Numerical Simulation and Experimental Investigation of Hydraulic Bulging for Carbon Steel Stainless Steel Bimetallic Tee Fittings

Literature Overview

This study by Fan Minyu, Huang Fang, Guo Xunzhong, Wang Wentao, Li Huaguan, and Tao Jie from the College of Material Science and Engineering at Nanjing University of Aeronautics and Astronautics, published in 2014 in the "Journal of Plasticity Engineering," addresses a significant fabrication challenge in the pressure vessel and piping industry: the production of bimetallic tee fittings by hydraulic bulging. The research is supported by the National Natural Science Foundation of China (Project No. 51205196), the Ministry of Education Doctoral Point Fund, and the Jiangsu Province Major Science and Technology Achievement Transformation Project. The work bridges the gap between numerical simulation and experimental validation for a process that is widely used in chemical, petrochemical, and power generation industries where corrosion resistance and structural strength must be combined in a single component.

Core Technical Points

The hydraulic bulging process involves inserting a plug into a tube or fitting and applying internal hydraulic pressure to expand the material plastically until it conforms to the desired shape. For bimetallic components consisting of a carbon steel base layer and a stainless steel cladding layer, the process presents unique challenges due to the different mechanical properties of the two layers. Carbon steel typically has a yield strength of 250–350 MPa and an elongation of 20–25 percent, while stainless steel (304 or 316) has a yield strength of 200–300 MPa and an elongation of 40–50 percent. The stainless steel layer is generally softer and more ductile, which means it tends to deform more than the carbon steel layer during bulging, potentially leading to delamination or excessive thinning of the cladding layer.

The authors employ a finite element simulation using the ABAQUS software with a user-defined material subroutine (UMAT) to capture the plastic deformation behavior of both layers. The simulation accounts for friction between the plug and the inner surface of the tee, the strain hardening behavior of both materials, and the contact constraints between the two layers. The key output parameters include the distribution of equivalent plastic strain, the thickness variation across the bulged section, and the stress state at the interface between the two layers.

Process Analysis and Engineering Implications

The experimental results confirm that the hydraulic bulging process can successfully form bimetallic tee fittings with acceptable dimensional accuracy and metallurgical integrity, provided that the bulging parameters are carefully controlled. The optimal bulging pressure is determined by the yield strength of the carbon steel base layer, and the process should be conducted at ambient temperature to avoid grain boundary sensitization of the stainless steel cladding layer. The authors recommend a bulging rate of 0.5–2.0 MPa/s to ensure uniform material flow and minimize the risk of localized thinning.

A critical finding is that the maximum thinning of the stainless steel cladding layer occurs at the apex of the bulged section, where the circumferential strain is highest. The authors demonstrate that by controlling the bulging pressure to not exceed 80 percent of the burst pressure of the carbon steel tube, the cladding layer thinning can be limited to less than 15 percent, which is within the acceptance criteria specified in ASME VIII Div.1 and GB/T 150. The interface between the two layers remains intact after bulging, as confirmed by shear bond strength tests that yielded values exceeding 150 MPa, well above the minimum requirement of 100 MPa specified in relevant standards.

Key Questions and Reflections

One significant question is the effect of repeated bulging cycles on the metallurgical integrity of the bimetallic interface. While the study focuses on single-cycle bulging, industrial applications may require multiple bulging operations to achieve the final geometry. Each cycle introduces additional plastic strain, which could accumulate at the interface and potentially lead to interfacial cracking or delamination. The authors suggest that future work should investigate the fatigue behavior of the bimetallic interface under cyclic bulging conditions.

Another reflection is the applicability of the simulation model to other bimetallic configurations, such as nickel-based alloy clad carbon steel tees or titanium clad steel fittings. The different combinations of mechanical properties would require recalibration of the material models and process parameters, but the general methodology of the study can be adapted. The work also highlights the importance of integrating numerical simulation with experimental validation in the development of new fabrication processes, as the simulation alone cannot capture all the physical phenomena, such as friction variations, material inhomogeneity, and process-induced residual stresses.

Summary

This research provides a comprehensive framework for the design and optimization of hydraulic bulging processes for bimetallic tee fittings. The combination of finite element simulation and experimental validation demonstrates that the process can produce high-quality components with acceptable cladding layer thinning and intact metallurgical bonds. For engineering practice, the key parameters to control are the bulging pressure, bulging rate, and the thickness ratio of the cladding layer to the base metal. The work is particularly valuable for engineers involved in the fabrication of corrosion-resistant piping components for chemical and petrochemical applications, where the integrity of the bimetallic interface is critical for long-term service performance.