Length standards for pipe elbows at different angles
Length standards for pipe elbows at different angles
Definition of long elbow and short elbow:
In pipeline engineering, an elbow is a connecting piece that bends a pipe at a certain angle. Pipe elbows are usually divided into long elbows and short elbows based on the ratio of the required bending radius to the pipe diameter. The radius of a long elbow is generally 1.5 times the diameter of the pipe, while the radius of a short elbow is 1 times the diameter of the pipe. The materials of elbow fittings include cast iron fittings, stainless steel fittings, alloy steel fittings, malleable cast iron fittings, carbon steel fittings, non-ferrous metals and plastics fittings.
Long elbow and short elbow length standards:
The length standard of pipe elbows means that under a certain length of straight pipe section, the length of the elbow should meet the standard requirements. According to relevant domestic and foreign standards and specifications, under the same pipe diameter and elbow radius, the length of the long elbow should be 1.5 times the pipe diameter, while the length of the short elbow should be 1 times the pipe diameter.
In addition, the calculation of the elbow length also needs to consider factors such as the angle and thickness of the elbow. Generally speaking, when the angle of the elbow is larger, its length will also increase. For elbows with the same angle, the length of the elbow with larger wall thickness will also increase.
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The difference in elbow length at different angles:
1. 45 degree elbow
The length of the 45-degree elbow is generally 1.5 times the diameter of the elbow. It is suitable for occasions where the pipe bending angle is small and can reduce the flow resistance of the fluid. In some environments with limited space, the floor space of the pipeline can also be reduced through the use of 45-degree elbows.
2. 90 degree elbow
The length of a 90-degree elbow is often twice the diameter of the elbow and is one of the most common types of elbows. Due to the right angle, the fluid flow resistance of the 90-degree elbow is relatively large. Therefore, in pipelines with higher flow requirements, it is recommended to use long arc elbows with a relatively large radius instead.
3. 180 degree elbow
A 180-degree elbow is also called a U-shaped elbow, and its length is generally 4 times the diameter of the elbow. The advantage is that the pipe can be bent in the opposite direction, but the disadvantage is that the fluid flow resistance is greater. When the space is sufficient, the flow requirements are not high, and a reverse elbow is required, you can choose to use a 180-degree elbow.
4. Unequal angle elbow
An unequal angle elbow refers to an elbow with a bend angle other than 45 degrees or 90 degrees. Its length is related to the size and shape of the bend angle and requires design calculations. In some situations where pipes have special layout or flow direction needs to be adjusted, unequal elbows can provide a better solution.
5. Long radius elbow
The length of a long-radius elbow is generally 3 times the diameter of the elbow, the radius is relatively large, and the radius of curvature is more than 1.5 times the diameter of the elbow. Its advantage is that the fluid flow resistance is small, and the fluid streamlines when flowing through the elbow are relatively smooth. It is suitable for occasions with high flow requirements, high flow speed, and high fluid viscosity.
The above are the lengths and characteristics of several common elbows. Choosing a suitable elbow should be comprehensively considered based on the specific usage scenario and the physical properties of the fluid.
Conclusion:
Pipe elbow is one of the commonly used connectors in pipeline engineering. The elbow length standard is a detail issue that requires great attention in pipeline engineering. Whether it is a long elbow or a short elbow, its length needs to comply with relevant standards and specifications to ensure the quality and safety of the pipe connection. When selecting and installing pipe elbows, comprehensive considerations need to be made based on specific circumstances to ensure the smooth progress of the pipe project.
