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Cold push forming process of elbow

With the development of shipbuilding, petrochemical and gas industries, the demand for steel pipe elbows used to transport liquid and gas in the pipeline system of these industries is gradually increasing. Improving the forming and manufacturing level and productivity of elbows is the main content for pipe fitting manufacturers to speed up the pace of technical transformation of pipe fittings. How fast, good and economical can elbow be manufactured? There are choices in the feasibility, economy and efficiency of the method.
Reducing elbow is a special pipe fitting that combines reducing joint and elbow into one and turns and changes diameter in the pipeline. Its application can reduce the use of one reducing joint or elbow. The process of manufacturing reducing elbow by using special hydraulic press for elbow forming is gradually mature.

Common forming methods and deformation characteristics of steel elbow

At present, the common processing methods for the forming and manufacturing of steel pipe elbows in China include casting method, stamping molding method, welding method and cold and hot pushing method.

  1. Stamping method: the method of bending the heated pipe blank with a die on a hydraulic press; This method is the earliest process used for pipe elbow bending, and it has been widely used in elbow production. However, the elbow produced by this method has uneven wall thickness and poor appearance quality. When processing large elbow, the manufacturing cost of die is high.
  2. Welding method: press two pipe elbow half shells with a die, and then weld them into elbows; This method has many subsequent processes, and the welding manufacturability of large thin-wall elbow is poor.
  3. Hot pushing method: use medium frequency induction to quickly heat the pipe fitting to improve its plasticity, adopt two-step hydraulic propulsion, expand the diameter and bend the horn mandrel, and push the pipe blank with smaller diameter into the forming process of larger diameter elbow; This hot pushing process has high productivity, many product specifications, strong continuity of production process and easy mechanized production. It is a more economical and effective one of various elbow processes. It has become the main method of producing elbow at present. However, when hot pushing carbon steel elbow, the elbow forming temperature is generally controlled between 750 ~ 950 ° C, which has high requirements for the red hardness of horn mandrel.
  4. Cold pushing method: the method of pressing the pipe blank into the mold of the bending cavity with a special hydraulic press at room temperature to form a pipe elbow; According to whether there is a mold core during forming, it can be divided into cored and coreless elbow production methods.

When the mandrel is not used in the bending process of steel pipe, elliptical deformation will occur in the cross section, large or small. The outer material in the deformation area of the pipe will be extended by tangential tension, the inner material will be shortened by tangential compression, the thinning and fracture of the outer wall, the thickening, wrinkling and cross-section distortion of the inner wall, as well as the rebound and its control after unloading, It is difficult to effectively solve the technical problem of pipe bending. The change of bending wall thickness of pipe blank is not only related to the diameter and wall thickness of pipe, It is also related to the relative bending radius [Relative bending radius RX = R / D (R is the bending radius, D is the outer diameter of the pipe) ]. The smaller the relative bending radius RX, the greater the deformation. In extreme cases, the bending process will be damaged and shape defects will occur. Especially when bending thin-walled pipes, the design should avoid using a smaller bending radius as far as possible.

Principle of special hydraulic press for cold forming of elbow

The mechanical structure of the special hydraulic press for cold push pipe bending is shown in Figure.1:

20211214234239 64402 - Cold push forming process of elbow

1. Lower cross beam; 2. Push cylinder; 3. Push cylinder lock nut; 4. Electrical box; 5. Hydraulic oil tank; 6. Connecting plate of oil tank column; 7. Travel switch mounting base; 8. Column; 9. Upper beam; 10. Fastening nut of mold locking cylinder; l1. Mold locking cylinder; 12. Connecting flange of mold locking cylinder sliding block; 13. Mold locking cylinder slider; 14. Workbench guide block; 15. Push the limit frame; 16. Core turning cylinder mechanism; 17. Stripper cylinder mechanism; 18. Fixing frame of stripper cylinder; 19. Gear sliding seat; 20. Lower the workbench; 21. Anchor bolts; 22. Foundation plate; 23. Push the guide block; 24. Push table

