AWS D1.6 — Structural Welding Code for Stainless Steel
AWS D1.6 is the structural welding code for stainless steel. It governs procedure qualification, welder testing, fabrication, and inspection for structural stainless steel components including austenitic, ferritic, duplex, and precipitation-hardened grades with strict interpass temperature controls to prevent sensitization and preserve corrosion resistance.
Key distinction: Unlike AWS D1.1 for carbon steel where preheat prevents hydrogen cracking, D1.6 controls maximum interpass temperature to prevent sensitization. For austenitic stainless steels (304, 316), interpass must not exceed 350°F (175°C). Preheat is only required to remove moisture.
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 limits interpass temperature to 350 degrees F maximum for austenitic stainless steel and 300 degrees F for duplex. This is the opposite of D1.1, which specifies minimum preheat. In stainless steel, excessive heat causes sensitization (chromium carbide precipitation) that reduces 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 sets the same 350°F (175°C) maximum interpass, but project specifications commonly restrict it further 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.
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 8 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.
How D1.6 Compares to Other AWS Structural Codes
D1.6 governs stainless steel with interpass temperature limits (350 degrees F max austenitic, 300 degrees F duplex). D1.1 governs carbon steel with minimum preheat requirements. D1.6 requires ferrite number control; D1.1 does not. Both provide prequalified WPS paths, but D1.6 has fewer prequalified options.
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.
| Aspect | D1.6 (Stainless) | D1.1 (Carbon Steel) |
|---|---|---|
| Base metals | Austenitic, ferritic, duplex, PH | Carbon and low-alloy steels |
| Interpass max | 350°F austenitic, 300°F duplex | Not code-limited (WPS-specific) |
| Primary concern | Sensitization, hot cracking | Hydrogen cracking |
| Filler metal | ER308L, ER309L, ER316L (A5.9) | A5.1/A5.18/A5.20 |
| Ferrite control | Required (FN measurement) | Not applicable |
| Prequalified WPS? | Yes (limited) | Yes (Clause 5) |
Related Standards Guides
Frequently Asked Questions
AWS D1.6 requires minimum preheat only to remove moisture from the joint surfaces — there is no mandatory preheat temperature table as exists in D1.1 for carbon steel. The critical thermal control is the maximum interpass temperature. For austenitic stainless steels (304, 316, 321), Clause 5.5.2 sets the maximum interpass at 350 degrees Fahrenheit (175 degrees Celsius). However, Clause 5 applies only to austenitic grades per Clause 1.4.7 — ferritic, duplex, martensitic, and PH grades require qualified WPS procedures per Clause 6, where interpass limits are set by the WPS or project specification. Project specifications for duplex grades commonly restrict interpass to 300 degrees Fahrenheit or lower.
Sensitization is the precipitation of chromium carbides at grain boundaries that occurs when austenitic stainless steel is held in the temperature range of 800 to 1500 degrees Fahrenheit (427 to 816 degrees Celsius) for extended periods. The chromium consumed by carbide formation depletes the chromium content adjacent to the grain boundaries below the 10.5% minimum needed for corrosion resistance, creating a narrow zone susceptible to intergranular corrosion. Controlling interpass temperature, using low-carbon grades (304L, 316L), and minimizing heat input are the primary methods to prevent sensitization during welding.
Austenitic grades (304, 316, 321) are the most common structural stainless steels. They are non-magnetic, have excellent corrosion resistance, and are susceptible to sensitization during welding. Ferritic grades (430, 409) are magnetic, have lower toughness, and are susceptible to grain growth and embrittlement in the heat-affected zone. Duplex grades (2205, 2507) contain roughly equal proportions of austenite and ferrite, providing higher strength and better stress corrosion cracking resistance than austenitic grades. Each family requires different welding parameters, filler metals, and thermal controls.
D1.1 covers carbon and low-alloy structural steel where hydrogen-induced cracking is the primary concern, requiring preheat up to 400 degrees Fahrenheit per Table 5.11. D1.6 covers stainless steel where sensitization, hot cracking, and phase balance are the primary concerns, requiring controlled maximum interpass temperatures rather than minimum preheat. D1.6 Clause 5 provides a prequalified WPS path for austenitic grades only (per Clause 1.4.7) — ferritic, duplex, martensitic, and PH grades require full WPS qualification under Clause 6. D1.6 also addresses ferrite number requirements for weld metal to prevent hot cracking, which has no equivalent in D1.1.
AWS D1.6 permits SMAW (shielded metal arc welding), GMAW (gas metal arc welding), FCAW (flux-cored arc welding), GTAW (gas tungsten arc welding), SAW (submerged arc welding), and plasma arc welding (PAW). GTAW is the most common process for critical stainless steel applications because it provides the lowest heat input and most precise control of the weld pool. GMAW with pulsed spray transfer is used for production applications. SAW is used for heavy sections but requires careful flux selection to avoid chromium depletion.