Why does duplex stainless require a lower maximum interpass temperature than austenitic grades?

Duplex stainless steels must maintain a balanced two-phase microstructure (roughly 50% austenite and 50% ferrite) to achieve their corrosion resistance and strength. High interpass temperatures slow the cooling rate, giving the weld and HAZ time to form sigma phase and other embrittling intermetallics. Sigma phase forms most rapidly in the 1,300–1,700°F [705–925°C] range. Most lean duplex grades cap interpass temperature at 300°F [150°C]; super duplex grades are stricter, often at 250°F [120°C] or lower as specified by the filler metal manufacturer.

What filler metals are used for welding duplex stainless under AWS D1.6?

Standard duplex (S31803/S32205) is typically welded with ER2209 wire for GTAW and GMAW, or E2209 covered electrodes for SMAW. Super duplex (S32750, S32760) requires higher-alloy fillers such as ER2594 or ER2507, which are overalloyed relative to the base metal to compensate for the dilution effects of welding. Using austenitic 308L or 316L filler on duplex stainless is not acceptable — it will produce weld metal with poor corrosion performance and incorrect phase balance.

Is AWS D1.6 the governing code for duplex stainless structural welding?

AWS D1.6 covers structural stainless steel welding, including duplex grades used in structural applications such as architectural elements, industrial platforms, chemical plant structures, and offshore components. For pressure vessels or process piping, ASME Section IX with appropriate P-Number assignments applies instead. Some projects reference both standards — AWS D1.6 for structural connections and ASME IX for pressure-retaining components. The project specification governs which code applies.

Does duplex stainless require postweld heat treatment?

Generally no — and in fact, PWHT of duplex stainless is typically prohibited or strictly controlled. Standard solution annealing (heating to 1,900–2,100°F [1,040–1,150°C] and water quenching) restores duplex microstructure after welding, but this is rarely practical for fabricated structural assemblies. The alternative is proper interpass temperature control and heat input management during welding, so the as-welded microstructure is acceptable without post-weld treatment. The WPS should explicitly state whether any heat treatment is specified and, if not, confirm that the WPS heat input and interpass limits are sufficient.

Duplex Stainless Steel WPS Under AWS D1.6: What Changes

Duplex stainless requires tighter interpass limits, nitrogen-bearing shielding gas, and phase-balance verification. Here's what your AWS D1.6 WPS and PQR must address.

Duplex stainless steel is increasingly specified for industrial structures, offshore platforms, chemical processing facilities, and architectural elements where the combination of high strength and corrosion resistance justifies its cost premium. Fab shops familiar with austenitic grades — 304, 316, 317 — encounter a different set of welding engineering challenges with duplex. The same processes work, the same AWS D1.6 code governs, but the WPS parameters that produce acceptable welds in austenitic grades will produce out-of-spec duplex welds if applied without adjustment.

The fundamental difference is microstructural. Austenitic stainless weld metal is single-phase; a small amount of delta ferrite is deliberately included to resist hot cracking, and the acceptable ferrite range is broad. Duplex stainless weld metal must be two-phase — roughly equal portions of austenite and ferrite — maintained by controlling heat input, interpass temperature, shielding gas chemistry, and filler metal selection. Stray from those parameters and you either get a weld with too much ferrite (brittle at low temperatures, lower corrosion resistance), too little ferrite (susceptible to hot cracking and stress corrosion cracking), or sigma phase and other intermetallic precipitates that severely reduce both ductility and corrosion performance.

Duplex grades and their weldability differences

Not all duplex stainless is the same, and the WPS requirements vary by sub-type:

Lean duplex (UNS S32101, S32304) — lower alloying than standard duplex, lower cost, easier to weld. Maximum interpass temperature typically 300°F [150°C]. Used in structural applications with moderate corrosion demands.