Fig.1 Structural diagram of special hydraulic press for cold push pipe bending
The upper beam 9, lower beam 1 and hydraulic oil tank 5 of the hydraulic press are welded and connected into the main frame by four columns 8; The push cylinder 2 is fastened in the mounting hole of the lower cross beam 1 with the lock nut 3, the push table 24 is directly connected with the piston of the push cylinder 2, the push table 24 is guided by the push guide block 23 fixed on the bed, and the push limit frame 15 is welded on the lower cross beam L along the push direction; The mold locking cylinder ll is fixed longitudinally in the mounting hole of the upper beam 9 with the locking nut 10, the mold locking cylinder slider 13 is connected with the piston of the mold locking cylinder ll, and the mold locking slider 13 is guided longitudinally along the column with the working guide block 14 installed on the slider; The main table 20 is fixed on the bed of the lower beam 1; The whole machine is connected and welded on the anchor support plate 22 with the anchor support, and then placed flat on the foundation with the anchor bolt 21.
The auxiliary device for core turnover and material removal of the hydraulic press includes a core turnover cylinder mechanism 16 and a material removal cylinder mechanism 17. The core turning oil cylinder mechanism is mainly composed of core turning gear rotating mechanism, core turning oil cylinder, auxiliary oil cylinder, etc. its function is to turn up the mold core installed on it and the pushed elbow pipe fittings sleeved on the mold core after the elbow pushing is completed. The stripper cylinder mechanism includes a stripper cylinder, a stripper slider mechanism, etc. its function is to withdraw the formed elbow workpiece on the turned up die core in the core turning mechanism.
The hydraulic system of elbow hydraulic press is mainly composed of axial piston pump, gear pump, two position cartridge valve system and five cylinders, such as die locking cylinder, pushing cylinder, core turning cylinder, core turning auxiliary cylinder and stripper cylinder.
The key to the work of the hydraulic system of the mandrel hydraulic press is that the die locking cylinder should maintain the closing pressure and lock the die up and down. At the same time, the pushing cylinder should push the workpiece with horizontal thrust. The period from the beginning to the end of pushing is the die locking stage of the die locking cylinder, so it should be ensured that the pushing can not punch the die; After completing the pushing work, the mold locking cylinder opens the mold, and the core turning cylinder machine forms 90. Turn up the mold core and workpiece, and the stripper cylinder mechanism takes off the workpiece from the mold core.

Forming process of carbon steel and stainless steel elbows cold pushed by special hydraulic press for elbow

The die assembly diagram of pushing equal diameter carbon steel elbows and stainless steel elbows with the special hydraulic press for elbow forming is shown in Figure 2.

20211214234703 47272 - Cold push forming process of elbow

1. Lower formwork; 2. Mold core; 3. Workpiece; 4. Guide post; 5. Push rod; 6. Upper mold

Fig.2 Assembly diagram of push equal diameter elbow die

The lower die 1 is installed on the fixing tool of the elbow mandrel hydraulic press Fig. 2, the assembly diagram of the push equal diameter elbow die, the upper die 6 is fixed on the working slider of the piston rod of the die locking cylinder with a pressure block, the push rod 5 is fixed on the working slider of the piston rod of the push cylinder, the die core 2 is connected with the core turnover mechanism, and the upper and lower dies are guided by the guide column 4. During pushing, the mold locking cylinder presses the upper mold 6 and the lower mold 1 up and down with the maximum mold locking force, and the pushing cylinder pushes the elbow with the horizontal thrust to push the rod 5, which also makes the workpiece sleeve tightly on the mold core 2. After pushing, the sliding block of the mold locking cylinder returns to open the mold, and the core turning mechanism is 90. Turn up the pushed intact workpiece 3 with the die core 2, and the stripper mechanism of the ‘hydraulic press withdraws the workpiece 3 sleeved on the die core from the core turning mechanism to complete the complete working process of elbow pushing. The die is designed as a die core 2 to support the pipe wall from the inside of the pipe blank, which has a certain effect on preventing the section distortion of the pipe blank.

