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What Is AWS D1.6?

AWS D1.6 governs structural welding of stainless steel, covering austenitic (304, 316), ferritic (430), duplex (2205, 2507), and precipitation-hardened (17-4PH) families. The primary welding concern is sensitization and hot cracking, not hydrogen cracking as in carbon steel.

AWS D1.6/D1.6M — Structural Welding Code — Stainless Steel — covers the welding of structural stainless steel components. The current edition is AWS D1.6:2017. It applies to stainless steel members and connections in structures subjected to design stress, including architectural applications, food processing equipment supports, chemical plant structural frameworks, water treatment facilities, and coastal or corrosive-environment structures where carbon steel is unsuitable.

Stainless steel welding is fundamentally different from carbon steel welding because the primary metallurgical concerns are sensitization (chromium carbide precipitation that destroys corrosion resistance), hot cracking (solidification cracking in fully austenitic weld metals), and maintaining the correct phase balance (in duplex grades). These concerns require thermal controls that are opposite in direction to carbon steel — instead of adding heat through preheat, stainless steel welding typically requires limiting heat input and controlling maximum interpass temperature.

The standard covers four major families of stainless steel, each with distinct welding metallurgy and different requirements for filler metal selection, thermal control, and post-weld treatment. The code is organized to address the specific concerns of each family while providing a unified framework for procedure qualification, welder qualification, and inspection.

Stainless Steel Families and Welding Behavior

Each stainless steel family has distinct welding requirements. Austenitic grades (304, 316) resist cracking but are susceptible to sensitization above 800 degrees F. Ferritic grades have limited weldability. Duplex grades require careful heat input control to maintain the austenite-ferrite balance. PH grades need post-weld aging.

Austenitic Stainless Steel (300 Series)

Austenitic grades including 304, 304L, 316, 316L, 321, and 347 are the most common structural stainless steels. They are non-magnetic, have excellent corrosion resistance, and are readily welded. The primary welding concern is sensitization — the precipitation of chromium carbides (Cr23C6) at grain boundaries when the material is held in the temperature range of 800 to 1,500°F (427 to 816°C). Sensitization depletes the chromium content adjacent to grain boundaries below the minimum 10.5% needed for the passive oxide film, creating a narrow zone vulnerable to intergranular corrosion.

The most effective control against sensitization during welding is using low-carbon grades (304L with 0.030% max carbon, 316L with 0.030% max carbon) that have insufficient carbon to form significant carbide precipitation. Stabilized grades (321 with titanium, 347 with niobium) provide alternative carbon control by forming preferential carbides that do not consume chromium. When standard grades (304, 316 with up to 0.08% carbon) must be welded, controlling heat input and interpass temperature becomes critical to minimize time in the sensitization range.

Ferritic Stainless Steel (400 Series)

Ferritic grades including 430, 409, and 439 are magnetic and have moderate corrosion resistance. They are used in structural applications where austenitic grades are too expensive and mild corrosion resistance is sufficient, such as automotive exhaust systems, architectural trim, and interior structural members. Ferritic stainless steels are susceptible to grain growth in the heat-affected zone during welding, which causes significant toughness reduction. Unlike austenitic grades that can be solution-annealed to restore properties, the grain growth in ferritic HAZ is largely irreversible. Low heat input and controlled interpass temperatures help minimize the grain growth zone width.

Duplex Stainless Steel

Duplex grades including 2205 (UNS S31803/S32205) and super duplex 2507 (UNS S32750) contain roughly equal proportions of austenite and ferrite phases. They offer higher strength than austenitic grades (approximately twice the yield strength of 316L) and superior resistance to stress corrosion cracking and pitting corrosion. Welding duplex stainless steel requires careful control of heat input and interpass temperature to maintain the critical phase balance. Excessive heat input promotes ferrite, while insufficient heat input prevents adequate austenite reformation. Duplex fabrication specifications commonly limit interpass temperature to 300°F (150°C) or lower to preserve the approximately 50/50 phase ratio. Note that D1.6 Clause 5 (prequalified WPS provisions) applies only to austenitic stainless steels per Clause 1.4.7 — ferritic, duplex, martensitic, and PH grades require WPS qualification per Clause 6, and their interpass limits are set by the qualified WPS or project specification rather than Clause 5.5.2.

