Multi-Pass Submerged Arc Weld Overlay: Microstructure and Performance Research
1. Definition and Fundamental Principles
Multi-pass submerged arc weld overlay (SAWO) is a specialized cladding technique in which multiple layers of alloy filler metal are deposited sequentially onto a base substrate using the submerged arc welding (SAW) process, with the arc shielded by a flux blanket. Unlike single-pass overlay methods, multi-pass SAWO involves the controlled deposition of transition layers, build-up layers, and final cap layers, each with potentially different filler metal compositions to manage dilution, metallurgical compatibility, and mechanical performance.
The fundamental principle governing multi-pass SAWO microstructure development is the interplay between thermal cycling, solidification rate, and alloy segregation. During multi-pass deposition, each subsequent pass is deposited onto a partially reheat-affected previous layer, creating a complex thermal history that directly influences grain morphology, phase distribution, and hardness profiles. The submerged arc process, characterized by high deposition rates (typically 20–60 kg/h), deep arc penetration, and relatively low cooling rates compared to GMAW or GTAW, produces columnar dendritic microstructures in the weld metal with potential for coarse grain formation if interpass temperature is not controlled.
The research study referenced in this entry focuses on systematic investigation of how the number of passes, interpass temperature, welding parameters, and filler metal selection collectively influence the final microstructural characteristics and mechanical properties of the multi-pass overlay. This knowledge base is critical for WPS development, procedure qualification, and ensuring reliable cladding performance in demanding industrial applications.
2. Category and Business Positioning
Within the technology portfolio of Cladding Technology Shanxi Co., Ltd., multi-pass submerged arc weld overlay occupies a distinct position in the weld overlay family of cladding technologies. The company's three primary technology routes are:
- TIG/MIG Weld Overlay — Precision-controlled processes suitable for thin overlays, complex geometries, and high-purity requirements
- Hydraulic Explosive Bonding — Solid-state joining producing metallurgical bonds without melting, ideal for thick cladding and dissimilar metal combinations
- Explosion Welding — High-velocity impact bonding for large-area cladding with excellent interface integrity
Multi-pass SAWO serves as a complementary and often primary route for heavy-duty overlay applications where deposition thickness (typically 3–25 mm), production throughput, and cost-efficiency are paramount. The research study on microstructure and performance provides the scientific foundation that bridges between process execution and engineering qualification, enabling the company to:
- Develop qualified welding procedure specifications (WPS) with confidence in microstructural outcomes
- Predict and control dilution across multiple pass sequences
- Optimize interpass temperature regimes for specific alloy systems
- Establish acceptance criteria grounded in metallurgical understanding rather than empirical trial-and-error
3. Technical Purpose and Value
The systematic research into multi-pass SAWO microstructure and performance serves several critical technical purposes:
3.1 Dilution Control and Gradient Management
In multi-pass overlay, dilution from the base metal into the overlay is a primary concern. The first pass typically experiences 20–40% dilution depending on the welding parameters and base material, while subsequent passes see progressively lower dilution (5–15%) as the preceding layer acts as a buffer. Understanding this dilution gradient allows engineers to:
- Select appropriate transition layer compositions (e.g., using 309L between carbon steel and 316L cap)
- Predict final overlay composition and properties
- Design pass sequences that achieve target hardness and corrosion resistance
- Minimize risk of dilution-induced property degradation in the first pass
3.2 Microstructural Evolution Across Passes
The thermal history of multi-pass deposition creates distinct microstructural zones:
- Weld metal zone (WM): Columnar dendritic structure with potential equiaxed grain formation in center of thick deposits
- Heat-affected zone (HAZ) of base metal: Grain growth, carbide precipitation, and potential softening or hardening depending on base composition
- Intercritical zone between passes: Partial melting and re-solidification creating mixed microstructural features
3.3 Performance Prediction and Optimization
The research enables quantitative prediction of key performance parameters including hardness distribution, tensile strength, impact toughness, corrosion resistance, and thermal fatigue resistance — all of which determine whether the overlay meets application-specific requirements.
