Zhuojin Pipe Fitting 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 in Welding (No. 12, pp. 2–5), from the Harbin Welding Research Institute, addresses the technical challenges of overlay welding stainless steel on the inner walls of thick-walled pressure vessels. This is a critical technology in the chemical, petrochemical, and nuclear industries where equipment must withstand both external pressure loading and internal corrosive media. The paper discusses technical performance requirements, overlay material selection, welding process selection, and key technical issues in overlay welding for such applications.
Technical Performance Requirements
Thick-walled pressure vessels (typically with wall thickness exceeding 30 mm) present unique challenges for inner-wall overlay welding:
- Bond strength: The overlay must maintain adequate bond strength to the base material under cyclic pressure loading and thermal cycling.
- Corrosion resistance: The overlay must provide a continuous, defect-free barrier against the internal corrosive medium.
- Mechanical compatibility: Thermal expansion mismatch between the overlay and base material can generate interfacial stresses during thermal cycling.
- Geometric constraints: Access to the inner wall of a pressure vessel is limited, requiring specialized welding positions and equipment.
- Residual stress management: Thick sections develop significant residual stresses during overlay welding that must be controlled to prevent cracking.
| Requirement | Specification | Verification Method |
|---|---|---|
| Bond strength | ≥ 200 MPa (peel test) | ASTM A377 or equivalent |
| Overlay thickness | Minimum 3 mm (typically 5–8 mm) | UT measurement |
| Dilution at interface | < 15% base metal in first pass | Metallographic analysis |
| Surface continuity | No porosity, cracks, or incomplete fusion | RT or PT inspection |
| Corrosion resistance | Pass ASTM A262 Practice E (intergranular) | Standard corrosion tests |
| Residual stress | < 150 MPa after stress relief | X-ray or hole drilling |
Overlay Material Selection
The selection of overlay material depends on the specific corrosive environment:
- Austenitic stainless steel (304, 316, 316L): Suitable for general chemical environments, mild acids, and oxidizing conditions.
- High-alloy austenitic (310, 904L): Required for aggressive reducing acids and high-temperature oxidation.
- Nickel-base alloys (625, C-276): Essential for severe corrosion environments such as concentrated sulfuric acid or hot chloride solutions.
- Duplex stainless steel (2205): Offers good combination of corrosion resistance and mechanical strength for moderate environments.
For thick-walled vessels, the first pass (transition layer) is critical. A low-dilution process or a transition filler metal (such as E309L for carbon steel to austenitic stainless steel transitions) is typically used to ensure adequate bond strength while maintaining corrosion resistance in subsequent passes.
Welding Process Selection and Key Technical Issues
Several welding processes are applicable for inner-wall overlay welding of thick-walled vessels:
| Process | Advantages | Limitations | Typical Application |
|---|---|---|---|
| GTAW (TIG) | Low dilution, precise control, high quality | Low deposition rate | First pass, thin overlays |
| GMAW (MIG) | High deposition rate, good efficiency | Higher dilution, requires shielding gas | Multi-pass overlays on flat surfaces |
| FCAW (flux-cored) | Good penetration, outdoor capability | Higher dilution, slag inclusions | Thick overlays, field repair |
| SAW (submerged arc) | Very high deposition rate, low dilution | Limited to horizontal surfaces | Large flat areas, multi-pass |
| Plasma arc | Low dilution, high current density | Equipment complexity | Thin, high-quality overlays |
Key technical issues identified in the literature include:
- Thermal input management: Thick sections require careful control of heat input to prevent excessive HAZ softening in the base material while ensuring adequate fusion of the overlay.
- Weld sequence planning: For large vessel interiors, the welding sequence must be designed to minimize distortion and manage cumulative thermal effects. A segmented approach with controlled overlap is recommended.
- Positional welding: Inner-wall overlay welding often requires welding in various positions (flat, horizontal, vertical, overhead). Process parameters must be adjusted for each position to maintain consistent quality.
- Post-weld stress relief: Thick-walled vessels with overlay deposits typically require stress relief annealing to reduce residual stresses. The temperature must be compatible with both the base material and the overlay alloy.
- Inspection challenges: Full inspection of inner-wall overlays is difficult due to access limitations. UT scanning from the outer surface or PT/MT inspection from the inner surface are commonly employed.
Engineering Practice Integration
In my experience with pressure vessel overlay welding, the most critical factor is ensuring complete fusion at the overlay/base interface. Incomplete fusion defects are the primary cause of overlay failure in service. Using a GTAW first pass with a 309L-type filler metal provides excellent fusion characteristics and creates a transition zone that accommodates the thermal expansion mismatch between carbon steel and austenitic stainless steel.
For vessels requiring thick overlays (8 mm or more), a multi-pass approach using GMAW or FCAW after the GTAW first pass provides an efficient balance of quality and productivity. The interpass temperature should be maintained below 150 °C for austenitic stainless steel overlays to prevent sensitization and intergranular corrosion susceptibility.
The integration of FMEA (Failure Mode and Effects Analysis) into the overlay welding process planning is highly recommended. Common failure modes include overlay spallation, intergranular cracking, and corrosion under deposit. Each failure mode should be addressed through specific process controls and inspection requirements.
Study Insights and Implications
This paper, though published in 1998, remains highly relevant to contemporary practice. The fundamental challenges of overlay welding thick-walled pressure vessels—dilution control, bond strength assurance, residual stress management, and inspection verification—have not changed significantly. However, modern advances in welding process control, real-time monitoring, and advanced NDT techniques (such as phased array UT) have improved the reliability of overlay welding operations. Engineers should recognize that overlay welding of pressure vessels is not merely a surface treatment but a structural modification that must be designed and executed with the same rigor as primary pressure-containing welds. The selection of overlay material, process parameters, and inspection protocols should be justified through a systematic engineering analysis that considers the entire service life of the vessel, including maintenance intervals and re-overlay possibilities.