CLADDING TECHNOLOGY SHANXI CO., LTDFinite Element Mechanical Analysis of Overlay Welding and Butt Welding in Bimetal Lined Composite Pipes
Literature Overview
Published in 2020 in the journal China Safety Science and Technology, this study by researchers from Southwest Petroleum University and China Petroleum Engineering Corporation addresses the structural integrity of bimetal lined composite pipes used in high-pressure oil and gas wellbores. The work combines finite element analysis (FEA) with experimental validation to evaluate the stress distribution and mechanical behavior at both the overlay weld interface and the butt weld joints connecting successive pipe segments. The research was supported by the National Natural Science Foundation of China (Grants 51974271 and U19A209).
Core Technical Points
Bimetal lined composite pipes typically consist of a carbon steel or low-alloy steel base pipe with an internal stainless steel or nickel-based alloy overlay layer deposited by weld overlay or explosive cladding. The critical engineering challenge lies in the butt weld joints, where the dissimilar metal interface must maintain both structural integrity and corrosion resistance under combined mechanical and thermal loading.
The FEA model employed in this study incorporated the following boundary conditions and material assumptions:
| Parameter | Value / Description |
|---|---|
| Base pipe material | 20# carbon steel (σ_y = 245 MPa, E = 206 GPa) |
| Overlay material | 304 stainless steel (σ_y = 205 MPa, E = 193 GPa) |
| Overlay thickness | 2–4 mm |
| Pipe outer diameter | 139.7 mm (5-1/2") |
| Pipe wall thickness | 9.17 mm |
| Internal pressure | 35–70 MPa |
| Temperature range | 20–150 °C |
| Butt weld filler | ER308L consumable |
Stress Distribution Analysis
The finite element results revealed several critical findings regarding stress concentration at the butt weld:
- Circumferential stress discontinuity: The mismatch in elastic modulus between the carbon steel base and stainless overlay creates a stress concentration factor of approximately 1.3–1.6 at the weld toe of the butt joint, particularly under internal pressure loading.
- Residual stress superposition: The residual stresses from the overlay welding process and the butt welding process interact constructively at the weld root, potentially exceeding the yield strength of the overlay material locally.
- Thermal stress contribution: At elevated operating temperatures (up to 150 °C), the differential thermal expansion between carbon steel (α = 12×10⁻⁶/°C) and austenitic stainless steel (α = 17×10⁻⁶/°C) introduces additional thermal stresses that are additive with the mechanical stresses.
- Creep-fatigue interaction: Under sustained high-pressure and temperature conditions, the stress relaxation in the overlay layer may reduce the overall hoop stress but simultaneously promote creep deformation at the weld interface.
Comparison of Overlay Methods on Mechanical Performance
The study compared the mechanical performance of composite pipes fabricated by different overlay methods:
| Overlay Method | Bond Strength (N/mm²) | Residual Stress (MPa) | Fatigue Life (cycles) |
|---|---|---|---|
| SAW overlay | 280–350 | 180–250 | 2.5×10⁵ |
| GTAW overlay | 300–380 | 150–220 | 3.2×10⁵ |
| Explosive cladding | 400–500 | 80–150 | 5.0×10⁵ |
| Hot-wire TIG | 310–390 | 120–180 | 3.8×10⁵ |
The results indicate that explosive cladding produces the lowest residual stresses and highest fatigue life due to the absence of thermal cycling, while GTAW overlay offers a good balance between processability and mechanical performance for field applications.
Engineering Practice and Code Compliance
From a code compliance perspective, the stress concentrations identified in the FEA analysis have direct implications for the design of butt welds in accordance with API 934 and ASME B31.4. The applicable stress concentration factor (SCF) should be incorporated into the fatigue assessment of composite pipe joints in pressure piping systems.
For fabrication, the following practical recommendations emerge from the study:
- The butt weld preparation should ensure full penetration of the overlay layer, requiring a generous root gap (3–4 mm) and appropriate backing ring geometry.
- Preheating to 100–150 °C is recommended to minimize residual stresses in the butt weld zone.
- Post-weld heat treatment (PWHT) at 620–650 °C for 2 hours per 25 mm of wall thickness effectively relieves residual stresses while maintaining the corrosion resistance of the stainless overlay.
- The overlay weld should be ground flush with the pipe surface before butt welding to ensure uniform weld geometry.
Key Questions and Reflections
A significant question arising from this work is how the FEA predictions scale to larger diameter pipes (e.g., 273 mm and above) used in offshore applications. The stress concentration factors may differ due to the change in geometric constraints and the increased influence of plastic deformation. Additionally, the study does not address the long-term degradation mechanisms such as chloride stress corrosion cracking (Cl-SCC) at the weld interface, which is a known concern for austenitic stainless overlays in chlorinated environments.
The interaction between hydrogen-induced cracking (HIC) in the carbon steel base and the residual stress field from overlay welding is another area requiring further investigation. In sour service applications (containing H₂S), the combined effect of hydrogen embrittlement and residual stress could compromise the structural integrity of the composite pipe.
Study Insights and Conclusions
This research provides valuable quantitative data for the design and assessment of bimetal composite pipe joints. The FEA approach, validated against experimental measurements, offers a reliable tool for predicting stress distributions under complex loading conditions. For engineering practice, the key message is that the butt weld in a composite pipe is not merely a mechanical connection but a critical metallurgical transition zone that requires careful design, fabrication, and inspection. The integration of FEA predictions with code-based design procedures represents a significant advancement in the reliable application of bimetal composite pipes in demanding oil and gas environments.