Zhuojin Pipe Fitting Co., LtdNumerical Simulation and Residual Stress Analysis of Multi-Pass Multi-Layer Surfacings Forming
Literature Overview
This paper, published in Mechanical Science and Technology in 2023, Volume 42, Issue 3, pages 408–414, presents a finite element analysis of the multi-pass multi-layer surfacing forming process for high-temperature alloys using Marc software. Authored by Zhang Xue, Xia Yufeng, Teng Hailao, Peng Mengxia, and Jin Li from Chongqing University, the study investigates the influence of surfacing thickness and surfacing path strategy on post-welding residual stress distribution. Funded by the National Natural Science Foundation of China (Grant 51775068), the work provides valuable insights for process optimization in thick overlay applications.
Core Technical Content
The numerical simulation employs a coupled thermo-mechanical analysis approach, which is essential for accurately predicting residual stress in welding processes. The thermal analysis calculates the temperature field evolution during the multi-pass multi-layer deposition, while the mechanical analysis uses the temperature-dependent material properties and plastic strain to compute the residual stress state. The Marc finite element software was selected for its advanced capabilities in handling the large deformation and complex material behavior associated with welding simulations.
Key Simulation Parameters and Results
| Parameter | Effect on Residual Stress |
|---|---|
| Increasing surfacing thickness | Increases transverse residual stress peak; changes local stress direction |
| Multi-directional stress state | Develops as thickness increases beyond optimal value |
| Interpass thermal effect | Causes stress release between layers due to temperature influence from adjacent welds |
| Sequential pass surfacing | Higher residual stress peaks, less uniform distribution |
| Sequential layer surfacing | More uniform stress distribution, lower residual stress peaks |
Analysis of Surfacing Thickness Effects
The study demonstrates that increasing the surfacing thickness leads to an increase in the transverse residual stress peak value. This is attributed to the cumulative thermal and mechanical effects of multiple passes, where each subsequent pass adds additional thermal strain to the already stressed material. More importantly, the increase in thickness causes a change in the direction of local stress in the surfacing layer, resulting in a multi-axial stress state rather than the predominantly uniaxial stress found in thinner coatings.
This finding has direct implications for the design of thick overlay coatings on pipe components. In applications such as corrosion-resistant overlay on pipeline elbows or high-temperature overlay on turbine components, the coating thickness must balance functional requirements (corrosion resistance, thermal barrier performance) against the risk of cracking induced by excessive residual stress. The study concludes that, within the range of thicknesses that satisfy service requirements, the surfacing thickness should not be made excessively large.
Interpass Stress Release Phenomenon
A particularly interesting finding is the observation of stress release between surfacing layers. This phenomenon occurs because the thermal field from subsequent welds partially relieves the residual stress established by previous welds. The degree of stress release depends on the interpass temperature and the spatial relationship between adjacent welds. This finding supports the practice of controlling interpass temperature in multi-pass welding, as it directly influences the final residual stress state.
Surfacing Path Strategy Comparison
The comparison between sequential pass surfacing and sequential layer surfacing reveals significant differences in residual stress distribution. Sequential pass surfacing, where all passes in a single layer are completed before moving to the next layer, produces higher residual stress peaks and less uniform stress distribution. Sequential layer surfacing, where the deposition follows a layer-by-layer progression with all passes in each layer completed before the next layer, produces more uniform stress distribution with lower residual stress peaks.
Path Strategy Comparison
| Aspect | Sequential Pass Surfacing | Sequential Layer Surfacing |
|---|---|---|
| Stress uniformity | Poor | Good |
| Peak residual stress | Higher | Lower |
| Thermal accumulation | Localized | Distributed |
| Distortion tendency | Higher | Lower |
| Processing complexity | Simpler path planning | More complex path planning |
| Recommended for | Thin coatings, simple geometries | Thick coatings, complex geometries |
The superiority of sequential layer surfacing can be attributed to the more uniform thermal history experienced by the coating material. In sequential layer surfacing, the thermal input is distributed more evenly across the coating volume, reducing thermal gradients and, consequently, residual stress gradients. This is particularly important for thick coatings where thermal gradients can be severe.
Engineering Practice Implications
For pipe and fitting manufacturers producing components with thick overlay coatings—such as high-pressure pipeline fittings with corrosion-resistant cladding, or heat exchanger tubes with thermal barrier coatings—the findings of this study provide actionable guidance for process optimization.
Process Optimization Recommendations
- Thickness optimization: Determine the minimum coating thickness that satisfies service requirements and avoid unnecessary thickness increases that elevate residual stress without providing additional functional benefit.
- Path strategy selection: For coatings exceeding two to three layers, adopt sequential layer surfacing to minimize residual stress and improve coating integrity.
- Interpass temperature control: Maintain interpass temperatures within the range that promotes stress release while avoiding excessive softening that could compromise the coating's mechanical properties.
- Stress relief consideration: For critical applications where residual stress must be minimized, plan for post-welding stress relief treatment, using the simulation results to predict the required temperature and duration.
Key Questions and Reflections
While the numerical simulation provides valuable predictive capabilities, several limitations should be acknowledged. First, the accuracy of the simulation depends on the material property database used, particularly the temperature-dependent mechanical properties and plastic strain behavior of the high-temperature alloy. Second, the simulation assumes a specific weld bead geometry and thermal profile, which may not exactly match the actual process conditions. Third, the study does not validate the simulation results against experimental residual stress measurements, which would be essential for establishing confidence in the predictions.
From my experience in pipe fitting manufacturing, I have observed that residual stress cracking in thick overlay coatings is a significant quality issue, particularly in high-strength steel and alloy components. The study's recommendation to limit coating thickness and adopt sequential layer surfacing aligns with our practical experience, where we have observed fewer cracking incidents in coatings produced with layer-by-layer deposition strategies.
Study Insights and Outlook
This study demonstrates the value of numerical simulation as a tool for process optimization in multi-pass multi-layer surfacing operations. The ability to predict residual stress distribution before physical trials can significantly reduce development time and cost, particularly for complex geometries where experimental characterization is challenging.
The findings regarding surfacing path strategy have broader implications for additive manufacturing processes, where the deposition sequence is a critical process parameter that directly influences part quality. As the boundary between traditional surfacing and additive manufacturing continues to blur, the process optimization principles established in this study will be increasingly relevant to both fields.
The integration of simulation-based process optimization with experimental validation should be the standard approach for developing new surfacing processes, particularly for critical applications in the oil and gas, power generation, and aerospace industries where coating failure can have severe consequences.