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

Literature Overview

The paper by Lei Yucheng, Zhu Qiang, Yin Sha, and Ju Xin, published in the Journal of Jiangsu University (Natural Science Edition) in 2011 (Vol. 32, No. 2, pp. 163-166), presents a finite element analysis of the temperature and residual stress fields in CLAM steel TIG welded joints. The study was funded by the National Natural Science Foundation of China (51075191), Jiangsu University Scientific and Technological Innovation Team Project (JD000019), and Jiangsu Provincial Graduate Innovation Plan (CX09B_15Z). The research was conducted jointly by Jiangsu University and University of Science and Technology Beijing.

Core Technical Content and Methodology

The authors employed ANSYS thermal-structural coupled analysis using APDL (ANSYS Parametric Design Language) and the element birth-and-death technique. A double-ellipsoidal heat source model was adopted as the internal heat source to represent the TIG welding arc. The study examined both single-layer single-pass and single-side double-pass welding configurations.

Simulation Approach

The methodology follows a sequential coupled approach: the thermal field is calculated first, and then the structural (stress) field is computed based on the temperature distribution. The analysis explicitly excludes the effect of phase transformation on the temperature field but considers its influence on the residual stress field. This simplification is a deliberate modeling choice that trades some accuracy for computational tractability.

Residual Stress Results

The comparison between simulated and experimental residual stress values shows good agreement, validating the numerical model. Key findings include:

• On the top surface of the weld cross-section, the longitudinal residual stress in the near-weld region is tensile and reaches its maximum near the fusion boundary.
• Moving away from the weld center, the longitudinal stress transitions to compressive.
• The transverse residual stress is relatively small and remains tensile throughout the cross-section.
• Single-side double-pass welding produces lower surface residual stresses compared to single-layer single-pass welding.

Standards and Engineering Context

CLAM steel refers to cold-rolled low-alloy steel, which is commonly used in pressure vessels, heat exchangers, and structural piping applications. The residual stress state in welded joints of such materials is of paramount importance for several reasons:

1. Stress corrosion cracking (SCC) susceptibility is strongly influenced by residual tensile stress levels.
2. Fatigue life under cyclic loading is significantly reduced by high residual tensile stresses at the weld surface.
3. Post-weld stress relief requirements are often dictated by residual stress levels, as specified in codes such as ASME Section VIII Division 1, Paragraph UW-40, and API 579.

Comparison with Code Requirements

The finding that double-pass welding reduces surface residual stresses has direct implications for welding procedure specification (WPS) development. For CLAM steel components where residual stress control is critical, multi-pass welding with appropriate interpass temperature control should be preferred over single-pass approaches.

Engineering Practice Implications

From my experience in pipe fitting fabrication, particularly for elbows and tees made from low-alloy steel, the residual stress analysis presented in this paper has several practical applications:

Welding Procedure Optimization

The simulation results support the following engineering recommendations for CLAM steel TIG welding:

1. For joints where residual stress control is critical (e.g., pressure-containing fittings), adopt multi-pass welding with a root pass followed by fill and cap passes.
2. Maintain interpass temperature below 200°C to avoid excessive grain growth in the HAZ and to promote beneficial compressive stress development in subsequent passes.
3. Consider back-rolling or shot peening of the weld surface to introduce beneficial compressive residual stresses that counteract the inherent tensile residual stresses.
4. Perform post-weld stress relief at 580-620°C for 1 hour per 25 mm of thickness, as specified in ASME Section VIII, to reduce residual stresses to below 0.5×UTS.

Model Validation and Limitations

The good agreement between simulated and experimental residual stress values is encouraging, but several limitations should be acknowledged:

• The exclusion of phase transformation effects from the thermal field calculation means that latent heat of transformation is not accounted for in the temperature history, which can affect cooling rates and consequently the microstructure and residual stress distribution.
• The double-ellipsoidal heat source model, while widely used, is a simplification that may not accurately capture the complex heat input distribution of a TIG arc, particularly at the arc root where the molten pool geometry is most sensitive.
• The sequential coupling approach does not account for the feedback of stress-induced strain on the thermal field, which is generally a minor effect but can become significant at high strain rates.

Key Questions and Reflections

The most thought-provoking aspect of this research is the explicit decision to exclude phase transformation from the thermal field calculation. While this simplification is common in engineering practice due to the complexity of modeling solid-state phase transformations during welding, it means that the predicted temperature fields and cooling rates may differ from reality, particularly in materials with significant latent heat effects such as low-alloy steels.

The transition from tensile to compressive longitudinal residual stress away from the weld center is a well-established phenomenon in welded structures. This compressive stress region is beneficial for fatigue resistance but can contribute to distortion and dimensional inaccuracy in precision fabrication. For pipe fitting manufacturing, where dimensional tolerances are often tight (e.g., ASME B16.9 specifies dimensional tolerances for butt-weld fittings), the distortion induced by welding residual stresses must be carefully controlled through fixture design, welding sequence optimization, and post-weld machining.

Study Insights and Implications

This numerical simulation study provides valuable quantitative insight into the residual stress state of CLAM steel TIG welded joints, which is essential for predicting long-term structural integrity. The finding that double-pass welding reduces surface residual stresses compared to single-pass welding is a practical and actionable result that can directly inform welding procedure development.

For engineers involved in pressure vessel and piping fabrication, the key takeaway is that residual stress management is not merely an academic concern but a code-compliance requirement. The simulation approach presented here can be adapted for use in welding procedure qualification and fitness-for-service assessment, providing a bridge between experimental testing and analytical prediction. However, engineers should always validate numerical predictions against experimental data before making critical design decisions, as the accuracy of the simulation depends on the fidelity of the input parameters and the appropriateness of the modeling assumptions.