Internal Wall Weld Overlay Technology for Pressurized Gasifier Vessels
1. Definition and Fundamental Principles
Internal wall weld overlay for pressurized gasifiers refers to the specialized metallurgical process of depositing corrosion-resistant and erosion-resistant alloy layers onto the inner surfaces of pressurized gasification vessels operating under extreme conditions—typically high temperature (1,300–1,600 °C), high pressure (2.0–8.0 MPa), and aggressive chemical environments containing H₂S, CO₂, water vapor, and molten slag. This overlay technology creates a functionally graded barrier between the structural base material (usually low-alloy or carbon steel such as 16MnR, 18MnMoNbR, or P91) and the corrosive gasification medium.
The fundamental principle relies on the dilution control of the overlay alloy into the base metal through precise heat input management, multi-pass layering, and dilution-resistant alloy chemistry. The overlay material—typically austenitic stainless steel grades such as 309, 309L, 310, or duplex 2205—migrates from the base steel through the transition layer to achieve a surface composition that resists sulfidation, carburization, and slag attack while maintaining adequate toughness and fatigue resistance under cyclic thermal loading.
For pressurized gasifiers specifically, the overlay must satisfy ASME Section VIII Division 1 or Division 2 pressure vessel requirements, ensuring that the weld overlay does not compromise the structural integrity of the pressure boundary. The overlay layer thickness is typically 3–6 mm total, applied in multiple passes with controlled interpass temperatures and heat input parameters.
2. Category and Business Positioning
2.1 Technology Classification
This technology falls squarely within the company's TIG/MIG Weld Overlay technology route, specifically under the subcategory of pressurized vessel internal surface cladding. It represents a high-value-added application requiring:
- ASME Section IX qualification of Welding Procedure Specifications (WPS) and Performance Qualification Records (PQR)
- NB (National Supervision of Special Equipment) manufacturing license compliance for pressure vessel fabrication
- Specialized knowledge of gasification process metallurgy and degradation mechanisms
2.2 Business Positioning
Pressurized gasifier overlay represents one of the highest-margin and technically most demanding segments of the company's weld overlay business. Key positioning factors include:
- Market entry barrier: Requires simultaneous qualification under pressure vessel codes (ASME/NB) and overlay-specific standards, creating significant competitive differentiation
- Customer stickiness: Once qualified for a gasifier OEM or EPC contractor, the relationship tends to be long-term due to the critical nature of the component
- Technical leverage: Mastery of pressurized gasifier overlay positions the company for adjacent applications including supercritical boilers, petrochemical reactors, and hydrogen production equipment
- Revenue concentration: Individual gasifier units can require 200–800 kg of overlay material with overlay labor rates significantly exceeding standard welding services
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Corrosion resistance: Achieve overlay surface composition ≥ 25% Cr and ≥ 10% Ni to resist sulfidation and oxidation at operating temperatures
- Erosion resistance: Provide hardness of 200–300 HV for the overlay surface to resist slag droplet impingement and gas particle erosion
- Thermal fatigue resistance: Maintain adequate toughness (CVN ≥ 47 J at service temperature) through the overlay-to-base transition zone
- Pressure boundary integrity: Ensure overlay welds meet full-penetration requirements without introducing defects that could initiate pressure boundary failure
3.2 Quantifiable Value Metrics
| Parameter | Without Overlay | With Proper Overlay | Value Addition |
|---|---|---|---|
| Base material cost | 18MnMoNbR (high alloy) | 16MnR (lower cost) + overlay | 15–25% material cost reduction |
| Service life | 3–5 years | 10–15 years | 2–3× life extension |
| Shutdown frequency | Annual inspection/replacement | 5-year inspection cycle | Reduced downtime costs |
| Replacement cost | Full vessel replacement | Local overlay repair | 60–80% repair cost savings |
4. Key Process and Implementation Points
4.1 Base Material and Overlay Material Selection
| Application Zone | Base Material | Transition Layer | Final Overlay Layer | Total Thickness |
