Zhuojin Pipe Fitting Co., LtdDynamic Simulation of Overlay Welding Thermal Stress Based on ANSYS Platform
Literature Overview
The paper authored by Wang Qiang and Li Donglin, published in 2004 in the Journal of Wuhan University of Technology (Transportation Science and Engineering), presents a finite element analysis approach to simulate the transient temperature field, stress field, residual stress, and deformation during the overlay welding process on flat plates. The study leverages the ANSYS software platform and its APDL (ANSYS Parametric Design Language) programming capability to implement a moving heat source model. This work falls within the classification TG455 (welding processes and equipment) and addresses a fundamental challenge in overlay welding: predicting and controlling residual stress and distortion without relying solely on empirical trial-and-error methods.
Core Technical Approach
The authors establish an appropriate finite element model to capture the dynamic evolution of thermal and mechanical fields during overlay welding. The key innovation lies in the use of APDL scripting to apply a moving heat source, which mimics the actual welding travel condition. This approach allows for the simulation of the transient temperature distribution throughout the weld pass, followed by the computation of thermal stresses and residual stresses upon cooling.
Moving Heat Source Model
The moving heat source is typically represented as a Gaussian or double-ellipsoidal distribution, where the heat flux intensity varies spatially and temporally as the welding torch traverses the workpiece. The APDL program automates the node-by-node activation of heat loads at each time step, effectively replicating the moving boundary condition. This is critical because the thermal history of each material point directly determines the phase transformation behavior, plastic strain accumulation, and ultimately the residual stress state.
Temperature Field and Stress Field Coupling
The simulation adopts a sequentially coupled thermal-mechanical approach. In the first stage, the transient heat conduction equation is solved to obtain the temperature distribution at each time increment. In the second stage, the temperature history is mapped onto the structural model, where the thermal strain is computed and the mechanical response is determined through an elasto-plastic constitutive law. The residual stress is extracted after the cooling phase when the structure is fully relaxed to ambient temperature.
Key Technical Parameters and Process Windows
| Parameter | Typical Value / Range | Remarks |
|---|---|---|
| Base material | Carbon steel plate (Q235/Q345) | Common overlay welding substrate |
| Welding process | SMAW or SAW overlay | Depends on application |
| Heat input | 15-35 kJ/mm | Influences HAZ width and dilution |
| Welding speed | 100-300 mm/min | Determines thermal cycle rate |
| Heat source power | 5-15 kW | Gaussian or double-ellipsoidal |
| Time step | 0.5-2 s | Ensures convergence of transient analysis |
| Mesh size near weld | 1-3 mm | Critical for stress gradient resolution |
| Material model | Elasto-plastic with thermal softening | Accounts for plastic deformation |
Engineering Practice Implications
From an engineering perspective, this study underscores the value of numerical simulation in optimizing overlay welding parameters before committing to physical trials. In the context of pipe fitting manufacturing, overlay welding is frequently employed to deposit corrosion-resistant or wear-resistant layers on carbon steel substrates. For example, stainless steel overlay on carbon steel pipe elbows or reducers for service in aggressive chemical environments requires precise control of dilution and residual stress to avoid cracking in the weld overlay or base metal HAZ.
The residual stress distribution predicted by the simulation can guide the design of post-weld stress relief procedures. For overlay welded pipe fittings, stress relief annealing temperatures typically range from 550 to 650 degrees Celsius for carbon steel substrates, but must be carefully controlled to avoid sensitization of stainless steel overlay layers. The simulation results can also inform the selection of preheating temperature and interpass temperature to reduce thermal gradients and minimize distortion.
Common Defects and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking in overlay layer | High residual stress, low ductility of overlay | Reduce heat input, increase preheat, post-weld stress relief |
| Excessive dilution | High heat input, thick overlay passes | Reduce current, increase travel speed, use dilution-resistant electrodes |
| Distortion | Asymmetric thermal input, constrained cooling | Symmetric welding sequence, back-plate clamping, stress relief |
| Porosity | Gas entrapment, poor flux coverage | Proper shielding gas flow, clean base metal, controlled travel speed |
Study Insights and Reflections
The 2004 publication represents an early but significant contribution to the computational modeling of overlay welding in the Chinese engineering community. The use of APDL for automated heat source application was particularly noteworthy at that time, as it reduced the manual effort required for time-step-by-time-step loading. However, the study is limited to flat plate geometry, which does not fully capture the curvature effects encountered in cylindrical pipe components. In modern practice, the simulation of overlay welding on curved geometries requires more sophisticated meshing strategies and often incorporates phase transformation models such as the Koistinen-Marburger equation to predict martensite formation in the HAZ.
The methodology described in this paper can be extended to the analysis of multi-pass overlay welding on pipe fittings, where the thermal interaction between successive passes significantly affects the final residual stress state. Engineers working on overlay welded pipe components should consider integrating such simulation tools into their process development workflow to reduce the number of destructive tests and accelerate qualification procedures.
In conclusion, this study provides a solid foundation for understanding the thermal-mechanical behavior of overlay welding through numerical simulation, and its methodology remains relevant for modern engineering applications involving overlay welded pipe fittings and components, where residual stress control is critical to service life and integrity.