The Bottom Line Up Front
In the world of welding, AWS D1.1 and AWS D1.6 aren’t just technical documents—they’re trusted guides, each crafted to protect the integrity of different materials and the projects that rely on them.
AWS D1.1 is the structural welding code for carbon and low-alloy steels, covering everything from building frameworks to bridges. AWS D1.6 is specifically designed for structural welding of stainless steel, addressing the unique metallurgical and corrosion-resistance requirements of stainless materials.
Using the wrong code can lead to structural failures, failed inspections, costly rework, and potentially catastrophic safety issues.
Quick Guide: Which Code Should You Use?
| Your Project Involves | Use This Code |
| Carbon steel structures, Low-alloy steel construction | AWS D1.1 |
| Stainless steel fabrication or Corrosive environments requiring | AWS D1.6 |
| Mixed materials (carbon + stainless) | Both codes apply* |
Understanding the Fundamentals
AWS D1.1: The Backbone of Structural Steel Welding
AWS D1.1, officially titled “Structural Welding Code – Steel,” is the most widely used welding standard in North America. Published by the American Welding Society, it provides comprehensive requirements for welding carbon and low-alloy steels in structural applications.
This code covers design, qualification procedures, fabrication, inspection, and acceptance criteria for welded structures ranging from buildings and bridges to offshore platforms.
Key characteristics of AWS D1.1
- Addresses carbon steel and low-alloy steel (typically with carbon content below 0.30%)
- Focuses on structural integrity and load-bearing capacity
- Widely adopted in construction, infrastructure, and manufacturing
- Emphasizes weld strength and ductility
AWS D1.6: The Specialist for Stainless Steel
AWS D1.6, “Structural Welding Code – Stainless Steel,” was developed specifically to address the unique properties and challenges of welding stainless steel structures. While it mirrors D1.1’s organisational structure for easier navigation, it contains significantly different technical requirements that reflect stainless steel’s distinct metallurgical behaviour.
Key characteristics of AWS D1.6
- Covers austenitic, ferritic, duplex, and martensitic stainless steels
- Addresses corrosion resistance as a primary concern
- Includes specific guidance on heat input control to prevent carbide precipitation
- Emphasizes post-weld surface treatment (passivation)
The Critical Distinction: Why Material Matters
This isn’t just an academic distinction. Use D1.1 procedures on stainless steel, and you risk chromium carbide precipitation along grain boundaries, destroying corrosion resistance. Apply D1.6’s restrictive heat input limits to carbon steel unnecessarily, and you’ll create brittle welds prone to hydrogen cracking.
The fundamental difference between these codes stems from the metallurgical properties of their respective base materials:
Carbon Steel (D1.1)
- Higher thermal conductivity (heat dissipates faster)
- May require preheating to prevent cracking
- Lower cost materials
- Susceptible to rust without protective coatings
Stainless Steel (D1.6)
- Lower thermal conductivity (heat concentrates in weld zone)
- Requires careful heat input control to avoid sensitisation
- Significantly higher material and welding costs
- Inherent corrosion resistance (when properly welded)
Five Key Differences That Impact Your Project
- Base Metal Composition Requirements
- Welding Consumable Specifications
- Thermal Control Parameters
- Corrosion Resistance Considerations
- Inspection and Acceptance Criteria
Base Metal Composition Requirements
Misidentifying your base material means applying the wrong welding parameters, leading to either inadequate mechanical properties or compromised corrosion resistance.
D1.1 Approach: Focuses on carbon content and carbon equivalent formulas to assess weldability. The code provides guidance for materials with carbon content typically below 0.30%, with special considerations for higher carbon levels.
D1.6 Approach: Emphasizes chromium and nickel content, which determine the stainless steel classification (austenitic, ferritic, duplex, or martensitic). Each classification has unique welding requirements. The code addresses potential issues like sigma phase formation in duplex stainless and martensite formation in certain grades.
Welding Consumable Specifications
A weld that’s structurally sound but has mismatched corrosion resistance will become the weak point in a stainless steel system, potentially leading to premature failure in service.