|---|---|---|---|---|
| Gasifier shell (cylindrical) | 18MnMoNbR / P91 | E309L (1 pass) | E310 / E310L (2–3 passes) | 4–6 mm |
| Gasifier head (hemispherical) | 18MnMoNbR / P91 | E309L (1 pass) | E309L + E310 (2–3 passes) | 3–5 mm |
| Nozzle internals | 15CrMo / P11 | E309L (1 pass) | E310 (2 passes) | 3–4 mm |
| Slag tap zone | 18MnMoNbR | E309L (1 pass) | E310 / Hastelloy C-276 (2–3 passes) | 5–8 mm |
4.2 Welding Procedure Parameters
| Parameter | Transition Layer (E309L) | Overlay Layer (E310L) |
|---|---|---|
| Welding process | GMAW (MIG) / GTAW (TIG) | GMAW (MIG) / GTAW (TIG) |
| Welding current | 180–220 A | 160–200 A |
| Welding voltage | 22–26 V | 20–24 V |
| Travel speed | 250–350 mm/min | 300–400 mm/min |
| Heat input | 0.8–1.2 kJ/mm | 0.6–1.0 kJ/mm |
| Interpass temperature | ≤ 150 °C | ≤ 100 °C |
| Shielding gas | Ar 98% + CO₂ 2% | Ar 98% + CO₂ 2% or 100% Ar |
| Wire diameter | 1.2–1.6 mm | 1.2–1.6 mm |
| Preheat temperature | 100–150 °C | Not required (if interpass controlled) |
4.3 Critical Implementation Sequence
- Surface preparation: Grind to bright metal with 40-grit abrasive; ensure Ra ≤ 12.5 μm; remove all contaminants by acetone cleaning within 4 hours of welding
- Base preparation groove: For thick sections, machine a shallow V-groove (1–2 mm depth, 60° included angle) to promote dilution control and mechanical interlock
- Transition layer deposition: Single pass of E309L with low heat input (0.8–1.0 kJ/mm); bead width controlled to 1.5× wire diameter maximum
- Overlay layer deposition: 2–3 passes of E310L with progressively reduced heat input; ensure full overlap (75% minimum) between adjacent beads
- Post-weld treatment: Cool to below 50 °C before moving to next section; PWHT only if required by base material code (typically 550–620 °C for 2 hours per inch of thickness)
- Surface finish: Final grind to Ra ≤ 6.3 μm for slag-contact surfaces; Ra ≤ 3.2 μm for gas-flow surfaces
4.4 Dilution Control Strategy
Dilution is the primary technical challenge in pressurized gasifier overlay. The transition from carbon steel (0–1% Cr) to the overlay (25–30% Cr) requires careful management:
- First pass dilution: Typically 40–60% base metal dilution; the resulting composition remains in the austenite-ferrite region
- Second pass dilution: Reduces to 15–25%; composition approaches the designed overlay chemistry
- Third pass dilution: Below 10%; surface composition meets specification requirements
- Verification method: Optical Emission Spectroscopy (OES) or XRF analysis at 25% depth intervals
5. Applicable Standards and Acceptance Criteria
5.1 Governing Codes and Standards
| Standard | Scope | Key Requirement |
|---|---|---|
| ASME BPVC Section VIII Div. 1 | Pressure vessel construction | Overlay welds shall not reduce wall thickness below minimum required |
| ASME BPVC Section IX | Welding qualification | WPS/PQR qualification for each overlay material combination |
| ASME BPVC Section II Part D | Welding consumables | E309L, E310L filler metal specifications |
| ASTM A240 | Stainless steel sheet/plate | Overlay material grade verification |
| GB/T 12466 | Carbon steel weld wires | Base weld consumable specifications |
| GB/T 985 | Weld groove dimensions | Overlay groove preparation geometry |
| GB/T 3323 | RT testing of welds | Acceptance level for overlay welds (Level II minimum) |
| GB/T 11345 | UT testing of welds | Acceptance level for overlay welds (Level B) |
| NB/T 47013 | Pressure vessel NDT | Special equipment NDT requirements |
| ISO 5817 | Weld quality levels | Quality Level B for overlay welds |
| ASTM A403 | Cast stainless steel | Overlay casting material reference |
| NACE MR0175 | H₂S service materials | Sulfide stress cracking resistance requirements |
| API 579-1/ASME FFS-1 | Fitness-for-service | Overlay repair assessment methodology |
5.2 NDT Acceptance Criteria
- Visual inspection (VT): 100% coverage; no cracks, porosity clusters, undercut > 0.5 mm, or incomplete fusion visible
- Penetrant testing (PT): 100% coverage of overlay surface; acceptance per ISO 17637 Level 2; no linear indications
- Ultrasonic testing (UT): 100% coverage; acceptance per GB/T 11345 Level B; no indications above acceptance threshold
- Hardness testing: 3 points per 100 mm length; overlay surface hardness 200–320 HV; transition zone gradient verified
- Chemical analysis: Surface composition ≥ 25% Cr, ≥ 10% Ni for E310L overlay; verified by OES at 0.5 mm depth
