When a structural application calls for austenitic stainless steel — architectural cladding, food-processing equipment supports, chemical plant walkways, marine structures — the welding engineer reaches for AWS D1.6 rather than AWS D1.1. The two codes share a family resemblance in structure, but stainless steel introduces metallurgical considerations that change how a WPS is written and how production welding is controlled.
Grade 304 and its molybdenum-bearing cousin 316 are the workhorses of structural stainless applications. This article covers what a WPS for these materials must address, why filler metal selection matters more than it does for carbon steel, and what CWI and QC managers need to watch in production.
Why AWS D1.6, Not D1.1
AWS D1.1 governs carbon and low-alloy steel. AWS D1.6 is the Structural Welding Code for Stainless Steel. When the base material is austenitic stainless, D1.6 is the applicable standard — unless the contract documents specifically reference D1.1, which would be unusual and potentially problematic given the different metallurgical requirements.
The structural stainless code shares D1.1's general framework: prequalified and tested procedure pathways, essential and nonessential variables, mechanical test requirements for PQR coupons, and welder qualification requirements. But the material-specific rules differ in important ways that affect every WPS written for 304 or 316.
For a summary of when to apply D1.6 versus D1.1, see When AWS D1.6 Applies Instead of D1.1 for Stainless Welds.
Base Metal Grouping Under AWS D1.6
AWS D1.6 groups stainless base metals by M-numbers (a classification system analogous to ASME P-numbers). The M-number grouping determines cross-qualification — whether a PQR conducted on one grade qualifies a WPS for a related grade within the same group.
Grade 304 (UNS S30400) and 304L (UNS S30403) fall in the same M-number group, so a PQR conducted on either qualifies the other within the code's thickness and position ranges. Grade 316 (S31600) and 316L (S31603) are similarly grouped.
Cross-qualification between the 304 group and the 316 group is not automatic. The addition of molybdenum in 316 changes the weld metal chemistry requirements and corrosion performance, and the two groups are treated separately for WPS qualification purposes.
Filler Metal Selection: ER308L and ER316L
The standard matching fillers for structural stainless work are:
- ER308L (AWS A5.9) for welding 304 and 304L
- ER316L (AWS A5.9) for welding 316 and 316L
The "L" designation limits carbon content to 0.03% maximum. This is not a convenience choice — it directly controls sensitization risk. In the temperature range of roughly 800–1500°F (430–815°C), austenitic stainless alloys can precipitate chromium carbides at grain boundaries. This depletes the grain boundary region of chromium and makes it susceptible to intergranular corrosion in service environments that would not affect the base metal. Using L-grade fillers reduces the available carbon and suppresses carbide formation during welding thermal cycles.
For most structural applications — architectural, light industrial, non-corrosive environments — sensitization is a secondary concern and standard ER308 without the L designation could be used. But L-grade filler is the default in most structural WPS specifications because the cost difference is minimal and the margin against sensitization is worth preserving.
ER309L is used for dissimilar metal welds — joining 304/316 stainless to carbon steel. The higher alloy content (24–26% Cr, 12–14% Ni) compensates for dilution from the carbon steel side and maintains austenitic weld metal composition. Writing a WPS for dissimilar base metals under AWS D1.6 requires careful attention to essential variable changes related to the A-number (chemical analysis grouping for weld metal).
Interpass Temperature: A More Critical Control Than Carbon Steel
For carbon steel welding under AWS D1.1, preheat is typically the more critical thermal parameter. For austenitic stainless, the interpass temperature limit is the dominant concern, and for opposite reasons.
Austenitic stainless steel has roughly 50% higher thermal expansion than carbon steel and significantly lower thermal conductivity. Heat builds up in the workpiece quickly during multi-pass welding, and dissipates slowly. This combination means:
- Distortion accumulates faster and is harder to correct after the fact
- Extended time in the sensitization temperature range increases chromium carbide precipitation risk
- Hot cracking susceptibility increases at elevated temperatures due to the austenitic microstructure
Standard industry practice, and what most WPS documents for 304/316 structural welding specify, limits interpass temperature to 350°F (175°C). Some specifications are more stringent — 250°F (120°C) — on precision or corrosion-critical applications.
Interpass temperature is measured with contact thermometers or temperature-indicating crayons at the weld joint, not far out in the base metal. The CWI's pre-pass check should confirm the joint has cooled to within range before the next pass begins. Production welders on stainless work often rotate between joints or use compressed air cooling (for non-sensitization-critical applications) to maintain interpass compliance.