D1.1 Specifications: Filler metals are selected primarily for matching tensile strength and impact properties. The code allows for some under-matching in certain applications where joint design compensates.
D1.6 Specifications: Filler metal selection is far more complex, requiring consideration of:
- Matching base metal composition for corrosion resistance
- Ferrite content control in austenitic welds (typically 4-10 FN to prevent hot cracking)
- Potential for galvanic corrosion between weld and base metal
- Operating temperature range of the structure
Thermal Control Parameters
Excessive heat input in stainless steel can cause chromium carbides to precipitate at grain boundaries (sensitization), creating chromium-depleted zones vulnerable to intergranular corrosion.
D1.1 Requirements:
- Preheat: Often required, calculated based on carbon equivalent and thickness (typically 50-400°F/10-204°C)
- Interpass temperature: Maximum limits to prevent grain growth (usually 550°F/288°C maximum)
- Heat input: Generally not restricted unless specified by the engineer
D1.6 Requirements:
- Preheat: Usually NOT required (except for martensitic grades); room temperature start is typical
- Interpass temperature: Strictly controlled maximum limits (often 350°F for austenitic grades)
- Heat input: Maximum limits specified to prevent sensitization and excessive ferrite formation
Corrosion Resistance Considerations
A stainless steel structure that’s properly welded per D1.1 mechanical requirements might still fail catastrophically in corrosive service because the weld zone’s corrosion resistance has been compromised.
D1.1 Focus: Corrosion protection is typically achieved through coatings, painting, or galvanizing after fabrication.
D1.6 Focus: Corrosion resistance is intrinsic to the material and must be preserved through proper welding. The code includes:
- Requirements for protecting the weld area from atmospheric contamination
- Backing gas requirements for full-penetration welds
- Post-weld cleaning and passivation procedures
- Guidance on avoiding carbon steel contamination
Inspection and Acceptance Criteria
A weld that passes D1.1 visual inspection might fail D1.6 requirements due to surface oxidation or heat tint that will compromise corrosion performance, even though structural integrity is adequate.
D1.1 Criteria: Focused primarily on structural integrity:
- Weld size and profile requirements
- Crack prohibition (zero tolerance)
- Porosity limits based on size and distribution
- Undercut and overlap limitations
- Radiographic acceptance standards for internal discontinuities
D1.6 Criteria: Includes all structural requirements(as above) PLUS:
- Surface finish requirements for corrosion resistance
- Tighter restrictions on surface imperfections that could initiate crevice corrosion
- Requirements for removal of heat tint and oxide layers
- Special inspection of backing removed areas
- Verification of proper post-weld cleaning
The Direct Comparison of D1.1 and D1.6
Comprehensive Comparison Table
| Parameter | AWS D1.1 | AWS D1.6 |
| Primary Material | Carbon & low-alloy steel | Stainless steel (all grades) |
| Carbon Content Focus | 0.30% typical maximum | Not the limiting factor |
| Preheating | Often required (50-400°F) | Usually not required |
| Max Interpass Temp | ~550°F typical | ~350°F for austenitic |
| Heat Input Control | Generally unrestricted | Strictly limited |
| Primary Concern | Mechanical strength | Corrosion resistance + strength |
| Filler Metal Selection | Strength matching | Composition + corrosion matching |
| Backing Gas | Not typically required | Required for full penetration |
| Post-Weld Treatment | Optional (stress relief) | Mandatory (cleaning/passivation) |
| Surface Finish | Functional | Critical for corrosion resistance |
| Heat Tint Removal | Not addressed | Required |
| Material Cost | Lower | 3-10x higher |
| Welding Speed | Faster (higher heat input) | Slower (controlled heat input) |
| Inspector Training | D1.1 specific | D1.6 specific + corrosion knowledge |
Process Flow Comparison
D1.1 Typical Workflow:
Material Identification → Preheat Calculation → Joint Preparation → Welding (WPS) → Cooling → Visual Inspection → NDT (if required) → Coating/Painting
D1.6 Typical Workflow:
Material Identification & Grade Verification → Joint Preparation (avoid carbon steel contamination) → Backing Gas Setup → Welding with Heat Input Monitoring → Controlled Cooling → Heat Tint Removal → Surface Cleaning → Visual Inspection → NDT (if required) → Passivation → Final Inspection
Notice: the D1.6 workflow has additional steps specifically addressing corrosion resistance preservation.