Comparison of three forming methods of steel elbow

See Table.1 for the production process comparison of elbow produced by special hydraulic press cold pushing, medium frequency heating pushing and hydraulic press stamping pressing method.

  • (1) The process of cold pushing production of carbon steel elbows and stainless steel elbows with special hydraulic press for elbows is as follows: sawing and blanking of multiple pipe blanks (the shape of blanking pipe blank is trapezoid), cold pushing mass production, turning and cutting of elbow groove. This push production method has the advantages of simple process, high production efficiency, low cost, high product accuracy, small grinding workload, low labor intensity, low cutting loss, small elbow machining allowance and good product quality, but the cutting shape is complex.

Table.1 Production process comparison of elbow

Name Special hydraulic press cold pushing production elbow Intermediate frequency heating push production elbow Hydraulic press stamping production elbow
Cutting Cut shape Sawing ports into trapezoidal ports Sawing vertical port Sawing ports into trapezoidal ports
Single piece cutting time About 0.38 man hours About 0.25 man hours About 0.38 man hours
Cutting loss large small large
Cutting cost high low high
Heating Heating method No heating Medium frequency heating Heating furnace heating
Heating time 0 short long
Mode of production Production efficiency high towel low
Total labor hours for single piece production About 0.5 man hours About 3.0 man hours About 7.3 man hours
Labour intensity small medium large
Single piece push production time About 0.15 man hours About 0.20 man hours About 0.25 man hours
Single piece production cost About: 20.00 yuan About: 36.00 yuan About: 75.00 yuan
Product quality Surface shape Smooth and good Smooth and good Unsmooth and poor
Wall thickness size comparison uniformity uniformity Uneven
Port margin small medium large
Dimensional accuracy comparison High precision High precision Low precision

  • (2) The process of medium frequency heating push production of carbon steel elbow is as follows: pipe blank sawing and blanking, multi piece installation, medium frequency heating continuous push production, pushing semi-finished product hydraulic press shaping, turning and cutting elbow groove. This method has high production efficiency, small grinding workload, general labor intensity, less cutting loss, small machining allowance of elbow, low production cost, general product quality and simple cutting shape, but it needs heating and correction.
  • (3) The process of forming elbow with hydraulic press is as follows: sawing and blanking of multiple pipe blanks (the shape of blanking pipe blank is trapezoid), heating in oil furnace, twice forming with hydraulic press, shaping, twice pyrotechnics rounding, manual grinding and turning. The production process of this elbow is complicated, the production efficiency is not high, the grinding workload is large, the cutting loss is large, and the production cost is also high.

Special hydraulic press for elbow cold pushing reducing elbow

( Φ 88.9/73 × 7.5) the forming process reducing elbow is a pipe elbow with different radii at both ends of the curved transition connection, in which the center line is a standard arc, the distance a from the center line of the large end D1 of the elbow to the small end D2 is equal, and the large and small ends are similar to an arc. See Fig. 3 for the part drawing of reducing elbow.

20211215000733 16915 - Cold push forming process of elbow

Fig.3 Schematic diagram of reducing elbow
(1) Determination of necking times
In the cold pushing forming process of reducing elbow, the blank is formed from a section of straight steel pipe into a reducing elbow, which has both bending and necking, and the deformation is relatively complex. The process influencing factors include the mechanical properties, deformation elongation Relative bending radius R / D, relative wall thickness T / DPI “, and necking coefficient (necking coefficient K refers to the ratio of pipe outer diameter after necking to that before necking, k = D2 / D1D2 – pipe outer diameter after necking; D1 – pipe outer diameter before necking) 。 The number of necking times of reducing elbow is determined by the necking coefficient. In order to ensure the relative necking quality, the larger the necking coefficient of single necking, the better. The minimum necking coefficient of non wrinkling and instability of pipe blank is inversely proportional to the thickness of pipe blank. The smaller the thickness is, the minimum necking coefficient shall be increased accordingly. For necking less than the minimum necking coefficient, secondary or multiple necking shall be adopted, Generally, it is more appropriate when the necking coefficient of single necking is 0.8. Φ 88.9/73X7. 5. The necking coefficient K of reducing elbow is 0.82, and the method of one-time necking forming is adopted.
(2) Reducing elbow( Φ 88.9/73X7. 5) Preparation of pushing blank

