Welding Residual Stress in CHS-CHS K-Joints of CFST Arch Bridges

Literature Overview

This paper by Fu Yisheng and colleagues from Chang'an University investigates how welding process parameters influence residual stress distributions in chord-chord (K-type) joints of circular hollow section (CHS) steel tube concrete arch bridges. The study focuses on the Shuangbao Bridge on the Xiangyu Expressway, employing thermally-sequenced indirect-coupled finite element analysis to simulate both the welding thermal field and residual stress field. The authors validated their numerical methodology against published experimental data before applying it to parametric studies of heat input, welding speed, preheating, and post-weld heat treatment (PWHT).

Core Findings and Technical Points

The study reveals several critical relationships between welding parameters and residual stress behavior:

• Residual stress is positively correlated with welding heat input and negatively correlated with welding speed.
• Preheating at 150–200°C reduces peak residual stress by more than 4%, but has negligible effect on the overall stress distribution pattern.
• PWHT at 550–650°C for 4 hours reduces welding residual stress by over 40%, with higher holding temperatures yielding more effective stress relief.
• The distribution pattern of residual stress remains fundamentally consistent across different heat inputs and welding speeds, differing primarily in magnitude.

Process and Standards Analysis

From a welding engineering perspective, the findings align with established principles in the heat-affected zone (HAZ) metallurgy. Lower heat input reduces the extent of thermal cycling and the volume of material subjected to phase transformation, thereby limiting the development of tensile residual stresses. Higher welding speed similarly reduces the duration of thermal exposure per unit length.

The recommended preheating range of 150–200°C is consistent with typical practices for carbon and low-alloy steels such as Q345 or Q390 (equivalent to ASTM A572 Gr. 50 or A992), which are commonly used in bridge construction. For thicker sections or higher-grade steels, preheating temperatures may need to be increased to prevent hydrogen-induced cracking. The PWHT temperature range of 550–650°C falls within the recommended range per ASME Section IX and GB/T 1942, which typically specifies 550–650°C for carbon and low-alloy steels to achieve effective stress relief without sensitization.

The paper's emphasis on the Shuangbao Bridge arch foot joint is significant because this region experiences the highest stress concentrations in the entire arch structure. The chord-chord intersection geometry creates complex multi-axial stress states that compound with welding residual stresses, potentially triggering fatigue cracking under cyclic loading.

Engineering Practice Integration

In practical bridge construction, the following measures should be adopted based on these findings:

1. Optimize welding procedures to minimize heat input while maintaining adequate penetration, such as using multi-pass welding with lower current and higher travel speed.
2. Apply preheating of 150–200°C for typical bridge steel grades to reduce hydrogen cracking risk and modestly lower peak residual stress.
3. Implement PWHT at 550–650°C for 4 hours on critical joints where fatigue performance is paramount, particularly at arch feet and crown regions.
4. Consider alternative stress relief methods such as vibration stress relief (VSR) or hammer peening for joints where PWHT is impractical due to site constraints.

The FMEA approach can be applied to identify the most critical joints in the arch bridge where residual stress mitigation is essential. Joints with high geometric discontinuity, thick plate sections, and high stress amplitude under service loads should be prioritized for PWHT or other stress relief treatments.

Key Questions and Reflections

The paper raises an important question: why does preheating have such a limited effect on residual stress reduction compared to PWHT? The answer lies in the fundamental mechanism of residual stress formation. Preheating primarily reduces the temperature gradient during welding, which helps control distortion and cracking but does not significantly alter the thermal strain distribution. PWHT, on the other hand, operates through stress relaxation mechanisms at elevated temperatures where the material yield strength is substantially reduced, allowing internal stresses to redistribute and relax.

Another reflection is that the study focuses on the numerical simulation validated by literature data rather than full-scale experimental testing. While this approach is cost-effective, the accuracy of residual stress predictions depends heavily on the accuracy of material property inputs, particularly the temperature-dependent yield strength and thermal expansion coefficient. Engineers should exercise caution when applying these results to specific projects without site-specific calibration.

Study Insights and Implications

This study provides a valuable quantitative basis for welding process optimization in CFST arch bridge construction. The key takeaway is that while reducing heat input and increasing welding speed are effective primary measures, PWHT remains the most powerful tool for residual stress mitigation, achieving reductions exceeding 40%. For critical bridge joints subject to fatigue loading, a combination of optimized welding procedures, appropriate preheating, and post-weld heat treatment should be considered as the standard practice. The findings also underscore the importance of residual stress management in the overall structural integrity and service life of steel tube concrete arch bridges, warranting its inclusion in quality control protocols and inspection plans for major bridge projects.