Zhuojin Pipe Fitting Co., LtdDrawing Process for Tee Pipe Manufacturing
Overview of the Study
The paper by Lin Weizhong from Guangzhou Petrochemical General Plant Construction and Installation Company, published in Piping Technology and Equipment (1998, Issue 2, pp. 25–29), provides a comprehensive practical guide to the manufacture of tee pipes by the drawing (拔制) process. The paper covers the manufacturing process, working principles, drawing die design, drawing force calculation, and critical issues to be addressed during the drawing operation. This is a highly practical paper that bridges the gap between theoretical forming mechanics and actual shop-floor manufacturing, and it remains relevant for engineers working on small-diameter tee fittings and custom pipe fittings.
Drawing Process Description and Working Principle
The drawing process for tee pipes involves pulling a pre-formed tee-shaped pipe blank through a die with the desired final cross-sectional profile. The process is analogous to wire or tube drawing but applied to a more complex geometry. The tee blank is typically pre-formed by bending a straight pipe and welding a branch pipe to create the tee shape, or by using a pre-bent tee blank from a specialized tee preforming machine.
The working principle relies on the plastic deformation of the pipe material as it is pulled through the die. The die constrains the outer surface of the pipe, causing the material to flow inward and reduce the wall thickness while maintaining or slightly reducing the outer diameter. The drawing force required is determined by the material flow stress, the die angle, the friction coefficient, and the cross-sectional area reduction.
The drawing process is particularly suitable for small-diameter tees (DN15 to DN50) made from stainless steel, copper, or carbon steel pipe, where the drawing force is manageable with standard drawing machines. For larger diameters, the drawing force becomes prohibitively large, and alternative forming methods such as hydroforming or forging are more appropriate.
Drawing Die Design
The drawing die is the critical tooling component in the drawing process. The die is typically made from tungsten carbide or tool steel (H13, D2) and consists of a bearing surface with a carefully designed die angle. The die angle for tee pipe drawing is typically 8–15 degrees for the straight sections and 15–25 degrees for the branch section, where the larger angle accommodates the more complex deformation.
The die profile must accurately reproduce the internal geometry of the tee, including the branch root fillet radius, which is typically 1.5–3 times the branch wall thickness. The fillet radius is critical for stress distribution and must be maintained within ±0.2 mm tolerance to ensure consistent quality. The die surface finish should be Ra 0.2–0.4 μm to minimize friction and surface defects on the drawn tee.
| Die Parameter | Recommended Value | Rationale |
|---|---|---|
| Die material | Tungsten carbide (Y6A) | High wear resistance and dimensional stability |
| Die angle (straight) | 8–12 degrees | Minimizes drawing force and surface defects |
| Die angle (branch) | 15–20 degrees | Accommodates complex deformation at branch root |
| Surface finish | Ra 0.2–0.4 μm | Reduces friction and prevents surface scratches |
| Fillet radius tolerance | ±0.2 mm | Ensures consistent stress distribution |
| Maximum cross-section reduction | 20–25% | Prevents excessive thinning and cracking |
Drawing Force Calculation
The drawing force is calculated using the formula F = σ_f × A_1 × (1 + μ × cot(α)) × ln(A_0 / A_1), where σ_f is the flow stress of the material, A_1 is the final cross-sectional area, μ is the friction coefficient between the pipe and die, α is the die angle, and A_0 is the initial cross-sectional area. For a typical stainless steel tee (AISI 304) with an initial outer diameter of 25 mm, wall thickness of 2 mm, and a cross-sectional reduction of 15%, the drawing force is approximately 15–25 kN, which is within the capacity of standard drawing machines.
The friction coefficient is a critical parameter that significantly affects the drawing force. For steel pipe drawn through a tungsten carbide die with graphite-based lubricant, the friction coefficient is typically 0.05–0.08. The use of calcium soap or hard soap lubricants can reduce this to 0.03–0.05, resulting in a 20–30% reduction in drawing force.
Critical Issues and Quality Control
The most common defects in drawn tee pipes include excessive wall thinning at the branch root, surface scratches from die wear, and dimensional inaccuracy at the branch opening. The wall thinning at the branch root is the most critical defect because it directly affects the pressure-bearing capacity of the tee. The thinning should not exceed 25% of the original wall thickness at the branch root for carbon steel and 20% for stainless steel.
Surface scratches are caused by die wear, lubricant breakdown, or foreign particles in the lubricant. These scratches can initiate stress concentrations and lead to premature fatigue failure in service. The die should be inspected and polished every 500–1000 drawn pieces, and the lubricant should be filtered and replenished every 200 pieces.
Dimensional accuracy is verified using plug gauges for the branch opening and calipers for the overall dimensions. The tolerance on the branch opening diameter is typically ±0.3 mm for carbon steel and ±0.2 mm for stainless steel, in accordance with ASME B16.9 requirements for small-diameter fittings.
Study Insights and Practical Recommendations
The paper by Lin Weizhong is a valuable practical reference for engineers and technicians working in small-batch or custom fitting manufacturing. The drawing process offers advantages over hydroforming for small-diameter tees because it requires simpler equipment, has shorter setup times, and can accommodate a wider variety of materials without specialized tooling.
The key insight from the paper is that the drawing process for tee pipes is fundamentally a multi-axial forming operation, and the process parameters must be carefully controlled to manage the non-uniform deformation. The branch root, being the region of highest strain, requires special attention in both die design and process parameter selection. The die angle at the branch should be larger than at the straight sections to accommodate the more complex material flow, but not so large as to increase the drawing force excessively.
For modern applications, the drawing process can be enhanced with online monitoring systems that measure the drawing force in real time and adjust the pulling speed to maintain a constant force. This approach prevents sudden increases in force that could cause cracking or die damage. Additionally, the use of finite element simulation to predict the deformation pattern and wall thickness distribution before the first drawing trial can significantly reduce trial-and-error and accelerate process development.
The study also underscores the importance of material selection for the drawing process. The material should have good ductility and strain hardening capacity to accommodate the non-uniform deformation without cracking. For carbon steel, grades such as 20# and 45# are suitable, while for stainless steel, AISI 304 and 316 in the annealed condition are preferred. Cold-worked or quenched materials are not suitable for the drawing process because they lack the ductility to accommodate the required deformation.
In summary, the drawing process for tee pipes is a practical and economical manufacturing method for small-diameter fittings, provided that the process parameters, die design, and quality control measures are carefully implemented. The insights from this paper remain relevant and applicable to modern manufacturing environments, and they provide a solid foundation for further process optimization and quality improvement.