Zhuojin Pipe Fitting Co., LtdInterlayer Stress Analysis and Process Optimization in Multi-Layer Metal Surfacing
Literature Overview
This paper by Xu Yan, Li Bingru, Bao Yang, Zhou Jianping, and Xue Ruilei, published in Foundry Technology (Vol. 38, No. 7, 2017, pp. 1713-1717), presents a finite element analysis-based study of residual stress and deformation in single-pass multi-layer overlay welding. The research was supported by the National Natural Science Foundation of China (Project No. 51665055), the Xinjiang Uygur Autonomous Region High Technology Research Program (Project No. 201113129), and the Xinjiang Youth Science and Technology Innovation Talent Training Project (Project No. gn2015yx008). The authors employed the Visual-weld software platform with the Sysweld solver to simulate the temperature field, stress field, and deformation field during multi-layer surfacing, with particular emphasis on the effects of welding speed and welding path on residual stress distribution. The optimized welding process was subsequently validated through machining experiments.
Core Technical Findings
The study identifies critical stress concentration zones and establishes quantitative relationships between process parameters and residual stress outcomes. The most significant finding is that interlayer junctions and arc start/stop locations constitute the primary stress concentration zones, representing the most vulnerable regions for cracking and delamination in multi-layer surfacing deposits. The authors further demonstrate that subsequent layers exert a stress-relief effect on previously deposited layers, which has important implications for surfacing sequence planning and interpass temperature control.
Residual Stress Distribution Patterns
The numerical simulation results reveal distinct residual stress patterns that vary systematically with position within the surfacing deposit:
| Location | Longitudinal Stress | Transverse Stress | Deformation | Risk Level |
|---|---|---|---|---|
| Interlayer junction | High tensile | High tensile | Significant | Critical |
| Arc start location | Very high tensile | Moderate tensile | Large | Critical |
| Arc stop location | Very high tensile | Moderate tensile | Large | Critical |
| Stable weld zone | Moderate tensile | Low tensile | Moderate | Moderate |
| Base plate near weld | Compressive | Compressive | Moderate | Low |
| Base plate far from weld | Negligible | Negligible | Negligible | Low |
The stress concentration at interlayer junctions arises from the thermal mismatch between the newly deposited hot layer and the previously solidified cooler layer. The rapid heating and cooling cycle at each interlayer interface generates differential thermal expansion and contraction, resulting in localized tensile stress concentrations. The arc start and stop locations are particularly problematic because the thermal input is discontinuous, creating sharp thermal gradients that translate into severe stress concentrations.
Effect of Welding Speed on Residual Stress
The study establishes a clear relationship between welding speed and residual stress components. As welding speed increases, the longitudinal residual stress in the weld zone increases while the transverse residual stress decreases. This inverse relationship can be understood through the thermal cycle theory: higher welding speeds produce narrower, more elongated heat-affected zones with steeper thermal gradients in the longitudinal direction but shallower gradients in the transverse direction. The practical implication is that welding speed selection involves a trade-off between longitudinal and transverse stress levels, and the optimal speed depends on the specific failure modes of concern for the application.
Effect of Welding Path on Residual Stress
One of the most practically valuable findings of this study is the demonstration that a reciprocating (back-and-forth) surfacing path significantly reduces residual stress in both the stable weld zone and the base plate compared to a unidirectional path. The reciprocating path achieves this by distributing the thermal input more evenly across the surfacing area, reducing the peak thermal gradient and thereby reducing the thermal stress amplitude. Additionally, the reciprocating path improves the quality of arc stop locations by allowing the arc to be terminated in a more thermally stable region rather than at the extreme edge of the deposit.
Process Optimization Recommendations
Based on the simulation results and experimental validation, the following process optimization recommendations emerge:
- Interlayer Temperature Control: Maintain interpass temperature within a controlled range to minimize thermal mismatch between layers while avoiding excessive softening of previously deposited layers.
- Arc Start and Stop Management: Implement back-step welding or arc-on-plate techniques at start and stop locations to redirect stress concentration away from critical deposit boundaries.
- Reciprocating Path Adoption: Use reciprocating surfacing paths to reduce overall residual stress levels and improve arc stop quality.
- Welding Speed Selection: Select welding speed based on the dominant failure mode of concern; lower speeds favor reduced longitudinal stress while higher speeds favor reduced transverse stress.
FMEA Analysis of Multi-Layer Surfacing
Applying Failure Mode and Effects Analysis (FMEA) to the findings of this study:
| Failure Mode | Potential Cause | Severity | Occurrence | Detection | RPN | Mitigation |
|---|---|---|---|---|---|---|
| Interlayer cracking | High interlayer stress | 9 | 7 | 6 | 378 | Reciprocating path, controlled interpass temp |
| Arc stop cracking | Stress concentration at stop | 8 | 8 | 5 | 320 | Back-step welding, arc-on-plate |
| Base plate distortion | Excessive transverse stress | 7 | 6 | 4 | 168 | Reciprocating path, lower heat input |
| Delamination | Stress mismatch at interface | 9 | 5 | 3 | 135 | Gradual transition layers, stress relief |
Engineering Practice Integration
For engineers involved in the manufacturing of surfaced pipe fittings, valve components, and other wear-resistant assemblies, this study provides a systematic framework for residual stress management. The identification of interlayer junctions and arc start/stop locations as critical stress zones should directly inform quality control procedures, including the selection of non-destructive testing locations and methods. Ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) should be directed at these critical zones to detect potential cracking or delamination.
The demonstration of the stress-relief effect of subsequent layers on previously deposited layers has important implications for surfacing sequence planning. In multi-layer surfacing operations, the order of layer deposition can be strategically planned to take advantage of this stress-relief mechanism, with layers deposited in a sequence that progressively reduces the residual stress in previously completed layers.
Study Insights and Reflections
This paper exemplifies the power of numerical simulation as a tool for understanding and optimizing complex welding processes. The use of the Visual-weld/Sysweld platform allows for detailed visualization of the temperature, stress, and deformation fields that are impossible to measure directly during welding. The authors' approach of systematically varying process parameters and analyzing their effects on residual stress provides a methodology that can be adapted to other surfacing applications.
A key insight from this study is that residual stress management in multi-layer surfacing is not merely a matter of post-weld stress relief but can be addressed during the welding process itself through careful selection of welding speed, path, and sequence. This in-process approach to stress management is more cost-effective and more reliable than relying solely on post-weld heat treatment.
The finding that subsequent layers relieve stress in previously deposited layers is particularly intriguing from a metallurgical perspective. This stress relief likely occurs through a combination of thermal softening and plastic deformation of the earlier layers during the thermal cycle of subsequent passes. The extent of this stress relief depends on the thermal cycle severity of subsequent passes, which in turn depends on welding parameters and interpass temperature.
In conclusion, this paper provides a comprehensive and practically valuable framework for residual stress analysis and process optimization in multi-layer metal surfacing. The identification of critical stress zones, the quantification of process parameter effects, and the demonstration of stress reduction through reciprocating paths all contribute to improved manufacturing quality and reliability. Engineers working on surfacing operations for critical pipeline components, pressure vessels, and wear-resistant fittings should incorporate these findings into their process design and quality assurance procedures.