CLADDING TECHNOLOGY SHANXI CO., LTDDeformation Control in Cladding of Thin-Walled Gasifier Quench Chamber Shells
Literature Overview
This 2021 paper by Zhang Yongguang and colleagues from Dongfang Electric Group Dongfang Boiler Co., Ltd., published in Chemical Design Communication, addresses a critical manufacturing challenge in coal-to-chemical gasification technology: controlling welding deformation during the overlay cladding of thin-walled quench chamber shells. The quench chamber is a key component in gasifier systems where hot syngas is rapidly cooled by direct contact with water, and the shell interior requires a hardfacing or corrosion-resistant cladding layer to withstand erosive and corrosive conditions.
Background and Technical Context
The quench chamber in modern gasifiers (such as those used in Shell SCGP, GE Topsoe, or East China Engineering Design Institute gasification processes) operates under severe conditions: high temperature, high pressure, and continuous water spray with abrasive gas-solid particles. The shell wall thickness is typically thin (12–25 mm) to accommodate the large diameter of the vessel (often 3000–5000 mm) while maintaining reasonable weight. The cladding layer, which may be a nickel-based alloy or a specific stainless steel, must be applied to the entire interior surface of the shell.
The combination of thin walls and large diameter creates a highly susceptible condition for welding distortion. Even moderate residual stresses from cladding can cause measurable out-of-roundness, which is unacceptable for pressure vessel integrity and for the fit-up of subsequent assembly operations.
Deformation Mechanism Analysis
The authors conduct a systematic analysis of deformation mechanisms during cladding of thin-walled shells:
- Longitudinal shrinkage — As the cladding weld cools, it contracts in the longitudinal direction, creating compressive stresses in the unwelded regions ahead of the weld and tensile stresses in the weld zone.
- Transverse shrinkage — The transverse contraction of the weld bead pulls the adjacent base metal inward, reducing the local diameter of the shell.
- Angular distortion — In multi-pass cladding, the asymmetric heat input from successive passes can cause the shell cross-section to deviate from a perfect circle.
- Global bending — When cladding is performed sequentially along the shell length, the accumulated residual stresses can cause the entire shell to bend, resulting in end-face tilt.
The paper presents experimental data showing that without countermeasures, the out-of-roundness can exceed the acceptance limits specified by GB/T 150 and NB/T 47002. For a shell with a nominal diameter of 4000 mm, the allowable out-of-roundness is typically limited to 0.5% of the diameter (20 mm), but in practice, tighter tolerances (10–15 mm) are required to ensure proper fit-up with the vessel head and internal components.
Countermeasures and Process Optimization
The study proposes a multi-pronged approach to deformation control:
| Countermeasure | Description | Effectiveness |
|---|---|---|
| Symmetric welding sequence | Weld from both ends toward the center, or use multiple simultaneous welding stations | Reduces global bending by 60–80% |
| Back-plate rigidity | Weld a temporary stiffening plate to the exterior of the shell | Reduces local distortion by 40–60% |
| Pre-deformation | Intentionally over-size the shell blank to compensate for expected shrinkage | Reduces net out-of-roundness to within tolerance |
| Low-heat-input process | Use short-arc SAW or multi-wire SAW with reduced current | Reduces peak temperature and shrinkage |
| Interpass cooling control | Maintain interpass temperature below 150 °C using active cooling | Limits thermal cycle and reduces residual stress |
The most effective strategy described in the paper combines symmetric welding sequence with a temporary back-plate. The back-plate is a ring-shaped steel plate that is tack-welded to the exterior of the shell at intervals of 500–800 mm. This plate provides sufficient rigidity to resist local distortion during cladding but is removed after the cladding is complete and the shell is inspected.
Process Parameters and Welding Procedure
The cladding process parameters recommended in the study are as follows:
| Parameter | Value |
|---|---|
| Base material | 16MnR or 15CrMoR |
| Shell wall thickness | 14–22 mm |
| Cladding alloy | 310SS or Inconel 625 (depending on service) |
| Cladding thickness | 3–5 mm |
| Welding process | Multi-wire SAW (3–4 wires) |
| Current | 500–800 A per wire |
| Voltage | 28–34 V |
| Travel speed | 300–500 mm/min |
| Number of passes | 2–3 |
| Preheat | 100–150 °C |
| PWHT | 590–620 °C, 2–4 h |
The use of multi-wire SAW is particularly effective for thin-walled applications because it distributes the heat input across multiple wires, reducing the peak temperature in any single location while maintaining a high deposition rate. This is advantageous for productivity as well as for distortion control.
Study Insights
This paper highlights an important principle in pressure vessel fabrication: deformation control must be planned from the design stage, not merely corrected after fabrication. The proposed back-plate technique is simple and cost-effective, but it requires additional handling and welding labor. Engineers should evaluate the cost-benefit trade-off between deformation control measures and post-fabrication correction (such as mechanical straightening or thermal straightening), which can be more expensive and may introduce additional residual stresses.
The study also underscores the importance of numerical simulation in predicting welding distortion. While the paper relies primarily on experimental data, the integration of finite element analysis (FEA) for residual stress prediction would allow for more precise optimization of welding sequences and countermeasures before actual production begins.