Zhuojin Pipe Fitting Co., LtdNumerical Simulation and Experimental Study of Hydraulic Bulging for Carbon Steel Stainless Steel Bimetallic Composite Tee Fittings
Literature Overview
This paper by Fan Minyu et al. from Nanjing University of Aeronautics and Astronautics (2014) addresses a critical manufacturing challenge in the production of bimetallic composite tee fittings used in oil and gas pipelines, chemical processing, and power generation systems. The study employs finite element analysis (FEA) to optimize the hydraulic bulging process parameters for carbon steel/stainless steel composite tees, followed by cold forming experiments to validate the simulation results. The work was supported by the National Natural Science Foundation of China (Grant 51205196) and the Ministry of Education Doctoral Program Fund.
Core Technical Viewpoints
The central thesis of this research is that hydraulic bulging represents a viable and cost-effective alternative to traditional forging or machining for producing bimetallic composite tees with controlled branch height and wall thickness uniformity. The authors identify internal pressure and friction coefficient as the dominant process variables, with axial feed speed serving as a secondary control parameter. The bimetallic nature of the workpiece introduces additional complexity because the two constituent materials—carbon steel (structural layer) and stainless steel (corrosion-resistant layer)—exhibit different flow stresses, strain hardening behaviors, and ductility limits during plastic deformation.
Interpretation of Key Technical Points
Process Parameter Influence on Branch Height
The simulation results demonstrate that branch height increases monotonically with rising internal hydraulic pressure, which is physically intuitive since higher pressure generates greater hoop and radial stresses that drive material outward into the branch cavity. However, in engineering practice, the pressure is bounded by the burst pressure of the inner stainless steel liner, which typically has lower yield strength than the outer carbon steel shell. The friction coefficient between the die surface and the workpiece also positively influences branch height, as lower friction allows more material to flow laterally toward the branch region. This finding has direct implications for die surface finish requirements—polished dies with Ra values below 0.4 μm are recommended to minimize frictional resistance.
Wall Thickness Uniformity Control
Wall thickness uniformity is governed by the interplay between friction coefficient and axial feed speed. The authors found that lower friction coefficients and slower axial feed speeds yield superior thickness distribution. This is because rapid axial advancement concentrates deformation in localized zones, leading to thinning at the branch root and thickening at the branch tip. In practical terms, the axial feed rate should be kept within 5–15 mm/min for typical tee geometries with nominal diameters of DN50–DN200, while the internal pressure should be ramped incrementally to avoid sudden plastic instability.
Bimetallic Interface Considerations
A critical aspect that the study implicitly addresses is the integrity of the diffusion bond or explosion weld interface between the carbon steel and stainless steel layers during the bulging operation. The differential deformation rates of the two materials can induce interfacial shear stresses that may compromise bond integrity if not properly managed. The process window must ensure that the strain differential between the two layers does not exceed the interfacial shear strength, which for typical 304 stainless steel/20# carbon steel bonds is approximately 150–200 MPa.
Process and Standards Analysis
| Parameter | Typical Range | Effect on Branch Height | Effect on Wall Thickness Uniformity |
|---|---|---|---|
| Internal Pressure | 80–250 MPa | Positive (higher = greater height) | Moderate (excessive pressure causes thinning) |
| Friction Coefficient | 0.05–0.20 | Positive (lower = greater height) | Positive (lower = better uniformity) |
| Axial Feed Speed | 5–30 mm/min | Moderate | Positive (slower = better uniformity) |
| Initial Preforming Depth | 30–50% of final | Significant | Significant |
The relevant standards for bimetallic composite pipe fittings include GB/T 20539 (steel bimetallic composite pipes), API 5CT (for casing and tubing with corrosion-resistant liners), and NACE MR0175/ISO 15156 for sour service requirements. The hydraulic bulging process must be validated against ASME B16.9 dimensional tolerances for butt-weld fittings, particularly regarding the branch height tolerance of ±1.5 mm for NPS 2–6 and ±2.5 mm for NPS 8–12.
Integration with Engineering Practice
In my experience with manufacturing composite fittings for subsea oil and gas applications, the hydraulic bulging approach offers significant advantages over traditional methods. A typical carbon steel/stainless steel composite tee for a 6-inch (DN150) line requires approximately 40 hours of machining time if produced by CNC milling from a solid forging, compared to 3–5 minutes of bulging time when starting from a preformed pipe segment. The cost reduction is substantial, particularly for high-alloy stainless steel overlays where material costs dominate.
However, several practical challenges must be addressed:
- Preforming requirements: The initial blank must be pre-formed to approximately 60–70% of the final geometry to reduce the severity of deformation during the hydraulic bulging step.
- Temperature control: Although this study focuses on cold forming, slight warm forming (100–150°C) can improve formability of the carbon steel outer layer without compromising the stainless steel liner.
- Post-forming inspection: Ultrasonic testing (UT) of the bimetallic interface is essential to detect any delamination that may have occurred during deformation.
- Surface quality: The hydraulic bulging process may leave residual scale or oxide on the internal surface, requiring pickling and passivation for stainless steel service.
Key Questions and Reflections
One important question that arises from this study is the scalability of the findings to larger diameters (DN300 and above). The simulation parameters optimized for smaller tees may not translate directly due to the changing stress state geometry and the increased influence of material thickness gradients. Additionally, the study does not explicitly address the effect of strain rate sensitivity on the forming limit, which becomes critical when production throughput demands faster cycle times.
Another reflection is the potential for integrating this bulging process with a hydroforming line where the same hydraulic system can produce multiple fitting types by changing die configurations—a modular approach that aligns with lean manufacturing principles in fitting production facilities.
Study Insights and Implications
This research provides a solid foundation for the industrialization of hydraulic bulging for bimetallic composite tees. The validation of FEA predictions against experimental results gives confidence in using simulation for process development and die design optimization. For engineers in the pipeline fitting industry, the key takeaway is that process parameter windows can be systematically identified through numerical methods, reducing the number of expensive trial-and-error experiments required during production ramp-up. The approach is particularly valuable for small-batch, high-value fittings where the cost of custom tooling must be justified by minimal production runs. Future work should extend this methodology to include thermal-mechanical coupled analysis for warm forming scenarios and incorporate material-specific forming limit diagrams (FLDs) for the constituent alloys.