Which code governs a weld joining A36 carbon steel to 304L stainless steel?

AWS D1.6 typically governs the joint because the stainless member is involved. The engineer of record specifies the applicable code, but when austenitic stainless is one of the base metals, AWS D1.6 is the appropriate specification. Your WPS should reference AWS D1.6 and note the dissimilar base metal combination.

What filler metal should I use for a carbon steel to 304L stainless joint?

AWS ER309L or ER309LSi is the standard choice. The 309L composition bridges the alloy gap: its higher chromium and nickel content compensates for dilution from the carbon steel side and maintains austenitic microstructure in the weld metal. For 316L stainless base, use ER309LMo to match the molybdenum content.

Do I need to preheat a carbon steel to stainless transition weld?

Preheat requirements come from the carbon steel side. If the carbon steel base metal requires preheat under AWS D1.1 (based on carbon equivalent and thickness), that minimum preheat still applies to the weld. The stainless side of the joint does not add a preheat requirement, but you must not exceed an interpass temperature of about 350°F (175°C) to avoid sensitization of the stainless heat-affected zone.

Can I post-weld heat treat a carbon-to-stainless dissimilar joint?

Generally no. PWHT that relieves stress in carbon steel (typically 1100–1200°F / 595–650°C) falls within the sensitization range for austenitic stainless (800–1500°F / 425–815°C), causing chromium carbide precipitation at grain boundaries and reducing corrosion resistance. Unless the stainless is a stabilized grade (321 or 347) or low-carbon L-grade with very short soak times, avoid post-weld heat treatment and use low-hydrogen procedures instead.

Carbon Steel to Stainless Steel Transition Joints: WPS Requirements

How to qualify and document WPS for carbon steel to austenitic stainless steel transition joints under AWS D1.6 and AWS D1.1, including filler metal selection, preheat, and PWHT restrictions.

Welding carbon steel structural members to austenitic stainless steel is common in water treatment plants, food processing facilities, chemical plant structures, and pharmaceutical equipment supports. The weld is at the intersection of two different codes, different material behaviors, and different failure modes. Getting the procedure right — and qualifying it correctly — keeps corrosion, cracking, and rejection at bay.

Which Code Governs?

AWS D1.1 covers carbon and low-alloy structural steel. AWS D1.6 covers austenitic, ferritic, and duplex stainless steels used in structural applications. When a weld joins a carbon steel member to an austenitic stainless member, neither code alone is sufficient.

In practice, the engineer of record designates the governing specification in the contract documents. The standard industry approach is to qualify under AWS D1.6, which contains provisions for dissimilar base metal joints involving stainless steel. Your WPS should explicitly name both base metals, reference AWS D1.6 as the governing code, and note the dissimilar combination on Annex M or equivalent documentation forms.

If the project specification references both codes (e.g., AWS D1.1 for all carbon steel work, AWS D1.6 for stainless or mixed joints), you need separate WPS documents — one for carbon-to-carbon joints under D1.1 and a separate WPS for the carbon-to-stainless joints under D1.6.

Filler Metal Selection

Filler metal choice for carbon-to-stainless transition joints is not intuitive. The instinct to match the stainless base metal — using ER308L for 304L, for example — is wrong. ER308L diluted by the carbon steel base metal produces a weld metal that can hot-crack or form martensite during cooling, depending on the ratio of dilution.

The correct approach uses an overalloyed austenitic filler:

ER309L — standard choice for 304/304L stainless to carbon steel. The 309L composition (23–25% Cr, 12–14% Ni) tolerates dilution from the carbon steel side while maintaining a fully austenitic weld with a small amount of delta ferrite (FN 3–12) to resist hot cracking.
ER309LSi — same as 309L with added silicon for improved arc stability and wetting in GTAW or short-circuit GMAW applications.
ER309LMo — required when the stainless base is 316 or 316L. The molybdenum addition maintains corrosion resistance in the weld metal despite dilution.

For SMAW (stick), the equivalent electrodes are E309L-15 (DCEP, suitable for all positions) and E309L-16 (AC or DCEP). Avoid E309-16 without the "L" designation when the stainless side requires low carbon for service corrosion resistance.

Verify that your filler metal is classified to AWS A5.9 (bare wire and strip) or AWS A5.4 (covered electrodes) and that your WPS references the specific classification. A change in filler classification from ER309L to ER309LMo is an essential variable under AWS D1.6 and triggers requalification.

Preheat: Apply It to the Carbon Steel Side

Preheat requirements in dissimilar joints come from the higher-preheat base metal — in this case, the carbon steel. If your carbon steel requires a minimum preheat per AWS D1.1 Table 5.3 or per calculated carbon equivalent, that preheat applies to the entire joint regardless of the stainless side.

Common combinations and practical considerations:

A36 (≤1 in.) to 304L: Minimum preheat is typically 32°F (0°C) — essentially just ambient, no active preheat required for thin material. Verify against actual carbon equivalent from the MTR.
A572 Gr 50 (1–2 in.) to 316L: Preheat to 150°F (65°C) minimum. Measure at least 3 in. from the joint on the carbon steel side.
A572 Gr 50 (>2 in.) or A36 heavy plate: Carbon equivalent may drive preheat to 200–300°F (95–150°C). Calculate per AWS D1.1 Annex I or use thermite/flame-heating with contact pyrometer verification.

