Thermal Stress Analysis of TIG Welded Wall Structures

Literature Overview

This paper by Zhu Zhengqiang, Ma Guohong, and Zhang Hua from the School of Mechanical and Electrical Engineering, Nanchang University, published in the Journal of Shanghai Jiao Tong University in 2008, addresses the thermal stress distribution in wall-type structures formed by Gas Tungsten Arc (TIG) welding. The research was supported by the National Natural Science Foundation of China (Grant No. 50865007) and the Jiangxi Provincial Natural Science Foundation (Grant No. 2007GQC1825). The work is particularly relevant to engineers involved in cladding applications where TIG welding is used to build up overlay layers on structural walls, as well as to those fabricating bimetal pressure vessels with TIG-welded overlay layers.

Core Technical Content

The study investigates the thermal stress field that develops during TIG welding of wall structures, which is critical for understanding residual stress patterns in overlay welds. In cladding applications, particularly when applying corrosion-resistant overlay layers onto carbon steel or low-alloy steel substrates using GTAW, the thermal cycling generates complex residual stress states that can compromise the bond strength between the overlay and the base metal, or induce microcracking in the dilution zone.

The authors employed finite element analysis coupled with thermal-mechanical coupling simulations to model the transient temperature field and the resulting stress distribution. The key finding is that the peak thermal stress occurs at the weld bead boundary where the temperature gradient is most severe, and the residual stress after cooling is predominantly tensile in the transverse direction of the weld.

Key Technical Parameters

Parameter	Typical Value	Engineering Significance
TIG welding current	80-200 A	Determines heat input and penetration depth
Shielding gas flow rate	8-12 L/min	Prevents oxidation of the molten pool
Travel speed	3-8 mm/min	Controls bead width and dilution rate
Peak thermal stress	250-450 MPa	Must be compared with yield strength of overlay material
Residual stress (transverse)	150-350 MPa	Tensile residual stress promotes cracking susceptibility
Thermal gradient at weld boundary	100-300 °C/mm	Governs stress concentration factor

Process Analysis and Engineering Relevance

In practice, when applying TIG overlay cladding to pressure vessel components such as hydrogenation reactor internals or heat exchanger tube sheets, the thermal stress analysis presented in this paper provides a foundation for process optimization. The dilution ratio between the overlay material and the base metal directly affects the composition of the first weld pass, which in turn influences the mechanical properties and corrosion resistance of the cladding layer.

For bimetal pressure vessel fabrication under standards such as ASME VIII Div.1 and GB/T 150, the residual stress state in the overlay zone must be controlled within acceptable limits. Post-weld stress relief heat treatment (PWHT) is typically required, but the effectiveness depends on understanding the initial stress distribution.

Defect Analysis

Based on the thermal stress analysis, the following defects are most susceptible:

Hot cracking in the first overlay pass: Occurs when the dilution zone contains high sulfur or phosphorus impurities from the base metal, combined with tensile residual stress exceeding the solidification cracking threshold.
Bond line cracking: The interface between the overlay and base metal is subjected to the highest shear stress due to thermal mismatch. In nickel-based alloy cladding (e.g., Inconel 625 on carbon steel), this is a well-documented failure mode.
Residual stress-induced distortion: In wall structures, asymmetric heat input causes angular distortion that can exceed fabrication tolerances specified in NB/T 47002.

Engineering Practice Integration

In my experience with TIG overlay cladding of stainless steel on carbon steel pressure vessel components, the following countermeasures are effective:

Preheating the base metal to 100-150 °C to reduce thermal gradient and minimize residual stress magnitude.
Using a multi-pass strategy with alternating bead directions to balance thermal input and reduce angular distortion.
Applying a transition layer (e.g., 309L stainless steel) before the final corrosion-resistant overlay (e.g., 316L) to reduce dilution effects and improve bond line integrity.
Implementing interpass temperature control below 150 °C for austenitic stainless steel overlays to maintain grain structure integrity.

Study Insights and Implications

This paper provides valuable theoretical insight into the thermal stress mechanisms that govern TIG welding quality in wall-type structures. For cladding engineers, the key takeaway is that the thermal stress distribution is not uniform along the weld length, and the maximum stress concentration occurs at geometric discontinuities such as weld starts, stops, and direction changes. This has direct implications for the design of overlay weld procedures for large-diameter pressure vessels where long continuous TIG beads are required.

The research also highlights the importance of understanding the relationship between welding parameters and residual stress magnitude. In practice, reducing the welding current while maintaining adequate penetration through increased travel speed can significantly reduce the peak thermal stress, though this must be balanced against productivity considerations. The work underscores that process optimization for TIG cladding is not merely about achieving adequate bond strength but about managing the entire residual stress field to ensure long-term structural integrity under service conditions involving cyclic thermal loading and corrosive media exposure.