Elbow manufacturing by hot push method

The hot push method of manufacturing elbows was developed by Hamburger Bohling GmbH around the sixties according to foreign information.

Yaang product specifications range from 21.7mm to 2033mm in steel, stainless steel, non-ferrous metals, and special alloys. Varieties include elbows, pipes, reducer, tee, cross, caps, flanges, sockets, etc.
The manufacturing process of the elbow made of steel pipe by the hot push method is as follows.

Steel pipe → Cutting off → Raw pipe treatment → Bending → Shaping → Sandblasting → Pipe end processing → Bevel processing → Inspection → Marking → Painting → Drying → Shipping

The hot push method has the advantages of small bending radius, uniform wall thickness, small ellipticity, simple mold, etc. It is especially suitable for manufacturing thin-walled elbows, and can automate the production of high quality elbows with high efficiency. So for decades in the world to get praise. This article on the thermal thrust method of manufacturing elbow, from the theory to explain.

Mechanism of manufacturing elbow by hot pushing method

The core of the hot push method for manufacturing elbows is the mandrel, as shown in Figure 1.
20220227033658 44249 - Elbow manufacturing by hot push method
Figure 1 Geometry of the mandrel
The mandrel consists of a cylindrical part (PA) and a bent part (PB). And the bending part consists of the enlarged conical part (PB-7) and the guiding part (PB-2). Bending part of the outer arc radius of r1, while the inner arc is with the elbow cross-sectional diameter from d0 to d1 gradually expanded on the inside, so that the elbow inside the compression surface by stretching and compensate for the outer stretch surface thinning, so as to get a uniform wall thickness distribution of the elbow, which avoids the general elbow wall thickness increase due to compression, while the outer side of the wall thickness due to stretching surface thinning shortcomings.
The increase in the diameter of the cross-section of the bending angle is limited to an α of between 45° and 55°. The diameter of the mandrel cross-section remains constant between α and β of 75° to 90° for guiding the elbow forming.
The diameter of the cylindrical section d. is slightly smaller than the inner diameter of the original tube, while the diameter d1 is equal to the inner diameter of the finished product. When manufacturing the elbow, the steel pipe in the cylindrical part of the section, must be heated to about 600 ℃, while the bending part is heated to 900 ℃.
For carbon steel, alloy steel elbow various nominal outside diameter, wall thickness and required thermal thrust, can be determined in accordance with Figure 2.
20220227034107 81098 - Elbow manufacturing by hot push method
Figure 2 thrust of hot push carbon steel elbow, alloy steel elbow

Features

When the finished product is made, the uniform wall thickness of the entire elbow is almost the same as that of the original tube (T=t). In addition, the total length of the outer arc of the elbow is exactly equal to the length of the original tube (l=L), as shown in Figure 3.

20220227034214 77442 - Elbow manufacturing by hot push method

Figure 3 Parameters of the original pipe and elbow
d: outer diameter; lpipe length; twall thickness; Vporiginal pipe volume; Douter diameter of elbow; Twall thickness of elbow; Llength of outer arc of elbow; Veelbow volume
1. Volume of the original pipe
20220227034310 86349 - Elbow manufacturing by hot push method(1)
2. Volume of the elbow
20220227034345 50628 - Elbow manufacturing by hot push method(2)
3. According to the law of volume invariance of plastic deformation of metal materials requires t = T
20220227034428 49320 - Elbow manufacturing by hot push method(3)
4. Elbow outer arc length
20220227034457 73701 - Elbow manufacturing by hot push method(4)
5. Require the elbow outer arc length L = original pipe length l, the (4) into (3) to organize the following.
20220227034532 48651 - Elbow manufacturing by hot push method
Because D>>T and d>>t
20220227034614 82344 - Elbow manufacturing by hot push method(5)

6. The relationship curve between the expansion rate and the bending radius is shown in Figure 4.

20220227034853 27302 - Elbow manufacturing by hot push method

Figure 4 Relationship between expansion rate and bending radius
When R = D times into the equation (5) to organize to obtain.
D=1.5d or d=D/1.5
Expansion rate 20220227035144 81660 - Elbow manufacturing by hot push method= 50%.
When R = 1.5D, substitute into equation (5) to organize the following
D=1.33d or d=D/1.33
Expansion rate 20220227035144 81660 - Elbow manufacturing by hot push method= 33%
When R = 2D, the substitution into equation (5) is organized as follows
D=1.25d or d=D/1.25
Expansion rate 20220227035144 81660 - Elbow manufacturing by hot push method= 25%
When R = 0.5D, substitute into equation (5) to organize the following
D=2d or d=D/2
Expansion rate 20220227035144 81660 - Elbow manufacturing by hot push method= 100%
7. Comparison with other methods

  1. The hot push method is a method that can continuously produce elbows, while other methods are single-piece production, so the production efficiency is high, the cost is low, and the labor intensity of workers is small.
  2. Thermal thrust method is based on the theoretical basis of the volume invariance law of plastic deformation of metal materials designed mandrel to manufacture elbows, while other methods are only through a simple mechanical deformation to achieve, so the former has the advantage of elbow wall thickness uniform and consistent, good quality, especially thin bending radius small elbow, other methods are sometimes unable to produce.

Conclusion

1. The smaller the elbow bend radius, the larger the difference between the elbow diameter and the diameter of the original pipe, the greater the expansion rate.
2. Elbow bending radius of a certain case, the expansion rate (D – d) / d only at a certain value to get a uniform and consistent wall thickness, otherwise the expansion rate decreases, the elbow inner wall thickness t2 > t1 (outer wall thickness). Expansion rate increases, the elbow inside wall thickness t2 < t1 (outside wall thickness).

3. According to the formula derived from the moment-free theory when the annulus (Figure 5) is subjected to a constant internal pressure P.

20220227035546 97047 - Elbow manufacturing by hot push method

Figure.5 ring pipe
20220227035908 57627 - Elbow manufacturing by hot push method(1)
20220227035926 42130 - Elbow manufacturing by hot push method(2)
Type 20220227040001 62504 - Elbow manufacturing by hot push method(1) foreign information called Lorentz factor, that is, the stress index or stress concentration coefficient of the ring pipe on the straight pipe.
When φ=0 (center surface)
20220227040154 43927 - Elbow manufacturing by hot push method (equivalent to the calculated stress in a straight pipe)
When φ = 20220227040229 34870 - Elbow manufacturing by hot push method(outside)
20220227040253 71215 - Elbow manufacturing by hot push method (less than the straight pipe stress)
When φ=-20220227040229 34870 - Elbow manufacturing by hot push method (inside)
σx = max σ = 20220227040440 11317 - Elbow manufacturing by hot push method (greater than the straight pipe stress)
According to this theory, the inside of the elbow stress index is greater than the outside. So the hot push method of manufacturing elbow no suitable original tube available, and to ensure that t1 = t2, the choice of approximate original tube diameter can only be slightly larger than the calculated diameter can not be reduced to meet t2 > t1, so that the elbow inside slightly thickened.

Source: China Pipe 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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