Solidification Behavior and Thermal-Mechanical Coupling Analysis of TIG Welded 304 Stainless Steel Pipe Joints

Literature Overview and Research Objectives

This 2023 study published in Precision Forming Engineering by Yang Haibo, Chen Yong, and colleagues from Nanjing Vocational University of Industry Technology and Jiangsu University of Science and Technology presents a comprehensive investigation into the welding of 2 mm thick 304 austenitic stainless steel pipe. The research addresses two critical engineering problems: the optimization of TIG welding parameters for thin-walled pipe joints and the prediction of residual stress fields through coupled thermal-mechanical simulation. The work was supported by the Jiangsu Provincial Natural Science Foundation (20KJB460015) and institutional research funds, reflecting the practical importance of thin-walled stainless steel pipe fabrication in chemical processing, pharmaceutical, and food processing industries.

Welding Process Optimization and Results

The researchers employed a systematic experimental approach to determine the optimal TIG welding parameters for 2 mm thick 304 stainless steel pipe. The optimization was based on three evaluation criteria: macroscopic weld appearance, microstructural characteristics, and microhardness distribution.

At the optimal parameters of 150 A and 66 cm/min, the weld joint achieved complete penetration with uniform and dense weld beads on both the front and back sides. This parameter combination represents a practical process window for shop-floor application in pipe fabrication facilities.

Microstructural and Mechanical Characterization

The metallographic analysis revealed a characteristic grain morphology typical of TIG welding in austenitic stainless steel. The weld center region, both in the upper and lower portions, exhibited equiaxed grain structures, with the lower region showing slightly larger grain sizes compared to the upper region. Near the fusion line, columnar grains were observed, consistent with the directional solidification pattern driven by the heat extraction geometry.

The U-shaped hardness distribution across the weld cross-section is a well-known phenomenon in stainless steel welding. The HAZ exhibits the highest hardness due to grain refinement and possible precipitation hardening from solution-strengthening elements. The weld metal shows moderate hardness, while the region near the fusion line represents the weakest zone with a hardness of only 145 HV. This soft zone is a potential initiation site for fatigue cracking and must be considered in the design of pressure vessels and piping systems.

Thermal-Mechanical Coupled Simulation

The numerical simulation employed a double-ellipsoid heat source model combined with a temperature-displacement coupling approach. This methodology allows for the accurate representation of the moving heat source and the sequential coupling of thermal and mechanical fields. The simulation results provide valuable insights into the residual stress distribution that cannot be easily measured experimentally in thin-walled components.

The simulation results showed good agreement with experimental measurements, validating the numerical model. The longitudinal residual stress transitions from compressive in the base metal to tensile toward the weld center, which is the primary driver of weld cracking and distortion. The transverse stress is compressive at the weld center and gradually approaches zero at the base metal edges. The radial stress exhibits minimal variation through the thickness, which is expected for thin-walled geometry.

Engineering Practice Integration

For engineers involved in stainless steel pipe fabrication, this study offers several practical insights:

1. The optimal welding parameters identified (150 A, 66 cm/min) provide a reliable starting point for process development on 2 mm thick 304 stainless steel pipe, reducing the trial-and-error cycle time.
2. The U-shaped hardness distribution indicates that the fusion line region is the weakest link in the joint and should be the focus of non-destructive testing (NDT) procedures, particularly ultrasonic testing (UT) or phased array ultrasonic testing (PAUT).
3. The residual stress predictions from the simulation can inform stress relief procedures, such as post-weld heat treatment or mechanical stress relief, to mitigate the risk of stress corrosion cracking (SCC) in corrosive service environments.
4. The coupled simulation approach provides a powerful tool for process optimization without extensive physical testing, which is particularly valuable for thin-walled components where experimental stress measurement is technically challenging.

Study Insights and Conclusions

This research effectively bridges the gap between experimental welding characterization and numerical simulation for thin-walled stainless steel pipe fabrication. The combination of experimental optimization and thermal-mechanical coupled simulation provides a comprehensive understanding of the weld quality and residual stress state. For piping engineers and welding specialists working in the chemical and pharmaceutical industries, this study demonstrates the value of integrating simulation-based analysis into the welding procedure qualification process. The identified soft zone near the fusion line and the predicted residual stress patterns should be incorporated into the design considerations for welded pipe joints, particularly in applications involving cyclic loading or corrosive media. The study underscores the importance of a systematic approach to welding process development that considers both metallurgical and mechanical aspects of joint integrity.