Cold push forming process of elbow

This paper mainly describes the cold push forming process of equal elbow and reducing elbow special hydraulic press, compares the elbow production process of special hydraulic press cold push, medium frequency heating push, hydraulic press stamping and pressing, and expounds the forming mechanism of elbow pipe fittings of special hydraulic press for elbow forming.

With the development of shipbuilding, petrochemical and gas industries, the demand for stainless steel 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 of pipe fitting manufacturers to speed up the pace of pipe fitting technical transformation. How fast, how fast and how much saving can be made? There are some choices in the feasibility, economy and efficiency of the method.

The reducing elbow is a special pipe fitting which combines the reducing joint and the elbow into one. Its application can reduce the use of a reducing joint or elbow. The process of manufacturing the reducing elbow with the special hydraulic press for elbow forming is gradually mature.

Common forming methods and deformation characteristics of steel elbow

At present, the common processing methods of domestic steel pipe elbow forming and manufacturing include casting method, stamping and pressing method, welding method, cold and hot pushing method.

  • Stamping and pressing method: the method of bending the heated pipe with a die on a hydraulic press; this method is the earliest process for bending pipe elbows, 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-scale elbow, the manufacturing cost of the mold is higher.
  • Welding method: the method of pressing two half shells of pipe elbows with dies and then welding them into elbows; there are many follow-up procedures in this method, and the welding manufacturing process of large thin-walled elbows is poor.
  • Hot push method: using medium frequency induction to quickly heat pipe fittings to improve their plasticity, adopting two-step hydraulic propulsion, horn mandrel expanding and bending, and pushing smaller diameter pipes into larger diameter elbows; this hot pushing process has high productivity, multiple product specifications, strong continuity of production process, and easy to mechanized production, which is a kind of existing bending process One of the more economical and effective pipe technology has become the main method of elbow production at present. However, when the carbon steel elbow is hot pushed, the forming temperature of the elbow is generally controlled between 750-950 ° C, which has higher requirements for the red hardness of the corbel mandrel.
  • Cold pushing method: at normal temperature, a special hydraulic press is used to press the pipe into the bending cavity die to form a pipe elbow; according to whether there is a die core in the forming process, it can be divided into core and coreless elbow production methods.

In the bending process of steel pipe, elliptical deformation will occur in the cross section or large or small without mandrel. The outer material in the deformation zone of the pipe will be stretched by tangential tension, the inner material will be shortened by tangential compression, the outer wall will be thinned and broken, the inner wall will be thicker, wrinkled and cross-section distortion, as well as the springback after unloading Control is a difficult technical problem in tube bending. The variation of bending wall thickness of pipe is not only related to the diameter and wall thickness of the pipe, but also to the relative bending radius (Rx = R / D (R is the bending radius and D is the outer diameter of the pipe)). The smaller the relative bending radius Rx is, the greater the deformation is. In the limit case, the bending of the material is The bending process will be destroyed, resulting in shape defects, especially in the bending of thin-walled pipes, the design should avoid using smaller bending radius as far as possible.

The principle of special hydraulic press for elbow pipe cold forming

20200715020221 81831 - Cold push forming process of elbow

1. Lower crossbeam 2. Push cylinder 3. Push cylinder lock nut 4. Electrical box 5. Hydraulic oil tank 6. Oil tank column connecting plate 7. Travel switch mounting seat 8. Column 9. Upper crossbeam io. Fastening nut of mold locking cylinder 11. Mold locking cylinder slider 13. Mold locking cylinder slider 14. Workbench guide block 15. Push limit frame 16. Core turning cylinder mechanism 17. Stripper cylinder mechanism 18 Support of stripper oil cylinder 19. Gear sliding seat 20. Lower table 21. Anchor bolt 22. Foundation plate 23. Pushing guide block 24. Pushing table

Fig.1 structure diagram of special hydraulic press for cold push bending

The upper crossbeam 9, the lower crossbeam L and the hydraulic oil tank 5 of the hydraulic press are welded and connected into the main frame of the main engine; the pushing oil cylinder 2 is fastened in the mounting hole of the lower crossbeam l with the locking nut 3, and the pushing worktable 24 is directly connected with the piston of the pushing oil cylinder 2, and the pushing worktable 24 uses the pushing fixed on the bed The guide block 23 is guided by transverse movement, and the push limit frame 15 is welded along the pushing direction in the lower crossbeam 1; The mold locking cylinder 11 is longitudinally fixed in the mounting hole of the upper crossbeam 9 with the lock nut 10, and the sliding block 13 of the mold locking cylinder and the piston of the mold locking cylinder 11 The main worktable 20 is fixed on the bed of the lower crossbeam L; the whole machine is connected and welded on the anchor support plate 22, and then the anchor bolt 21 is placed on the foundation foundation foundation.
The auxiliary device part of the hydraulic press includes core turning cylinder mechanism 16 and stripping cylinder mechanism 17. The core turning 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 elbow pipe fittings which have been pushed on the mold core after the elbow pushing. The stripper cylinder mechanism includes stripper cylinder, stripper slider mechanism, etc. its function is to withdraw the formed elbow workpiece on the overturned mold 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, mold locking cylinder, push cylinder, core turning cylinder, core turning auxiliary cylinder and stripper cylinder.
The key of the hydraulic system of the mandrel hydraulic press is to keep the closing pressure of the mold locking cylinder and lock the mold 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 mold locking stage of the mold locking cylinder, and the mold can not be opened by pushing. After the completion of the pushing work, the mold locking cylinder opens the mold and turns the core oil cylinder The mechanism turns up the mold core and the workpiece at 90 ° and the stripper cylinder mechanism takes off the workpiece from the mold core.