4. Key Process and Implementation Points
4.1 Welding Parameter Optimization
| Parameter | Typical Range | Effect on Microstructure | Effect on Performance |
|---|---|---|---|
| Welding Current | 300–700 A | Higher current → deeper penetration → increased dilution → coarser grains | Higher current → lower hardness in overlay due to dilution |
| Travel Speed | 200–600 mm/min | Faster speed → thinner weld bead → faster cooling → finer grains | Faster speed → higher hardness but potential lack of fusion |
| Wire Feed Speed | 150–400 mm/min | Higher feed → thicker deposit per pass → slower cooling | Affects deposit thickness uniformity and porosity rate |
| Interpass Temperature | 100–300°C (material dependent) | Higher IT → slower cooling → coarser grains → reduced hardness | Lower IT → higher residual stress → potential cracking |
| Flux Type | Rutile / Basic / Agglomerated | Affects arc stability, slag composition, and weld metal chemistry | Basic fluxes → lower S/P → better toughness |
| Number of Passes | 2–6 passes typical | More passes → more thermal cycles → potential grain coarsening in lower layers | More passes → better dilution control in cap layer |
4.2 Pass Sequence Design
The layer-by-layer design of multi-pass overlay follows established metallurgical principles:
- Base preparation: Machining to remove surface contaminants, ensuring flatness within 0.5 mm/m, and preheating if required by material specification
- Transition layer (Pass 1): Filler metal selected for metallurgical compatibility between base and overlay (e.g., 309L for austenitic stainless on carbon steel)
- Build-up layers (Passes 2–n-1): Target alloy composition, with dilution effects diminishing with each successive pass
- Cap layer (Final pass): May use a different filler metal to achieve specific surface properties (hardness, wear resistance, corrosion resistance)
4.3 Microstructural Characterization Methods
The research study employs standard metallurgical characterization techniques to validate process outcomes:
- Optical microscopy (OM): Grain structure, phase distribution, inclusion morphology
- Scanning electron microscopy (SEM): Fine-scale microstructural features, crack initiation sites
- X-ray diffraction (XRD): Phase identification (ferrite/austenite ratio, carbide types)
- Energy dispersive spectroscopy (EDS): Elemental mapping, dilution quantification
- Hardness testing: Vickers hardness profiles across overlay thickness and into base metal
- Chemical analysis: Spectrometric analysis of overlay composition at different depths
4.4 Performance Testing Protocol
| Test Property | Standard Method | Typical Acceptance Criteria | Pass/Transitional Layer Variation |
|---|---|---|---|
| Hardness | ASTM E92 (Vickers) | Per specification (e.g., 350–450 HV for overlay) | First pass typically lower due to dilution; cap layer meets target |
| Tensile Strength | ASTM E8/E8M | ≥ Base metal tensile strength | Overlay tensile ≥ 90% of specified minimum |
| Impact Toughness | ASTM E23 (Charpy V-notch) | ≥ 27 J at -20°C (typical) | May decrease in first pass HAZ due to grain growth |
| Ferrite Content | ASTM E1255 / ASTM E1925 | 5–35 FN (for duplex/austenitic) | Affects cracking resistance during welding |
| Corrosion Resistance | ASTM G48 (pitting) / ASTM G59 (intergranular) | No corrosion in specified medium | Cap layer must meet full requirement; transition layer may be exempt |
| Interpass Cracking | Visual / Magnetic Particle (ASTM E709) | No cracks > 0.5 mm | Controlled via interpass temperature and preheat |
5. Applicable Standards and Acceptance Criteria
5.1 Welding Procedure Standards
- ASME Section IX, Part Q: Qualification of welding procedures and welders for pressure equipment — governs WPS/PQR development for multi-pass overlay on pressure vessels and piping
- ASME Section II, Part D: Specifications for welding consumables — filler metal qualification
- GB/T 985.1-2008: Welding procedure qualification test methods — Chinese national standard for WPS qualification
- NB/T 47014-2011: Procedure qualification of welding for pressure vessels — Chinese industry standard specific to pressure equipment
- ISO 15614-1:2017: Qualification testing of welding procedures for metallic materials — international standard for WPS qualification
5.2 Inspection and Acceptance Standards
- ASTM E709-2016: Magnetic particle testing method for welds — surface defect detection
- ASTM E165-2018: Penetrant testing method — surface-breaking defect detection
- ASTM E230/E230M-2018: Radiographic testing of welds — volumetric defect detection
- ASTM E1444-2019: Ultrasonic testing of welds — internal defect detection
- ASME Section V: Nondestructive examination methods and acceptance criteria
- GB/T 3323-2005: Radiographic testing of welds — Chinese standard
- GB/T 11345-2013: Ultrasonic testing of welds — Chinese standard
5.3 Material and Performance Standards
- ASTM A517/A517M: Submerged-arc welding electrodes — electrode specifications
- ASTM A555/A555M: Submerged-arc welding fluxes — flux specifications
- ASTM A240/A240M: Chromium and chromium-nickel stainless steel plates — overlay material specifications
- NACE MR0175/ISO 15156: Materials for H₂S environments — for oil and gas applications
- ASTM G48-2019: Pitting and crevice corrosion testing of stainless steels
