Forming technology of metal bend

Bends are widely used in metal pipelines. There are many forming methods for bends, including groove wheel rolling method, induction heating simmering method, mold pressing method, step-by-step forming method and numerical control bending method. Bends are also made of many materials, including ferrous metals and non-ferrous metals, including carbon steel, alloy steel, stainless steel, copper, aluminum, titanium, zirconium, nickel and their alloys. Different forming methods can be adopted for bends of different materials, specifications and batches according to their processability, price and processing cost. This paper introduces various forming methods and forming principles of metal bends, and compares their advantages and disadvantages. Reasonable selection of bend forming method can improve product quality, reduce cost, improve production efficiency and create better social and economic benefits.

In industrial production installations such as petrochemicals, electric power, metallurgy, boilers, etc., pipelines are used in large quantities for the transport of gases, liquids or fluids with solid particles. Pipelines are composed of pipes, fittings and pumps and valves [1]. Pipes include straight pipes and bends, and bends are also important in pipelines. Bends and elbows are different in that elbows do not carry straight sections at both ends, and elbows with straight sections are bends. Bend and elbow forming methods are some of the same, often can be borrowed and transformed. This article describes the classification of the bend forming method, the elbow forming method is described in a separate article.

Carbon steel bends are used in large quantities in industry, but other materials are also gaining a broader and broader market for bends. These metals are: alloy steel, stainless steel, copper, aluminum, titanium, zirconium, nickel and its alloys, and so on. The forming methods of different materials of bends are different.

Bend forming method

The bend is at the ends of the elbow with a straight section, the bend schematic diagram shown in Figure 1. The main parameters of the bend are the outside diameter D, wall thickness T, bend radius R, straight section length L1 and L2. Since the bend has straight sections at both ends, the bend is manufactured from the same outside diameter of the straight pipe. The outer and inner arcs of the bending part of the bend are the same length before the deformation, and the outer arc is longer than the inner arc after the bend. Deformation process in the outer arc metal evacuation and the inner arc metal aggregation, according to the principle of constant quality, so the bend can not do equal wall thickness. The biggest problem in the manufacture of the bend is to avoid bending part of the ellipse too large and the outer arc thinning. Technical standards require wall thickness changes of no more than 12.5%. The traditional bending method of force as shown in Figure 2b. The outer arc of the billet heating deformation area under tension, the inner arc under pressure, when the outer arc thinning, the inner arc thickening. So the traditional method of processing requires the processing of the bend bending radius R can not be too small, cold bending R / D> 4, hot bending R / D> 3.5, when the bend can be controlled ellipticity < 8%, thinning rate < 15%. When changing the force method (as shown in Figure 2c schematic) to apply an auxiliary pressure on the axis of the bend, so that the outer arc is not subject to tensile stress, then the inner arc will be subject to greater compressive stress, the thickness of the outer arc of the bent bend remains unchanged, the inner arc also becomes thicker, with a straight section of the bend is not possible to achieve equal wall thickness. Bend and the elbow is different, because there is no straight section, deformation of the billet ends are not restricted, the use of appropriate deformation methods can be done when the wall thickness.
Bend can be processed continuous or three-dimensional space elbow, thus reducing the ring weld, bend than the elbow of the pipe weld less, saving the corresponding welding, testing and other costs; welding heat-affected zone is not in the bending part of the pipe, thus improving the reliability of safe operation of the pipeline. In addition, in the pipeline construction bend pipe alignment than elbow alignment is easy, thus facilitating construction. Bend compared with the elbow advantage is less welded seam, high pressure resistance, the disadvantage is that the production efficiency is much lower than the elbow, bend wall thickness uniformity is poor, standardization of small scale, production and application scale is small, but in recent years the bend forming technology has gained a lot of progress.
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Fig.1 Schematic diagram of bending pipe
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Fig.2 Force diagram during bending pipe: (a) the heated region of the pipe; (b) the stress state of the pipe by traditional bending pipe method; (c) the stress state of the pipe with loaded subsidiary axes (c) the stress state of the pipe with loaded subsidiary axial stress

