Zhuojin Pipe Fitting Co., LtdFinite Element Analysis of Residual Stress in Bimetallic Lined Pipe Overlay and Butt Welding
Literature Overview
This paper, published in China Safety Science and Technology (2020, Vol. 16, No. 11, pp. 53-58), presents a comprehensive finite element analysis of residual stress distribution in bimetallic lined composite pipes after overlay welding and butt welding operations. The research was conducted by Southwest Petroleum University's State Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering, with industrial collaboration from PetroChina Engineering Construction Co., Ltd. Southwest Branch. Funded by the National Natural Science Foundation of China (Grants 51974271 and U19A209), the study focuses on Φ89(8+2) mm lined composite pipes, which are widely used in oil and gas production systems where corrosion resistance and structural integrity are both required.
Modeling Approach and Methodology
The researchers established a transient thermomechanical coupled axisymmetric mechanical model based on thermal-mechanical coupling theory. The model encompasses both the overlay welding of the lining layer and the subsequent butt welding of the V-groove circumferential weld seam. The axisymmetric assumption is justified by the circumferential continuity of the weld seam and the uniformity of the lining layer thickness around the pipe circumference.
Model Parameters and Boundary Conditions
| Parameter | Value / Description |
|---|---|
| Pipe OD × WT | Φ89 mm × 10 mm (8 mm base + 2 mm lining) |
| Weld preparation | V-groove butt weld |
| Thermal model | Transient heat conduction with moving heat source |
| Mechanical model | Elasto-plastic with thermal expansion coupling |
| Material model | Temperature-dependent properties for base and lining materials |
| Symmetry | Axisymmetric (2D axisymmetric) |
The model captures the sequential welding process: first the overlay welding of the inner lining, followed by the butt welding of the pipe joint. This sequence is critical because the residual stress state from the overlay weld serves as the initial condition for the butt welding analysis.
Key Results and Stress Analysis
The simulation results reveal that the welding process generates substantial residual stresses in the composite pipe, with peak values reaching 463.47 MPa. This magnitude is significant when compared to the yield strength of typical carbon steel pipe materials (approximately 245-355 MPa for API 5L grades), indicating that significant plastic deformation occurs during welding.
Residual Stress Distribution Before and After Heat Treatment
| Location | Before Heat Treatment (MPa) | After Heat Treatment (MPa) | Reduction |
|---|---|---|---|
| Butt weld weld metal | Up to 463.47 | Significantly reduced | ~90% |
| Transition zone near butt weld on base pipe | High | Moderate | ~90% |
| Secondary overlay weld start point | Highest residual | 46.63 (peak) | ~90% |
| Overlay weld metal | High | Low | ~90% |
The heat treatment process (likely stress-relief annealing at approximately 600-650 °C for carbon steel) effectively eliminates approximately 90% of the residual stress. However, three distinct high-stress regions persist after heat treatment:
- Within the butt weld weld metal — The weld metal retains residual stress due to its heterogeneous microstructure and the constrained cooling from the surrounding base metal.
- Transition zone on the base pipe near the butt weld — This region experiences complex stress interactions between the thermal contraction of the butt weld and the geometric discontinuity at the lining/base interface.
- Secondary overlay weld start point — This location exhibits the highest residual stress (46.63 MPa) after heat treatment, attributed to the stress concentration at the weld start crater and the geometric discontinuity where the overlay weld terminates.
Engineering Practice Implications
The persistence of stress at the secondary overlay weld start point is particularly concerning from a fatigue and stress corrosion cracking perspective. In oil and gas production environments, where the lining material provides corrosion protection, any residual tensile stress at the lining interface can accelerate crack initiation and propagation. The secondary overlay weld start point represents a location where the lining continuity is interrupted, creating a potential pathway for corrosive media to reach the base pipe.
FMEA Analysis of Residual Stress Risks
| Failure Mode | Cause | Effect | Severity | Detection Method |
|---|---|---|---|---|
| Stress corrosion cracking | Residual tensile stress + corrosive environment | Lining failure, base pipe corrosion | High | UT/MT inspection |
| Fatigue cracking | Cyclic loading + residual stress | Cracks at weld start points | Medium | RT/PT inspection |
| Lining delamination | Thermal mismatch stress | Loss of corrosion protection | High | UT thickness mapping |
| Distortion | Uneven residual stress | Misalignment at joints | Medium | Visual/geometric inspection |
For engineering practice, the following recommendations emerge from this analysis:
- Implement a post-weld heat treatment (PWHT) cycle specifically designed for the composite pipe configuration, considering the different thermal properties of the base and lining materials.
- Apply additional local stress relief (such as low-frequency vibration stress relief or shot peening) at the secondary overlay weld start point to further reduce the residual stress.
- Design the overlay weld termination sequence to avoid stress concentration at the start point, possibly by using a back-step welding technique or a gradual taper at the overlay weld ends.
- Incorporate the predicted residual stress distribution into the fatigue life assessment of the composite pipe, particularly for applications subject to cyclic pressure loading.
Study Insights and Reflections
This research demonstrates the critical importance of integrated process simulation in composite pipe manufacturing. The sequential nature of the overlay and butt welding operations creates a complex stress history that cannot be adequately captured by analyzing each operation independently. The finding that heat treatment eliminates 90% but not 100% of residual stress underscores the need for additional stress relief measures at critical locations.
The axisymmetric model simplification is appropriate for circumferential welds but may not capture stress concentrations at longitudinal seams or at locations where the pipe geometry is non-uniform. Future work should consider three-dimensional models that include the actual pipe geometry, including any bends, reducers, or fittings in the piping system.
The predicted peak residual stress of 463.47 MPa, while high, is within the range typically observed in full-penetration butt welds of thick-walled pipes. The more concerning finding is the persistent 46.63 MPa at the overlay weld start point after heat treatment, which, although lower in magnitude, may be more damaging in corrosive environments due to its location at the lining interface.
This work provides valuable quantitative data for process optimization and quality control in composite pipe fabrication. Engineers should use the predicted stress distributions as a baseline for validating their own manufacturing processes and for developing inspection protocols targeted at the identified high-stress regions.