Zhuojin Pipe Fitting Co., LtdWelding Residual Stress in Steel Tube Welded Hollow Sphere Joints
Literature Overview
This research by Jiao Jinfeng, Jia Pengpeng, Jia Yue, and Lei Honggang (2020, Journal of Guangxi University, Natural Science Edition, Vol. 45, No. 6, pp. 1259-1266) investigates the welding residual stress distribution in steel tube welded hollow sphere joints, commonly referred to as tube-sphere joints. The study was supported by the National Natural Science Foundation of China (51578357), the Shanxi Provincial Graduate Joint Training Base Talent Cultivation Project (2016JD11), and the Taiyuan University of Technology School Team Fund Project (2014TD043). The research combines numerical simulation using ABAQUS with experimental validation using the blind hole method.
Joint Description and Engineering Significance
Steel tube welded hollow sphere joints are widely used in space structures, transmission towers, and large-span roof systems. These joints connect multiple steel tubes to a hollow spherical node, creating a complex three-dimensional weld configuration. The welding residual stress in these joints is of critical importance because it affects the fatigue life, buckling resistance, and overall structural integrity of the connection.
The residual stress distribution in tube-sphere joints is complex due to the geometric discontinuity at the tube-sphere interface, the different thermal expansion rates of the tube and sphere materials, and the multi-directional welding sequence. Understanding this distribution is essential for predicting the long-term performance of the structure.
Numerical Simulation Methodology
ABAQUS Welding Interface (AWI)
The welding process was dynamically simulated using the ABAQUS Welding Interface (AWI). This approach allows for the sequential simulation of each weld pass, capturing the thermal history and mechanical response of the joint at each stage. The dynamic simulation accounts for the sequential heat input and cooling, which is critical for accurately predicting the residual stress state.
Key Simulation Parameters
| Parameter | Description | Typical Value |
|---|---|---|
| Heat source model | Moving heat source representing welding arc | Gaussian distribution |
| Thermal properties | Temperature-dependent material properties | From material database |
| Mechanical properties | Plasticity model with temperature dependence | From material database |
| Weld sequence | Order of weld passes | Based on fabrication plan |
| Boundary conditions | Constraint conditions simulating fabrication setup | Fixed supports at base |
Simulation Results: Heat-Affected Zone
The numerical simulation revealed that the heat-affected zone (HAZ) on the steel tube side exceeds 500°C over an area approximately three times larger than that on the hollow sphere side. The temperature in the heated zone on the tube side is approximately twice that on the sphere side. This asymmetry is attributed to the different thermal mass and geometry of the tube versus the sphere.
The larger HAZ on the tube side has significant implications for the residual stress distribution. The greater thermal expansion and contraction on the tube side generates higher residual stresses, which can be detrimental to the fatigue performance of the joint.
Experimental Validation: Blind Hole Method
The blind hole method was employed to measure the residual stress distribution in a physical specimen. This technique involves drilling a small hole in the material and measuring the strain relaxation using strain gauges arranged around the hole. The measured strains are then converted to residual stresses using calibration factors.
The simulation results showed good agreement with the experimental measurements, validating the reliability of the numerical model. This agreement confirms that the AWI-based simulation approach is suitable for predicting the residual stress distribution in tube-sphere joints.
Residual Stress Distribution Zones
The residual stress field was divided into three zones for detailed analysis:
| Zone | Location | Residual Stress Characteristics |
|---|---|---|
| Zone 1 | Near weld joint on tube | Highest tensile stress, exceeds yield strength |
| Zone 2 | Transition region | Moderate tensile to compressive stress |
| Zone 3 | Away from weld | Compressive stress, lower magnitude |
The maximum residual stress occurs near the weld joint on the steel tube side and exceeds the nominal yield stress of the material. This is a critical finding because it indicates that the material in this region has undergone plastic deformation during welding, which can reduce the fatigue life and buckling resistance of the joint.
Residual Stress Distribution Model
Based on the analysis results, the authors established a residual stress distribution model for the steel tube in tube-sphere joints. This model provides a practical tool for engineers to estimate the residual stress state without performing full-scale numerical simulations. The model can be used in fatigue life assessments and structural integrity evaluations.
Engineering Practice Implications
For welding engineers, the key takeaway is that the tube-sphere joint requires careful welding procedure control to minimize residual stresses. Strategies include:
- Optimizing the weld sequence to balance heat input from different directions
- Using post-weld stress relief treatment, such as thermal stress relief or vibration stress relief
- Applying preheat and interpass temperature control to reduce thermal gradients
- Considering the use of low-hydrogen welding consumables to minimize hydrogen-induced cracking
For structural engineers, the residual stress distribution model enables more accurate fatigue life predictions and buckling assessments of space structures using tube-sphere joints. The knowledge that the maximum residual stress exceeds the yield stress near the weld joint should be incorporated into the design safety factors.
Study Reflections and Outlook
This research provides valuable insights into the welding residual stress behavior of tube-sphere joints, which are critical components in many structural applications. The combination of numerical simulation and experimental validation offers a reliable approach for predicting residual stress distributions in complex welded joints. The residual stress distribution model developed in this study is a practical tool for engineering design.
Future research should extend to the effect of residual stresses on the fatigue behavior of tube-sphere joints under various loading conditions. The interaction between residual stresses and external loads should also be investigated to provide a more comprehensive understanding of the structural performance. Additionally, the effectiveness of different stress relief methods should be evaluated to provide practical guidance for reducing residual stresses in tube-sphere joints.
The research also highlights the importance of welding procedure optimization in the fabrication of space structures. The welding engineer must balance productivity with the need to control residual stresses, and the structural engineer must account for the residual stress state in the design and assessment of these joints.