Wrinkle problem of stainless steel elbow and its preventive measures
Wrinkle problem of stainless steel elbow and its preventive measures
In order to prevent the problem of wrinkles when cold bending stainless steel, first from the standardization of pipe fitting manufacturing perspective to examine and assess the advantages of cold bending methods and their reasonable range of applications, and then analyze the occurrence of wrinkles when cold bending stainless steel “thin” wall conditions, influencing factors, and through the analysis of two typical cases. The analysis points out that the essence of the wrinkle wave is the arch belly compression plastic deformation zone of the “compression bar” instability buckling phenomenon. According to the steel pipe to/do and Ro/do decision of the bend “thin” wall degree, the reasonable setting of mandrel type and other mold processing and installation accuracy to determine the “gap”, in order to mechanically enhance the “compression bar “Compressive stability, which is necessary to prevent wrinkles guarantee. Stainless steel pipe manufacturing standards allow to and do tolerance zone is often greater than the “thin” wall bend necessary for the mold gap, sometimes in small batches of multiple specifications of stainless steel bend wrinkles, but as long as the grasp of the “thin” wall degree, pay attention to the mandrel structure, mold gap and reasonable adjustment of the bending parameters, stainless steel cold bending pipe when the problem of wrinkled waves can be solved.
Stainless steel pipe application process will inevitably involve the elbow or bend. In the manufacturing process of stainless steel elbow or manufacturing process of stainless steel bend, it will be accompanied by uneven wall thickness, wrinkle wave and rebound, roundness distortion and other problems, which are of considerable concern [1-18]. occurred in a two shipyard users of Φ88.9mm × 3.05mm specification 304 steel pipe and Φ60mm × 3.5mm specification 316L pipe bend wrinkle wave problem is extremely typical.
This study examines the occurrence of stainless steel pipe bend wrinkle wave conditions, influencing factors and the prevention of ways based on the two cases that occurred to explore the relevance of stainless steel pipe manufacturing scale tolerance and bend wrinkle wave. The text also describes the advantages and disadvantages of various methods of elbow manufacturing, the state of standardization, to clarify the reasonable range of application of different types of bends and considerations. Hope to promote the progress of stainless steel tube manufacturing and application level with users.
Elbow production methods, standardization and rational selection
(45 ° elbows, 90 ° elbows and 180 ° elbows) Elbow is an indispensable pipe steel application of the most commonly used joint parts (fittings), its standardization and standardized intensive production is an important feature of the pipeline steel pipe and other non-pipe pipe difference, domestic and foreign The market can be directly procured in accordance with national pipeline standards to a variety of pipe specifications of the elbow, but the tube specifications of the elbow can only be self-made.
China and the United States stainless steel pipe elbow manufacturing standards are compared in Table 1. it is important to note that China’s standards for pipe fittings established in accordance with Japanese standards as a model is actually only equivalent to the U.S. pipe fittings scale standard ASME B16.9-. the U.S. pipe fittings scale standard ASME B16.9- does not distinguish between manufacturing methods. Nor does it discuss in detail the various details of the manufacturing methods involved. The U.S. standard divides these steel-related details into a variety of additional pipe fittings product standards and the corresponding general pipe material standards, and discusses them in detail. Table 1 lists only the products related to stainless steel fittings and their related general material standards governed by. Considering the shape of elbows and pipe fittings and the manufacturing process is not simpler than pipe steel, and our current way of distinguishing between the two types of pipe fittings only with or without welds may be somewhat inappropriate, so special standards for the manufacture of stainless steel pipe fittings should be established to meet the needs of the current rapid development of stainless steel pipe.
As seen in Table 1 stainless steel elbow manufacturing standards comparison, the U.S. standards in a variety of elbow manufacturing methods mainly cold bending or hot bending, molding + welding, forging + cutting process 3 ways, the following analysis of the advantages and disadvantages of these manufacturing methods and their scope of application.
Table.1 China and the United States stainless steel pipe elbow manufacturing standards comparison
Bending method
Elbow method is a simple and commonly used 45 ° ~ 180 ° elbow and even coil manufacturing method, but not the only ideal or best elbow manufacturing method.
The advantages of the bend method
(1) Simple process equipment, from the simplest manual bending molds to fully automatic digital program control hydraulic cold bending machine are very mature, practical and convenient.
(2) Can be determined within a certain range according to the actual needs of the bending radius, bending orientation and bending angle, the most suitable for doing space three-dimensional multi-angle bending tube structure parts. Therefore, aircraft, automotive exhaust system is the earliest application area of stainless steel bends.
