Bulging of reducing tee

In this paper, a series of experimental studies on high pressure bulging of reducing tee are carried out, the main influencing factors of reducing tee forming are analyzed, and reasonable technological parameters are obtained. The results show that pure paraffin is selected as the swelling medium and the forming result is ideal. Geometry is also an important parameter; The forming effect is better when the axial pressure is 17MPa~ 22MPa and the balance pressure is 3.5mpa ~ 4.5mpa.

At present, many manufacturers of thin-walled tees mostly adopt the traditional manufacturing process: two straight pipes with different diameters are inserted and welded together, as shown in Figure 1.

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FIG.1 Reducing tee
The main problems of tee made by this process are as follows: welding hole is easy to appear, which leads to high reject rate; Large number of parts, low assembly precision and poor surface quality; Increased stamping, assembly and other procedures, production efficiency is low; The inner surface of pipe fitting is not smooth, the fluid resistance is big and so on. However, the three-way pipe manufactured by inner high pressure bulging technology has the following advantages:

  • ① The pipe surface is beautiful;
  • ② High quality and high reliability;
  • ③ The inner surface is smooth and the fluid resistance is small;
  • ④ The processing procedure is reduced and the production efficiency is improved;
  • ⑤ The cost has been reduced by about 25%.

The internal high pressure bulging process can form tee at one time, which obviously overcomes many problems brought by welding process. Internal high pressure bulging is one of the main production methods of multi pass pipe fittings.
In view of the above advantages of the internal high pressure bulging technology, with the continuous maturity of the technology, the technology has been widely used in aerospace, automobile, oil and other industries. Therefore, the technology of internal high pressure bulging has broad development space.

Bulging principle of reducing tee

Firstly, the blank through pipe is filled with solid medium, and then put into the mold. Then, the left and right punches exert certain pressure on the medium, so that the tube blank metal will plastic deformation under the action of internal pressure; as the deformation continues, the punch starts to contact the end face of the tube blank and applies axial pressure to push the metal to flow to the branch cavity of the female die, so as to supplement the shortage of material in the bulging area, and ensure that After the branch pipe reaches a certain height and contacts with the balance punch, the balance punch exerts radial back pressure on the bulging area, and the balance always contacts with the branch pipe in the backward process, so that the metal in the bulging area is in a state of high hydrostatic pressure deformation and the length of the branch pipe is increased.

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FIG.2 Principle of high pressure expansion in reducing tee
1. Upper die 2. Right punch 3. Balance punch 4. Lower die 5. Left punch

Experimental study on bulging of reducing tee

Experimental condition

The experiment was carried out on a 30t axial compression bulging test machine developed by Henan University of science and technology, as shown in Fig. 3. The main mechanical properties of brass are shown in Table 1.
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FIG.3 30T axial compression bulging testing machine
Table 1 Main mechanical parameters of brass /MPa

Tensile strength

Elastic limit

Yield limit

Modulus of elasticity





Because paraffin has both solid and fluid properties, it can generate a uniform force field inside the tube. Compared with the liquid medium, there is no sealing problem, and it is easy to fill and compact. In addition, paraffin wax can be recycled, which reduces the cost. However, the strength and hardness of paraffin are relatively low, and the strength and hardness can be improved by adding an appropriate amount of graphite. Meanwhile, graphite also has lubricating effect. Therefore, paraffin + graphite powder is selected as the swelling force transfer medium in the experiment. In order to analyze the influence of paraffin and graphite on the forming performance of three-way tube bulging, three kinds of mixed force transfer media with different proportions of paraffin and graphite were selected, as shown in Table 2.
Table.2 Expansion medium ratio


Name of the medium

Volume ratio


Pure paraffin



Paraffin: graphite



Paraffin: graphite


Analysis of experimental results

FIG. 4 shows the forming of reducing tees with bulging medium of paraffin and graphite in different proportions. After analysis, the height of branch tube inflated by pure paraffin was 7mm, the height inflated by paraffin/graphite = 1:1 was 5.8mm, and the height inflated by medium ratio of 4:1 was 6.5mm. This is due to the increase of graphite proportion, the hardness and strength of the mixed swelling medium are increased, and the fluidity is gradually reduced, thus affecting the fluidity of metal materials.

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Fig.4 forming results of tee tubes with different bulging media

It can be seen from Fig. 4 that the height of the branch pipe obviously can not meet the requirements. On the premise of meeting the requirements of parts, the outer diameter of branch pipe is increased from 12mm to 14mm. The experimental results show that the diameter of branch pipe is 14 mm and the bulging height is 12.2 mm in Fig. 5A and 7 mm in Fig. 5B. This is due to the increase of die cavity hole, the reduction of bulging pressure, and the improvement of metal fluidity, thus the forming quality of tee has been improved.

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Fig.5 forming results of tees with different branch diameters
(a) The outer diameter of branch pipe is 14mm (b) the outer diameter of branch pipe is 12mm
By adjusting the pressure of balance cylinder, that is, from the original 8Mpa to 3.5Mpa, the height of branch pipe is further increased to 15.7mm. This is because reducing the balance cylinder pressure will reduce the applied radial back pressure F3 correspondingly, thus improving the material stress state in the deformation zone, that is, the reasonable matching of internal pressure, axial pressure and radial back pressure can improve the formability of tee and increase the limit height of branch pipe. Figure 6 shows the qualified forming parts of tee.

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Fig. 6 formed tee


  • (1) Different proportion of paraffin and graphite powder were selected as bulging media, and different bulging results were obtained, among which pure paraffin was better. However, due to the relatively small strength, paraffin wax is easy to overflow in the experimental process. Further optimization of bulging medium composition, search for ideal medium materials and Research on friction characteristics in bulging process are still the key to the development and application of bulging technology.
  • (2) The diameter of branch pipe is also one of the main factors affecting the formability of reducing tee. On the premise of meeting the requirements of parts, changing its size reasonably is also one of the key factors for the success of the experiment. In this paper, the reference standard of branch pipe outer diameter is less, which is a pity of experiment. Therefore, finite element simulation analysis is needed to find out the reasonable outer diameter of branch pipe.
  • (3) The reasonable matching of internal pressure, axial pressure and radial back pressure is an important factor for the success of this experiment, and it is also a key parameter to be explored in the technology of internal high pressure bulging.

Authors: Li Changkun, Wen Tong, Liu Yajun, song Zhizhen

Source: China Reducing Tee Manufacturer – Yaang Pipe Industry (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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forming results of tee tubes with different bulging media - Bulging of reducing tee
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Bulging of reducing tee
In this paper, a series of experimental studies on high pressure bulging of reducing tee are carried out, the main influencing factors of reducing tee forming are analyzed, and reasonable technological parameters are obtained.
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