Grey Areas and How to Handle Them
Situation 1: Dissimilar Metal Joints
When you need to join carbon steel to stainless steel:
- Both D1.1 and D1.6 apply
- Use stainless steel filler metals (typically 309L or 312 for carbon-to-stainless transitions)
- Follow D1.6 heat input restrictions
- Apply the more stringent inspection requirements
- Consider galvanic corrosion potential and isolate if necessary
Situation 2: Non-Structural Stainless Components
If stainless steel components are non-structural (architectural only):
- D1.6 may not be contractually required
- However, following D1.6 practices ensures corrosion performance
- At minimum, use D1.6 welding techniques and surface finishing
- Document your approach clearly
Situation 3: Overlay Welding
When applying stainless steel overlay to carbon steel for corrosion protection:
- Base material is carbon steel (D1.1 governs structural attachments)
- Overlay is stainless (D1.6 governs overlay technique and quality)
- Typically requires specialized WPS development
- May reference both codes or specialized overlay specifications
Situation 4: Repair Welding
When repairing existing structures:
- Identify original construction code
- Match or exceed original requirements
- If original code version is outdated, use current version but document any conflicts
- For stainless repairs, always follow D1.6 heat input limits regardless of original code
Common Mistakes and How to Avoid Them
Using D1.1 Qualified Welders on Stainless Steel
The mistake: Assuming that welding is welding, and a skilled D1.1 welder can handle stainless steel.
The consequence: Excessive heat input causes sensitization, destroying the corrosion resistance in the heat-affected zone. Within months, the structure shows intergranular corrosion at weld seams.
Prevention strategy:
- Implement strict qualification tracking by code and material
- Provide code-specific training highlighting the differences
- Use visual identifiers (different colored welding caps or badges for D1.1 vs. D1.6 qualified welders)
- Never allow “just this once” exceptions
Neglecting Post-Weld Cleaning and Passivation
The mistake: Treating stainless steel welds like carbon steel, skipping specialized cleaning.
The consequence: Heat tint (chromium-depleted oxide layer) remains on the surface, creating a vulnerable zone for corrosion initiation. The weld area corrodes preferentially, defeating the purpose of using stainless steel.
Prevention strategy:
- Include post-weld cleaning as a mandatory WPS step
- Train welders and inspectors on proper cleaning techniques (pickling pastes, electropolishing, or mechanical methods)
- Verify passivation through water break test or other methods
- Don’t release fabrications from the shop without proper surface treatment
Applying Carbon Steel Preheat Practices to Stainless
The mistake: Preheating stainless steel because “that’s how we always do it” from carbon steel experience.
The consequence: Excessive heat accumulation extends time at sensitization temperature range (800-1500°F), promoting chromium carbide precipitation throughout larger areas of the heat-affected zone.
Prevention strategy:
- Educate welding supervisors on fundamental metallurgical differences
- Post clear guidelines in fabrication areas: “Stainless Steel – NO PREHEAT (unless martensitic grade)”
- Question any preheat recommendation for austenitic or duplex stainless
- When in doubt, consult the WPS or a welding engineer
Mixing Codes Within a Single Project
The mistake: Having some welders work to D1.1 and others to D1.6 on the same structure without clear material segregation.
The consequence: Documentation confusion, potential for wrong procedures on wrong materials, inspection difficulties, and liability issues.
Prevention strategy:
- Clearly mark materials in the shop (paint codes, tags, or dedicated storage areas)
- Color-code WPS documents (blue folders for D1.1, red for D1.6, for example)
- Assign specific welders to specific material types during fabrication
- Implement verification checkpoints before welding begins
Ignoring Heat Input Documentation for D1.6
The mistake: Not recording actual heat input during stainless steel welding, assuming “it looked right.”
The consequence: No way to verify compliance with WPS limits, potential rejection during third-party inspection, and risk of hidden sensitization that will cause field failures.