- Macrograph examination: Cross-section sample showing full bond, no unmelted inclusions, proper layer progression
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause | Consequence | Control Measure |
|---|---|---|---|
| Cracking in transition zone | Excessive heat input causing HAZ softening + residual stress | Pressure boundary failure | Limit heat input ≤ 1.2 kJ/mm; controlled interpass temperature; post-weld stress relief if required |
| Weld decay (intergranular corrosion) | Chromium carbide precipitation in sensitized zone | Reduced corrosion resistance | Use low-carbon grades (E309L, E310L); limit interpass to ≤ 150 °C; avoid PWHT above 425 °C on overlay |
| Incomplete bonding | Insufficient heat input or surface contamination | Overlay delamination under thermal cycling | Minimum heat input 0.6 kJ/mm; surface preparation within 4 hours; gas flow verification |
| Excessive dilution | Too few passes or excessive heat input | Surface composition below specification | Mandatory minimum 2-pass overlay; OES verification at 0.5 mm depth |
| Hot cracking | Low melting eutectic phases in overlay | Surface cracking under cooling | Proper wire composition verification; adequate shielding gas; avoid welding on cold base |
6.2 Process Risks
- Geometric tolerance deviation: Gasifier shell curvature requires constant adjustment of welding angle; control through fixture design and welder training
- Thermal distortion: Internal overlay creates asymmetric heating; control through balanced weld sequences and back-of-vessel cooling
- Access limitations: Internal gasifier access requires specialized positioning equipment; plan weld sequence for maximum accessibility
- Contamination ingress: Hydrogen from moisture or oil causes porosity; control through gas-dry consumable storage and pre-weld surface heating
6.3 Quality Assurance Controls
- WPS qualification: Separate WPS for each base-overlay combination; PQR with full NDT and mechanical testing
- Welder certification: ASME Section IX or ISO 9606-1 qualified for overlay welding specifically
- First article inspection: Full destructive testing on qualification coupon before production welding
- In-process monitoring: Heat input logging, interpass temperature recording, gas flow verification at each weld start
- Final verification: 100% PT + UT + hardness mapping + OES composition verification
7. Application Across the Company's Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Application)
This is the principal technology route for pressurized gasifier internal wall overlay. The MIG process (GMAW) is preferred for cylindrical shell sections where deposition rate efficiency is critical, while TIG (GTAW) is specified for:
- Nozzle internal surfaces where access is restricted
- Repair applications on existing gasifiers in service
- Final surface finish passes requiring maximum bead control
- Thin-wall sections where heat input must be minimized
The company's qualification portfolio for pressurized gasifier overlay should include WPS/PQR combinations covering:
| WPS ID | Base Material | Overlay Sequence | Process | Applicable Code |
|---|---|---|---|---|
| WPS-GAS-001 | 18MnMoNbR | E309L + E310L (3 passes) | GMAW | ASME VIII Div.1 / NB |
| WPS-GAS-002 | P91 (9Cr-1Mo-V) | E309L + E310L (3 passes) | GTAW | ASME VIII Div.1 / NB |
| WPS-GAS-003 | 15CrMo | E309L + E310 (2 passes) | GMAW | GB/T 150 / NB |
| WPS-GAS-004 | 18MnMoNbR | E309L + E310 + C-276 (3 passes) | GTAW | ASME VIII Div.1 / NB |
7.2 Hydraulic Explosive Bonding (Complementary Application)
While hydraulic explosive bonding is not the primary route for internal gasifier overlay, it serves a complementary role in the following scenarios:
- Large-scale pre-cladding: For new gasifier fabrication, hydraulic explosive bonding can produce clad plate (steel/stainless) for the gasifier shell, reducing the overlay thickness requirement from 5–6 mm to 2–3 mm
- Repair of heavily corroded sections: Where overlay thickness exceeds 8 mm, hydraulic bonding of a stainless steel plate followed by a thin overlay finish provides a more economical solution
- Slag tap area reinforcement: The combination of hydraulic bonded duplex steel plate + TIG overlay provides superior slag erosion resistance for the most severely degraded zone
The integration strategy involves bonding a 2–3 mm stainless steel backing plate to the internal surface via hydraulic explosive bonding, then applying 1–2 passes of E310L overlay via TIG for surface finish and metallurgical bonding assurance.
