Zhuojin Pipe Fitting Co., LtdResidual Stress Distribution in Narrow-Gap Welding of Austenitic Stainless Steel Pipes and the Effect of Diameter-to-Thickness Ratio
Literature Overview
This research, published in Hot Working Technology in 2023 by Yan Jialing and colleagues from Datang Boiler Pressure Vessel Inspection Center, China Datang Corporation, Foshan University of Science and Technology, and Xi'an University of Technology, addresses a critical issue in the welding of austenitic stainless steel pipes: the distribution of residual stresses following narrow-gap multi-pass welding. The study focuses on 316L austenitic stainless steel pipes with dimensions of 273 mm diameter by 28 mm wall thickness, a specification commonly encountered in power generation and petrochemical applications. The research employs both contour method experimental testing and finite element numerical simulation to characterize the residual stress field, and further investigates the influence of the diameter-to-thickness ratio (R) on the circumferential and axial residual stress distributions.
Core Technical Findings
Residual Stress Distribution Characteristics
The study reveals that the circumferential residual stress in the weld zone exhibits a "bending-type" distribution across the wall thickness. Specifically, the weld region near the inner wall and its adjacent areas are in a compressive stress state, while the outer wall region is in a tensile stress state. This pattern is consistent with the thermal-mechanical behavior expected during multi-pass welding, where each subsequent pass subjects the previously deposited metal to reheating and plastic deformation. The axial residual stress also follows a similar bending-type distribution, with compressive stresses near the inner wall and tensile stresses near the outer wall.
The contour method experimental results and the finite element simulation results show good agreement in terms of the distribution trend of circumferential stresses within the weld zone. This validation is important because it confirms the reliability of the numerical model for parametric studies that would be impractical to conduct experimentally for every possible geometric configuration. The contour method, which involves cutting the specimen at the plane of interest and measuring the deformation that occurs upon release of residual stresses, provides a direct measurement of the through-thickness stress distribution that is difficult to obtain by other non-destructive means.
Influence of Diameter-to-Thickness Ratio
The parametric study reveals that, at constant wall thickness, increasing the diameter-to-thickness ratio (R) leads to the following changes in the residual stress field:
| Parameter | Effect of Increasing R |
|---|---|
| Circumferential tensile stress zone | Slightly increases in extent |
| Circumferential compressive stress zone | Decreases in extent |
| Axial compressive stress near inner wall | Decreases in magnitude |
| Axial tensile stress near outer wall | Decreases in magnitude |
| Through-thickness axial stress profile | Shifts toward self-balancing state |
The trend toward a self-balancing axial stress state at higher R values is particularly significant. A self-balancing stress state means that the integral of the axial stress across the wall thickness approaches zero, which is favorable for minimizing the risk of stress-corrosion cracking and for reducing the sensitivity of the weld to residual stress-related fatigue failure. This finding has direct implications for the design of large-diameter stainless steel pipe spools, where the diameter-to-thickness ratio is inherently higher.
Process Analysis and Welding Metallurgy
Narrow-Gap Welding Process Considerations
Narrow-gap welding, often implemented using pulsed GTAW or hybrid processes, is a preferred method for thick-walled stainless steel pipes because it minimizes the number of passes, reduces heat input per pass, and limits the dilution ratio between the base metal and filler metal. For 316L stainless steel pipes with a 28 mm wall thickness, the narrow-gap technique typically requires 6-12 passes depending on the gap preparation and process parameters. Each pass introduces a localized thermal cycle that contributes to the cumulative residual stress field.
The residual stress distribution observed in this study is consistent with the thermal contraction theory of welding residual stresses. During welding, the weld metal and the adjacent heat-affected zone (HAZ) experience rapid heating followed by cooling. The cooling phase generates compressive stresses as the contracting weld metal is constrained by the cooler surrounding material. As subsequent passes are deposited, the previously deposited metal is reheated and partially relieved, but the cumulative effect of multiple thermal cycles produces the characteristic bending-type stress distribution.