Grooved wheel rolling method

Grooved wheel rolling bending is one of the most original and widely used bending method, grooved wheel rolling bending schematic diagram shown in Figure 3. Groove wheel rolling bending method uses the same diameter as the finished elbow straight pipe as a billet, the billet is locked in a semi-circular groove with a garden die plate or sector die plate [2], with a roller or with a module to apply pressure-guided bending, straight pipe in a grooved die plate deformed by pressure into a bend. The traditional grooved wheel rolling method is subjected to pure bending load in the cross-section of the billet, with the outer arc under tensile stress and the inner arc under compressive stress, resulting in thinning of the outer arc and thickening of the inner arc.
Long-term production practice to improve the process, the creation of a variety of force methods, such as: pulling, bending, bending, rolling, bending, vibration impact bending and collapsing angle bending, etc., thus improving the groove wheel rolling method of stress state, adjusting the force of various parts of the billet, so that it is subject to tension, compression and bending. A typical method is to bend the tube loaded with an axial pressure so that the outer arc is not subject to tensile stress, which avoids the outer arc thinning, while improving the bend ellipticity [3]. However, the forming principle of the bend determines the total bending moment remains the same, when the inner arc compressive stress is greater and the inner arc thickening is more serious. Axially applied groove wheel rolling method for mechanical cold bending with an axial top-heading device [4] and the addition of lasso in the tube and so on [5]. The technical keys to the grooved wheel rolling method are the method of applying force and the method of filling the inner cavity, which involves the design and manufacture of the core head. Coreless bends generally result in ovalization of the bend cross-section, and the traditional method is filled with sand, which is sometimes unsatisfactory and prone to cause damage to the pipe wall. The method of filling the inner cavity with a flexible medium is to apply a constant pressure to the tube, and the method is shown in Figure 4. Flexible media include rubber, gas and liquid. Adding an inner core is also a common method, as shown in Figure 5. The inner cores are cylindrical, spherical [6], claw [7], and chain, flexible shaft, and ball-and-socket joints [7]. For large-size bends a combination of steel rods and steel rings can be used as the inner liner core, as shown in Figure 6. Grooved wheel roll bending is different from the induction heating simmer bending method (see paper 1.2) because it is deformed under pressure in a die plate with grooves, and adjusting the groove cross-section curve is the key to improving the yield of the bend [8].
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Fig.3 Diagrammatic sketch of rolling depression with drop center wheel and bend with die pressing parts: (a) bending press by roller; (b) bending press by (b) bending press by die block; (c) bending press of low diameter radius bending pipe
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Fig.4 Filling flexibility mediator in bore during bending pipe
The advantage of groove wheel rolling bending is simple equipment, less investment, applicable specifications, more suitable for the manufacture of small bends, both ends can be retained any length of straight pipe section. General groove wheel rolling bend requires cold bending R / D> 4, hot bending R / D> 3.5, but the groove wheel rolling method to take special measures can also be manufactured R = 1.5 ~ 3D bend. DN25 below the small size elbow with conventional bend production method is more difficult, you can use the groove wheel rolling method first made elbow, and then remove the two straight section, cut out the elbow. The disadvantage of the groove wheel rolling bending method is that the operation is complicated, sometimes difficult, easy to cause excessive thickening of the abdomen of the bend, excessive thinning of the back, the finished product shape is poor, poor quality assurance.
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Fig.5 The used mandrel during bending pipe
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Fig.6 The used core during bending large size pipe

Induction heating simmer bending method

In 1969, Japan’s first high wave industrial company (DHF) applied for a patent [9,10]. With the development of the induction heating simmer bending method technology, manufacturers make technical improvements to the method. The induction heating simmer bending method uses a billet with the same diameter as the finished elbow, and one end of the billet is clamped by the clamp after it is pierced from the heating ring, and the clamp moves along a circular trajectory, and the billet is processed into a bend, as shown in Figure 7. When bending large diameter pipe billets, the clamp moves on a circular rail. Small-diameter pipe bending, the clamp locked in the iron arm around the center of the circle for circular motion.
Traditional induction simmer bending method, the billet in the heating circle of the ring body deformed by axial bending forces, the inner arc axial compression, the outer arc axial stretching, so that the inner arc of the bent pipe thickening, the outer arc thinning. This method is called quite a lot, such as: hot bending method, intermediate frequency bending, hot push bending, induction bending, intermediate frequency local heating bending, etc., but these calls can not exactly reflect the essence of the meaning of the bending forming process, elbow forming expansion push bending method also uses intermediate frequency induction heating and pushing into the shape called, the two are difficult to distinguish. The original meaning of simmering means that the billet is constantly fed into the small heating zone bending, and induction simmer bending method of heating does not necessarily use medium-frequency heating, and sometimes high-frequency heating can be used [11].
The process parameters that affect the induction simmer bending method include heating temperature, heating zone width, pushing force, propulsion speed, method of applying auxiliary force, cooling part, cooling method, cooling zone width and cooling speed. Induction simmer bending method is suitable for the manufacture of R / D> 3.5 thick-walled bend, the ends can be retained at any length of straight pipe section. The inverse bending distance method is used to apply an axial pressure to the front part of the billet so that the outer arc is subjected to zero force [5,12,13,14], or the heating zone width is reduced to less than 1.5 T [15], or an iron core is added to the inside of the inner arc to increase the temperature of the inner arc [15], or the temperature of the outer arc is reduced, etc. Various methods are used to avoid thinning of the outer arc, improve roundness, and improve the quality of the bend [14]. The use of expansion joints can prevent the generation of cracks during bending [16].
Methods such as reverse bending distance have been developed to produce 1.3D<R<3D bends, called small radius of curvature bends (or sharp bends, small R bends). As small radius bends with R<3DN are widely used in power plant pipelines, a large number of technological developments and equipment improvements have successfully solved the difficult problems of thinning rate and cross-sectional ellipse on the outer arc side of the bend. Russia’s Ural Heavy Machinery Plant, Japan’s Daiichi Takami Co., Ltd. and the Netherlands Cojefex have manufactured large small radius of curvature bending equipment widely used [17]. Domestic production of large medium frequency pipe bending machine. Induction simmer bending method can also be manufactured large spiral coil [18].

