Solidification Behavior and Thermo-Mechanical Coupling Analysis of 304 Stainless Steel Thin-Wall Pipe TIG Weld Joints

Literature Overview

This 2023 study by Yang Haibo and colleagues from Nanjing Vocational University of Industry Technology, Jiangsu University of Science and Technology, and Jiangsu Maritime Vocational Technical College presents a comprehensive investigation of TIG welding on 2 mm thick 304 stainless steel pipes. The research combines experimental optimization of welding parameters with finite element simulation of the thermal and stress fields, addressing the practical challenge of stress measurement difficulties in thin-wall pipe joints. Funded by the Jiangsu Provincial Natural Science Research Project (20KJB460015) and related institutional grants, this work bridges the gap between laboratory experimentation and numerical prediction for thin-section stainless steel pipe fabrication.

Welding Parameter Optimization

The researchers systematically varied welding current and travel speed to identify optimal parameters for complete penetration with good bead geometry on 2 mm 304 stainless steel pipe. The optimal parameters identified were a welding current of 150 A and a travel speed of 66 cm/min, which achieved full penetration with uniform, dense weld beads on both the front and back surfaces.

Microstructural Analysis

The microstructural examination reveals distinct zones within the weld joint:

• Weld center (upper region): Equiaxed grain morphology, indicating grain boundary melting and nucleation at the liquidus isotherm.
• Weld center (lower region): Equiaxed grains slightly larger than the upper region, suggesting differences in cooling rate between the front and back surfaces of the pipe.
• Fusion line vicinity: Columnar grain structure, characteristic of directional solidification from the fusion boundary into the weld pool.

The presence of equiaxed grains throughout the weld center suggests either high grain refinement (possibly due to pipe geometry promoting multiple nucleation sites) or significant grain boundary melting during welding. The slight difference in grain size between upper and lower regions reflects the asymmetric heat dissipation inherent in pipe welding — the back surface typically experiences faster cooling due to the smaller thermal mass and potential air cooling.

Hardness Distribution and Thermo-Mechanical Simulation

The microhardness distribution exhibits a characteristic U-shape pattern:

The HAZ hardness matching the base metal is typical for austenitic stainless steels, which do not undergo phase transformations during welding. The lower weld center hardness reflects the complete dissolution and re-solidification of the microstructure, producing a relatively soft, single-phase austenite. The minimum hardness at the fusion line is attributed to the presence of a narrow partially melted zone where incomplete dissolution creates a soft, locally over-heated region with coarsened grains.

The numerical simulation employed a double-ellipsoidal heat source model with temperature-displacement coupling. Key simulation results include:

• Longitudinal residual stress: Transitions from compressive stress in the base metal to tensile stress at the weld center, reflecting the constraint imposed by the cooler surrounding material on the contracting weld metal.
• Transverse residual stress: Compressive at the weld center, gradually approaching zero toward the base metal.
• Radial stress: Minimal variation, consistent with the thin-wall geometry where through-thickness gradients are small.

The agreement between simulated and measured stress trends validates the simulation approach for predicting residual stress in thin-wall pipe joints where direct measurement is impractical.

Engineering Practice Integration

For pipe fabrication engineers, this study provides several actionable insights:

• The optimal parameter window (150 A / 66 cm/min) for 2 mm 304 SS pipe is narrow and requires careful calibration. Deviations can lead to incomplete penetration or excessive burn-through.
• The U-shaped hardness profile indicates that the fusion line region is the mechanically weakest zone. In applications subject to cyclic loading or corrosion, this region warrants special attention during quality assurance.
• The residual stress pattern (tensile longitudinal at weld center) has implications for stress corrosion cracking susceptibility. Post-weld stress relief may be necessary for applications in chloride-containing environments, particularly for piping systems governed by ASME B31.3 or similar codes.
• The simulation approach offers a practical alternative to strain gauge measurements for thin-wall components, enabling process qualification without extensive destructive testing.

Study Insights and Conclusions

This work effectively demonstrates the value of combining experimental metallurgy with numerical simulation for characterizing weld joints in thin-wall stainless steel piping. The identification of the fusion line as the weakest microstructural zone, combined with the prediction of tensile longitudinal residual stress at the weld center, highlights the importance of considering both microstructural and macroscopic stress factors when evaluating joint integrity. For production environments, the simulation methodology could be integrated into welding procedure qualification programs to reduce reliance on destructive testing while maintaining confidence in joint performance. The findings reinforce the principle that even in materials like 304 stainless steel that are generally considered weldable, careful parameter optimization and post-weld evaluation remain essential for ensuring reliable performance in piping applications.