Heat-Affected Zone Considerations
For 316L austenitic stainless steel, the HAZ is characterized by a relatively narrow region of microstructural change compared to ferritic or martensitic steels. The austenitic structure is retained in the HAZ, but grain growth and potential sensitization (chromium carbide precipitation at grain boundaries) can occur depending on the thermal cycle. The residual stress field interacts with these microstructural features to influence the long-term performance of the weld, particularly in terms of stress-corrosion cracking susceptibility in chloride-containing environments.
Engineering Practice Implications
Residual Stress Management Strategies
Based on the findings of this study, the following strategies can be employed to manage residual stresses in narrow-gap welded stainless steel pipe assemblies:
- Post-weld heat treatment (PWHT): A stress-relief annealing cycle at approximately 425-450 degrees Celsius for 316L stainless steel can significantly reduce residual stresses. However, PWHT must be carefully controlled to avoid sensitization, which can reduce corrosion resistance.
- Shot peening or ultrasonic impact treatment: These surface treatments can introduce beneficial compressive residual stresses on the surface, which can offset the tensile stresses near the outer wall and improve fatigue performance.
- Optimized welding sequence: The sequence of weld passes can be designed to minimize the peak residual stress levels by ensuring that each new pass is deposited in a location that promotes the relaxation of previously generated stresses.
- Interpass temperature control: Maintaining interpass temperatures within a specified range (typically below 150 degrees Celsius for 316L) helps control the thermal gradients and reduces the magnitude of residual stresses.
Quality Control Integration
For quality control purposes, the residual stress distribution should be considered in the following ways:
- Non-destructive testing: While conventional NDT methods such as radiographic testing (RT) and ultrasonic testing (UT) are effective for detecting volumetric and surface defects, they do not directly measure residual stresses. Magnetic particle testing (MT) can reveal surface cracks that may be induced by high residual stresses, but it is not a direct stress measurement technique.
- Corrosion testing: Residual stresses in the tensile regime, particularly near the outer wall, can increase susceptibility to stress-corrosion cracking (SCC) in chloride-containing environments. Post-weld corrosion testing, such as the ASTM G48 or ASTM G108 protocols, should be considered for critical applications.
- Design margin: The residual stress field should be accounted for in the structural design by applying appropriate stress concentration factors or by incorporating the effect of residual stresses into the fatigue assessment per relevant codes such as EN 1993-1-9 or DNV-RP-C203.
Key Questions and Reflections
The study raises an important question about the extrapolation of residual stress findings from a single pipe size (273 mm by 28 mm) to other pipe geometries. While the parametric study of the diameter-to-thickness ratio provides valuable insights, the absolute stress magnitudes may vary with pipe size, welding procedure, and filler metal selection. In practice, each welding procedure should be validated through either experimental measurement or validated numerical simulation before being applied to production welding.
Another consideration is the interaction between residual stresses and the operating conditions of the pipe. In service, the pipe is subjected to internal pressure, external loads, thermal cycling, and potentially corrosive media. The residual stress field superimposes on the service stress field, and the combined effect can be either beneficial (compressive residual stresses offsetting tensile service stresses) or detrimental (tensile residual stresses adding to tensile service stresses). A comprehensive assessment of the combined stress state is essential for the integrity evaluation of welded stainless steel pipe assemblies.
From a personal perspective, the finding that higher diameter-to-thickness ratios lead to a more self-balancing axial stress distribution is particularly encouraging for the design of large-diameter stainless steel pipe spools. This trend suggests that the residual stress challenges associated with large-diameter pipes may be somewhat self-mitigating in terms of axial stress balance, although the circumferential stress distribution still requires attention.
Summary
This study provides a comprehensive characterization of the residual stress field in narrow-gap welded 316L austenitic stainless steel pipes, validated through both contour method testing and finite element simulation. The bending-type distribution of circumferential and axial stresses across the wall thickness, with compressive stresses near the inner wall and tensile stresses near the outer wall, is consistent with thermal-mechanical theory and has important implications for stress-corrosion cracking susceptibility and fatigue performance. The parametric study of the diameter-to-thickness ratio reveals that higher R values promote a more self-balancing axial stress state, which is favorable for structural integrity. These findings should inform welding procedure development, post-weld treatment strategies, and quality control protocols for stainless steel pipe welding in critical applications.