Zhuojin Pipe Fitting Co., LtdMulti-Directional Loading Forming of Tee Fittings and Die Design Optimization
Literature Overview
The paper by Li Suli, Zhang Zhimin, and Dong Ziru (2009), published in Hot Working Technology (ISSN 1001-3814, Vol. 38, No. 9), investigates the forming process of tee fittings using multi-directional loading techniques and presents corresponding die design solutions. The research was funded by the National Natural Science Foundation of China (Project No. 50575213) and conducted by Shaanxi National Defense Industry Vocational and Technical College, Shanxi Integrated Precision Forming Engineering Technology Research Center, and Taiyuan Nanfang Heavy Vehicle Manufacturing Co., Ltd. The work employs MSC-Superform finite element software to simulate and compare three multi-directional loading strategies.
Core Technical Approach and Multi-Directional Loading Strategies
Tee fitting forming is inherently challenging due to the complex three-dimensional geometry involving simultaneous material flow in two orthogonal directions. Conventional single-direction forging or pressing often results in incomplete forming, excessive material thinning at critical regions, or high residual stresses. Multi-directional loading addresses these limitations by distributing the forming force across multiple axes, promoting more uniform material deformation and reducing localized strain concentrations.
The authors compared three distinct loading strategies:
| Loading Strategy | Description | Advantage | Disadvantage |
|---|---|---|---|
| Multi-directional synchronous loading | All directions loaded simultaneously at the same rate | Shortest forming time | Complex die mechanics, high peak force |
| Multi-directional step loading | Directions loaded in defined sequential steps | Balanced forming quality | Moderate cycle time |
| Multi-directional sequential loading | One direction completed before the next begins | Simplest die design | Risk of asymmetric deformation |
The comparative analysis focused on time-velocity curves and extrusion force profiles during the forming process. The step-loading approach emerged as the optimal strategy, providing a favorable balance between forming quality and process efficiency.
Finite Element Simulation Results and Die Design
The MSC-Superform simulation revealed that multi-directional step loading produces the most uniform strain distribution across the tee geometry, with reduced thinning at the branch junction and main pipe intersection. The extrusion force curves showed that step loading avoids the peak force spikes associated with synchronous loading while maintaining adequate material flow in all directions.
For the die design, the authors proposed a multi-cavity configuration with independent actuation for each loading direction. Key die design parameters include:
- Die cavity surface finish: Ra ≤ 0.4 μm to minimize friction during hot forming
- Die corner radius: Optimized to promote material flow into the branch pipe region
- Release angle: 5° to 8° to facilitate part ejection
- Material preheating temperature: Dependent on steel grade, typically 900–1100 °C for carbon steel tees
The die material selection should consider the operating temperature and service life requirements. For carbon steel tee forming, hot-work die steels such as H13 (4Cr5MoSiV1) or 3Cr2W8V are appropriate, with appropriate heat treatment to achieve a hardness of 48–52 HRC.
Engineering Practice Integration and Reflections
From a manufacturing engineering perspective, this research highlights several important considerations for industrial implementation of multi-directional forming for tee fittings. First, the die design complexity increases significantly with multi-directional loading, requiring precision machining of conformal cavities and reliable multi-axis actuation systems. The tolerance requirements for die cavity surfaces are stringent, typically ±0.05 mm for critical regions.
Second, the selection of loading strategy must be validated through physical trials, as finite element simulations, while valuable, cannot fully capture phenomena such as die friction, material anisotropy, and thermal gradients. The step-loading approach recommended by the authors should be implemented with careful control of inter-step dwell times and temperature maintenance between steps.
Third, quality control considerations include dimensional inspection of formed tees against ASME B16.9 or EN 10253 tolerances, surface integrity assessment for cracks or laps, and mechanical property verification. Non-destructive testing (NDT) methods such as magnetic particle inspection (MT) for surface defects and ultrasonic testing (UT) for internal discontinuities are essential for production quality assurance.
The research contributes to the broader field of precision forging technology by demonstrating that multi-directional loading strategies can be systematically optimized through numerical simulation. The practical significance lies in enabling the production of complex tee geometries that are difficult or impossible to achieve through conventional single-direction forging, thereby reducing subsequent machining requirements and material waste. Future work should address the scalability of multi-directional forming to larger tee sizes and the integration of process monitoring systems for real-time quality control during production.