Precipitation-Hardened Stainless Steel

PH grades including 17-4PH (UNS S17400) and 15-5PH (UNS S15500) achieve high strength through age-hardening heat treatments. These grades are used in structural applications requiring both corrosion resistance and high strength, such as aerospace structural components and high-performance architectural elements. Welding PH grades requires matching the heat treatment condition to the welding procedure — welding in the solution-treated condition followed by aging provides the best results. Welding in the aged condition causes overaging in the HAZ with significant strength loss.

Thermal Control in D1.6

D1.6 Clause 5.5.2 limits interpass to 350°F for austenitic stainless (the only grades prequalified under Clause 5 per 5.1). Duplex and ferritic interpass is per the qualified WPS under Clause 6 — project specs commonly limit duplex to 300°F or lower. This is the opposite of D1.1, which specifies minimum preheat. In stainless, excessive heat causes sensitization (chromium carbide precipitation) reducing corrosion resistance.

The thermal control approach in D1.6 is fundamentally different from D1.1. Where D1.1 requires minimum preheat to slow cooling and prevent hydrogen cracking, D1.6 requires maximum interpass temperature limits to prevent sensitization and maintain phase balance. The minimum preheat in D1.6 is simply to remove moisture from the joint surfaces — typically requiring only that the metal be above the dew point, with no specific temperature mandated for most austenitic grades.

For austenitic grades, the maximum interpass temperature is 350°F (175°C). This limit ensures that the cumulative time at sensitization temperatures is minimized across multiple weld passes. In practice, welders must pause between passes and allow the weldment to cool before depositing the next pass. Temperature measurement is typically by contact thermometer or temperature-indicating crayon applied at least 1 inch from the weld toe.

For duplex grades, D1.6 Clause 5 does not apply (Clause 5.1 scopes prequalification to austenitic only). Duplex WPSs require qualification per Clause 6, and interpass temperature is controlled by the qualified WPS and producer recommendations. Project specifications commonly limit duplex interpass to 300°F (150°C) or even 250°F for critical applications. The lower limit reflects the sensitivity of the austenite-ferrite phase balance to cumulative heat exposure. Heat input must also be controlled within a specific band — too low prevents adequate austenite reformation, too high promotes detrimental sigma phase formation.

Distortion Control on Stainless

Stainless steel distorts more aggressively than carbon steel during welding. The thermal expansion coefficient of austenitic grades is higher and the thermal conductivity is lower — heat input does not dissipate from the joint, and the hotter region wants to expand more per degree of temperature rise. The result is that a stainless weldment will pull, twist, and warp through fabrication unless the welding sequence is controlled deliberately. D1.6 codifies this with explicit sequence and distortion-control mandates in Clause 7.

D1.6 §7.7.3 — Distortion Control Program

Per §7.7.3, when shrinkage or distortion is expected to affect the end use of the fabrication, the Contractor shall prepare a welding sequence and distortion control program, and the Engineer shall evaluate it before welding begins. This is mandatory clause-body language, not commentary. For long fabricated members (8 ft and longer), thin sections, or tight-tolerance work, a distortion control program is the default expectation.

Sequencing — Balance the Applied Heat

Per §7.7.2, insofar as practical, all welds shall be made in a sequence that will balance the applied heat of welding while the welding progresses. In practice, this means welds on opposite sides of a joint are alternated rather than completed in one direction; cleats and stiffeners welded around a frame in a star or skip pattern rather than a continuous sweep; and groups of joints especially sensitive to shrinkage are identified on the drawings. Long fillets on stainless plate are typically run as backstepped or skip welds rather than continuous.

Martensitic Exception — Continuous Welding under Restraint

Per §7.7.5, welding of martensitic materials where conditions of severe external shrinkage restraint are present shall be welded continuously to completion, or to a point that ensures freedom from cracking before the joint cools below the minimum preheat and interpass temperatures. This is the opposite of the skip-welding pattern used for austenitic grades — martensitic stainless cracks under restraint if cooled mid-weld.