7.3 Explosion Welding (Specialized Application)
Explosion welding finds application in pressurized gasifier technology primarily in:
- Clad pipe fabrication for gasifier internals: Gas distributor pipes, gas outlet nozzles, and quench water nozzles can be fabricated from explosion-welded clad pipe (carbon steel core + stainless overlay) with overlay welding of internal surfaces
- Repair of gasifier internals: Detachable internal components (gas distributors, dip pipes) can be repaired by explosion welding replacement sections rather than full component replacement
- Research and development: Explosion welding enables rapid prototyping of new overlay material combinations for gasifier service without requiring full WPS qualification for each trial
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building Impact
- NB license enhancement: Pressurized gasifier overlay capability demonstrates the company's ability to perform weld overlay on pressure boundaries, strengthening the NB manufacturing license scope
- ASME stamp qualification: Successful delivery of gasifier overlay projects supports ASME Section VIII "U" or "S" stamp applications
- OEM qualification: Gasifier OEMs (GE/Alstom, Siemens/Doosan, Shell/GTL, Siemens Energy) require specific supplier qualification; this technology entry establishes the technical basis for OEM-approved vendor status
- WPS library expansion: Each gasifier project adds qualified WPS/PQR combinations that extend the company's qualification portfolio across base materials, overlay materials, and joint configurations
8.2 Product Delivery Value
- Complete gasifier internal protection package: The company can offer a complete overlay solution covering shell, heads, nozzles, and internals—providing single-source accountability for the customer
- Repair and refurbishment services: In-service gasifier overlay repair extends vessel life by 10–15 years, creating recurring revenue from existing customer base
- Performance guarantee capability: With qualified WPS and proven field performance, the company can offer overlay performance guarantees (minimum life, corrosion rate limits)
8.3 Customer Value Proposition
| Customer Concern | Company Solution | Quantified Value |
|---|---|---|
| Gasifier unplanned shutdown | Qualified overlay extending service life to 15 years | ¥50–200 million per avoided shutdown |
| Full vessel replacement cost | Overlay repair at 20–30% of replacement cost | ¥300–800 million savings per vessel |
| Corrosion monitoring burden | Overlay with guaranteed performance reducing inspection frequency | 50% reduction in inspection costs |
| Material cost optimization | Lower-grade base + overlay vs. full alloy vessel | 15–25% material cost reduction |
| Compliance and certification | ASME/NB qualified overlay with full traceability | Eliminates regulatory risk and delay |
9. Conclusions and Forward-Looking Recommendations
The internal wall weld overlay technology for pressurized gasifiers represents a strategically critical capability for Cladding Technology Shanxi Co., Ltd. It sits at the intersection of high technical complexity, significant revenue potential, and strong customer loyalty. The technology requires:
- Systematic WPS qualification covering all relevant base material/overlay combinations under ASME Section IX and NB requirements
- Investment in specialized NDT equipment including phased array UT for overlay thickness and bond quality verification, and portable OES for in-situ composition analysis
- Development of integrated solutions combining TIG/MIG overlay with hydraulic bonding and explosion welding for comprehensive gasifier internal protection
- Establishment of field performance databases tracking overlay degradation rates, repair history, and service life outcomes to build empirical performance guarantees
- Pursuit of OEM qualification with major gasifier manufacturers to establish preferred supplier status in the gasification equipment supply chain
By systematically developing this capability, the company positions itself as a differentiated provider in the high-value pressure vessel overlay market, leveraging its multi-route technology portfolio (TIG/MIG, hydraulic bonding, explosion welding) to deliver comprehensive solutions that no single-route competitor can match.