Hydraulic Bulging Simulation and Experiment of Nuclear-Grade 304 Stainless Steel Ultra-Thin-Wall Tee

Literature Overview

Published in the Journal of Plasticity Engineering in 2015, this study by Ma Fuye, Liu Zhongli, Wang Liu'an, Shen Wei, and Guo Xunzhong from Nanjing University of Aeronautics and Astronautics and the Nuclear and Radiation Safety Center of the Ministry of Environmental Protection addresses the challenging problem of forming nuclear-grade 304 stainless steel ultra-thin-wall tees using hydraulic bulging technology. The research is supported by multiple funding sources including the National Natural Science Foundation of China (51205196) and the China Postdoctoral Science Foundation (2013M531347).

Core Technical Content

The study focuses on the hydraulic bulging forming process for 304 stainless steel ultra-thin-wall tees, which are critical components in nuclear power plant systems. The ultra-thin wall thickness presents unique challenges for forming, including susceptibility to wrinkling at low pressures and excessive thinning or cracking at high pressures. The research combines finite element simulation with experimental validation to establish process parameters that achieve acceptable forming quality.

The simulation investigates three primary process parameters: punch extrusion speed, internal pressure, and fillet radius. Each parameter is varied systematically to determine its effect on the forming outcome, including wall thickness distribution, branch height, and defect formation.

Interpretation of Technical Points

Wall Thickness Distribution

The simulation reveals a characteristic wall thickness distribution pattern in the formed tee:

The minimum wall thickness at the branch top is a critical quality parameter. In nuclear applications, the minimum wall thickness must be sufficient to maintain structural integrity under design pressure and to resist stress corrosion cracking. The wall thickness uniformity is also important for consistent corrosion resistance, as thinner regions may have different microstructural characteristics due to higher strain levels.

Effect of Punch Extrusion Speed

The punch extrusion speed has a non-monotonic effect on branch height. At low speeds, the branch height increases with speed because the faster extrusion promotes more material flow into the branch region. However, beyond an optimal speed, the branch height decreases because the rapid extrusion causes excessive strain localization and potential instability. This non-monotonic behavior is a manifestation of the competition between material flow and strain localization in the forming process.

Effect of Internal Pressure

The internal pressure is the most critical parameter for controlling forming defects. The study identifies a clear pressure window:

• Below a minimum threshold: wrinkling occurs because the internal pressure is insufficient to stabilize the pipe wall against compressive hoop stresses
• Within the optimal range: good forming quality with acceptable wall thickness distribution
• Above a maximum threshold: severe thinning at the branch top, potentially leading to cracking

The wrinkling threshold is particularly important for ultra-thin-wall components because the pipe wall is more susceptible to buckling under compressive stresses. The critical pressure for wrinkling prevention is related to the material's yield strength, the pipe geometry, and the boundary conditions.

Effect of Fillet Radius

The transition fillet radius between the main pipe and branch pipe has a significant effect on forming quality. Larger fillet radii result in:

• Higher maximum branch height due to smoother material flow paths
• Improved wall thickness uniformity due to reduced strain concentration
• More uniform stress distribution during forming

The fillet radius is a geometric parameter that is determined during the design stage and cannot be easily modified during forming. Therefore, the optimal fillet radius should be selected based on the forming capability of the material and equipment, as well as the functional requirements of the fitting.

Engineering Practice Integration

For nuclear-grade applications, the forming process must meet stringent quality requirements that go beyond conventional industrial fittings. The following considerations are particularly important:

The hydraulic bulging process is particularly attractive for nuclear-grade 304 stainless steel tees because it is a cold forming process that does not involve welding. This eliminates concerns about welding-induced sensitization, weld decay, and stress corrosion cracking in the weld zone. However, the forming process itself can introduce residual stresses and strain hardening that must be evaluated for fitness-for-service.

The experimental validation of the simulation results is essential for establishing confidence in the process parameters. The study demonstrates that when the internal pressure and axial feed speed are properly matched, it is possible to form ultra-thin-wall 304 stainless steel tees without wrinkling or cracking. This validates the simulation-based process design approach and provides a basis for scaling the process to production.

From a quality assurance perspective, the forming process parameters must be tightly controlled and documented. The process should be qualified according to the applicable nuclear qualification procedures, with witness tests performed at representative parameter settings. The forming equipment should be calibrated and maintained to ensure consistent parameter delivery.

Study Insights and Reflections

This study demonstrates the feasibility of hydraulic bulging for nuclear-grade ultra-thin-wall tees, which is a significant finding given the stringent quality requirements for nuclear components. The combination of finite element simulation and experimental validation provides a rigorous basis for process development that is essential for nuclear qualification.

One important insight is the non-monotonic effect of punch extrusion speed on branch height. This type of behavior is common in forming processes where competing mechanisms create an optimal operating window. Engineers must be aware of such behaviors and avoid assuming monotonic relationships between process parameters and outcomes.

The study also highlights the importance of fillet radius in determining forming quality. In nuclear applications, the fillet radius has both structural and functional significance. A larger fillet radius reduces stress concentration factors, which is beneficial for fatigue resistance and stress corrosion cracking resistance. The finding that larger fillet radii improve forming quality is therefore doubly beneficial for nuclear applications.

The challenge of forming ultra-thin-wall components remains significant. The narrow process window between wrinkling and cracking requires precise control of pressure and displacement parameters. Future developments in real-time process monitoring and adaptive control could improve the consistency of ultra-thin-wall forming by adjusting parameters in response to actual forming conditions.

Reference Value and Outlook

This study provides a validated process development methodology for nuclear-grade ultra-thin-wall tee forming. The established process window and quality criteria can serve as a basis for qualification of similar components. Future research should investigate the effects of forming parameters on the microstructure and mechanical properties of the formed tees, particularly the strain hardening behavior and residual stress distribution. The integration of forming simulation with stress corrosion cracking assessment would provide a more comprehensive fitness-for-service evaluation for nuclear applications.