Hot Die Drawing Forming Process for Large-Diameter Tee Pipes

Literature Overview

The paper by Zhang Zhiyuan, Zhao Baoguang, and Wang Huan from Zhengzhou University of Light Industry, published in Manufacturing Technology and Machine Tools (2014, No. 1, pp. 121–124), introduces a novel hot die drawing process for forming large-diameter tee pipes. The process involves pre-cutting an elliptical hole in the pipe blank, heating the surrounding area to 1200°C, and then using hot drawing dies on a vertical hydraulic press to form the branch pipe. This study represents an innovative approach to overcoming the limitations of traditional tee manufacturing methods.

Process Description and Technical Parameters

The traditional manufacturing methods for large-diameter tees—hot pressing, hydraulic expansion, and welding—each have significant limitations. Hot pressing requires large reduction ratios and specialized equipment, hydraulic expansion is limited by material formability, and welded tees introduce weld quality concerns. The hot die drawing process proposed in this study offers a middle ground that leverages existing equipment while achieving good forming quality.

Process Parameter	Value / Range	Function
Pre-cut hole shape	Elliptical	Reduces material resistance during drawing
Heating temperature	1200°C	Achieves sufficient plasticity for drawing
Heating method	Localized (around pre-cut hole)	Minimizes thermal distortion of surrounding pipe
Forming equipment	Vertical hydraulic press	Provides axial drawing force
Die type	Hot drawing dies (upper and lower)	Shapes the branch pipe during drawing
Material	Carbon steel seamless pipe	Standard pipeline material

Process Mechanism and Advantages

The process exploits the principle that localized heating reduces the yield strength of the steel significantly, allowing plastic deformation under relatively low forces. At 1200°C, the yield strength of carbon steel drops to approximately 30–50 MPa, compared to 250 MPa at room temperature. This dramatic reduction in flow stress enables the drawing operation to proceed with standard hydraulic press capacities.

The elliptical pre-cut hole serves multiple purposes:

It reduces the amount of material that needs to be displaced during drawing.
It provides a natural stress relief path that prevents cracking.
It defines the initial geometry of the branch pipe, reducing die complexity.

Comparative Analysis with Traditional Methods

Method	Advantages	Limitations	Applicable Diameter Range
Hot pressing	High production rate, good dimensional accuracy	Requires large press capacity, high energy consumption	DN50–DN500
Hydraulic expansion	Low material waste, good surface finish	Limited by material formability, wall thinning concerns	DN25–DN300
Welded (plate fabrication)	Unlimited size, flexible design	Weld quality critical, residual stress, lower fatigue life	DN100–DN2000+
Hot die drawing (this study)	Uses existing equipment, good forming quality, reduced energy	New process requiring qualification, localized heating control	DN100–DN600

Engineering Practice Considerations

For implementing this process in production, the following considerations are critical:

Thermal control: The localized heating must be precisely controlled to avoid excessive thermal gradients that cause distortion. Infrared or induction heating with thermocouple feedback is recommended.
Die design: The drawing dies must account for material flow patterns and include sufficient clearance for the elliptical hole. Die materials should be heat-resistant (e.g., H13 hot work tool steel).
Post-forming treatment: Stress relief annealing is essential to eliminate residual stresses from the thermal cycle. Temperatures of 550–650°C for 2 hours per 25 mm thickness are typical.
Quality assurance: Full NDT (UT, MT) and dimensional inspection per ASME B16.9 or EN 10253 are required.
Process qualification: A comprehensive qualification program including forming trials, mechanical testing, and pressure testing must be completed before production use.

Study Insights and Implications

This study presents a creative solution to the manufacturing challenge of large-diameter tees, particularly valuable for manufacturers with existing hydraulic press infrastructure who lack specialized tee-forming equipment. The process innovation demonstrates that combining simple operations—pre-cutting, localized heating, and drawing—can achieve results comparable to more complex forming methods. However, the process requires careful thermal management and die engineering to achieve consistent quality. For production implementation, a pilot program with detailed process documentation and statistical quality control is strongly recommended before scaling up to full production volumes.