- ASTM G59-2016: Intergranular corrosion testing of austenitic stainless steels
5.4 Acceptance Criteria Framework
Acceptance of multi-pass SAWO overlays follows a tiered approach:
- Visual inspection: Surface quality, bead profile, no undercut > 0.5 mm, no surface porosity
- NDT: Volume of defects per ASME Section V or applicable code; no continuous linear indications
- Dimensional verification: Overlay thickness within ±0.5 mm of specified minimum; flatness per drawing
- Metallurgical evaluation: Dilution within specified limits; microstructure consistent with WPS expectations
- Mechanical testing: Hardness, tensile, and impact properties meeting specification requirements
- Corrosion testing: Where required by service conditions, immersion or potentiodynamic testing
6. Common Risks and Controls
6.1 Cracking Risks
| Cracking Type | Cause | Control Measures |
|---|---|---|
| Hot cracking (solidification) | Low ferrite content in austenitic overlay; high sulfur/phosphorus; excessive restraint | Control ferrite content (5–35 FN); use low-sulfur consumables; minimize restraint; proper preheat |
| Cold cracking (hydrogen-induced) | High hydrogen content; high carbon equivalent base metal; rapid cooling | Use low-hydrogen flux; preheat and control interpass temperature; post-weld heat treatment; limit base metal CE |
| Reheat cracking | High-strength low-alloy steels; sensitized microstructure; thermal stress during PWHT | Control PWHT heating/cooling rates; avoid sensitizing temperature range; proper material selection |
| Lamellar tearing | Thick plates with unfavorable inclusion alignment; high transverse restraint | Use clean steel (low S); control plate rolling direction; minimize restraint; proper groove design |
6.2 Dilution-Related Risks
- Insufficient dilution control in first pass: Results in overlay composition outside specification — controlled by using transition layer with appropriate alloy composition
- Over-dilution of overlay properties: Hardness or corrosion resistance below minimum — controlled by reducing penetration depth (lower current, faster travel speed)
- Under-dilution causing brittleness: Pure overlay alloy without base metal mixing may be too brittle — controlled by maintaining minimum dilution
6.3 Porosity and Inclusion Risks
- Gas porosity: Caused by flux moisture, base metal contamination, or atmospheric contamination — controlled by flux drying, surface preparation, and proper shielding
- Solid inclusions: Slag inclusions from improper flux coverage or overlapping passes — controlled by proper technique and interpass cleaning
- Oxide inclusions: From oxidized wire or base metal — controlled by proper flux chemistry and surface preparation
6.4 Residual Stress Management
Multi-pass deposition accumulates residual stresses that can affect dimensional stability, fatigue life, and stress corrosion cracking susceptibility. Controls include:
- Optimized pass sequencing (balanced deposition patterns)
- Post-weld stress relief heat treatment (typically 550–650°C for 1–2 hours per 25 mm thickness)
- Interpass temperature control to manage thermal gradients
- Peening of cap layer where permitted by specification
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Integration
Multi-pass SAWO research findings directly inform TIG/MIG overlay procedures developed by the company. Key integration points include:
- Dilution prediction models: Developed from SAWO research, adapted for GMAW/GTAW with different heat input characteristics
- Transition layer design: Alloy selection principles validated in SAWO research applied to precision TIG/MIG transition layers
- Microstructural expectations: Understanding of grain structure evolution guides acceptance criteria for both processes
- Multi-pass sequencing: For thick overlays where TIG/MIG is used, the same pass-by-pass dilution management applies
Typical applications where TIG/MIG overlay benefits from SAWO research insights include: nuclear-grade piping cladding (ASME Section III), thin-wall vessel overlays, repair cladding in service, and high-purity stainless steel overlays where contamination control is critical.
7.2 Hydraulic Explosive Bonding Integration
While hydraulic explosive bonding (HEB) operates on fundamentally different principles (solid-state joining without melting), the multi-pass SAWO research contributes to the technology portfolio in several ways:
- Interface characterization: Metallurgical evaluation techniques developed for SAWO overlays are applied to characterize HEB bond interfaces
- Performance benchmarking: Mechanical and corrosion properties of HEB bonds are compared against welded overlay benchmarks
- Hybrid process development: Some applications combine HEB bonding with weld overlay for thick cladding packages where explosive welding provides the bulk bond and welding provides surface finishing
- Material compatibility data: Research on alloy combinations in SAWO informs material pair selection for HEB
Applications where HEB and SAWO complement each other include: thick cladding (10–50 mm) where HEB provides the base bond and SAWO provides surface finish; dissimilar metal combinations where explosive bonding avoids dilution entirely; and large-area cladding where HEB is more economical.
