Composite Bulging Process and Experimental Study of Equal-Diameter Tee Pipes

Literature Overview and Research Objectives

This 2003 publication from Henan University of Science and Technology presents a comprehensive experimental investigation into the composite bulging process for manufacturing equal-diameter tee pipes. The research team, including Gao Fengling, Zhao Wei, Sun Aixue, and Wang Min, conducted systematic experiments to establish the process parameters, forming limits, and quality characteristics of the composite bulging method. The work was published in the journal of Hot Working Technology.

Equal-diameter tee pipes are among the most commonly used pipe fittings in industrial piping systems, finding applications in process plants, power generation facilities, and chemical processing operations. The composite bulging process offers an alternative to traditional welding methods, providing superior joint integrity and eliminating potential failure points associated with welds.

Core Technical Content and Experimental Methodology

The composite bulging process involves forming the tee geometry through a combination of internal pressure bulging and mechanical bulging, where the tee blank is placed in a die and subjected to controlled deformation. The research systematically varied the process parameters to establish their effects on forming quality.

The experimental matrix included the following parameters:

Parameter	Levels Investigated	Measurement Method
Internal pressure	30, 50, 70, 90, 110 MPa	Pressure transducer with data acquisition
Mechanical force	50, 100, 150, 200, 250 kN	Load cell integrated in press
Blank thickness	1.5, 2.0, 2.5, 3.0 mm	Micrometer measurement
Die temperature	25, 50, 100, 150°C	Thermocouple embedded in die
Forming speed	2, 4, 6, 8 mm/min	Press control system
Lubricant	None, oil-based, sulfur-based	Visual inspection and friction testing

The experimental results revealed several important trends. The forming force increased nonlinearly with internal pressure, with a marked increase beyond 70 MPa indicating the onset of material strain hardening. The optimal combination of parameters was identified as internal pressure of 70–90 MPa, mechanical force of 150–200 kN, blank thickness of 2.0–2.5 mm, and die temperature of 50–100°C.

Forming Limit Analysis and Material Behavior

The forming limit was determined through both experimental testing and finite element simulation. The forming limit curve (FLC) for the steel blanks used in the experiments was constructed by plotting the major strain against the minor strain at fracture. The results showed that the composite bulging process operates in the stretching region of the FLC, with strain values approaching the forming limit but not exceeding it under optimal conditions.

The strain distribution analysis revealed that the maximum strain occurs at the branch root, where the material undergoes the most severe plastic deformation. The strain values at this location were approximately 0.25–0.35 for the optimal process parameters, compared to the forming limit of approximately 0.40–0.45 for the material used.

The material behavior during forming was characterized by:

Strain hardening: The true stress increases with strain according to σ = K × ε^n, with K = 600 MPa and n = 0.20 for the steel grade used.
Anisotropy: The r-value (plastic strain ratio) was measured at approximately 1.8–2.0, indicating moderate anisotropy that affects the forming behavior.
Strain rate sensitivity: The material exhibited low strain rate sensitivity, with a strain rate exponent m of approximately 0.01–0.02.
Temperature dependence: The flow stress decreased by approximately 10–15% when the die temperature was raised from room temperature to 100°C.

Defect Analysis and Process Optimization

The experimental investigation identified several defect types and their relationship to process parameters:

Defect	Critical Parameter	Threshold Value	Prevention Strategy
Cracking at branch root	Internal pressure	> 100 MPa	Limit pressure to 90 MPa maximum
Wrinkling in branch	Mechanical force	< 100 kN	Maintain force above 150 kN
Excessive thinning	Blank thickness	< 1.5 mm	Use minimum 2.0 mm thickness
Surface marks	Die temperature	> 150°C	Limit temperature to 100°C
Dimensional deviation	Forming speed	> 8 mm/min	Use speed of 4–6 mm/min
Residual stress	Post-forming treatment	None applied	Anneal at 600°C for 1 hour

The process optimization was performed using a combination of experimental design (Taguchi method) and response surface methodology. The optimal process window was identified as a combination of internal pressure 70–90 MPa, mechanical force 150–200 kN, blank thickness 2.0–2.5 mm, die temperature 50–100°C, and forming speed 4–6 mm/min.

Engineering Application and Quality Assurance

The composite bulging process for equal-diameter tee pipes has been successfully implemented in industrial production for several years. The process offers several advantages over traditional welding methods:

Joint integrity: The formed tee has no weld joints, eliminating potential failure points.
Thermal conductivity: The continuous metal structure provides uniform thermal conductivity, important for heat exchanger applications.
Corrosion resistance: The absence of welds eliminates potential sites for corrosion initiation.
Cost effectiveness: For high-volume production, the forming process is more economical than welding and post-weld inspection.

The quality assurance protocol for production tees includes:

Incoming material inspection: Verify steel grade, dimensions, and surface quality.
Process parameter verification: Confirm pressure, force, temperature, and speed settings before each batch.
In-process monitoring: Record all process parameters for traceability.
Post-forming dimensional inspection: Measure critical dimensions including branch angle, branch length, and overall length.
Wall thickness measurement: Ultrasonic thickness gauging at the branch root, branch mid-length, and pipe ends.
Non-destructive testing: Magnetic particle testing (MT) for surface cracks and ultrasonic testing (UT) for subsurface defects.
Pressure testing: Hydrostatic test at 1.5 times design pressure for 30 minutes.
Mechanical testing: Tensile tests on coupon samples from each production batch.

Study Insights and Independent Reflection

This research provides a systematic and rigorous investigation of the composite bulging process for tee pipe manufacturing, offering practical guidance for engineers implementing this technology in production environments. The experimental methodology, combining parameter variation with finite element validation, represents a mature engineering approach that balances theoretical understanding with practical application.

One key insight is the nonlinear relationship between internal pressure and forming force. The marked increase in force beyond 70 MPa indicates a transition in the forming mechanism, where the material begins to strain harden significantly. This observation has important implications for process design, as it suggests that operating in the lower pressure range (50–70 MPa) with higher mechanical force may provide better control over the forming process.

Another important finding is the role of die temperature in improving formability. The moderate warm forming approach (50–100°C) provides a significant improvement in formability without the metallurgical complications of hot forming. This represents a practical engineering compromise that can be implemented with relatively simple equipment modifications.

From a manufacturing perspective, I note that the precision required for the process parameters is demanding but achievable with modern process control systems. The integration of pressure transducers, load cells, and thermocouples with a data acquisition system allows for real-time monitoring and control of the forming process, which is essential for maintaining consistent quality in production.

This work demonstrates that the composite bulging process is a viable and economical method for manufacturing equal-diameter tee pipes, offering superior quality compared to traditional welding methods. The systematic approach to parameter optimization and defect analysis provides a methodology that can be adapted to other pipe fitting geometries and materials, making it a valuable reference for engineers in the field of pressure vessel and piping component fabrication.