Peening for Shrinkage Stress (Intermediate Layers Only)

Per §7.18.1, peening may be used on intermediate weld layers for control of shrinkage stresses in thick welds to prevent cracking or distortion. No peening shall be done on the root or surface layer of the weld or the base metal at the edges of the weld. Peening tools must have a minimum 1/8 in [3 mm] radius per §7.18.3, and the Engineer shall specify the required preheat (if any) and interpass temperatures prior to peening per §7.18.4.

Heat Straightening Temperature Limits

Per §7.14, heat straightening of distorted members is permitted with Engineer approval. The clause states that heat straightening temperatures should not exceed 600°F (315°C) for ferritic, martensitic, or duplex stainless steels; 800°F (430°C) for austenitic stainless steels; and the aging temperature for precipitation-hardening stainless steels — advisory language ("should"), not a hard mandatory cap. The Engineer is responsible for evaluating the effect of the heat on corrosion resistance of stainless steels and external stresses of the fabrication before approving heat straightening.

Shop-Floor Practice

For long stainless angles or tight-tolerance members, fabrication shops typically follow three practical disciplines beyond the code requirements: (1) tack heavily and brace the part with cleats every 10 inches before any production weld is run; (2) run a 12-inch sample coupon of the actual joint configuration before committing to a long production weld, to verify the distortion-control sequence works in this specific weldment; (3) push back to engineering on geometry-vs-fabrication trade-offs — a 3/4 inch stainless angle 8 feet long with single-bevel and outside fillet is a fab-shop edge case, and the right answer is sometimes to procure a hot-rolled angle rather than build one from plate.

Filler Metal Selection and Ferrite Control

D1.6 requires matching or overmatching filler metals from AWS A5.9 (ER308L, ER309L, ER316L). Ferrite number (FN) measurement is required to verify adequate ferrite content in austenitic welds — typically FN 3 to FN 10 for crack resistance. Insufficient ferrite increases hot cracking susceptibility.

Filler metal selection in D1.6 must account for matching corrosion resistance, achieving adequate strength, and controlling weld metal microstructure. For austenitic stainless steel, the filler metal typically matches the base metal composition (308L filler for 304L base, 316L filler for 316L base). However, the filler metal must also produce a weld deposit with controlled ferrite content to prevent hot cracking.

Ferrite number (FN) is a critical weld metal property in austenitic stainless steel welding. A small amount of delta ferrite (typically 3 to 10 FN) in the weld metal disrupts the continuous grain boundary network and prevents solidification hot cracking. Fully austenitic weld metals (zero ferrite) are highly susceptible to hot cracking. D1.6 requires the filler metal manufacturer to certify the ferrite number range, and the WPS must specify the required FN range for the application.

For dissimilar metal joints between stainless steel and carbon steel, D1.6 addresses the filler metal compatibility requirements. Typically, a high-alloy filler (309L or 312) is used to bridge the composition difference and ensure adequate corrosion resistance on the stainless steel side. The dilution of carbon steel into the weld pool must be considered when predicting the weld metal composition and ferrite content.

Stainless steel welding demands a qualified welding procedure that addresses sensitization, interpass temperature limits, and alloy-specific shielding requirements. Each procedure requires qualification testing that validates the WPS with mechanical testing and, where specified, corrosion testing for the alloy family. For austenitic grades, stress relief after welding is typically needed only to dissolve precipitated carbides or address stress corrosion cracking — D1.6 Annex G provides detailed PWHT guidance by stainless type.

Surface Cleaning and Heat Tint Acceptance

AWS D1.6 mandates specific surface cleaning rules that are stainless-steel-specific and often misunderstood on the shop floor. The code is simultaneously strict (stainless wire brush only, iron-free abrasive wheels per §7.20) and flexible (heat tint acceptance is Engineer-specified per Commentary C-7.4.3, not a universal threshold).

Mandatory After-Welding Cleanup — §7.20 and §7.20.2

Per §7.20.2, slag shall be completely removed from all finished welds. All welds and adjacent base metals shall be cleaned by brushing or other suitable means after welding is completed. The parent clause §7.20 adds the stainless-domain rules: when brushes are used, brush wires shall be made of stainless steel, and grinding shall be done with iron-free abrasive wheels. Carbon steel brushes and carbon-steel-contaminated grinding wheels are not acceptable.