Standard duplex (UNS S31803, S32205) — the most common grade in structural and process applications. S31803 and S32205 are nearly identical in composition (S32205 has slightly tighter ranges); they share the same filler metals and WPS parameters. Maximum interpass temperature typically 300°F [150°C] per most filler metal manufacturer recommendations.

Super duplex (UNS S32750, S32760) — higher alloy content (chromium, molybdenum, nitrogen), higher PREN (pitting resistance equivalent number). More stringent WPS requirements: lower maximum interpass temperature (often 250°F [120°C] or lower), stricter heat input windows, and typically requires higher-alloy filler metals that are deliberately overalloyed to compensate for dilution. Most demanding to qualify and most sensitive to parameter deviations.

The WPS must identify the specific UNS designation of the base metal, not just "duplex stainless." A WPS qualified on S31803 does not automatically cover S32750 — the base metal change is an essential variable under AWS D1.6.

Essential variables under AWS D1.6 for duplex WPS qualification

AWS D1.6 lists essential variables for stainless steel WPS qualification. For duplex grades, the variables that most commonly trigger requalification in practice:

Base metal specification change. Changing from standard duplex to super duplex, or from one UNS designation to another not covered by the qualified range, requires requalification. The filler metal, shielding gas, and parameter envelope that produces acceptable phase balance in S31803 does not necessarily produce the same result in S32750.

Filler metal classification change. ER2209 to ER2594 is a classification change requiring requalification. Even within the duplex family, filler metals are not interchangeable for qualification purposes.

Shielding gas composition change. The addition or removal of nitrogen from the shielding gas, or a change in nitrogen percentage beyond the qualified range, is an essential variable. Nitrogen content in the shielding gas directly affects austenite reformation in the weld metal and HAZ — this is not a minor process parameter.

Heat input change beyond qualified range. For duplex stainless, heat input has a tighter acceptable window than for austenitic grades. Too low, and there is insufficient time for austenite to reform from the initially ferritic weld metal — weld metal ferrite content exceeds 70%, which compromises toughness and corrosion resistance. Too high, and you risk sigma phase formation and HAZ sensitization. The WPS must define both minimum and maximum heat input, not just a maximum.

For a comparison with how AWS D1.6 manages other stainless welding qualifications versus AWS D1.1 structural steel requirements, see When AWS D1.6 Applies Instead of AWS D1.1 for Stainless Structures.

Nitrogen in shielding gas: why it matters for duplex

This is the most counterintuitive aspect of duplex WPS development for welders and CWIs trained on austenitic stainless or carbon steel. In GTAW of austenitic stainless, argon shielding gas is standard. For duplex stainless GTAW, most WPS procedures specify a mixture such as 98% Ar / 2% N₂ or 97% Ar / 3% N₂.

The reason: nitrogen is an austenite stabilizer. When duplex stainless solidifies from the melt, it first forms ferrite, then partially transforms to austenite as it cools. Nitrogen in the shielding gas enters the weld pool and promotes that austenite reformation. Without nitrogen addition, the weld metal and HAZ can contain ferrite fractions above 70% — acceptable for some applications but outside the target range for duplex that must meet corrosion performance specifications.

For FCAW-G and GMAW, nitrogen is sometimes incorporated in the shielding gas, or the filler metal manufacturer formulates the wire chemistry to compensate without nitrogen addition — check the filler metal manufacturer's WPS guidance before mixing shielding gases. Self-shielded FCAW (FCAW-S) is generally not recommended for duplex stainless.

The WPS must state the shielding gas composition, mixture tolerance, and flow rate. It is not sufficient to specify "argon" for duplex GTAW — the nitrogen percentage is an essential variable.

Rule library based on AWS D1.1:2025; verify against your governing edition. For stainless structural applications, AWS D1.6 governs — confirm the applicable edition with your project specification.

Ferrite measurement and phase balance verification

The PQR for a duplex WPS should include ferrite measurement of the weld metal and HAZ. AWS D1.6 or the project specification may require this; even when not explicitly required, it is standard practice for qualifying a duplex WPS because ferrite content is the primary indicator of phase balance.

