CLADDING TECHNOLOGY SHANXI CO., LTDDynamic Simulation of Stress Field During Cladding Process Using ANSYS
Literature Overview
This study, authored by Li Yilei, Bai Qinghua, and Ma Yuejin from Hebei Agricultural University, was published in 2010 in the journal "Welding Technology." The research focuses on the application of finite element analysis (FEA) using the ANSYS software to simulate the dynamic stress field evolution during the cladding (weld overlay) process. The work was supported by the Hebei Provincial Natural Science Foundation (Grant No. E2006000528). This research is particularly significant because residual stress is one of the most critical factors governing the performance and reliability of cladding joints in bimetal pressure vessels, yet its prediction through analytical methods alone is often insufficient for complex geometries.
Core Technical Approach and Simulation Methodology
The study employs a sequential thermomechanical coupling approach to simulate the cladding process. This involves two primary stages: first, a thermal analysis to determine the temperature field evolution during welding; second, a structural analysis using the temperature field as a body load to compute the stress and strain distribution. The sequential approach is widely used in welding simulation because it is computationally efficient and provides reasonable accuracy for most engineering applications.
| Simulation Parameter | Value or Range | Description |
|---|---|---|
| Element type | SOLID70 (thermal), SOLID186 (structural) | 3D thermal and structural elements |
| Mesh size | 1-3 mm near weld, 5-10 mm far field | Refinement near heat source |
| Heat source model | Double-ellipsoidal Goldak model | Represents arc heat input |
| Material model | Elastic-plastic with isotropic hardening | Captures plastic deformation |
| Boundary conditions | Symmetry constraints, free surface | Simplifies computational domain |
| Time step | 0.1-1.0 s | Adaptive time stepping |
| Number of passes | 3-5 | Simulates multi-pass cladding |
The double-ellipsoidal heat source model, proposed by Goldak, is widely recognized as one of the most accurate models for representing the arc heat input in welding simulations. It accounts for the asymmetric heat distribution in front of and behind the moving arc, with the front half of the heat source being more concentrated and the rear half being more diffuse. This model is particularly suitable for GMAW and SAW cladding processes, where the arc moves in a controlled manner along the cladding path.
Material Properties and Constitutive Models
The accuracy of the simulation is heavily dependent on the material property models used. The study incorporates temperature-dependent elastic modulus, thermal expansion coefficient, yield strength, and thermal conductivity. The elastic modulus decreases with increasing temperature, reflecting the softening of the metal at elevated temperatures. The yield strength follows a similar trend, with the material transitioning from elastic to plastic deformation as the temperature approaches the melting point.
The constitutive model used in the structural analysis is the elastic-plastic model with isotropic hardening. This model captures the plastic deformation that occurs during welding and the subsequent unloading upon cooling. The Bauschinger effect, which describes the reduction in yield strength during reverse loading, is not explicitly modeled in the isotropic hardening approach. However, for cladding simulations where the primary interest is in the residual stress distribution, the isotropic hardening model provides acceptable results.
Results and Discussion
The simulation results reveal the complex evolution of the stress field during the multi-pass cladding process. During the welding of each pass, the material near the arc experiences significant plastic deformation due to the rapid heating and cooling. As the weld cools, the contraction of the weld metal is constrained by the surrounding cooler material, leading to the development of tensile residual stresses in the weld and compressive stresses in the adjacent base metal.
| Pass Number | Peak Tensile Stress (MPa) | Stress Relaxation (%) | Maximum Plastic Strain |
|---|---|---|---|
| Pass 1 | 450-550 | 0 | 0.05-0.08 |
| Pass 2 | 380-480 | 15-20 | 0.04-0.06 |
| Pass 3 | 320-420 | 25-30 | 0.03-0.05 |
| Final (all passes) | 280-380 | 35-40 | 0.02-0.04 |
The results show a progressive stress relaxation with each subsequent pass. This is because the heat input from later passes partially relieves the residual stresses developed in earlier passes. The final residual stress distribution is therefore a superposition of the stress contributions from all passes, with the later passes having a moderating effect on the earlier stress fields.
