CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on CMT Cladding Deformation of Boiler Water Wall Tubes
Literature Background and Context
Published in 2018 by researchers from Shenyang Ligong University, this paper examines the deformation behavior of boiler water wall tubes during cold metal transfer (CMT) cladding operations. Boiler water wall tubes are critical components in utility boilers, forming the primary heat transfer surface and the first barrier against water and steam. Corrosion and wear damage to these tubes can lead to catastrophic failures, making cladding repair a vital maintenance technology. The CMT process, a variant of gas metal arc welding (GMAW), is known for its low heat input and minimal spatter, making it particularly suitable for cladding thin-walled tubular components. However, the deformation induced by the welding thermal cycle remains a significant concern for maintaining the geometric integrity and structural performance of the water wall tubes.
Thermal Deformation Mechanisms and Analysis
The authors conducted both experimental and numerical studies to understand the deformation mechanisms during CMT cladding of water wall tubes. The experimental setup involved cladding stainless steel overlay on carbon steel tubes with an outer diameter of 51 mm and a wall thickness of 5.5 mm. The tubes were mounted on a fixture that simulated the actual installation conditions of the water wall. Deformation was measured using laser displacement sensors at multiple points along the tube circumference and length.
| CMT Parameter | Value | Effect on Deformation |
|---|---|---|
| Wire Diameter | 0.8 mm | Lower heat input reduces angular distortion |
| Wire Feed Speed | 5–8 m/min | Higher speed increases deposition rate but may increase deformation |
| Travel Speed | 150–300 mm/min | Higher speed reduces heat input per unit length |
| Shielding Gas | Ar + 2% CO₂ | Optimizes arc stability and penetration |
| Layer Thickness | 1.0–1.5 mm | Thinner layers minimize thermal distortion |
The numerical analysis employed a coupled thermo-mechanical finite element model to simulate the temperature field and stress-strain distribution during cladding. The results showed that the maximum angular distortion occurred at the end of the cladding bead, where the thermal gradient was most pronounced. The peak temperature in the base metal reached approximately 650 °C, which is below the austenitization temperature of the carbon steel, indicating that the thermal cycle is relatively mild. However, the repeated heating and cooling cycles during multi-pass cladding led to cumulative plastic strain, resulting in measurable distortion.
Control Strategies and Process Optimization
The authors proposed several strategies to control deformation during CMT cladding. The most effective approach was the use of a multi-pass, multi-directional cladding strategy, where each pass was offset from the previous one to distribute the thermal input more uniformly. This strategy reduced the maximum angular distortion by approximately 35 percent compared to a single-direction cladding approach. Additionally, the use of a backing ring to provide mechanical support during cladding was found to be effective in preventing ovalization of the tube cross-section.
Another key finding was the importance of the interpass temperature control. The authors recommended maintaining the interpass temperature between 100 and 150 °C to balance the competing demands of crack prevention and distortion control. Temperatures below 100 °C increased the risk of cold cracking, while temperatures above 150 °C accelerated oxidation and increased deformation. The use of a low-heat-input CMT process with a wire feed speed of 6 m/min and a travel speed of 250 mm/min provided the best compromise between deposition rate and deformation control.
Engineering Relevance and Practical Considerations
The study provides valuable guidance for the repair and maintenance of boiler water wall tubes in power plants. The authors emphasize that the deformation tolerance must be considered in the design of the cladding process, particularly for tubes that are part of a rigid structure where distortion could lead to misalignment or stress concentration. The recommended post-cladding straightening procedures, including local heating and mechanical correction, were validated through experimental verification.
This research highlights the importance of integrating process monitoring and adaptive control in cladding operations. The ability to measure and control deformation in real time is essential for ensuring the quality and reliability of repaired water wall tubes. Engineers should consider the specific geometry and material properties of the tubes when developing cladding procedures, and should not rely solely on generic process parameters.