The reducing elbow is formed by reducing the diameter and bending deformation along the bending empty channel, which inevitably produces some distortion of section shape. The higher the strength of the blank, the greater the hardness, and the greater the required deformation force. Take the measures of complete annealing of the pipe blank to improve the plasticity and reduce the generation of deformation defects. 20# steel Φ 88.9/73 × 7.5 the blanking of the blank pushed by the reducing elbow adopts the sawing method. See Fig. 4 for the process dimensions and shapes of single piece and two pieces of blank.

20211215000942 77671 - Cold push forming process of elbow

Fig.4 Blank opening shape
(3) The choice of surface roughness, heat treatment hardness, pushing lubrication and pushing speed of the die
The surface of elbow tube blank and die cavity, even after fine processing, is also composed of countless uneven convex teeth and pits. When the load increases, the friction between the convex tooth and the pit will intensify, increasing the deformation force. Therefore, the mold cavity should have high surface finish and heat treatment hardness, and add lubricant to reduce the friction between the pushed tube blank and the mold. Φ 88.9/73X7. 5 carbon steel and Φ 88.9/73 × 4. When pushing the stainless steel reducing elbow, the cavity of the pushing die is processed by numerical milling and polishing, so that the heat treatment hardness of the die is greater than HRC60 and the surface roughness reaches the finish finish finish; Graphite powder is used as lubricant for cold extrusion of carbon steel pipe blank; Lubricating grease is selected for cold pushing lubrication of stainless steel pipe blank. The pushing speed of reducing elbow has a great influence on the forming quality of workpiece, 20# steel Φ 88.9/73X7. 5. During pushing, the no-load pushing speed selected by the hydraulic press design is 19mm / s, and the actual average pushing speed is lower than 5mm / s.
(4) Pushing and die assembly process of reducing elbow

Figure 5 shows the assembly diagram of reducing elbow die.

20211215001042 93488 - Cold push forming process of elbow

1. Lower formwork; 2. Workpiece; 3. Push rod; 4. Upper formwork; 5. Guide post

Fig.5 Assembly diagram of reducing elbow die

The lower die 1 is installed on the fixed workbench of the elbow hydraulic press, the upper die 4 is fixed on the working sliding block of the piston rod of the die locking cylinder with a pressure block, the push rod 3 is fixed on the working sliding block of the piston rod of the push cylinder, and the upper and lower dies are guided by guide posts 5. During pushing, the clamping cylinder presses the upper die 4 and the lower die 1 up and down with the maximum clamping force. The pushing cylinder pushes the elbow horizontally with the pushing rod 3, and uses the shoulder of the pushing rod to position. After pushing in place, the sliding block of the clamping cylinder returns to open the die to complete the whole working process of pushing the elbow. The defects of the pushed workpiece port are within the reserved cutting allowance of the elbow. Push manufactured 20# carbon steel Φ 88.9/73 × 7.5 and Φ 88.9/73 × 4. The product shape of stainless steel reducing elbow is shown in Figure 6.

20211215001330 88972 - Cold push forming process of elbow

Figure.6 Shape of reduced elbow formed by push

Conclusion

It is more suitable to manufacture small and medium-sized carbon steel and stainless steel elbows by using the special hydraulic press for elbow forming for core cold pushing production. The formed elbow pipe has uniform wall thickness, good quality and high production efficiency; Reducing elbow fittings can also be made coreless by elbow forming hydraulic press.

Authors: Xiang Yanping, Zong weiqi, Zhang Youwu

Source: China Elbow Manufacturer – Yaang Pipe Industry (www.epowermetals.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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