If you're welding stainless steel and your WPS cites AWS D1.1, the WPS is wrong. AWS D1.1 explicitly scopes itself to carbon and low-alloy steels. Stainless steels — austenitic 300-series (304, 316, etc.), ferritic 400-series, duplex — are covered by AWS D1.6 Structural Welding Code—Stainless Steel.
The WPS templates look similar. The content underneath is materially different.
What D1.6 covers (and D1.1 doesn't)
AWS D1.6 covers:
- Austenitic stainless steels (304, 304L, 316, 316L, 321, 347)
- Ferritic stainless steels (430, 439)
- Martensitic stainless steels (410, 420)
- Duplex stainless steels (2205, 2304, super-duplex)
- Some clad and bimetallic applications
D1.6's essential-variable table parallels D1.1's Table 6.6 but with stainless-specific variables: ferrite content, carbide precipitation control, intermetallic phase considerations on duplex.
What changes on a stainless WPS
Three structural differences from a carbon-steel WPS:
1. Filler selection
Carbon-steel filler matching is by strength — match the base metal's UTS. Stainless filler matching is by chemistry and ferrite content:
- 304L base metal → ER308L filler (slightly higher Cr/Ni to account for dilution)
- 316L base metal → ER316L filler
- 321 base metal → ER347 filler (Nb stabilized to prevent carbide precipitation)
- 2205 duplex → ER2209 filler (matched duplex chemistry)
Cross-matching to a lower grade is generally not allowed.
2. Heat input control
Austenitic stainless has high thermal expansion and low thermal conductivity. High heat input causes:
- Distortion
- Carbide precipitation (sensitization) in HAZ
- Loss of corrosion resistance
D1.6 essential variables tighten the heat-input requalification trigger compared to D1.1. Maximum interpass temperature is also typically lower (often 350°F vs 500°F for carbon steel).
3. Back-purge requirements
For open-root GTAW on stainless pipe, the inside of the joint must be purged with inert gas to prevent oxidation ("sugaring") of the root pass. This is mandatory on most stainless WPSs:
- Purge gas: 100% Argon, 5–15 CFH
- Purge time: until oxygen content drops below 0.5% (verified with an oxygen analyzer)
- Maintained: throughout the root pass
A stainless WPS that doesn't specify back-purge is incomplete.
Common pitfalls when migrating from D1.1 to D1.6
- Citing D1.1 essential-variable tables on a stainless WPS. The tables don't apply. Use D1.6 equivalents.
- Carbon-steel filler on stainless base. A36 + E7018 + 304 base would galvanically corrode and lose stainless properties. Direct fail.
- Heat input from a D1.1 PQR. Doesn't transfer to D1.6 essentials. Requalify under D1.6.
- Omitting back-purge on open-root pipe. Production welds will show sugaring; corrosion resistance gone.
- Treating ferritic and duplex as if they were austenitic. They're not. Each has its own filler matching and heat-input window.
Cross-coverage and dual-qualification
Some shops do both carbon steel under D1.1 and stainless under D1.6. The CWI and the welder qualifications transfer — same AWS certification — but the WPS library is separate. You can't share PQRs across codes.
A few practical recommendations:
- Color-code the WPS binders or folders (D1.1 = blue, D1.6 = green)
- Label-print the code citation prominently on every WPS header
- Train welders on the back-purge discipline before assigning stainless work
- Stock the matching consumables in separate, clearly-labeled bins to prevent grabbing carbon-steel filler for a stainless job
When to qualify under both codes
A few applications need dual qualification — for instance, a carbon-to-stainless transition piece. In those cases:
- Carbon-steel side qualifies under D1.1
- Stainless side qualifies under D1.6
- The dissimilar-metal transition itself usually qualifies under a third reference (like AWS D10.10 or a custom client specification)
It's three procedures, three PQRs, three WPSs.