Ferrite Number (FN) is measured using a calibrated ferrite scope (magnetic instrument) per AWS A4.2. For duplex stainless, the target FN range in production weld metal is typically 30–70 FN, though the exact range depends on the grade, filler metal, and project specification. A FN reading below 30 suggests excessive austenite (hot cracking risk increases); above 70 suggests insufficient austenite reformation (toughness and corrosion resistance decrease).

Ferrite scope measurement on actual weld coupons during PQR testing confirms that the WPS parameters produce the required phase balance. If the ferrite reading falls outside the acceptable range, the WPS parameters — heat input, shielding gas composition, interpass temperature — must be adjusted and the PQR rerun.

For background on ferrite number measurement and its application to stainless weld procedures, see Ferrite Number in Stainless Steel WPS: AWS D1.6 Requirements.

Interpass temperature monitoring in production

On an austenitic stainless WPS, interpass temperature control typically focuses on a maximum limit (often 350°F [175°C] to limit distortion and sensitization). On a duplex WPS, the maximum interpass limit is both stricter and more consequential — exceeding it risks sigma phase precipitation that cannot be remediated in a fabricated assembly without solution annealing.

In practice, this means longer cooling delays between passes than welders are accustomed to on carbon steel or austenitic stainless. On a thick duplex section (say, 1-in [25 mm] plate), welders may need to pause for 5–10 minutes between passes. The CWI must verify compliance — checking with a contact thermocouple or temperature-indicating crayon at the location specified in the WPS, not just on a convenient flat surface away from the weld zone.

The production parameter log for duplex welds should record the measured interpass temperature for each pass or at defined intervals. This documentation is part of the quality record and may be reviewed during AISC certification audits or client source inspections.

PQR testing requirements for duplex stainless

Beyond the standard AWS D1.6 PQR mechanical tests (transverse tensile, bend tests), duplex stainless qualifications frequently include additional testing specified by the project or owner:

CVN impact testing. Many structural specifications require Charpy V-notch testing of duplex weld metal and HAZ at service temperature. Super duplex grades in cold-service applications may require testing at −40°F [−40°C] or colder. The interpass temperature and heat input limits in the WPS have a direct effect on CVN results — this is one reason the parameter window is tight.

Corrosion testing. For process structures where corrosion performance is critical, the owner may require immersion or electrochemical corrosion testing of the weld coupon. This is not a standard AWS D1.6 requirement but is common in chemical plant and offshore structural specifications.

Ferrite measurement. As described above — typically included in the PQR test record to document that the WPS parameters produce the correct phase balance.

Common WPS deficiencies on duplex stainless projects

The gaps most frequently found in duplex WPS packages during third-party review:

Interpass temperature limit not stated on the WPS. The WPS lists preheat (typically "none" or 50°F minimum ambient) but omits the maximum interpass temperature. Without that limit, the welder has no documented constraint, and the CWI has no basis for rejecting a joint where interpass temperature was exceeded.

Shielding gas specified as "argon" without nitrogen content. The WPS says "Argon shielding gas" — acceptable for austenitic stainless, not for duplex. The nitrogen percentage must be on the WPS.

Filler metal switched from ER2209 to an austenitic 308L or 316L without requalification. This happens when the purchasing department substitutes available material. The change produces a fundamentally different weld microstructure and voids the qualification.

Ferrite measurement omitted from PQR. The PQR records tensile and bend results but does not include ferrite scope readings. When the client's inspector asks for evidence that the WPS produces the correct duplex phase balance, there is no data.

For structural stainless applications that require a fully audit-ready WPS package — with the essential variables, PQR test records, and production parameter log formats in one place — the WPS documentation system at wpswelding.com supports AWS D1.6 stainless procedures alongside the AWS D1.1:2025 library.