CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Welding Technology for Inner Walls of Thick-Walled Pressure Vessels
Literature Overview
The paper by Wang Jiachun, Zhu Qi, and Sun Dunwu (1998), published by the Harbin Welding Research Institute, addresses the challenging problem of applying corrosion-resistant overlay layers to the inner walls of thick-walled pressure vessels. This is a critical engineering challenge encountered in the fabrication of hydrogenation reactors, ammonia synthesis loops, and other high-pressure equipment where the internal environment is highly corrosive while the external structure must maintain pressure-bearing capacity. The study presents systematic process development, quality control methods, and engineering solutions for this demanding application.
Technical Challenges and Core Issues
Geometric and Access Constraints
The inner wall of a thick-walled pressure vessel presents unique challenges that distinguish it from conventional overlay welding applications:
- Limited access: Internal surfaces of large-diameter vessels require specialized equipment positioning, often involving remotely operated welding heads or manually guided torches in confined spaces.
- Thermal management: The thick base metal (typically 40–150 mm wall thickness) acts as a massive heat sink, resulting in very high cooling rates at the weld interface that can promote brittle martensitic transformation in the heat-affected zone (HAZ).
- Residual stress accumulation: Multi-pass overlay welding on curved internal surfaces generates complex residual stress states that must be managed to prevent distortion and cracking.
- Quality verification: Non-destructive testing (NDT) of internal overlay welds is more difficult due to access limitations and the curved geometry that complicates ultrasonic testing.
Material Compatibility Requirements
The base metal for thick-walled pressure vessels is typically a low-alloy steel such as 16MnR (GB standard), 15CrMoR, or 12Cr1MoV, while the overlay material must provide corrosion resistance against the specific process environment. Common overlay materials include:
| Overlay Material | Application Environment | Typical Thickness | Standards Reference |
|---|---|---|---|
| 304/304L stainless steel | General corrosion, dilute acids | 3–10 mm | NB/T 47014, ASME IX |
| 316/316L stainless steel | Chloride-containing environments | 3–10 mm | NB/T 47014, ASME IX |
| Inconel 625 | High-temperature oxidation, aggressive chemicals | 3–8 mm | ASME IX, API 934 |
| Hastelloy C276 | Strong acids, reducing environments | 3–6 mm | ASME IX, API 934 |
| Monel 400 | Hydrofluoric acid, hydrochloric acid | 3–6 mm | ASME IX |
Process Development and Technical Solutions
Welding Process Selection
The study evaluates several welding processes for inner wall overlay application:
Submerged Arc Welding (SAW):
- Advantages: High deposition rate (3–5 kg/h), deep penetration, excellent weld quality
- Limitations: Requires flux recovery system in confined spaces, limited positional capability
- Application: Suitable for large-diameter vessels where the overlay head can be positioned on the internal surface
Gas Metal Arc Welding (GMAW) / Flux-Cored Arc Welding (FCAW):
- Advantages: Good positional capability, moderate deposition rate (1.5–3 kg/h)
- Limitations: Requires gas shielding equipment inside the vessel
- Application: Versatile choice for most vessel geometries
Electroslag Welding (ESW):
- Advantages: Very high deposition rate, low dilution (5–10%), uniform microstructure
- Limitations: Requires special equipment, limited to specific geometries
- Application: Best suited for vertical overlay on large-diameter vessels with internal access
Plasma Transferred Arc (PTA) Powder Cladding:
- Advantages: Very low dilution (5–15%), precise composition control, smooth surface finish
- Limitations: Lower deposition rate, higher equipment cost
- Application: Ideal for critical applications requiring precise overlay chemistry
Recommended Process Parameters
| Parameter | SAW | GMAW/FCAW | ESW | PTA |
|---|---|---|---|---|
| Current | 400–600 A | 200–400 A | 800–1500 A | 100–300 A |
| Voltage | 28–36 V | 25–35 V | 30–40 V | 18–25 V |
| Travel speed | 150–300 mm/min | 200–400 mm/min | 50–100 mm/min | 200–500 mm/min |
| Pre-heat | 150–250°C | 150–250°C | 200–300°C | 100–200°C |
| Interpass temp | <250°C | <250°C | <300°C | <200°C |
| Dilution | 20–35% | 25–40% | 5–15% | 5–15% |
Layer Design and Pass Sequencing
For thick overlay layers (8–15 mm), a multi-pass sequence is essential. The study recommends the following approach:
- First pass (transition layer): Use a low-dilution consumable with composition intermediate between base metal and final overlay material to prevent cracking and ensure good metallurgical bonding.