(3) Elbow straight pipe section length is not limited, or can reduce the number of pipe ring welding head as much as possible.
(4) Plasticity of austenitic stainless steel seamless pipeor austenitic stainless steel welded pipe at room temperature can be achieved by cold bending, and can be used directly in the static load and general (uniform corrosion) environment without heat treatment. But the plasticity of the poor ferritic, martensitic stainless steel pipe requires heat treatment, only the bending radius is larger (Ro ≥ 2.5do) without heat treatment. Stainless steel tube after cold bending whether the need for heat treatment of the relevant European standard basis is shown in Table 2 [2].
Table.2 stainless steel tube after cold bending whether the need for heat treatment based on the relevant European standards
Note: According to EN-4: in section 7.2 and table 7.2.2-1 summary.
(5) Bending method formed by the elbow wall thickness is not uniform, the relevant literature can be found in many different bend wall thickness calculation formula, but basically in line with the law of this change: that is, the arch back (elbow outside) when reducing the wall thickness, the arch belly (elbow inside) when increasing the wall thickness. The theoretical variation of wall thickness of the bend without mold constraint plastic deformation and its calculation formula are shown in Figure 1.
According to the analysis of literature [11] introduced the approximate wall thickness extremes and arch back, arch belly thinning and thickening the relative amount of the calculation formula is as follows.
Figure.1 Schematic wall thickness theoretical variation of the plastic deformation bend without accounting for mold constraints and its calculation formula.
In the formula:
- tex – arch back (outer) wall thickness extremes.
- tin – arch belly (inside) wall thickness extremes.
- to – the original wall thickness.
- △ tex – arch back relative thinning extremes.
- △ tin – arch belly relative thickness extremes; △ tin – arch belly relative thickening extremes.
- Ro-bending radius.
- do – outside diameter of the pipe.
For conventional pipes subjected to internal pressure, in accordance with the stress calculation arch back, arch belly of the required wall thickness and allowable incremental approximation is as follows [19].
In the formula:
- texr – the minimum net wall thickness required by the arch back when the pipe is subjected to internal pressure (excluding allowances and tolerances).
- tinr – the minimum net wall thickness required for the arch belly when the pipe is subjected to internal pressure (without allowance and tolerance).
- Δtexr-allowable wall thickness increment in the arch back.
- Δtinr – arch belly allowable wall thickness increment.
In accordance with the formula (1) to formula (4) to make the Ro/do-Δt/to curve, as shown in Figure 2. As seen in Figure 2, the wall thickness increase or decrease caused by the bend is very close to the internal pressure pipe force requirements, that is, under the conditions of the bend although it will cause uneven wall thickness, but does not affect the use of the bend.
Figure.2 in accordance with the formula (1) to formula (4) to make the Ro/do-Δt/to curve
The disadvantages of the bending method
(1) In dynamic conditions such as time-varying stress, i.e., alternating stress, the above bend method causes uneven wall thickness, which is not allowed. The elbow under time-varying stress conditions should be equal-thickness elbow with the wall thickness value required by the arch belly for the minimum elbow wall thickness value [19]. Many studies have proved that the wall thickness thinning and roundness distortion caused by the elbow method can contribute to the acceleration of stress and creep stress rate, which in turn affects the dynamic stability [20-22].
(2) Unless special measures such as additional internal pressure and sand filling are taken, there may be additional roundness distortion in the bent section. The literature [23] points out that the degree of roundness distortion μ also depends mainly on Ro/do.
In the formula:
- dmax-maximum outer diameter of the bent pipe curved section.
- dmin – the minimum outer diameter of the bend section.
Usually, the bend produces roundness distortion. In order to limit the μ within 10% or lower value, it is necessary to control the Ro / do above 2.
The correlation between the roundness of the bend section and Ro/do and the allowable values of the European standard are shown in Figure 3 [23]. Figure 3 is based on EN-4: in Figure 7.4.1.1 redrawn, the original does not specify to/do and steel material and other conditions, but also does not specify 20/(Ro/do) inverse curve presumption basis, the author believes that to increase when the υ curve will shift down.