Interpass temperature limit: AWS D1.6 requires a maximum interpass temperature for austenitic stainless to prevent sensitization of the heat-affected zone. The typical limit is 350°F (175°C) for 304L and 316L. Monitor interpass on both sides of the joint between passes. Rule library based on AWS D1.1:2025; verify against your governing edition.

Qualifying the Procedure

Under AWS D1.6, a dissimilar base metal WPS must be qualified by test (PQR) unless the joint meets prequalified status under AWS D1.6 Clause 5. Most carbon-to-stainless combinations are not prequalified and require PQR testing.

The PQR test plate must use the actual base metal combination — A36 to 304L, for example — not a surrogate. Mechanical tests follow AWS D1.6 requirements for the stainless side, which include:

Transverse tensile test — minimum tensile strength must equal the lower of the two base metal minimums. For A36-to-304L, that's typically the A36 side (58 ksi minimum).
Side-bend tests — for plate ≥3/8 in. thick, side bends at 2T bend radius per AWS D1.6. Face and root bends for thinner material.
Fillet weld fracture test — if qualifying fillet welds, nick-break test on the fillet weld macrosection.

Delta ferrite measurement (Ferrite Number) is required by many project specifications for austenitic welds. Measure with a calibrated ferritescope on the completed weld. A FN of 3–12 is the typical acceptable range for 309L on carbon-to-304L joints.

PWHT: Usually Not Acceptable

Post-weld heat treatment (PWHT) that benefits carbon steel — stress relief at 1100–1200°F (595–650°C) — falls squarely in the sensitization range for austenitic stainless steel (800–1500°F / 425–815°C). In this temperature range, chromium carbides precipitate at austenite grain boundaries, depleting the adjacent grain boundary region of chromium below 12% and making it susceptible to intergranular corrosion, especially in wet service environments.

If PWHT is specified by the structural engineer for the carbon steel component and the joint involves stainless, options include:

Use L-grade stainless (304L, 316L) with carbon ≤0.03% and limit PWHT time-at-temperature to minimize chromium carbide precipitation. Consult with a metallurgist on acceptable soak times.
Use stabilized grades (321, 347) which resist sensitization via titanium or niobium stabilizers.
Redesign the joint to allow PWHT of the carbon steel component before the stainless connection is made, with field welding of the stainless afterward (no subsequent PWHT).
Use buttering — apply 309L butter passes on the carbon steel side, complete any PWHT on the carbon steel assembly, then make the final joint-to-stainless weld without further PWHT.

The buttering technique deserves attention for heavily restrained joints. By applying 309L weld butter to the carbon steel faying surface in the shop — before the stainless member is present — the final weld is made between 309L butter and the stainless base metal. The remaining preheat and PWHT requirements at that point apply only to the carbon steel/butter interface, which has already been treated.

Documentation Requirements on the WPS

Your WPS for a carbon-to-stainless joint must explicitly state:

Both base metal designations: e.g., "ASTM A572 Gr 50 to ASTM A240 Type 304L"
AWS D1.6 as governing specification
Filler metal: AWS ER309L, classification per AWS A5.9
Minimum preheat (from carbon steel requirement) and maximum interpass temperature (from stainless requirement — typically 350°F)
No PWHT (or specific PWHT restrictions with justification)
Ferrite Number acceptance range if specified by the project

See the AWS D1.6 stainless structural WPS overview for the broader D1.6 qualification framework, and dissimilar base metal WPS under AWS D1.1 for the code selection logic when the carbon-only code might still apply. The when D1.6 governs instead of D1.1 article covers the jurisdiction decision in more detail.

If you need to track these dissimilar WPS procedures across multiple projects with proper essential variable control, see our pricing page for the WPS management tools available in each plan.

Common Failures in Dissimilar Joint Qualifications

A few patterns show up repeatedly in pre-delivery audits on carbon-to-stainless joints:

Wrong filler metal on the WPS: Using ER308L (correct for 304L-to-304L) on a carbon-to-304L joint. The PQR may have passed mechanical testing, but the delta ferrite content in the weld metal may be marginal and the weld is susceptible to hot cracking on future production work.

Missing interpass temperature limit: The WPS shows the carbon steel preheat minimum but omits the stainless interpass maximum. An inspector monitoring production has no documented limit to enforce.

Missing base metal call-out on MTR: The stainless mill test report must confirm the plate is 304L (not 304H, which has higher carbon). Using 304H in a wet-service application negates any benefit of PWHT avoidance.

Incorrect qualification range for base metal thickness: AWS D1.6 qualification ranges for dissimilar joints follow the stainless side thickness rules in most cases. Confirm that the PQR test plate thickness qualifies the production range you need, especially for thin stainless plate welded to heavy carbon structural sections.

Bringing in a Certified Welding Inspector with D1.6 experience before the PQR test — not after — prevents these issues from showing up in the client's pre-shipment audit.