7.3 Explosion Welding Integration
Explosion welding and multi-pass SAWO represent two ends of the cladding technology spectrum, and the research on SAWO microstructure provides critical context for the company's explosion welding operations:
- Wavy interface analysis: The distinctive wavy interface produced by explosion welding is characterized using the same SEM/EDS techniques developed for SAWO research
- Post-explosion weld overlay: Some explosion-welded components require additional weld overlay for surface preparation or repair — SAWO expertise is directly applicable
- Performance comparison: Understanding SAWO microstructure and properties provides baseline data against which explosion welding performance is evaluated
- Process selection criteria: The research helps establish when explosion welding is preferred over multi-pass welding based on thickness requirements, dilution sensitivity, and interface quality needs
7.4 Technology Selection Matrix
| Application Requirement | TIG/MIG Overlay | Multi-Pass SAWO | Hydraulic Explosive Bonding | Explosion Welding |
|---|---|---|---|---|
| Overlay thickness | 0.5–5 mm | 3–25 mm | 1–30 mm | 1–10 mm |
| Dilution control | Low (5–15%) | Variable (10–40% first pass) | Zero | Zero |
| Geometry flexibility | High (complex shapes) | Medium (flat/large surfaces) | Medium (flat/curved) | Medium (flat/curved) |
| Production rate | Low-Medium | High | Medium | Medium |
| Cost efficiency | Low volume/high value | High volume/medium cost | Medium volume | Medium-high volume |
| Interface quality | Fusion bond | Fusion bond | Mechanical interlock | Mechanical interlock |
8. Contribution to Qualification Building and Customer Value
8.1 WPS Qualification Support
The microstructure and performance research directly supports the company's welding procedure qualification program by:
- Providing scientific basis for WPS parameter selection rather than purely empirical approaches
- Enabling prediction of overlay properties before full-scale qualification testing
- Reducing the number of trial welds needed for PQR development
- Supporting qualification to multiple codes (ASME, NB, GB) with unified process understanding
- Establishing traceability between process parameters and final product performance
8.2 Product Delivery Reliability
The research translates into tangible delivery benefits:
- First-time-right production: Reduced rework rates through improved process control
- Consistent quality: Standardized procedures based on metallurgical understanding
- Reduced inspection burden: Confidence in process capability reduces need for 100% destructive testing
- Accelerated delivery: Optimized parameters maximize deposition rate while maintaining quality
- Scalability: Process knowledge transfers across production scales and equipment
8.3 Customer Value Proposition
For customers requiring multi-pass overlay cladding, the company's research-backed approach delivers:
- Verified performance: Mechanical and corrosion properties backed by systematic metallurgical research
- Code compliance: Procedures qualified to ASME, NB, GB, and ISO standards
- Technical documentation: Complete traceability from raw materials through process parameters to final product properties
- Problem-solving capability: Ability to diagnose and resolve overlay performance issues in service
- Technology transfer: Capability to train customer personnel and support in-service applications
9. Research Methodology and Knowledge Management
The study notes referenced in this capability entry represent a systematic approach to knowledge acquisition and dissemination within the organization. Key elements of the methodology include:
- Literature review: Comprehensive survey of published research on multi-pass SAWO microstructure, including international journals and industry standards
- Experimental validation: Controlled welding trials with systematic variation of parameters and comprehensive characterization
- Data analysis: Correlation of process parameters with microstructural features and mechanical properties
- Model development: Establishment of predictive relationships for dilution, hardness, and phase composition
- Knowledge documentation: Structured recording of findings for organizational knowledge base and WPS development
- Continuous improvement: Integration of new findings into existing procedures and training programs
10. Conclusion and Forward Outlook
Multi-pass submerged arc weld overlay microstructure and performance research forms a critical knowledge pillar within the company's technology portfolio. The systematic understanding of how process parameters influence microstructural evolution and final product properties enables:
- Confident WPS development across multiple qualification codes
- Predictive process control for consistent product quality
- Informed technology selection across the company's three primary routes
- Technical authority in customer interactions and bid preparation
- Continuous improvement of existing procedures and development of new capabilities
As the company expands into new markets (nuclear, offshore, hydrogen energy, advanced manufacturing), the metallurgical knowledge base developed through this research becomes increasingly valuable as a foundation for innovative process development and competitive differentiation. The integration of SAWO research insights with TIG/MIG precision overlay, hydraulic explosive bonding, and explosion welding creates a comprehensive cladding technology platform capable of addressing the full spectrum of industrial cladding requirements.