Thermal Stress Analysis of TIG Welded Wall-Forming Structure

Literature Overview

This 2008 study by Zhu Zhengqiang, Ma Guohong, and Zhang Hua from the School of Mechanical and Electrical Engineering at Nanchang University, published in the Journal of Shanghai Jiao Tong University, presents a finite element analysis (FEA) of thermal stresses in a wall-forming structure produced by TIG welding. The research was funded by the National Natural Science Foundation of China (50865007) and the Jiangxi Provincial Natural Science Foundation (2007GQC1825). The work addresses a practical engineering challenge: the prediction and management of residual stresses in thin-walled structures fabricated by TIG welding, which is a common technique in the construction of pressure vessel internals, heat exchanger components, and structural panels.

Thermal Stress Mechanisms in TIG Welding of Wall Structures

The TIG welding process generates a highly localized heat source that creates steep temperature gradients in the surrounding material. As the weld pool solidifies and the surrounding material cools, differential thermal contraction produces residual stresses that can reach or exceed the yield strength of the base material in the weld zone. In wall-forming structures, where thin plates are welded together to form a three-dimensional geometry, the constraint conditions are complex and the resulting stress state is multiaxial.

The key mechanisms driving residual stress development in this context include:

1. Thermal gradient effect: The rapid heating and cooling cycle creates a temperature differential between the weld zone and the surrounding material, leading to constrained contraction upon cooling.
2. Geometric constraint: The wall-forming geometry imposes boundary conditions that prevent free thermal expansion and contraction, particularly at weld intersections and along the support boundaries.
3. Phase transformation effects: In low-alloy steels, the phase transformation during cooling (austenite to ferrite and pearlite) can produce volume changes that contribute to residual stresses.
4. Plastic deformation: The high temperature gradients cause localized plastic deformation in the weld zone, which is locked in upon cooling as residual stresses.

FEA Model Parameters and Results

The FEA results revealed several important patterns. The maximum residual stresses were concentrated at the weld root and near the wall intersections, where geometric constraints were most severe. The longitudinal residual stresses (parallel to the weld direction) were predominantly tensile in the weld zone and compressive in the surrounding material, consistent with the classical residual stress pattern in welded joints. However, the transverse residual stresses exhibited a more complex distribution, with alternating tensile and compressive zones depending on the welding sequence and the specific geometry of the wall-forming structure.

Implications for Bimetal Pressure Vessel Fabrication

The findings of this study have direct relevance to the fabrication of bimetal pressure vessels, where thin-walled overlay layers are deposited on thicker base plates. In clad-plate pressure vessels, the residual stresses in the overlay layer are a critical design consideration. High tensile residual stresses in the overlay layer, combined with a corrosive service environment, can significantly accelerate stress corrosion cracking (SCC) initiation and propagation. For example, in 316L stainless steel overlay layers on carbon steel pressure vessels operating in chloride-containing environments, tensile residual stresses exceeding 100 MPa can reduce SCC resistance by orders of magnitude.

Residual Stress Management Strategies

Study Insights and Reflections

The study demonstrates the value of coupled thermo-mechanical FEA in predicting residual stress distributions in complex welded geometries. However, the accuracy of such predictions depends critically on the quality of the material property data, particularly the temperature-dependent constitutive models. In practice, obtaining accurate high-temperature material properties for overlay alloys such as Inconel 625 or Hastelloy C276 is challenging, and the resulting predictions may have significant uncertainty.

The study also highlights an important gap between numerical prediction and experimental validation. While FEA provides detailed spatial distributions of residual stresses, experimental measurement techniques such as neutron diffraction, X-ray diffraction, and hole-drilling strain gauge methods are limited in their ability to measure through-thickness stress distributions in complex geometries. For engineering practice, a pragmatic approach combining FEA predictions with targeted experimental measurements at critical locations provides the best balance of accuracy and practicality.

The research underscores the importance of welding sequence optimization in minimizing residual stresses in wall-forming structures. By carefully planning the welding sequence to allow progressive thermal contraction rather than constraining it, significant reductions in peak residual stresses can be achieved. This principle is directly applicable to the fabrication of clad-plate pressure vessels, where the welding sequence of the base plate, transition layer, and overlay layer must be carefully coordinated to minimize the risk of overlay layer cracking and delamination.