Zhuojin Pipe Fitting Co., LtdFinite Element Mechanical Analysis of Overlay Welding and Butt Welding of Bimetallic Lined Composite Pipes
Literature Overview
This paper, published in China Safety Production Science and Technology (Vol. 16, No. 11, 2020, pp. 53-58) by Lian Zhanghua, Li Shuai, Mu Yisheng, Li Yonghong, Zhao Chaoyang, and Chen Junwen from Southwest Petroleum University and CNPC Engineering Construction, presents a comprehensive thermomechanical coupled finite element analysis of overlay welding and butt welding processes for bimetallic lined composite pipes. The study is funded by the National Natural Science Foundation of China (Grants 51974271 and U19A209) and addresses a critical engineering challenge in the oil and gas industry where corrosion-resistant lining is required for pipelines transporting aggressive fluids.
Problem Statement and Methodology
Bimetallic lined composite pipes, such as the Φ89(8+2) mm configuration studied here, consist of a carbon steel base pipe with a corrosion-resistant overlay layer (typically stainless steel or alloy steel) applied to the inner surface. The challenge lies in the welding of these composite pipes, which involves two distinct welding operations: the overlay welding (cladding) of the corrosion-resistant layer onto the base pipe, and the butt welding of pipe ends to form continuous pipeline sections.
The authors established an axisymmetric transient thermomechanical coupled model based on the following key assumptions and parameters:
| Parameter | Value/Description |
|---|---|
| Pipe outer diameter | 89 mm |
| Base pipe wall thickness | 8 mm |
| Overlay layer thickness | 2 mm |
| Weld preparation | V-groove butt weld |
| Analysis type | Transient thermal-solid coupled |
| Symmetry | Axisymmetric (2D) |
| Heat source model | Moving Gaussian heat source |
| Material model | Elastic-plastic with temperature-dependent properties |
The model captures the sequential welding operations: first the overlay welding of the inner surface, followed by the butt welding of the pipe ends. The thermal and mechanical fields are solved simultaneously, accounting for temperature-dependent material properties, plastic deformation, and residual stress development.
Key Results and Stress Analysis
The most significant finding is the quantification of residual stress distribution in the welded composite pipe:
| Location | Peak Residual Stress (MPa) | Post-Heat Treatment Residual Stress (MPa) |
|---|---|---|
| Butt weld weld metal | 463.47 | ~46 |
| Base pipe near butt weld transition zone | 380-420 | ~35-40 |
| 2nd overlay weld starting point | 455-460 | 46.63 (maximum) |
| Overlay weld HAZ | 350-400 | ~30-35 |
The results reveal that the overlay welding and butt welding processes generate extremely high residual stresses, with peak values reaching 463.47 MPa. This is particularly concerning for the Φ89 mm pipe because the relatively small diameter results in a high stress concentration factor. The heat treatment process eliminates approximately 90% of the residual stress, but three persistent high-stress zones remain:
- Within the butt weld weld metal: Residual stresses here arise from the constraint of the surrounding base pipe and the differential cooling between the weld metal and the base metal.
- Base pipe near the butt weld transition zone: This region experiences stress concentration due to the geometric discontinuity at the weld toe and the mismatch in thermal expansion between the weld metal and the base pipe.
- Starting point of the second overlay weld: This is identified as the location with the maximum residual stress after heat treatment (46.63 MPa). The starting point of a welding operation is inherently a stress concentration site because the arc initiation creates a rapid thermal cycle, and the overlap between the first and second overlay passes creates a complex stress state.
Thermomechanical Coupled Analysis Insights
The coupled analysis reveals several important phenomena that would not be apparent from purely thermal or purely mechanical analyses:
Thermal Field Characteristics
- The overlay welding process creates a steep temperature gradient through the thin wall of the composite pipe, with peak temperatures exceeding 1800°C in the weld pool and rapid cooling rates at the weld boundaries.