Figure.3 bend section roundness and Ro/do correlation and European standard allowable values
(3) After the completion of the bend, in the direction of the bending angle will show a certain amount of rebound, the size of the bend with the material, Ro / do and other related. Therefore, in the actual bending process, a certain amount of overbending angle is usually increased to eliminate the impact of rebound [6-10]. The size of the overbending angle is determined based on empirical values or simulation analysis.
(4) In Ro/do smaller, to/do lower “thin” wall pipe cold bending or bender structure is not suitable or improperly adjusted, the arch belly will appear wrinkles, wrinkles will not only affect the appearance of the bend, but also affect the fluid transfer performance of the bend. For stainless steel piping, wrinkle wave will also damage its corrosion resistance, reducing the service life of stainless steel piping. It is evident that the study of wrinkle wave defects should receive particular attention [1, 4, 5, 11-18]. The effect of the degree of cold working on the magnetic phase content and corrosion rate of 304L steel is shown in Table 3 [1].
Table.3 Effect of the degree of cold working on the magnetic phase content and corrosion rate of 304L steel
Notes.
- ① %CW is the degree of cold working, %WW is the degree of hot working, with weld bending specimens as shown in Figure 4.
- ② The annual corrosion amount derived from the test method in ASTM A262C.
Although theoretically speaking, the use of bends with wrinkled waves will produce certain safety hazards and should be rejected. But this is not the case, from the implementation of EN-4: to date, still provides that as long as the wrinkle wave height (h) and wave distance (a) control within a certain range, the bend with wrinkles can still be used. EN-4: standard provides for the measurement of the allowable value of the bend wrinkles as shown in Figure 5.
Figure.4 Table 3 bending specimens of 304L stainless steel with welded seams
Figure.5 EN-4: standard in the bend wrinkle allowable value measurement method
EN-4: standard on the bend pipe wave height (h) and wave distance (a) the specific requirements are as follows.
In the formula:
- h – the average height of adjacent wave crests, h = 0.5 (do2 + do4) – do3.
- a – the spacing of adjacent wave crests.
This can be seen in the application of the European pipeline is not very strict control of the bend wrinkle wave.
Cold bend stainless steel elbow application considerations
- (1) Different types of stainless steel pipe allowed after cold bending directly after the application of the bending radius ratio is different, more than the given limit must be heat treatment of the bend, and then can be used.
- (2) Cyclic dynamic load, high temperature, stress corrosion media environment application of austenitic stainless steel pipe cold elbow must also be heat treatment, and preferably solution heat treatment. It should be particularly noted that although ASTMSA403/SA403M, ASTMSA815/SA815M standard allows the use of bending method to manufacture bends, but must be supplied in a heat-treated state, while ASTMSA182/SA182M standard is not allowed to use the bending method to manufacture bends, see Table 1 for details.
- (3) Large diameter pipe usually do not use cold elbow elbow, the reason: ① bender power and cost is larger; ② large diameter stainless steel pipe to/do is very small, bending difficulty, especially to avoid wrinkle wave is very difficult. Φ219mm or more stainless steel pipe has rarely used the cold bending method to manufacture bends.
Steel pipe local heating manufacturing bend
Stainless steel tube heated to thermoforming temperature for bending can reduce the mechanical force required when bending, the hot bending method is suitable for thick-walled pipe bending, can also be used for Ro/do smaller steel pipe bending. Hot bending method of manufacturing bends not only requires heating processes, but also must be filled with sand in the steel pipe, more procedures, complex operations, and the forming of the bend must also be solid solution or annealing treatment. Therefore, the use of integral heating to manufacture bends is not commonly used.
The literature [24] describes a method for hot bending U-shaped elbows that is only partially heated within 120° of the bend’s arch abdominal position. This method can reduce the amount of thinning of the arch back of the elbow and is particularly suitable for U-shaped bends with small radius of curvature of (Ro/do) <1.5. The heat bending method for locally heated Φ50mm×7mm mild steel pipe is shown in Figure 6.