Ferrite Number: Weld Metal Microstructure Control
One parameter that appears on stainless WPS documents that carbon steel fabricators seldom encounter is the ferrite number (FN). Weld metal in austenitic stainless steel is not fully austenitic — a small proportion of delta ferrite is intentionally maintained in the weld microstructure because it suppresses hot cracking.
Fully austenitic weld metal is crack-sensitive. Delta ferrite, present at 3–10 FN in the weld metal, interrupts the continuous austenite grain structure and reduces the hot crack susceptibility associated with impurity segregation at grain boundaries.
The AWS A5.9 filler metal classification for ER308L targets a ferrite number in the range of approximately 5–10 FN under standard welding conditions. The exact FN achieved depends on the dilution rate, which is why process and parameter changes are controlled as essential variables — different dilution changes the weld metal ferrite content.
For applications where sensitization or ferrite content is formally verified, a weld metal chemistry analysis or magnetic feritscope measurement can be performed on the PQR test coupon. This is more common in pressure vessel and piping work under ASME Section IX than in structural work under AWS D1.6, but it applies when owner specifications require it.
Essential Variables Under AWS D1.6
AWS D1.6's essential variable list differs from AWS D1.1:2025 Table 6.6 in ways that reflect stainless metallurgy. Key essential variables for welding 304 and 316 include:
Base metal M-number group: Changing from an M-number group qualified by the PQR (e.g., from 304 group to 316 group) requires a new PQR. The weld metal chemistry, dilution, and corrosion behavior differ.
Filler metal classification: Changing filler metal type (e.g., from ER308L to ER316L, or to ER309L for dissimilar work) is essential. The weld metal chemistry and ferrite number are directly tied to filler classification.
Current type and polarity: DCEP vs. DCEN vs. AC produces different arc energy distributions and penetration profiles. As with AWS D1.1, this is essential.
Shielding gas composition: For GTAW and GMAW, shielding gas composition is essential. Stainless steel is sensitive to contamination from oxygen and nitrogen in the shielding gas, and the gas mix affects arc stability, weld bead appearance, and oxidation at the weld surface.
Heat input range: While not always framed as a standalone essential variable, changes in heat input that alter the thermal cycle significantly — particularly extending time in the sensitization temperature range — can affect qualification validity. On corrosion-critical applications, heat input is often logged and controlled.
Welding Process Selection for 304/316 Structural Work
GTAW (TIG): The preferred process for root passes on CJP groove welds in stainless piping and tube connections. Provides precise heat control, no slag, and excellent fusion on thin material. Travel speed is slower than GMAW, making heat input and distortion management more demanding.
GMAW (MIG): Practical for structural welds on plate and structural sections. Spray transfer mode is common for stainless, producing clean, high-deposition welds. Short-circuit transfer introduces more spatter and is less common on austenitic stainless. Shielding gas is typically 98% Ar / 2% O₂ or 98% Ar / 2% CO₂ — pure CO₂ is not used for stainless.
SMAW: Used in field repair and maintenance welding on stainless where gas shielding is impractical. Electrode classifications E308L-15/16 and E316L-15/16 per AWS A5.4 are the matching rods. SMAW on stainless is slower, more prone to slag inclusions, and produces more distortion than GMAW or GTAW.
FCAW-G: Flux-cored wires with gas shielding are available for stainless in the E308LT and E316LT classifications per AWS A5.22. Deposition rate is higher than SMAW and competitive with GMAW, making it attractive for production structural fabrication.
Production Controls That Matter
Beyond the WPS, production welding on 304 and 316 structural components requires:
- Segregation from carbon steel: Stainless steel must be handled, stored, and fabricated separately from carbon steel to prevent iron contamination. Iron particles embedded in the stainless surface cause rust staining and pitting in service. Dedicated stainless grinding wheels, brushes, and handling equipment are standard.
- Back purge for root passes: On structural tube and pipe connections, back purging the root side with argon prevents oxidation of the root weld. Root oxidation ("sugaring") reduces corrosion resistance and is a common rejection cause on stainless pipe welds.
- Post-weld passivation: AWS D1.6 does not require it, but many owner specifications call for chemical passivation of completed welds to restore the passive chromium oxide layer that the welding thermal cycle may have disrupted.
For WPS documentation tools that cover AWS D1.6 stainless in addition to AWS D1.1:2025 carbon steel procedures, see WeldingWPS.com pricing. For comparison of duplex stainless welding requirements, see Duplex Stainless Steel Weld Procedures Under AWS D1.6.
Rule library based on AWS D1.1:2025. AWS D1.6 governs structural stainless steel welding — verify your governing edition against the contract documents and applicable AHJ.