Type mold compression bending method [4]

Type mold pressure bending method is 90º elbow forming process, can also be used for the formation of bends, as shown in Figure 8. The type die press bending method uses the upper and lower forming die together after the bend is formed. The core structure is complex and difficult to manufacture is the key to its technology. Type die press bending method has the advantage of using conventional equipment, type die press bending method of manufacturing elbows can be equal wall thickness, but the type die press bending method of manufacturing bends with straight section is not pure bending deformation, easy to cause the bend oval too large and back excessive thinning, to forming failure. Wall thickness is not uniform when the need to roll the ball shaping (through the ball orthopedic). Model press bending method is not suitable for cold stamping, requiring the billet at the right temperature for slow pressing, multi-pass deformation to avoid defects. Model press bending method compared with other bending preparation methods, is an inefficient production method.
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Fig.7 Schematic illustration of bending pipe made by induction heating method
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Fig.8 Diagrammatic sketch of bending press with section mould

Stepwise forming method [19]

The schematic illustration of the stepwise forming method is shown in Fig. 9. Because the bending radius of the pipe fittings is very small, it is bound to produce serious cross-sectional distortion, in order to reduce the cross-sectional distortion, the bending part of the pipe fittings before the top bending to be anti-deformation processing. That is, let the elbow before bending, and after bending the opposite of the cross-sectional distortion (shown in Figure 1). After the anti-deformation of the bend using a grooved wheel roll bending method to create a smaller bend radius bend, that is, R / D = 1.5, and then placed in the extruder die extrusion into a small radius bend [4,20]. These methods often result in poor roundness of the bend and require ball shaping (through-ball orthopedic), usually by passing a 0.85DO inspection ball. This method is a complement to other methods, is inefficient, and is only suitable for small quantities of production.

CNC pipe bending [21,22]

CNC bending is an advanced plastic processing technology developed by combining traditional manual bending with machine tools and CNC technology. Its working principle is bending die fixed in the machine tool spindle, one end of the pipe billet is clamped by the clamping die, the outer arc side of the pipe billet has a pressure block, the inner arc side has an anti-wrinkle block, the tube has a core head and core rod. After the bending die is rotated, the billet is bent around the die. Rotation speed and the position of each die is controlled by CNC. CNC pipe bending is highly accurate and efficient [23]. The pipe after three-dimensional bending is not limited to a plane, but can be distributed in a three-dimensional space (see Figure 10). Stereo bending can be done using various methods described in this paper, but with great difficulty. Large-diameter three-dimensional bending machine mainly relies on imports [24], domestic in the development, there are products made [25]. CNC pipe bending can accurately and reliably achieve three-dimensional bending.
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Fig.9 Diagrammatic sketch of forming with step-by-step method
CNC tube bending technology is a three-dimensional coordinate control system [26,27]. CNC tube bending equipment is an automatic processing equipment that can bend a straight tube into a specific shape in space with four mechanisms: feeding mechanism, bending mechanism, rotating mechanism and complementary bending mechanism [21]. At present, foreign CNC pipe bending technology is more advanced, each mechanism is driven by servo motors, computer control, with the advantages of accurate positioning of the bend, good forming quality, easy to replace the product. CNC pipe bending uses a variety of methods for filling the inner cavity, including CNC adjustment of the inner core position, which plays a great role in improving the quality of pipe bending. Traditional manual pipe bending, worker labor intensity, bending is not in place, the quality is not high, poor product diversity, can not adapt to the market. CNC pipe bending requires investment in equipment and technical training. The CNC pipe bender manufactured by WALLACE COAST Machinery Company in the United States can form small radius bends [3].

