Residual Stress Distribution in Narrow Gap Multi-Pass Welding of Austenitic Stainless Steel Pipes and the Effect of Diameter-to-Thickness Ratio

Literature Overview

This 2023 study by Yan Jialing and co-authors, published in Hot Working Technology, addresses a critical issue in the fabrication of austenitic stainless steel piping systems: the residual stress distribution in circumferential welds produced by narrow gap multi-pass welding. The research combines contour method experimental testing with finite element numerical simulation on a φ273 mm × 28 mm 316L austenitic stainless steel pipe. Funded by the National Natural Science Foundation of China (Project No. 51575251) and additional provincial grants, this work provides valuable insights for engineers designing and inspecting high-temperature and corrosion-resistant piping systems in power generation and petrochemical applications.

Methodology and Technical Approach

The study employs a dual methodology that is particularly well-suited to residual stress characterization. The contour method is a destructive measurement technique that determines residual stress by measuring the relief deformation of a sectioned component, while finite element simulation provides a non-destructive means of analyzing stress distributions under varying geometric parameters.

The selection of 316L stainless steel is significant because this grade is widely used in nuclear power plants, chemical processing facilities, and food-grade piping systems where resistance to pitting corrosion and intergranular corrosion is paramount. The low carbon content of the "L" grade reduces sensitization risk during welding, but residual stresses remain a critical concern for stress corrosion cracking (SCC) susceptibility.

Key Findings on Residual Stress Distribution

The most important finding is the "bending-type" distribution of both hoop stress and axial stress through the weld thickness. Near the inner wall, the weld and adjacent regions experience compressive stresses, while the outer wall region is under tensile stress. This distribution pattern is consistent with the thermal gradient that develops during welding, where the inner surface heats up first and expands, then cools and contracts, placing the outer surface in tension.

The finite element parametric study reveals that as the diameter-to-thickness ratio (R) increases while wall thickness remains constant, the hoop tensile stress zone slightly increases while the compressive stress zone decreases. More importantly, the axial stress values near both the inner and outer walls decrease, causing the through-thickness axial stress profile to transition toward a "self-equilibrating" stress state.

This transition toward self-equilibration is particularly significant for engineers concerned with stress corrosion cracking. A self-equilibrating stress state implies that the maximum tensile stress is lower than in a non-self-equilibrating state, which reduces the driving force for SCC initiation. This is especially relevant for 316L piping operating in chloride-containing environments, where even moderate tensile residual stresses can initiate intergranular stress corrosion cracking.

Process and Standards Analysis

The narrow gap welding process used in this study is governed by standards such as ASME Section IX, EN ISO 15614, and NB/T 47014 in China. The process requires precise control of the root pass to establish the gap width, followed by successive fill passes. Each pass introduces its own thermal cycle and residual stress pattern, which superimpose on the existing stress field.

For 316L stainless steel, the welding procedure specification must address several critical parameters:

1. Heat input control: Excessive heat input promotes sensitization even in low-carbon grades, while insufficient heat input leads to incomplete fusion and lack of penetration.
2. Interpass temperature: Typically limited to below 150°C to minimize thermal cycling and reduce residual stress buildup.
3. Post-weld heat treatment: Solution annealing at 1050-1100°C is often required to relieve residual stresses and restore corrosion resistance, but this may not be feasible for large-diameter piping already installed in service.
4. Mechanical post-weld treatment: Shot peening or hammer peening of the weld surface can introduce beneficial compressive residual stresses that counteract the tensile stresses identified in this study.

The "bending-type" stress distribution has direct implications for non-destructive testing strategy. Since tensile stresses concentrate at the outer surface, surface-breaking defects on the outer diameter are more likely to be critical from an SCC perspective. Engineers should prioritize external surface inspection using methods such as magnetic particle testing or eddy current testing, supplemented by phased array ultrasonic testing (PAUT) for volumetric defect detection.

Engineering Practice Integration

For power plant and petrochemical piping systems, the findings of this study have several practical applications. First, the diameter-to-thickness ratio effect suggests that larger diameter pipes (higher R) may actually benefit from slightly more favorable axial stress distributions, which should be considered when evaluating the risk of SCC in piping networks. Second, the compressive stress region near the inner wall is important for internal corrosion assessment, as compressive stresses generally inhibit crack initiation.

The contour method validation of finite element results provides confidence in using simulation for design optimization. Engineers can now leverage numerical models to predict residual stress distributions for piping geometries that are impractical to test experimentally, particularly for large-diameter spools where sectioning would be prohibitively expensive.

Study Insights and Reflections

This paper exemplifies the value of combining experimental validation with computational parametric studies in welding research. The contour method, while destructive and labor-intensive, provides the essential validation data that lends credibility to numerical predictions. The parametric study of diameter-to-thickness ratio effects addresses a practical design question that is often overlooked: how does geometry influence the residual stress state in a way that affects long-term durability?

One area for further investigation is the interaction between residual stresses and in-service thermal cycling. The static residual stress distribution documented in this study will evolve under repeated thermal loading, potentially through creep relaxation or cyclic plasticity. Understanding this evolution is critical for assessing the long-term integrity of welded piping joints in service.

The practical significance of this research cannot be overstated. In power generation and petrochemical industries, stress corrosion cracking of stainless steel piping represents a major failure mode, and residual stress management is a key mitigation strategy. This study provides the quantitative basis for optimizing welding procedures and post-weld treatments to minimize the tensile residual stress component that drives SCC initiation.