Figure.6 Heat bending method of locally heated Φ50mm×7mm mild steel pipe
Molding and welding method
When making an elbow by the molded welding method, the steel plate is first cold pressed or hot molded into two elbow parts of 45°, 90° and 180° symmetry, then the two splices are welded to form an elbow of 45°, 90° and 180°, and finally the elbow is heat treated to make the final product. A pressure bending machine structure sketch is shown in Figure 7. Molded welded elbow production method is the only method of manufacturing elbows in GB/T-, and also ASTMSA403/SA403M, ASTMSA815/SA815M and ASMEB16.9 standard provisions can be used to make elbows.
Advantages of the molded welding method
The advantages of the welding method:
- ① The arch back of the bend joint, the arch belly without obvious wall thickness differences, bend joints without roundness variation and wrinkle waves and other phenomena;
- ② Different heat treatment state supply according to the steel, a wide range of applications.
Figure.7 a pressure bender structure schematic diagram
The disadvantages of the molded welding method
- (1) Each specification of the elbow radius is only long and short two.
- (2) Elbow straight section length is very short, the pipe installation, if the design is not reasonable will increase the number of ring welds.
- (3) Non-professional production, the cost is higher.
- (4) High welding quality requirements. Welding is best done with automated GTAW or PAW without filler metal, single-sided once-formed double-sided welding. Only the use of GTAW or PAW welding elbow can be exempted from filming or ultrasonic inspection, but must be strictly visual inspection, because GTAW or PAW welding weld corrosion resistance is better. If you use other welding methods or add filler metal to weld the elbow, it must be X-ray film or ultrasonic inspection, which is clearly defined in ASTM SA403/SA403M and ASTM SA815/SA815M and other standards.
Molded welding method of application
(1) Applicable to all kinds of stainless steel, especially austenitic stainless steel. But less applicable to the poor weldability of martensitic stainless steel.
(2) Applicable to large diameter thin-walled pipes. For to/do <2% of the 5S, 10S wall thickness series (see Table 4) should be preferred, wall thickness in 6 to 7mm below the 40S, 80S wall thickness series is also applicable.
(3) The choice of elbow, must be based on the pipeline working temperature, pressure and media environment to determine the elbow wall thickness and steel, and according to the category of steel and elbow manufacturing procedures and other requirements of special attention to the elbow heat treatment state. H-level austenitic stainless steel elbow and other pipe fittings for hot forming, must be made separately after forming solution annealing treatment, welding must also be the final solution annealing treatment. Solution annealing treatment, must ensure that the heating temperature, holding time and subsequent rapid cooling in line with the relevant requirements. The American standard refers to the “manufacturing process of heat treatment can not replace the final solution annealing treatment” is also this meaning.
Table.4 ANSI/ASME B36.19 stainless steel pipe standard size specifications and its comparison with the European standard, the national standard analysis ①
Notes.
- ① According to ANSI/ASME B36.19 preparation, delete the original table in units of outside diameter and wall thickness; to/do is the author added to the analysis of data, to the data in parentheses for the national standard or European standard closest data; ② for GB/T.
- ② For GB/T- can be found in the corresponding outer diameter specifications, data in brackets for the EN ISO- data.
(4) Die welding method of welding, if you use the welding method of adding filler wire, we must pay attention to the selection of appropriate welding materials by steel type to ensure the quality of welding.
Forging machine processing elbow
Forging machine processing elbow is the optimal solution for high-temperature applications of stainless steel elbow, is the only elbow conduit joint manufacturing method specified in ASTM SA182/SA182M, ASTM SA403/SA403M and ASTM SA815/SA815M manufacturing methods allowed.
Advantages of forged machined elbow:
- ① Seamless pipe, better material uniformity, high temperature corrosion resistance or creep resistance is easy to control;
- ② Especially suitable for martensitic and H grade austenitic stainless steel thick-walled pipes with poor welding performance.