Heat Tint — Engineer-Specified, Not a Universal Threshold

Per Commentary C-7.4.3, the acceptable level of discoloration (heat tint) from welding or heat treatment should be specified by the Engineer or in contract documents. Heavy levels of weld discoloration indicating poor gas coverage are generally unacceptable, but even light levels may be unacceptable for some applications. The normal stainless steel surface oxide (chromium oxide) does not affect weld quality — only excessive surface oxides or contamination-driven discoloration require attention.

Inspector Failure-Mode Hierarchy

In practice, inspectors evaluating a stainless weld check failure modes in order of severity: (1) penetration and fusion, (2) gas coverage quality (indicated by extreme discoloration), (3) heat tint level against the Engineer's spec, and (4) surface brushing completeness. This hierarchy reflects how experienced CWIs prioritize D1.6 inspection — it is not in the code text. If the Engineer has not specified a heat tint acceptance level, the default is Commentary C-7.4.1's "not adversely affected" language (referenced by C-7.4.3).

Calling a light chromium-oxide tint a "reject" on a CJP joint where the Engineer's spec is silent invokes a threshold D1.6 does not set. Conversely, ignoring heavy blue-purple discoloration that indicates poor gas coverage may hide a root-cause failure.

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For inspection acceptance criteria across defect types, see the visual weld inspection guide. For the carbon steel equivalent, see the AWS D1.1 guide.

How D1.6 Compares to Other AWS Structural Codes

D1.6 governs stainless steel with interpass temperature limits (350°F max for austenitic per Clause 5.5.2; duplex and ferritic per qualified WPS). D1.1 governs carbon steel with minimum preheat requirements. D1.6 requires ferrite number control; D1.1 does not. D1.6 prequalifies austenitic only (Clause 5.1) — all other stainless families require Clause 6 qualification.

D1.6 vs D1.1 (Carbon Steel)

D1.1 governs carbon and low-alloy structural steel where the metallurgical priority is preventing hydrogen-induced cracking through mandatory preheat (Table 5.11, up to 400°F). D1.6 governs stainless steel where the priority is preventing sensitization through controlled maximum interpass temperatures (350°F for austenitic per Clause 5.5.2). D1.6 Clause 5 provides a prequalified WPS path, but only for austenitic grades per Clause 1.4.7 — ferritic, duplex, martensitic, and PH grades require full WPS qualification under Clause 6. Carbon steel welding emphasizes adequate fusion and strength; stainless steel welding must also preserve corrosion resistance, which is the entire reason for using stainless steel.

D1.6 vs D1.2 (Aluminum)

Both D1.2 and D1.6 share the characteristic that preheat must be limited rather than increased. D1.2 limits aluminum preheat to 250°F to prevent strength loss; D1.6 limits austenitic stainless interpass to 350°F per Clause 5.5.2 to prevent sensitization. Both codes address hot cracking (solidification cracking) as a primary concern, though the metallurgical mechanisms differ. D1.6 provides a prequalified WPS path for austenitic grades only (Clause 5, per Clause 1.4.7); D1.2 requires all procedures to be qualified by testing.

Related Standards Guides

• AWS D1.1 — Structural Steel
• AWS D1.2 — Aluminum
• AWS D1.3 — Sheet Steel
• AWS D1.4 — Reinforcing Steel
• AWS D1.8 — Seismic Supplement
• AWS D1.9 — Titanium
• ASME Section IX — Welding Qualification
• API 1104 — Pipeline Welding
• CSA W59 — Welded Steel Construction
• CSA W47.1 — Company Certification
• ASME IX P-Numbers Guide
• ASME IX F-Numbers Guide
• NDE Requirements
• Pipeline Welding Requirements
• D1.1 Preheat Calculator
• D1.4 Rebar Preheat Calculator
• D1.5 Bridge Preheat Calculator
• AWS D1.5 — Bridge Welding Code
• AS/NZS 1554 — Structural Steel Welding (Australian Standard)