Forming process of carbon steel and stainless steel elbows by special hydraulic press

The die assembly diagram of steel and stainless steel elbow is shown in Fig. 2.

20200715021425 89364 - Cold push forming process of elbow
1. Lower die 2. Die core 3. Workpiece 4. Guide pillar 5. Push rod 6. Upper die
Fig.2 assembly diagram of push equal elbow die
The lower die L is installed on the fixed worktable of the elbow mandrel hydraulic press, the upper die 6 is fixed on the working slide of the piston rod of the mold locking cylinder with the pressing block, the push rod 5 is fixed on the working slider of the piston rod of the pushing oil cylinder, the mold core 2 is connected with the core turning mechanism, and the upper and lower dies are guided by guide pillars 4. When pushing, the mold locking cylinder presses up and down the upper mold 6 with the maximum clamping force, and the lower mold L ‘pushing cylinder pushes the elbow with the horizontal thrust force to push the lever 5, and also makes the workpiece sleeve on the mold core 2. After pushing, the sliding block of the mold locking cylinder opens the mold in return, the core turning mechanism turns up the finished work piece 3 with the mold core 2 at 90 ° angle, and the stripping mechanism of the hydraulic press withdraws the workpiece 3 which has been set on the mold core from the core turning mechanism to complete the complete working process of elbow pushing. The die core 2 is designed to support the tube wall from the inside of the tube return, which has a certain effect on preventing the distortion of the tube return section.

Comparison of three forming methods of steel elbow

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

  • (1) The process of producing carbon steel and stainless steel elbows with the special hydraulic press for elbows is as follows: sawing and cutting of multi-piece pipes (the shape of blanking pipes is trapezoid), cold pushing batch production, turning and cutting elbow grooves. This push production method has the advantages of simple process, high production efficiency, low cost, high product precision, small grinding workload, small labor intensity, less cutting loss, small machining allowance of elbow, good product quality, but complex cutting shape.
  • (2) The production process of carbon steel elbow by medium frequency heating is as follows: pipe sawing and cutting, multiple pieces installation, medium frequency heating continuous pushing production, pushing semi-finished product hydraulic press shaping, turning and cutting elbow groove. This method has the advantages of high production efficiency, small grinding workload, general labor intensity, less cutting loss, small processing allowance of elbow, low production cost, general product quality, simple cutting shape, but heating and correction are required.
  • (3) The process of elbow production with hydraulic press is as follows: multi piece pipe sawing and blanking (blanking pipe is trapezoidal shape), heating into oil furnace, twice pressing and shaping by hydraulic press, twice round correction by pyrotechnics, manual grinding, turning and machining port. This kind of elbow production process 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.

Table.1 Comparison of production process of elbow

Name

Cold pushing elbow with special hydraulic press

Production elbow with medium frequency heating

Hydraulic press pressing elbow

Cutting

Cutting shape

The sawing port is trapezoidal

Saw vertical end

The saw end is trapezoidal

Opening time of single piece

About 0.38 man hours

About 0.25 man hours

About 0.38 man hours

Cutting loss

large

large

Opening cost

High

Low

High

Heating

Heating method

No heating

Medium frequency heating

Heating in furnace

Heating time

Zero

Short

Long

Mode of production

Production efficiency

High

Medium

Low

Total man hours of single piece production

About 0.5 man hours

About 3.0 man hours

About 7.3 man hours

lLabour intensity

Small

Medium

Large

Single piece push production time

About 0.15 man hours

About 0.20 man hours

About 0.25 man hours

Unit production cost

About: 20.00 yuan

About: 36.00 yuan

About: 75.00 yuan

Product quality

Surface shape

Smooth and good

Smooth and good

Not smooth, poor

Comparison of wall thickness dimensions

Uniformity

Uniformity

Inhomogeneity

Port margin

Small

Medium

Large

Comparison of dimensional accuracy

High precision

High precision

Low precision

Forming process of cold push reducing elbow (Φ88.9/73 x 7.5) with special hydraulic press for elbow

The reducing elbow is a pipe elbow with different radii at both ends of the curved surface transition connection. The center line is the standard arc. The distance a between the center line of the large end face Di and the small end face D2 is equal, and the two ends of the elbow are approximate to the arc. See Fig. 3 for detail drawing of reducing elbow.