The disadvantages of forging machine processing elbow:
- ① Long manufacturing process;
- ② Forging usually requires machining to achieve surface finishing requirements;
- ③ The highest cost. Forging machine processing elbow method is suitable for nuclear power plants and other thick-walled pipe elbow manufacturing with particularly high requirements for operational reliability.
Bend wrinkling causes and preventive and control measures
The manufacture of stainless steel bends can be used in a variety of forms, such as roll bending machine, bending die rotation machine is the most common model. Here to bend around the die rotating machine for analysis and discussion.
The basic composition of the bending bending machine
The basic structure of the bending bender is shown in Figure 8. As seen in Figure 8, the bending bender is mainly composed of rotatable bending module, compression block, clamping block and other parts. When bending, the steel pipe to be bent clamped in the bender between the clamping block and clamping block, so that the clamping block to the appropriate pressure on the steel pipe after the rotation of the bending module, with the clamping block from the initial position 1 to 2, 3, 4 position, the steel pipe can be bent.
Figure.8 The basic structure of the bending bender
Figure.9 Two types of bending benders with boost
The structure of the two types of bending benders with boost is shown in Figure 9. Comparison between Figure 9 and Figure 8 can be seen, with the basic structure of the bending bender with a boost and the main difference between Figure 8 are two points: ① compression block or the end of the steel pipe to increase a booster (indirectly through the compression block boost model is older, but is still widely used in domestic production, its rationality is worth exploring); ② the initial position of the compression block is close to the initial position of the clamping block. It can be seen that the compression block on the winding bender with boost presses the steel pipe between the compression block and the module from the beginning, while the compression block in Figure 8 has a certain distance between it and the clamping block at the beginning. The literature [24] specifies that the clamping block should be long enough to press the tube to be bent and move with it, and states that “there should be no sliding between it and the tube”. But when there is a boost, the compression block and the steel tube may slide between. With the compression block together with the other half of the steel pipe support sliding block (wiperdie) is always in a relative slip with the steel pipe. To make the sliding block (i.e., Figure 9, the anti-wrinkle block) to play the role of anti-wrinkle, it is necessary to thin its front end and close to the tangent point of the bending die and pipe, this distance should be controlled at 3.2 to 13mm [24], otherwise it will not play the role of anti-wrinkle. Typically, the slip block (or anti-wrinkle block) of the stainless steel bend is made of aluminum bronze. The sliding block in Figure 8 plays a certain role in support, but the anti-wrinkle block in Figure 9 plays a very small role in support, so some factories to reduce costs on the bender to remove the anti-wrinkle block.
Bending bender mandrel and its type
Bending benders are equipped with mandrels (see Figure 8 and Figure 9), in fact, only when the wall thickness of the steel pipe is relatively thin mandrels will be used. Table 5 lists the minimum bending radius of cold bent steel pipe without mandrel for different wall thicknesses and outside diameters [24].
According to the analysis of the data given in Table 5, to/do ≥ (3.6-4)% of the steel pipe cold bending can be used without a mandrel, and the larger the value of to/do, the smaller the Ro/do value of mandrel-free cold bending. The literature [24] does not specify its applicable steel grades, but I believe that the austenitic stainless steel tube this data is roughly referable.
Table.5 different wall thickness and outside diameter without mandrel cold bending steel pipe minimum bending radius
Notes.
- ① Table do for the outside diameter, Ro for the minimum bending radius, Ro/do for the relative bending radius, to/do for the relative steel wall thickness.
- ② The table is based on the literature [24], page 464 Table 3 data preparation, where Ro/do and to/do for the analysis given by the author, the calculation and analysis results show that the data listed in the table ;
- ③ The constraints of steel grades are not given in the original table.
- ④ The constraints in this table are mainly wrinkle prevention and roundness distortion.