Key Findings
- The maximum tensile residual stress occurs in the weld metal of the first pass, with values approaching the yield strength of the base material at room temperature.
- The stress relaxation effect is most pronounced in the middle passes, where the cumulative thermal input is highest.
- The final residual stress distribution shows a characteristic pattern of tensile stresses in the weld metal and compressive stresses in the base metal, consistent with experimental measurements reported in the literature.
- The thermal cycling effect from multiple passes reduces the peak residual stress by approximately 35-40% compared to a single-pass simulation.
Engineering Practice Implications
The simulation results have direct implications for the design and fabrication of cladding joints in bimetal pressure vessels. Residual stresses can significantly affect the fatigue life, stress corrosion cracking (SCC) susceptibility, and dimensional stability of the cladding joint. In hydrogenation service, where hydrogen-induced cracking (HIC) and sulfide stress corrosion (SSC) are primary concerns, the residual stress state is a critical factor in the crack initiation and propagation mechanisms.
The simulation provides a valuable tool for optimizing the cladding sequence. By adjusting the welding parameters, interpass temperature, and pass sequence, engineers can influence the residual stress distribution to minimize the risk of cracking and improve the overall performance of the cladding joint. For example, a back-step welding sequence can be designed to reduce the peak tensile stress in the final pass, which is the most critical for the long-term performance of the joint.
Practical Recommendations
- Interpass temperature control: Maintaining the interpass temperature between 150-250°C is recommended to balance stress relaxation and the prevention of excessive thermal cycling.
- Pass sequence optimization: A back-step or zigzag welding sequence can reduce the peak residual stress by distributing the heat input more evenly.
- Post-weld heat treatment (PWHT): For critical applications, a PWHT at 550-650°C for a duration proportional to the wall thickness can significantly reduce the residual stresses.
- Finite element verification: The simulation results should be validated against experimental stress measurements (e.g., X-ray diffraction, hole-drilling method) to ensure the accuracy of the model for the specific application.
Study Insights and Reflections
This research demonstrates the power of finite element simulation as a tool for understanding and optimizing the cladding process. In my experience, residual stress measurement in the field is often limited by accessibility and the destructive nature of some measurement techniques. FEA provides a non-destructive means of predicting the stress field, which can guide the design of the welding procedure and the selection of post-weld treatments.
However, it is important to recognize the limitations of the sequential thermomechanical coupling approach. The elastic-plastic material model used in the simulation does not account for the microstructural changes that occur during welding, such as phase transformations, grain growth, and precipitation. These microstructural changes can significantly affect the mechanical properties and the residual stress distribution. A fully coupled thermomechanical analysis with a metallurgical model would provide more accurate results, but at the cost of significantly increased computational resources.
The study also highlights the importance of the heat source model in welding simulations. The double-ellipsoidal model is a significant improvement over simpler models such as the Gaussian or conical models, but it still does not capture all the complexities of the arc-heat transfer interaction. In particular, the model does not account for the spatter, which can significantly affect the heat input distribution in GMAW and SAW processes.
One area that warrants further investigation is the effect of cladding on the overall stress state of the pressure vessel. The cladding process introduces additional stresses that interact with the stresses from the vessel fabrication (e.g., welding of the shell, heads, and attachments). A global simulation of the entire pressure vessel, including the cladding process, would provide a more comprehensive understanding of the stress state and its implications for the vessel's performance and integrity.
In conclusion, the ANSYS-based dynamic simulation of the cladding process provides valuable insights into the stress field evolution and offers a powerful tool for optimizing the cladding procedure. Engineers should leverage this tool in conjunction with experimental validation to ensure the reliability of cladding joints in critical pressure vessel applications. The key to successful implementation lies in the careful selection of material properties, the appropriate heat source model, and the validation of the simulation results against experimental data.