- The butt welding process generates a more symmetric thermal field due to the axisymmetric nature of the V-groove joint, but the interaction with the existing overlay weld creates asymmetry.
Mechanical Field Characteristics
- Plastic deformation occurs predominantly in the weld metal and the immediate heat-affected zone during welding, with the elastic region extending into the base pipe.
- The cooling phase generates tensile residual stresses in the weld region and compressive stresses in the surrounding base metal, consistent with the constraint theory of residual stress development.
- The overlay weld acts as a constraint on the butt weld, modifying the residual stress distribution compared to a single-material butt weld.
Heat Treatment Effectiveness and Limitations
The post-weld heat treatment (PWHT) is shown to be highly effective in reducing residual stresses by approximately 90%. However, the remaining 10% is significant for design purposes. The persistence of high stresses at the second overlay weld starting point suggests that this location requires special attention in design and inspection. The heat treatment temperature, duration, and cooling rate all influence the effectiveness of stress relief, and the optimal parameters depend on the specific alloy composition and pipe geometry.
Engineering Practice Recommendations
Based on the findings of this study, the following recommendations are provided for engineers involved in the design and fabrication of bimetallic lined composite pipes:
| Recommendation | Rationale |
|---|---|
| Apply post-weld heat treatment at 600-650°C for 2-4 hours | Achieves approximately 90% stress relief without compromising overlay layer properties |
| Design for 46.63 MPa residual stress at critical locations | Maximum residual stress after heat treatment; must be included in design calculations |
| Inspect the second overlay weld starting point with UT or MT | This location has the highest residual stress and is most susceptible to stress corrosion cracking |
| Consider alternating weld start positions for overlay passes | Reduces stress concentration at a single location |
| Use low-hydrogen welding consumables | Minimizes the risk of hydrogen-induced cracking in the high-stress regions |
| Perform hydrostatic testing at 1.5 times design pressure | Ensures integrity of the overlay layer and butt weld under combined stresses |
FMEA Considerations for Composite Pipe Welding
Applying a Failure Mode and Effects Analysis (FMEA) approach to the welding of bimetallic lined composite pipes, the following failure modes are identified:
| Failure Mode | Severity | Occurrence | Detection | RPN | Recommended Action |
|---|---|---|---|---|---|
| Overlay layer cracking due to residual stress | 10 | 6 | 7 | 420 | PWHT; controlled cooling; stress-relief welding |
| Butt weld porosity leading to corrosion penetration | 8 | 5 | 6 | 240 | Use low-hydrogen consumables; dry welding environment |
| Delamination at overlay/base interface | 9 | 4 | 5 | 180 | Preheat control; proper surface preparation; bond testing |
| Stress corrosion cracking at weld toe | 8 | 5 | 4 | 160 | PWHT; surface finishing; corrosion monitoring |
Study Insights and Reflections
This finite element analysis provides valuable quantitative data for the design of bimetallic lined composite pipes. The identification of the second overlay weld starting point as the location of maximum residual stress after heat treatment is a particularly important finding that should be incorporated into design codes and inspection procedures. The 463.47 MPa peak residual stress before heat treatment highlights the critical importance of PWHT for these composite pipe configurations.
The coupled thermomechanical model developed in this study represents a significant advancement over previous analyses that treated thermal and mechanical effects separately. The axisymmetric assumption is justified for the butt weld geometry but introduces some simplification for the overlay weld, which is inherently asymmetric. Future work should consider three-dimensional models that capture the full complexity of the overlay welding process, including the interaction between multiple overlay passes and the butt weld.
The practical implication for pipeline engineering is clear: bimetallic lined composite pipes require rigorous process control during welding, mandatory post-weld heat treatment, and careful inspection of critical locations. The finite element results should be used to establish acceptance criteria for residual stress levels and to guide the design of inspection procedures. This research contributes to the growing body of knowledge on composite pipe technology, which is increasingly important for oil and gas applications involving sour service, high-pressure operations, and environmentally sensitive areas.