Numerical Simulation of Temperature and Stress Fields in CLAM Steel TIG Welding Joints

Literature Overview

This research by Lei Yucheng and colleagues from Jiangsu University and University of Science and Technology Beijing, published in the Journal of Jiangsu University in 2011, presents a comprehensive numerical simulation of the temperature and residual stress fields in TIG weld joints of CLAM steel. The study was supported by the National Natural Science Foundation of China and Jiangsu University innovation programs. The authors employed ANSYS thermal-structural coupling technology with APDL parametric programming and birth-death element techniques to simulate both single-pass and single-side double-pass butt welds, using a double-ellipsoid heat source model to represent the TIG arc. The simulation results were validated against experimental measurements of residual stress, demonstrating good agreement between predicted and measured values.

Core Technical Findings

The primary contribution of this paper is the establishment of a validated numerical model for predicting residual stress distributions in CLAM steel TIG welds. The simulation approach separates the analysis into two stages: first, the temperature field is computed considering only the thermal effects of the welding process, and second, the residual stress field is calculated using the temperature field data as input, while neglecting the influence of phase transformation on the stress field. This simplification, while a limitation, provides a practical framework for residual stress prediction that can be readily implemented in engineering practice.

The key findings regarding residual stress distribution are as follows: on the upper surface of the weldment, the longitudinal residual stress in the near-weld region is tensile and reaches its maximum near the fusion line, transitioning to compressive stress at locations further from the weld. The transverse residual stress is comparatively smaller in magnitude and remains tensile throughout. Notably, the single-side double-pass weld exhibits lower surface residual stresses than the single-pass weld, which has direct implications for distortion control and fatigue performance.

Numerical Modeling Approach

The numerical methodology employed in this study is of considerable practical value for engineers who need to predict residual stress in welding applications without resorting to extensive experimental testing. The following table summarizes the key modeling parameters and their roles:

The double-ellipsoid heat source model is a well-established approach for representing the TIG arc in numerical simulations. The model divides the heat input into two ellipsoidal volumes, one in front of the arc and one behind, to account for the asymmetric heat distribution caused by the arc's directional travel. This is particularly important for TIG welding, where the arc is relatively stable and the heat input is concentrated near the tungsten electrode tip.

The birth-death element technique is a practical approach for modeling multi-pass welding or progressive weld deposition. In this study, it was used to simulate the single-side double-pass weld, where the second pass is deposited after the first pass has partially cooled. This technique allows the simulation to account for the thermal history of each pass, which is critical for accurate prediction of residual stress in multi-pass welds.

Residual Stress Analysis and Engineering Significance

The residual stress distribution patterns identified in this study are consistent with well-established welding metallurgy principles, yet the quantitative predictions provide valuable engineering data. The tensile longitudinal residual stress near the fusion line arises because the weld metal, upon cooling, contracts and is restrained by the surrounding cooler base metal. This restraint generates tensile stresses in the weld and compressive stresses in the far-field region, maintaining overall force equilibrium. The maximum stress near the fusion line is a result of the steep thermal gradient in this region, which creates the largest differential contraction.

The finding that the single-side double-pass weld exhibits lower surface residual stresses than the single-pass weld is particularly significant. This result can be attributed to the additional thermal cycles imposed by the second pass, which partially relieve the residual stresses generated by the first pass through thermal relaxation mechanisms. In engineering practice, this finding supports the use of multi-pass welding for stress-sensitive applications, such as pressure vessels, pipelines, and structural components where fatigue performance is critical.

The neglect of phase transformation effects in the stress analysis is a recognized simplification. In CLAM steel, which is a low-carbon alloy steel, phase transformation during cooling can contribute to residual stress through volume changes associated with austenite-to-ferrite transformation. However, for low-carbon steels with relatively low transformation temperatures, the contribution of phase transformation to residual stress is often secondary to the thermal contraction effects, making the simplified approach acceptable for engineering estimation purposes.

Study Insights and Practical Recommendations

This paper demonstrates the power of numerical simulation as a tool for welding process optimization and quality prediction. For steel pipe manufacturing, where residual stress directly influences stress corrosion cracking susceptibility, fatigue life, and dimensional accuracy, the ability to predict residual stress distributions through simulation is invaluable. The validated model presented here can serve as a template for developing similar models for other steel grades and welding processes used in pipe production.

The comparison between single-pass and multi-pass welding configurations has direct relevance to pipe manufacturing practice. In the production of medium to thick-walled pipes, multi-pass welding is often necessary to achieve full penetration and adequate weld metal volume. The finding that multi-pass welding reduces surface residual stresses supports the practice of using multiple passes with adequate interpass cooling, rather than a single high-current pass, for stress-sensitive applications.

However, engineers should be mindful of the limitations of this simplified approach. For high-carbon or high-strength steels where phase transformation plays a significant role in residual stress generation, a more comprehensive model that includes transformation plasticity and transformation-induced stress would be necessary. Additionally, the model does not account for plastic deformation during welding, which can be significant for low-strength materials at high thermal input levels.

In the context of pipe fitting manufacturing, where welds are often subjected to subsequent forming operations such as bending or rolling, the residual stress state can interact with the forming-induced plastic deformation to produce unexpected distortion or cracking. The numerical simulation approach presented in this study can be extended to model these sequential processes, providing engineers with a predictive tool for optimizing the entire manufacturing sequence.

In conclusion, this study provides a validated numerical framework for predicting residual stress in CLAM steel TIG welds, demonstrating that multi-pass welding configurations can reduce surface residual stresses compared to single-pass configurations. The methodology and findings are directly applicable to steel pipe and fitting manufacturing, where residual stress management is critical for ensuring structural integrity and service life.