CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Numerical Simulation of Dissimilar Metal Flat Cladding Between Low-Alloy Steel and Stainless Steel
Literature Overview and Background
The numerical simulation of dissimilar metal flat cladding between low-alloy steel and stainless steel is a critical area of research for the design and fabrication of bimetal pressure vessels and heat exchangers. The cladding process involves the metallurgical bonding of a corrosion-resistant stainless steel layer to a structural low-alloy steel substrate, creating a composite material that combines the strength of the base metal with the corrosion resistance of the cladding layer. However, the significant differences in thermal and mechanical properties between the two metals lead to complex thermal and stress fields during the cladding process, which can result in residual stresses, distortion, and even cracking. Numerical simulation provides a powerful tool for predicting these phenomena and optimizing the cladding process parameters to achieve a high-quality bond without defects. This study note examines the finite element modeling approach, key findings, and engineering implications of the numerical simulation of dissimilar metal flat cladding.
Numerical Modeling Approach
The numerical simulation of the cladding process is typically performed using a coupled thermo-mechanical finite element analysis (FEA). The model consists of two main components: a thermal analysis to predict the temperature distribution during the cladding process, and a mechanical analysis to predict the residual stresses and deformation resulting from the thermal cycling. The key modeling assumptions and parameters include:
| Parameter | Value / Description |
|---|---|
| Base metal | 16MnR (low-alloy steel, σb = 490 MPa) |
| Cladding material | 304 stainless steel (σb = 520 MPa) |
| Cladding thickness | 3 mm |
| Base plate thickness | 20 mm |
| Welding process | ESW (electroslag welding) overlay |
| Heat input | 15–25 kJ/mm |
| Travel speed | 100–200 mm/min |
| Preheat temperature | 150–250°C |
| Thermal conductivity (16MnR) | 45 W/m·K (25°C), temperature-dependent |
| Thermal conductivity (304 SS) | 16 W/m·K (25°C), temperature-dependent |
| Thermal expansion (16MnR) | 12×10⁻⁶ /°C |
| Thermal expansion (304 SS) | 17×10⁻⁶ /°C |
| Elastic modulus (16MnR) | 206 GPa |
| Elastic modulus (304 SS) | 193 GPa |
The thermal analysis employs a moving heat source model to simulate the progression of the welding torch or slag pool along the cladding line. The heat source is typically modeled as a double-ellipse (Goldak model) to account for the asymmetric heat distribution in front of and behind the moving arc. The boundary conditions include convective heat transfer from the exposed surfaces and radiative heat loss from the top surface. The mechanical analysis is performed after the thermal analysis, using the temperature-dependent material properties and the plasticity model to predict the residual stresses and deformation.
Key Findings from Simulation
The numerical simulation reveals several important findings regarding the thermal and mechanical behavior of the dissimilar metal cladding process. First, the temperature distribution is highly asymmetric due to the difference in thermal conductivity between the base metal and the cladding layer. The stainless steel cladding layer, with its lower thermal conductivity (16 W/m·K compared to 45 W/m·K for 16MnR), retains more heat, resulting in a higher peak temperature and a wider heat-affected zone in the cladding layer compared to the base metal. The peak temperature in the cladding layer reaches approximately 1450°C, while the peak temperature in the base metal is approximately 1200°C at a distance of 5 mm from the weld centerline.
The residual stress distribution is also asymmetric. The maximum tensile residual stress in the cladding layer reaches 350–400 MPa, while the maximum compressive residual stress in the base metal reaches -200 to -250 MPa. The residual stress in the cladding layer is significantly higher than that in the base metal due to the higher thermal expansion coefficient of the stainless steel (17×10⁻⁶ /°C compared to 12×10⁻⁶ /°C for 16MnR). This difference in thermal expansion creates a mismatch stress at the interface, which can be detrimental to the bond strength and the resistance to intergranular corrosion.
The simulation also predicts the distortion of the cladded plate. The maximum angular distortion at the free edge of the plate is approximately 2.5° for a 200 mm wide plate with a 3 mm cladding layer. This distortion is primarily caused by the differential thermal contraction between the cladding layer and the base metal during cooling. The distortion can be reduced by applying a back-up plate on the opposite side of the base metal, which constrains the lateral expansion and reduces the angular distortion to approximately 1.0°.
Effect of Process Parameters on Residual Stress and Distortion
The simulation is used to investigate the effect of various process parameters on the residual stress and distortion. The results show that increasing the preheat temperature from 150°C to 300°C reduces the peak residual stress by approximately 15–20%, but increases the distortion by 10–15%. This is because the higher preheat temperature reduces the thermal gradient and the associated thermal stress, but also increases the overall thermal expansion. The optimal preheat temperature for minimizing both residual stress and distortion is approximately 200–250°C.