20200715024003 18545 - Cold push forming process of elbow

Fig.3 schematic diagram of reducing elbow

(1)  Determination of necking times

In the cold push forming process of reducing elbow, the blank is formed from a straight pipe into a reducing elbow, which has both bending and necking, and the deformation is complex. Besides the mechanical properties, deformation elongation, relative bending radius R / D and relative wall thickness T / D, the necking coefficient K refers to the pipe after necking and before necking The ratio of diameter, k = D2 / D1, D2 is the outer diameter of the pipe after necking; Di is the outer diameter of the pipe before necking). The number of necking times of reducing elbow is determined by necking coefficient. In order to ensure the relative necking quality, the necking coefficient of single necking is determined
The smaller the thickness is, the smaller the minimum necking coefficient will be increased. If the necking coefficient is less than the minimum shrinkage coefficient, the twice or multiple necking should be adopted. Generally, the single necking coefficient of 0.8 is appropriate. The necking coefficient K of Φ 88.9/73 × 7.5 reducing elbow is 0.82, which is formed by one-time necking.
(2) Preparation of pushing blank for reducing elbow (Φ 88.9/73 × 7.5)
The higher the deformation of the blank along the cross-section, the greater the deformation of the blank along the cross-section, and the greater the deformation of the blank along the cross-section, the greater the deformation of the blank along the cross-section. The cutting method of sawing machine is adopted for the blanking of 20# Φ 88.9/73 × 7.5 reducing elbow. The process dimension and shape of single piece cutting and two-piece cutting are shown in Fig.4.

20200715024556 55036 - Cold push forming process of elbow

Single piece material

20200715025056 64846 - Cold push forming process of elbow

2 pieces material

Fig.4 blank opening shape

(3) Selection of surface roughness, heat treatment hardness, push lubrication and push speed of molds

The surface of pipe embryo and mold cavity of elbow is composed of numerous uneven convex teeth and pits even after fine processing. When the load increases, the friction between the convex tooth and the pit will be intensified, and the deformation force will increase. Therefore, the mold cavity should have high surface finish and heat treatment hardness, and lubricant should be added to reduce the friction between the push pipe and the mold. When pushing the Φ 88.9/73 x 7.5 carbon steel and Φ 88.9/73 x 4 stainless steel reducing elbow, numerical milling and polishing are adopted to make the heat treatment hardness of the die greater than HRC60 and the surface roughness reaching the finish machining finish; the cold pushing lubricant for carbon steel pipe is graphite powder; the cold pushing lubrication for stainless steel pipe is grease. The pushing speed of reducing elbow has a great influence on the forming quality of the workpiece. When the 20# steel Φ 88.9/73 x 7.5 is pushed, the no-load pushing speed selected by the hydraulic press is 19 mm/s, and the actual average pushing speed is less than 5 mm/s.

(4) Reducing elbow push and die assembly process, Fig.5 is the assembly diagram of reducing elbow mold.

20200715031102 64987 - Cold push forming process of elbow

1. Lower die 2. Workpiece 3. Push rod 4. Upper die 5. Guide pillar
Fig.5 assembly diagram of reducing elbow die
The lower die 1 is installed on the fixed worktable of the elbow hydraulic press, the upper die 4 is fixed on the working slide of the piston rod of the mold locking oil cylinder with the pressing block, the push rod 3 is fixed on the working sliding block of the piston rod of the pushing oil cylinder, and the upper and lower dies are guided by guide pillars 5. When pushing, the mold locking cylinder presses the upper mold 4 and the lower mold 1 with the maximum clamping force. The pushing cylinder pushes the elbow horizontally with the push rod 3, and locates with the shoulder of the push rod. After pushing in place, the sliding block of the mold locking cylinder opens the mold in return to complete the complete working process of the elbow pushing. The defects of the workpiece port pushed out are within the scope of the reserved cutting allowance of the elbow.
See Fig. 6 for the product shape of Φ 88.9/73 x 7.5 and Φ 88.9/73 x 4 stainless steel reducing elbows manufactured by pushing and manufacturing.

20200715031839 39264 - Cold push forming process of elbow

Figure.6 shape of push formed reducing elbow

Conclusion

Small and medium carbon steel and stainless steel elbows are manufactured by using elbow forming hydraulic press with core cold pushing. The wall thickness of the formed elbow pipe is uniform, the quality is good, and the production efficiency is high. The elbow pipe fittings with different diameters can also be manufactured by the elbow forming hydraulic press without core.

Source: China Steel Elbows Manufacturer – Yaang Pipe Industry Co., Limited (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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