CLADDING TECHNOLOGY SHANXI CO., LTDDynamic Process Experiment and Numerical Analysis of Overlay Angular Deformation
Literature Overview
This 2007 paper by Liu Chuan, Wang Rui, and Zhang Jianxun of Xi'an Jiaotong University, published in the Journal of Xi'an Jiaotong University, addresses a fundamental challenge in overlay welding: the prediction and control of angular deformation. The research was supported by the National Natural Science Foundation of China (Project No. 50475093) and the Ministry of Education Doctoral Program Special Research Fund (Project No. 20030698018). Angular deformation is one of the most problematic distortion modes in overlay welding, particularly for thick-walled components where the overlay is applied to one surface only. The paper combines experimental measurement with finite element analysis to provide a comprehensive understanding of the deformation mechanism and its evolution during the welding process.
Deformation Mechanism in Overlay Welding
Angular deformation in overlay welding arises from the asymmetric heat input and the resulting asymmetric thermal strain field. When the overlay is applied to the top surface of a plate, the top surface experiences greater thermal expansion and subsequent contraction than the bottom surface. This differential strain causes the plate to curl upward, with the angle of curl increasing with the number of overlay passes and the thickness of the overlay layer.
The deformation can be decomposed into three primary mechanisms:
- Thermal bending: Caused by the temperature gradient through the plate thickness, which induces a bending moment.
- Residual stress relaxation: As the weld cools, the compressive residual stresses in the base metal near the weld can relax, contributing to additional deformation.
- Plastic strain accumulation: In thick plates or with high heat input, plastic deformation occurs in the base metal, leading to permanent angular distortion.
| Deformation Component | Relative Contribution (Typical) | Dependence on Parameters |
|---|---|---|
| Thermal bending | 40–60% | Strongly dependent on heat input and plate thickness |
| Residual stress relaxation | 20–35% | Dependent on plate thickness and yield strength |
| Plastic strain | 10–25% | Dependent on heat input and base metal yield strength |
The authors developed a three-dimensional finite element model using the coupled thermo-mechanical approach, incorporating a nonlinear material model that accounts for temperature-dependent elastic-plastic behavior, thermal expansion, and the Bauschinger effect. The model was validated against experimental measurements obtained from a strain gauge system and a laser displacement sensor.
Experimental Methodology
The experimental program involved overlay welding of stainless steel (308L) onto carbon steel (Q345R) plates with thicknesses of 20, 30, and 40 mm. The overlay was applied using submerged arc welding (SAW) with a wire diameter of 1.6 mm and a travel speed of 400 mm/min. The welding current was maintained at 350–400 A, and the arc voltage at 28–32 V. The overlay thickness was varied from 3 to 10 mm by adjusting the number of passes.
The angular deformation was measured at multiple points along the plate length using strain gauges bonded to the top and bottom surfaces. The measurements were taken at three stages: immediately after welding (hot state), after cooling to room temperature (cold state), and after stress relief heat treatment. The results showed that the angular deformation increased with overlay thickness and decreased with plate thickness, following a relationship that can be approximated as:
θ ∝ (q × t_overlay) / (E × t_base²)
where θ is the angular deformation, q is the linear heat input, t_overlay is the overlay thickness, t_base is the base plate thickness, and E is the elastic modulus.
Numerical Simulation Results
The finite element analysis revealed that the angular deformation develops in two distinct phases:
- During welding: The plate curls upward as the weld pool advances, with the maximum deformation occurring at the trailing edge of the weld. The deformation rate is highest during the first few passes and decreases as the overlay thickness increases.
- During cooling: Additional deformation occurs as the weld cools, with the total deformation being 15–30% greater than the hot-state deformation. This additional deformation is attributed to the relaxation of compressive residual stresses in the base metal.
The simulation also predicted the distribution of residual stresses through the plate thickness. The top surface (overlay surface) develops compressive residual stresses of 100–200 MPa, while the bottom surface develops tensile residual stresses of 150–300 MPa. The neutral axis shifts toward the bottom surface as the overlay thickness increases, which is consistent with the observed angular deformation.
Process Optimization Strategies
Based on the experimental and numerical results, the authors propose several strategies to control angular deformation:
| Strategy | Effectiveness | Practical Feasibility |
|---|---|---|
| Backing weld on the opposite surface | High – reduces asymmetry by 30–50% | Moderate – requires additional setup and time |
| Reduced heat input per pass | Moderate – reduces thermal gradient | Moderate – may require more passes |
| Sequential welding from center outward | Moderate – reduces free edge curl | High – easily implemented |
| Mechanical restraint (clamping) | High – prevents deformation during welding | Moderate – may introduce additional residual stresses |
| Post-weld stress relief | Low – does not correct angular deformation | High – standard practice |
The most effective strategy identified by the authors is the combination of a backing weld and sequential welding from the center outward. This approach can reduce angular deformation by 40–60% compared to conventional single-surface overlay welding without a backing weld.
Engineering Practice Integration
In my experience with pressure vessel fabrication, angular deformation control is one of the most challenging aspects of overlay welding. The paper's findings are directly applicable to the fabrication of hydrogenation reactors, where the overlay is typically applied to the inner surface of a thick-walled vessel. The recommended approach of using a backing weld is particularly relevant for large-diameter vessels, where the curvature of the shell can exacerbate angular deformation.
A practical approach that we have adopted in our shop is the use of a "balanced overlay" technique, where the overlay is applied alternately to the top and bottom surfaces in thin layers. This approach maintains the symmetry of the thermal field and significantly reduces angular deformation, although it requires more passes and longer fabrication time.
Key Reflections
This paper makes a significant contribution to the understanding of angular deformation in overlay welding through the combined use of experimental measurement and numerical simulation. The finite element model developed by the authors provides a powerful tool for predicting deformation and optimizing process parameters. However, the model assumes a perfectly homogeneous material and may not capture the full complexity of real overlay welds, where the microstructure and properties vary significantly through the overlay thickness. Future work should focus on incorporating more realistic material models and accounting for the effects of welding sequence on deformation.