat tint, no weld discoloration, no free iron contamination at goods-in or at site handover.
• For 316L service in chloride-bearing fluids, a final water-of-elution test on the inside surface to confirm passivation effectiveness.

A weld that looks dirty has not been pickled correctly, regardless of what the MTC says. The cost of rejecting and re-treating at site is several times the cost of getting it right at the bench.

Five Weld Defects a Buyer Can Spot from Across the Site

Most of the disputes between a piping contractor and an inspector happen on a weld that one party said was acceptable and the other said was not. The list of issues a non-welder can still recognize is short and useful:

1. Heavy heat tint on the root face

Straw, blue, or grey colour on the inside of a root pass means oxygen was present during welding. The joint is technically passable on tensile and bend tests, but the chromium-depleted layer is the first place to fail in service. The fix is re-pickling at minimum, re-cut and re-weld at worst.

2. Undercut at the cap

A groove at the toe of the weld where the base metal has been melted away. Undercut is a stress concentrator and is a hard reject on most project specifications. The cause is usually excessive current or too-fast travel — both correctable in the WPS.

3. Porosity clustered at the start or stop

Random scattered porosity at the cap is usually cosmetic. Clustered porosity at the start crater is a lack of fusion / lack of penetration. Clustered porosity fails RT and is the first thing the radiographic interpreter flags.

4. Rust spots appearing within 48 hours of passivation

Almost always free-iron contamination. The grinder used on the line was previously used on carbon steel. Strip the affected area, re-grind with a dedicated stainless disc, and re-passivate.

5. Distortion beyond the spool drawing tolerance

Austenitic stainless has high thermal expansion and low thermal conductivity, so it distorts more than carbon steel under the same heat input. The standard fix is balanced welding (skip or backstep technique) and copper backing bars. A spool that comes off the bench visibly out of square is a fabrication-planning problem, not a welder problem.

A Short Welding Checklist for the Next Stainless RFQ

1. Lock the base pipe grade (TP304/L, TP316/L, TP321/H, duplex) and the matching AWS filler on the PO. Reject "equivalent" filler substitutions at quotation stage.
2. Require a WPS qualified to ASME Section IX, with heat input range, interpass cap, and purge gas as essential variables.
3. Specify argon back-purge, dew point < -40 °C, oxygen < 50 ppm at the root, and pre-purge volume based on pipe diameter.
4. Require dedicated stainless grinding discs and stainless-only wire brushes, with a sign-off log at the fabrication shop.
5. Require pickling and passivation per ASTM A380 / A967 on every shop and field weld, with certificate.
6. Require the inspection scope (RT, UT, PT, MT, ferrite count for duplex) on the PO, not in the email.
7. Match the alloy system across the line — same 316L pipe, same 316L fittings, same F316L flanges, same trim on the connected valves.
8. Order the gasket, stud bolt and nut set with the spool, so the bolted joint does not arrive from a different shipment.