Zhuojin Pipe Fitting Co., LtdComposite Expansion Process and Experimental Study of Equal-Diameter Tee Pipes
Literature Overview
The paper by Gao Fengling, Zhao Wei, Sun Aixue, and Wang Min from Henan University of Science and Technology, published in Hot Working Technology (2003, Vol. 32, No. 6, pp. 19–20), presents a composite expansion process for forming equal-diameter tee pipes from seamless pipe blanks in a single operation. The study introduces the principles of composite expansion, determines key process parameters, designs the die structure, and presents experimental results and analysis. This approach represents a material-efficient alternative to traditional multi-step forming processes.
Composite Expansion Process Principles
Composite expansion combines the advantages of conventional hydraulic expansion with the benefits of rubber pad forming. The process uses a polyurethane rubber pad as a flexible die that conforms to the tee shape while providing uniform pressure distribution. The seamless pipe blank is placed between the rubber pad and a rigid die, and internal pressure is applied to expand the pipe wall outward, forming the tee geometry.
| Process Element | Specification | Function |
|---|---|---|
| Pipe blank | Seamless carbon steel pipe | Base material for tee forming |
| Flexible die | Polyurethane rubber pad | Provides uniform pressure, conforms to complex geometry |
| Rigid die | Steel die cavity | Defines final tee shape and dimensions |
| Internal pressure | Hydraulic fluid (high pressure) | Drives material expansion |
| Forming temperature | Room temperature or warm (up to 300°C) | Enhances formability |
Key Process Parameters
The success of the composite expansion process depends on the optimization of several interrelated parameters:
- Expansion ratio: The ratio of final wall thickness to original wall thickness, typically controlled between 0.75 and 0.85 for carbon steel to prevent bursting.
- Pressure profile: The pressure-strain curve must be carefully controlled to ensure uniform material flow. A ramped pressure profile is preferred over step loading.
- Rubber pad hardness: Polyurethane rubber hardness in the range of 40–60 Shore A provides the optimal balance between conformability and pressure transmission.
- Die clearance: The gap between the rubber pad and the rigid die must be controlled to ±0.05 mm to ensure accurate dimensional reproduction.
- Lubrication: A suitable lubricant reduces friction and prevents galling, improving surface finish and reducing required forming forces.
Experimental Results and Analysis
The experimental program involved forming equal-diameter tee pipes from seamless pipe blanks with various parameters. Key observations include:
- Dimensional accuracy: The composite expansion process achieved dimensional tolerances within ±0.5 mm for critical dimensions, meeting ASME B16.9 requirements for standard tees.
- Wall thickness distribution: The thinnest wall thickness occurred at the branch-pipe intersection, with thinning ratios of 15–20% of the original thickness, well within acceptable limits.
- Surface quality: The polyurethane rubber pad produced a smooth surface finish without the die marks or scratches associated with metal die expansion.
- Residual stress: Residual stresses were lower than those in conventional hydraulic expansion, attributed to the more uniform pressure distribution from the rubber pad.
Comparison with Other Forming Methods
| Method | Material Utilization | Surface Quality | Production Rate | Equipment Cost | Quality Consistency |
|---|---|---|---|---|---|
| Hot pressing | Moderate (high waste) | Good | High | High | Good |
| Conventional hydraulic expansion | High (low waste) | Fair (die marks) | Moderate | Moderate | Moderate |
| Composite expansion (this study) | High (low waste) | Excellent (rubber pad) | Moderate | Low–Moderate | Good |
| Welded fabrication | Low (weld material) | Poor (weld seams) | High | Low | Variable |
Engineering Practice Recommendations
For manufacturers considering composite expansion for tee production:
- Select polyurethane rubber with appropriate hardness and temperature resistance based on the forming temperature.
- Implement real-time pressure monitoring with automated control to prevent over-expansion.
- Conduct die clearance checks before each production batch to ensure dimensional accuracy.
- Perform full NDT inspection (UT, MT) on all produced tees to detect any subsurface defects.
- Maintain rubber pad replacement schedules based on cycle count, as hardness changes with use.
- Apply post-forming stress relief if the application requires low residual stress.
Study Insights and Implications
The composite expansion process offers a compelling combination of material efficiency, surface quality, and equipment flexibility for tee pipe manufacturing. The use of polyurethane rubber as a flexible die element is particularly advantageous for complex geometries where traditional metal dies would be expensive to manufacture and maintain. The experimental results demonstrate that the process can produce tees meeting industry standards, making it a viable alternative for medium-volume production runs. Engineers should note that the process is most suitable for equal-diameter tees, and adaptation to unequal-diameter or large-diameter applications would require additional die design and process development work. The study provides a solid foundation for further process optimization and industrial implementation.