The effect of welding speed is also significant. Increasing the welding speed from 100 mm/min to 200 mm/min reduces the peak temperature and the heat-affected zone width, resulting in a lower residual stress (by approximately 10–15%) but a higher stress gradient. The higher welding speed also reduces the distortion by approximately 20%, as the heat input per unit length is lower. However, the higher welding speed may compromise the bond strength if the cooling rate is too fast, leading to excessive hardness and cracking in the heat-affected zone.
| Parameter Variation | Peak Residual Stress (MPa) | Angular Distortion (°) | Bond Strength (MPa) |
|---|---|---|---|
| Base case (150°C preheat, 150 mm/min) | 380 | 2.5 | 280 |
| 300°C preheat, 150 mm/min | 320 | 2.9 | 265 |
| 150°C preheat, 200 mm/min | 330 | 2.0 | 290 |
| 200°C preheat, 180 mm/min | 340 | 2.1 | 285 |
| 200°C preheat, 150 mm/min + back-up plate | 300 | 1.0 | 295 |
Validation and Comparison with Experimental Results
The numerical simulation results are validated by comparing the predicted residual stress and distortion with experimental measurements obtained from strain gauge measurements and digital image correlation (DIC). The predicted peak residual stress in the cladding layer (380 MPa) is within 10% of the measured value (350 MPa), and the predicted angular distortion (2.5°) is within 15% of the measured value (2.2°). The good agreement between the simulation and experimental results confirms the accuracy of the model and its suitability for process optimization.
The simulation is also used to predict the effect of post-weld stress relief treatment on the residual stress distribution. A stress relief annealing at 600°C for 2 hours reduces the peak residual stress by approximately 60%, from 380 MPa to 150 MPa. However, the stress relief treatment may cause some distortion due to the differential thermal contraction between the cladding layer and the base metal during heating and cooling. The simulation predicts an additional angular distortion of 0.5° after stress relief, which is acceptable for most applications but may need to be considered for precision applications.
Engineering Practice Integration
The numerical simulation provides valuable guidance for the design and fabrication of dissimilar metal cladded plates. The following recommendations are derived from the simulation results:
- Use a preheat temperature of 200–250°C to balance residual stress and distortion.
- Use a welding speed of 150–180 mm/min to achieve a good balance between bond strength and distortion.
- Apply a back-up plate on the opposite side of the base metal to reduce distortion.
- Perform a post-weld stress relief treatment at 600°C for 2 hours to reduce residual stress.
- Control the interpass temperature below 250°C to prevent excessive softening of the base metal.
The simulation also highlights the importance of the cladding layer thickness. For a 3 mm cladding layer on a 20 mm base plate, the residual stress in the cladding layer is approximately 380 MPa. If the cladding layer thickness is increased to 6 mm, the residual stress increases to approximately 450 MPa, which may exceed the yield strength of the stainless steel and cause plastic deformation. Therefore, for thick cladding layers, a multi-pass approach with intermediate stress relief steps is recommended.
Key Questions and Reflections
Several important questions arise from the numerical simulation study. First, the long-term stability of the residual stress distribution under cyclic thermal loading (as in pressure vessel operation) is not fully understood. The simulation predicts a certain residual stress state after welding and stress relief, but the evolution of this stress state under cyclic thermal loading may lead to stress relaxation or stress redistribution, which could affect the bond strength and the resistance to intergranular corrosion. Second, the effect of microstructural evolution on the mechanical properties of the cladding layer and the heat-affected zone is not fully captured by the current model. The transformation of austenite to martensite in the heat-affected zone of the low-alloy steel, and the precipitation of carbides in the stainless steel cladding layer, can significantly affect the hardness and toughness of the materials, and these effects should be incorporated into the model for more accurate predictions.
The study also raises questions about the applicability of the simulation to other cladding processes, such as submerged arc welding (SAW) and electroslag welding (ESW). The heat source model and the boundary conditions may need to be adjusted for different welding processes, and the accuracy of the model for each process should be validated through experimental comparison. Additionally, the simulation assumes a perfectly bonded interface between the cladding layer and the base metal, but in practice, the bond quality may vary due to defects such as lack of fusion, slag inclusion, or porosity. The effect of these defects on the residual stress distribution and the bond strength should be investigated through parametric studies.
Study Insights and Implications
The numerical simulation of dissimilar metal flat cladding between low-alloy steel and stainless steel provides a powerful tool for understanding the thermal and mechanical behavior of the cladding process and for optimizing the process parameters to achieve a high-quality bond. The key insight from this study is that the residual stress and distortion in the cladded plate are primarily governed by the difference in thermal expansion coefficients between the two metals, and that these effects can be mitigated through careful selection of process parameters and the use of back-up plates and stress relief treatments.
For engineers involved in the design and fabrication of bimetal pressure vessels and heat exchangers, the simulation results provide valuable guidance on the selection of appropriate process parameters and the design of fabrication procedures. The simulation can also be used to predict the residual stress state in complex geometries, such as curved surfaces and nozzles, where experimental measurement is difficult. This capability is particularly valuable for the design of critical components in nuclear power plants, chemical processing plants, and oil and gas facilities, where the integrity of the cladding layer is essential for long-term reliable operation.
In conclusion, the numerical simulation of dissimilar metal flat cladding represents a significant advancement in the field of bimetal product manufacturing, providing a predictive tool that can reduce the need for costly trial and error experimentation and improve the quality and reliability of cladded components.