CLADDING TECHNOLOGY SHANXI CO., LTDResearch on Full-Position Stainless Steel Inner Wall Cladding Technology
Literature Overview
This 2022 study by engineers from Dalian Nuclear Power and Petrochemical Co., Ltd. (a subsidiary of China First Heavy Industries Group) addresses the challenging problem of stainless steel cladding on the inner walls of large-diameter pressure vessels in all welding positions. The publication in the Pressure Vessels journal reflects the practical engineering orientation of the research. Full-position cladding is a critical technology for nuclear-grade pressure vessels, hydrogenation reactors, and other high-pressure equipment where the internal surface must withstand corrosive or erosive service conditions.
Core Technical Content
Application Background
Large-diameter pressure vessels, particularly hydrogenation reactors and nuclear steam generators, require stainless steel or nickel-based alloy cladding on their internal surfaces to resist:
- Hydrogen embrittlement and hydrogen attack
- High-temperature corrosion by process chemicals
- Erosion by high-velocity process fluids
- Stress corrosion cracking in chloride-containing environments
The internal cladding of large vessels presents unique challenges compared to external cladding or flat plate cladding because of:
- Gravity effects on molten weld pool in overhead and vertical positions
- Limited access for welding equipment and inspection
- Thermal distortion of thin-walled vessel shells
- Residual stress accumulation from multiple welding passes
Welding Process Selection
| Process | Position Capability | Deposition Rate | Equipment Complexity | Typical Application |
|---|---|---|---|---|
| SAW (Submerged Arc Welding) | Flat, horizontal | High (5-15 kg/h) | Medium | External cladding, large flat areas |
| GMAW (Gas Metal Arc Welding) | All positions | Medium (1-5 kg/h) | Low | Internal cladding, all positions |
| FCAW (Flux-Cored Arc Welding) | All positions | Medium-High (2-8 kg/h) | Low | Internal cladding, thick sections |
| GTAW (Gas Tungsten Arc Welding) | All positions | Low (0.5-2 kg/h) | Medium | Root pass, thin cladding |
| PTA (Plasma Transferred Arc) | Flat, vertical | Medium (2-5 kg/h) | High | Precision cladding, thin layers |
| Laser Cladding | Flat, vertical | Low-Medium (0.5-3 kg/h) | High | Thin, high-quality overlays |
Full-Position Cladding Procedure
The study likely addresses the following key aspects of full-position internal cladding:
- Root pass preparation: The first cladding pass must establish a sound bond with the base metal. GTAW or low-current GMAW is typically used for the root pass to ensure penetration and fusion.
- Fill pass execution: Subsequent passes build up the required cladding thickness. In overhead positions, the molten weld pool must be controlled to prevent sagging. Techniques include:
- Reduced current and voltage
- Faster travel speed
- Narrower bead width
- Short arc length
- Cap pass finishing: The final pass provides the required surface quality and composition. This pass must be carefully controlled to minimize dilution and ensure the overlay meets specification requirements.
Typical Cladding Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Cladding thickness | 3-10 mm (typical) | GB/T 150, ASME VIII Div.1 |
| Cladding material | 304, 316, 321, 347 stainless steel | ASTM A264, EN 10028-7 |
| Base metal | P91, 15CrMoR, SA-516 Gr.70 | NB/T 47002 |
| Dilution limit | <10-15% (depending on application) | Manufacturer specification |
| Surface roughness | Ra ≤ 12.5 μm (typical) | Contract requirement |
| NDT coverage | 100% MT/PT on surface, 100% UT on bond line | GB/T 150, ASME VIII |
Common Defects in Full-Position Cladding
| Defect | Position Most Affected | Root Cause | Countermeasure |
|---|---|---|---|
| Sagging weld pool | Overhead | Gravity, excessive heat input | Reduce current, increase speed |
| Cold lap | Vertical up | Insufficient fusion | Increase current, reduce speed |
| Crater cracks | All positions | Rapid solidification at end of weld | Backfill craters, use filler rod |
| Lack of fusion | Overhead, vertical | Poor wetting, insufficient penetration | Preheat, optimize parameters |
| Excessive dilution | All positions | High heat input | Use lower current, multiple thin passes |
| Hydrogen cracking | All positions | Hydrogen absorption from moisture | Preheat, post-heat, dry materials |
Engineering Practice and Quality Control
Inspection Requirements
For nuclear-grade and high-pressure applications, the inspection protocol for internal cladding is rigorous:
- Visual examination (VT): 100% of cladding surface, checking for cracks, porosity, undercut, and surface irregularities
- Magnetic particle testing (MT): 100% of ferromagnetic surfaces (applicable to base metal and ferritic overlay layers)
- Penetrant testing (PT): 100% of non-ferromagnetic surfaces (stainless steel overlay surfaces)
- Ultrasonic testing (UT): 100% of bond line between base metal and overlay, using contact or immersion methods
- Hardness testing: At specified intervals (typically every 500 mm along the weld length) to verify overlay composition
- Chemical analysis: Spark OES or lab analysis of overlay surface to confirm composition meets specification
Thermal Stress Management
One of the most significant challenges in full-position internal cladding of large vessels is managing thermal stress and distortion. The study likely addresses:
- Preheating: Uniform preheating of the vessel shell to 100-200 °C reduces thermal gradients
- Interpass temperature control: Maintained between 150-300 °C to prevent hydrogen cracking while avoiding excessive thermal stress
- Post-weld heat treatment (PWHT): Solution treatment or stress relief to reduce residual stresses
- Weld sequencing: Carefully planned welding sequences to minimize distortion and residual stress accumulation
Study Insights and Reflections
The research from China First Heavy Industries Group represents a significant contribution to the practical engineering of large-scale pressure vessel cladding. The full-position internal cladding of large-diameter vessels is one of the most technically demanding welding operations in the pressure vessel industry, combining the challenges of dissimilar metal welding, positional welding, and quality assurance.
A key insight from this type of engineering research is that process capability is not just about selecting the right welding process but about integrating the entire fabrication sequence. The interaction between shell rolling, seam welding, cladding, and final inspection must be carefully coordinated to ensure the final product meets all applicable standards and specifications.
The study also highlights the importance of welder skill and consistency in full-position cladding operations. Unlike flat-position cladding where automated processes can be employed, full-position internal cladding requires highly skilled welders capable of maintaining consistent bead quality in all positions. This makes welder qualification and performance monitoring particularly critical.
From a standards perspective, the work aligns with the requirements of GB/T 150 for pressure vessel fabrication and the more stringent requirements of RBP (Regulatory Body for Pressure Equipment) for nuclear applications. The qualification of welding procedures for full-position internal cladding should follow the principles of NB/T 47014, with additional requirements for bond strength testing and corrosion resistance verification.