Prevention strategy:
- Require heat input calculations and recording for all D1.6 welds
- Formula: Heat Input (kJ/in) = (Voltage × Amperage × 60) / (1000 × Travel Speed in in/min)
- Use welding data loggers for critical applications
- Train welders to recognize the “feel” of proper heat input for their process
- Make heat input verification part of in-process inspection
Quick Selection Guiding Checklist
Use this step-by-step decision tool to determine which code applies to your project:
Step 1: Identify Your Base Material
- [ ] Carbon steel or low-alloy steel → Proceed to Step 2 (likely D1.1)
- [ ] Stainless steel (any grade) → D1.6 applies
- [ ] Both materials in the same structure → Both codes apply (see Step 4)
- [ ] Unsure of material type → STOP: Verify material via mill certs or PMI testing before proceeding
Step 2: Confirm Structural Application (for carbon/low-alloy steel)
- [ ] Load-bearing structural application → D1.1 applies
- [ ] Non-structural welding → D1.1 may not be contractually required, but best practice is to follow it
- [ ] Check contract specifications → Does the contract specify AWS D1.1? If yes, it governs.
Step 3: Assess Corrosion Environment
Even if you’ve determined D1.1 applies based on material:
- [ ] Severe corrosive environment → Consider if stainless steel (D1.6) would be more appropriate for long-term performance
- [ ] Moderate environment with coating protection → D1.1 with proper coating system is appropriate
- [ ] Benign environment → D1.1 is cost-effective choice
Step 4: Handle Mixed Material Projects
If your project involves both carbon steel AND stainless steel:
- [ ] Segregate materials clearly in drawings and shop fabrication
- [ ] Assign specific WPS for each material type
- [ ] Verify welder qualifications match the material they’re welding
- [ ] At dissimilar metal joints: Use stainless filler metal and follow D1.6 heat input limits
- [ ] Apply the more stringent requirements when both codes could apply
Step 5: Verify Qualification Requirements
- [ ] For D1.1 work: Confirm welders hold current D1.1 qualifications in the required position and process
- [ ] For D1.6 work: Confirm welders hold current D1.6 qualifications for the specific stainless grade and process
- [ ] Check continuity: Verify no gap > 6 months since last welding in the qualified process
- [ ] Document qualifications: Maintain accessible records for inspector review
Step 6: Confirm Inspection Resources
- [ ] CWI or qualified inspector available who is familiar with the applicable code
- [ ] For D1.6: Inspector understands stainless steel-specific requirements (heat tint, passivation, surface finish)
- [ ] NDT resources appropriate for code requirements (RT, UT, MT, PT)
- [ ] Testing lab access for procedure qualification if developing new WPS
Step 7: Review Documentation Requirements
- [ ] Welding procedures (WPS) are qualified per the applicable code
- [ ] Procedure qualification records (PQR) are available and valid
- [ ] Welder qualification records are current and properly maintained
- [ ] Contract specifications are reviewed and code compliance is clearly understood
- [ ] For D1.6: Post-weld treatment procedures are documented
Step 8: Final Confirmation
- Can you clearly state which code governs each welded joint?
- Yes → Proceed
- No → Review Steps 1-4
- Do you have qualified welders available for the required code?
- Yes → Proceed
- No → Plan for qualification testing
- Are inspection resources adequate for the selected code?
- Yes → Proceed
- No → Arrange for appropriate inspection support
- Do cost estimates reflect the correct code requirements?
- Yes → Proceed
- No → Revise estimates (D1.6 is significantly more expensive)
- Is everyone on the project team clear about which code applies?
- Yes → You’re ready to proceed
- No → Hold a project kickoff meeting to align the team
Our Support is Always Here
Whether your project requires carbon steel or stainless steel welding, selecting the correct AWS standard is essential for ensuring compliance, quality, and long-term reliability.
XDL provides full support for AWS D1.1 and D1.6 procedure development, WPS/PQR qualification, third-party certification, and production welding control. If you need help reviewing your drawings, preparing documentation, or qualifying new procedures, we are ready to assist.
Contact us now: inquiries@xdlmachinery.com