- Intermediate passes: Build up the bulk of the overlay thickness using the target overlay material.
- Final pass (surface pass): Apply a single pass with controlled parameters to achieve the required surface finish and ensure proper composition at the wear/corrosion surface.
The total overlay thickness should be designed considering the maximum expected wear or corrosion rate over the vessel's design life, plus a minimum of 1.5 mm additional thickness to accommodate machining and inspection requirements.
Quality Control and Inspection
Non-Destructive Testing Requirements
| Inspection Method | Purpose | Coverage | Acceptance Criteria |
|---|---|---|---|
| Visual Testing (VT) | Surface defects, undercut, porosity | 100% | No undercut > 1 mm, no surface cracks |
| Penetrant Testing (PT) | Surface-breaking cracks | 100% of overlay surface | No linear indications |
| Magnetic Particle Testing (MT) | Surface/near-surface cracks | 100% of overlay and HAZ | No cracks or linear indications |
| Ultrasonic Testing (UT) | Bond strength, internal defects | 100% at weld root | No lack of fusion, no cracks > 2 mm |
| Radiographic Testing (RT) | Internal porosity, inclusions | 10–100% depending on criticality | Per NB/T 47013 or ASME V |
| Hardness Testing | Microstructure verification | 3 points per 100 mm | Within specified range |
Bond Strength Verification
The bond strength between the overlay layer and the base metal is critical for pressure vessel safety. The study emphasizes that:
- Ultrasonic testing of the weld root is mandatory to detect lack of fusion or cracks at the critical interface
- The overlay layer must achieve full fusion with the base metal; no unmelted base metal should remain at the interface
- Dilution at the first pass should be controlled to ensure the overlay material composition is maintained sufficiently deep to provide the required corrosion resistance
Engineering Practice and Case Study
Hydrogenation Reactor Overlay Application
A typical application described in the study involves a 12Cr1MoV hydrogenation reactor with 80 mm wall thickness requiring a 6 mm Inconel 625 overlay on the internal surface. The process sequence implemented was:
- Surface preparation: Grinding of the internal surface to remove scale and ensure clean, sound base metal
- Pre-heat: Induction heating to 250°C uniformly across the vessel interior
- First pass: FCAW with Inconel 625 flux-cored wire, 2 mm deposited thickness, 300 A, 32 V, 250 mm/min
- Intermediate passes: 3 passes of Inconel 625 FCAW, building to 5 mm total thickness
- Surface pass: Single pass with controlled parameters for smooth finish
- Post-weld heat treatment: Stress relief at 620°C for 4 hours (coordinated with vessel PWHT schedule)
- Inspection: UT of weld root (100%), PT of overlay surface (100%), hardness testing
The resulting overlay achieved bond strength exceeding 180 MPa, hardness of 210–240 HV, and passed intergranular corrosion testing per ASTM A263.
Key Reflections and Study Insights
The most significant insight from this research is that inner wall overlay of thick-walled pressure vessels is fundamentally a thermal management problem. The massive thermal mass of the thick base metal creates extreme cooling rates that can produce undesirable microstructures in both the weld root and the HAZ. The solution requires not only proper pre-heat but also careful attention to the sequence of passes, with subsequent passes providing beneficial re-heating of the critical root region.
Another important observation is the economic consideration of overlay thickness. Excessive overlay thickness increases cost significantly (particularly for nickel-based alloys) and may introduce thermal stresses that compromise the pressure vessel's structural integrity. The study advocates for careful engineering analysis to determine the minimum required overlay thickness based on corrosion/wear rate calculations, with a safety margin that accounts for inspection intervals and maintenance access requirements.
The paper also highlights the importance of integrating overlay welding into the overall pressure vessel fabrication sequence. Overlay should typically be performed before final welding of vessel heads or nozzles, to allow proper stress relief of the entire assembly. Performing overlay after final assembly welding can introduce additional residual stresses that may exceed acceptable limits.
Reference Value and Outlook
This research provides a comprehensive engineering framework for the challenging application of inner wall overlay on thick-walled pressure vessels. The systematic approach to process selection, parameter optimization, and quality verification can be directly applied to similar industrial applications. The emphasis on thermal management and dilution control represents the core technical challenge that must be addressed in all such applications. With the increasing demand for corrosion-resistant pressure vessels in the petrochemical, hydrogen energy, and nuclear industries, the technologies described in this study remain highly relevant and continue to evolve with advances in welding consumables and process automation.