Discussion

For different materials, specifications, batch of bends, different forming methods can be used according to their processability, price and processing costs. Small-diameter (DN100 or less) bends are suitable for bending with groove wheel rolling method, large-diameter (DN65 or more) bends are suitable for induction simmer bending method to produce, batch hours can be bent with type mold pressure bending method. In general, the bend requires a bending radius R can not be too small, cold bending R / D> 4, hot bending R / D> 3.5. Such as the production of sharp bends (1.5 < R / D < 3.5), you can use the groove wheel rolling method and induction simmer bending method of production of bends, the application of an axial pressure so that the outer arc is not tensile stress; bend batch hours, but also available step-by-step forming method. CNC pipe bending requires a larger investment is suitable for professional pipe bending workshop or manufacturer.
Bend in the production application of the largest amount of carbon steel and low-alloy steel pipe, suitable for less corrosive occasions. Followed by alloy steel and stainless steel tubes, stainless steel tubes for weak acids, alkalis, salt occasions, the cost is 10 times higher than ordinary steel tubes. Many non-ferrous metals, such as: copper [28,29], aluminum, titanium, zirconium, nickel and its alloys are gaining more and more widespread applications, also applied to the pipe, including elbows and bends. Titanium alloy pipes are suitable for transporting salt and organic acid media such as brine, wet chlorine gas, organic anhydride and other very warm and weakly acidic or corrosive salts and organic acids. Nickel and nickel alloy pipes are suitable for conveying strong alkaline media such as NaOH. Pure zirconium tubes are suitable for harsh conditions in more acidic environments, and zirconium alloy tubes are used on nuclear reactors. The methods discussed in this paper are applicable to a variety of metallic materials, but should be processed at different temperatures due to differences in the materials’ own processing properties. Carbon and alloy steels have good deformation properties from room temperature to high temperature, and their bending temperatures are from room temperature to 900°C. Titanium, zirconium and its alloys room temperature plastic deformation capacity is poor, high flexural strength ratio, high temperature strength changes with the temperature, the processing temperature range is narrow, the material itself is chemically active, easy to oxidize at high temperatures, and easy to bond with the steel mold, so the general use of 300 – 750 ℃ for bending. Also take anti-oxidation, anti-bonding and other measures. Nickel and nickel alloys and stainless steel room temperature plasticity is better, but the work hardening index is high, high temperature strength is very high, suitable for processing at room temperature or above 900 ℃ temperature. The bending temperature of aluminum is 300 ℃ or less, copper bending temperature is 600 ℃ or less. The method chosen to ensure easy methods, low cost, stable quality of the finished product, good economic efficiency, to be considered in a comprehensive manner. The cost is too high, when the manufacturing value exceeds the value of use, the processing method is no longer good will not have the use of meaning. Bend the batch of small, but also low production efficiency, low overall cost approach. In short, according to the material, specifications (outside diameter, wall thickness), batch size and the principle of various elbow forming methods and advantages and disadvantages, the appropriate means of processing can be used.
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Fig.10 Schematic of numerical control bending pipe

Conclusion

  • (1) The bend forming methods are groove wheel rolling method, induction heating simmer bending method, mold pressure bending method, step-by-step forming method and CNC bending. Bending pipe forming is dependent on the bending stress in the section.
  • (2) Compared with the elbow, the advantage of the bend is less weld seam, good pressure resistance; disadvantage is the poor uniformity of the wall thickness of the bend, standardization of small scale, production and application scale is small, but in recent years the bend forming technology has gained great progress.
  • (3) According to the material of the bend produced, specifications (outside diameter, wall thickness), batch size and the principle of various elbow forming methods and advantages and disadvantages, the appropriate processing means can be used to improve product quality, reduce costs and improve production efficiency.
  • (4) In recent years, many non-ferrous metals (copper, aluminum, titanium, zirconium, nickel) and their alloys, etc. are gaining wider and wider application, also applied to the pipeline, including elbows and bends. Their bending methods are similar to carbon steel, but the bending process should be selected according to the properties of the material.

Authors: Jun Chen, Wensen Duan, Haiying Yang

Source: China Pipe Bend Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.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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