Zhuojin Pipe Fitting Co., LtdNumerical Simulation of Dissimilar Metal Surfacing Between Low-Alloy Steel and Stainless Steel
Literature Overview
This paper by Jiang Xiaohua, Dai Deping, Cai Jianpeng, and Deng De'an from Chongqing University (2016, Hot Working Technology, Vol. 45, No. 9, pp. 180-183) presents a finite element analysis (FEA) study of thermal and residual stress distributions in dissimilar metal surfacing welds between low-alloy steel substrates and stainless steel overlay layers. The research was supported by the National Natural Science Foundation of China (Grant No. 51275544) and directly addresses challenges encountered in nuclear power equipment manufacturing, where dissimilar metal welds are ubiquitous due to the diverse material requirements imposed by complex operating environments.
Core Technical Methodology
The authors developed a thermomechanical coupled FEA model using ABAQUS software to simulate the surfacing welding process. The key technical elements include:
| Modeling Parameter | Description |
|---|---|
| Heat source model | Gaussian surface heat source with moving heat source |
| Material model | Thermal-elastic-plastic constitutive law |
| Boundary conditions | Convective and radiative heat loss from free surfaces |
| Welding sequence | Multi-pass surfacing with progressive deposition |
| Mesh strategy | Adaptive remeshing for moving weld pool |
The Gaussian surface heat source model was validated against actual weld pool morphology observed in production, demonstrating good agreement between simulation and reality. This validation is essential for establishing confidence in the residual stress predictions.
Key Findings and Their Significance
Three principal findings emerge from the simulation:
- Residual stress discontinuity at the interface: Due to the significant difference in thermal expansion coefficients between low-alloy steel (typically α ≈ 12-13 × 10⁻⁶ /°C for 12Cr1MoV or P91) and stainless steel (α ≈ 17-18 × 10⁻⁶ /°C for 310 or 309), a pronounced stress discontinuity develops at the dissimilar metal interface. This is a critical finding for nuclear component design, as stress concentrations at such interfaces can initiate fatigue cracking or promote stress corrosion cracking (SCC).
- Annealing effect of subsequent passes: Multi-pass surfacing exhibits a beneficial annealing effect where later passes partially relieve the residual stresses formed during earlier passes. This has direct implications for welding sequence planning—strategic selection of pass order and direction can be used to minimize peak residual stresses.
- Weld pool morphology prediction: The model accurately predicts the geometry of the molten pool, which is important for understanding dilution rates and the resulting composition gradient at the interface.
Integration with Engineering Practice
For nuclear power equipment, dissimilar metal welds (DMWs) are a well-known challenge. The ASME Code and various industry standards (such as RCC-M in France) address DMWs through specific requirements for preheat, interlayer temperature, and post-weld heat treatment (PWHT). The simulation results reinforce several established practices:
- PWHT necessity: The high residual stresses predicted at the interface underscore the importance of post-weld heat treatment to relieve these stresses. Typical PWHT for P91/310 combinations involves 760°C for 1-2 hours per 25 mm thickness.
- Welding sequence optimization: The annealing effect suggests that a planned welding sequence (such as back-step welding or balanced welding) can reduce overall residual stress levels.
- Interface composition control: Dilution from the base metal into the overlay can create intermediate phases (such as sigma phase or brittle intermetallics) that degrade service performance. The simulation's weld pool predictions can guide consumable selection to minimize harmful dilution.
Study Insights and Reflections
The paper's strength lies in its rigorous numerical methodology and the clear physical insights it provides into a phenomenon that is difficult to measure experimentally. However, the study has limitations that practitioners should consider. The model assumes material properties that are temperature-dependent but may not fully capture the complex phase transformations occurring during cooling (such as martensitic transformation in the low-alloy steel HAZ). Additionally, the study focuses on flat plate geometry, whereas actual nuclear components often involve complex geometries with constraints that amplify residual stresses. Future work should incorporate phase transformation modeling and validate against experimental measurements (such as X-ray diffraction or hole-drilling strain gauge methods) on actual dissimilar metal welds. The practical takeaway for engineers is clear: numerical simulation is a powerful tool for understanding and predicting DMW behavior, but it must be used in conjunction with experimental validation and code-based design criteria.