Hydraulic Composite Forming Technology for Stainless Steel Tee Fittings

Literature Overview

This paper, authored by Feng Sule, Xu Yongchao, Zhao Tao, Guan Yajuan, and Xu Aijie from the Shanghai Aerospace Precision Machinery Research Institute and Harbin Institute of Technology, was published in Forging Technology in 2018 (Volume 43, Issue 1, pages 72-77). The study presents the application of hydraulic composite forming technology for manufacturing spherical tee fittings from 1Cr18Ni9Ti stainless steel, replacing the conventional CNC milling approach with a more efficient and material-saving process.

Core Technical Content

Process Overview

The hydraulic composite forming process combines two sequential operations:

1. Liquid-assisted deep drawing (充液拉深): The initial forming stage where liquid pressure assists the drawing of the blank into the die cavity.
2. Internal high-pressure bulging (内高压胀形): The subsequent bulging stage where internal hydraulic pressure expands the drawn part to achieve the final spherical tee geometry.

The final product is a spherical tee fitting with an internal radius of SR90 mm, fabricated from 1Cr18Ni9Ti stainless steel.

Mechanical Model Development

The authors established mechanical models for both process stages to analyze the influence of key process parameters on forming quality:

Key Experimental Results

The numerical simulation of chamber pressure revealed that at 40 MPa hydraulic pressure, the minimum wall thinning rate after liquid-assisted drawing is 27.5%, with the thinnest wall occurring at the punch radius region. The trial production of the spherical tee fitting achieved a minimum wall thickness of 1.59 mm at the flange edge, and the product passed hydraulic strength and air-tightness reliability tests.

Process Analysis and Technical Points

Material Formability Considerations

1Cr18Ni9Ti (equivalent to ASTM A240 321) is a stabilized austenitic stainless steel with excellent formability and corrosion resistance. Key material properties relevant to forming:

• Tensile strength: ~520 MPa
• Yield strength: ~205 MPa
• Elongation: ≥40%
• Work hardening exponent (n-value): ~0.50
• Strain rate sensitivity (m-value): ~0.02

The high n-value provides favorable strain hardening behavior that promotes uniform deformation during forming, while the low m-value indicates limited rate sensitivity, requiring careful control of forming speed.

Friction Effects on Forming Quality

The friction coefficient between the blank and die surface is a critical parameter in hydraulic composite forming. Higher friction coefficients lead to:

• Increased material strain at the punch radius region
• More severe wall thinning near the punch fillet
• Potential cracking risk at high strain locations
• Non-uniform wall thickness distribution

The study's finding that the thinnest wall occurs at the punch radius region is consistent with established forming theory, where the combination of tensile stress from material flow and bending stress at the die radius creates the highest strain state.

Hydraulic Pressure Optimization

The optimization of hydraulic pressure at 40 MPa represents a balance between:

• Sufficient pressure for material flow: Lower pressures result in inadequate material feeding, leading to excessive thinning and potential failure.
• Avoidance of excessive bulging: Higher pressures cause over-expansion, reducing wall thickness below acceptable limits and potentially causing wrinkling.
• Process efficiency: Optimal pressure minimizes the number of forming cycles required, reducing production time.

Engineering Practice Integration

Comparison with Conventional Manufacturing

Quality Control Considerations

For hydraulic composite formed tee fittings, the following quality control measures are essential:

1. Dimensional inspection: Verification of spherical geometry (SR90 mm) and wall thickness distribution using coordinate measuring machines (CMM) and ultrasonic thickness gauges.
2. Hydraulic strength testing: Pressure testing to confirm structural integrity, typically at 1.5 times the design pressure.
3. Air-tightness testing: Leak testing to verify sealing capability for pressure-containing applications.
4. Surface inspection: Visual and penetrant testing (PT) for surface defects such as cracks, laps, and folds.
5. Material verification: Positive material identification (PMI) to confirm 1Cr18Ni9Ti composition.

Standards Compliance

The manufactured tee fittings should comply with relevant standards:

• ASTM A403/A403M: Forged and rolled seamless butt-welding steel pipe fittings (reference for material properties)
• ASME B16.9: Butt-welding pipe fittings (dimensional requirements)
• GB/T 12459: Forged steel butt-welding pipe fittings (Chinese standard)
• NB/T 20002: Nuclear grade piping components (if applicable for nuclear service)

Key Questions and Reflections

1. Fatigue life implications: The hydraulic composite forming process creates a different residual stress state compared to forged or machined fittings. The compressive surface residual stress from bulging may improve fatigue life, but this requires experimental validation.
2. Scale-up challenges: The successful trial of SR90 mm tees raises questions about scalability to larger diameters. The process parameters (hydraulic pressure, drawing ratio, bulging force) would need to be re-optimized for different sizes.
3. Heat treatment requirements: While 1Cr18Ni9Ti has good formability without intermediate annealing, the strain hardening from forming may require consideration for final mechanical properties, particularly for high-pressure applications.
4. Weldability considerations: The formed tee fitting will require welding to adjacent piping. The deformation history and residual stress state from hydraulic forming should be evaluated for weldability and potential effects on weld quality.

Study Insights and Implications

This study demonstrates the significant potential of hydraulic composite forming technology for manufacturing complex tee fittings from stainless steel. The 60% improvement in material utilization and 8-day reduction in manufacturing time represent substantial economic benefits, particularly for high-value stainless steel components. The process also offers the advantage of producing parts with favorable residual stress states that may enhance fatigue performance. For engineering practice, this technology provides a viable alternative to conventional machining for complex geometry fittings, with particular applicability to aerospace and high-pressure systems where material efficiency and performance are critical. The established mechanical models and process parameter optimization framework provide a foundation for further development and industrialization of hydraulic composite forming for pipe fittings.