- The nomogram for selecting the mandrel according to the relative wall thickness of the steel pipe (to/do) and the relative bending radius (Ro/do) is shown in Figure 10 [24]. The nomogram shown in Figure 10(a) can be used to determine the matching conditions for different to/do and Ro/do in mandrelless cold bending. For example, when to/do = 20%, Ro/do ≥ 1 are available for coreless cold bending; but if to/do ≤ 10%, Ro/do ≥ 1.5 in order not to use coreless cold bending; if to/do ≤ 5%, Ro/do ≥ 3.5 can also be coreless cold bending; and if to/do ≤ 4%, Ro/do > 4 can still be coreless cold bending.
Bending bender used mandrel has a rigid or flexible bendable two types of various types. Mandrel structure type shown in Figure 11, which Figure 11 (a) and Figure 11 (b) for the rigid mandrel, Figure 11 (c) and Figure 11 (d) flexible mandrel, Figure 11 (e) for rectangular steel pipe cold bending with a flexible mandrel. Figure 11 (a) and Figure 11 (c) is a general mandrel that has been widely used in China. The rigid shaped end mandrel of Fig. 11 (b) is used for cold bending of elliptical tubes. The steel cable core type flexible mandrel in Figure 11 (d) has greater spatial freedom, and its applicability is worth further exploration and research.
The purpose of using mandrel is to prevent the arch belly from wrinkling wave, and to reduce the arch back thinning and bending section of the pipe out of roundness. Flexible mandrel is generally used for t / d value is much lower thin-walled steel pipe. Figure 10 (a) shown in the nomogram can also be used to determine the structural requirements of the mandrel, for example: when to/do = 3.3, Ro/do = 4, rigid mandrel can be used; Ro/do = 3, the need to use a single spherical flexible mandrel; and Ro/do = 2.0 must be used multi-spherical flexible mandrel. Fig. 10 (b) shows the Nodal diagram can further determine the number of balls or segments of spherical or segmented flexible mandrels required for bending 90° and above elbows. It can be seen that the flexible mandrel is more suitable for to/do ≤ 3.3% of the thin-walled pipe cold bending, to/do the smaller the number of balls or segments required should be more. The softer the material of the thin-walled tube cold bending more demanding with a spherical flexible mandrel [24].
It is important to note that in stainless steel pipe standards, usually only to/do distinguish thin-walled tubes (to/do ≤ 3%) or ultra-thin-walled tubes (to/do ≤ 2%), but in the discussion of bending, the value of to/do must also be linked to Ro/do to distinguish the so-called “thin” wall concept. The average practical bending radius for thin-walled cold bending with spherical mandrels and wrinkle-resistant sliders is shown in Table 6 [24].
Figure.10 According to the relative wall thickness of the steel pipe (to/do) and the relative bending radius (Ro/do) to select the mandrel of the No-mode diagram
Figure.11 Structure type of mandrel
The analysis of the average practical bending radius data for a single spherical mandrel given in Table 6 shows that: for the steel tube with to=0.89mm, do=75mm, the bendable radius is 381mm, Ro/do=5.1, to/do=1.2%; but when do=13mm, Ro=13mm, Ro/do=1.0, to/do=6.8%. This means that the bending radius is very small, to/do = 6.8% should still be regarded as a “thin” wall bend. Analysis of the data in Table 5 and Table 6 found that can be used as a cold bending steel pipe when selecting the mandrel “thin” wall assessment value: b > 25%, it is appropriate to use no mandrel cold bending; b = 12.6% to 25%, it is appropriate to use a rigid mandrel cold bending; b < 12.6%, it is appropriate to use a flexible spherical mandrel cold bending. And according to the analysis in Figure 10 (a), the corresponding b values should be b > 17.5%, b = 10.5% to 17.5% and b ≤ 10.5%, respectively. There is a certain difference between the two values, which may be due to the lack of completeness or continuity conditions of the data in Tables 5 and 6, as well as the calculation based on the speed, pressure, structure and other parameters of the bender is not exactly the same. It can be assumed that the value of b given in Figure 10(a) is more reasonable, but the choice given has only a relative significance and is not an absolute cut-off. It will be pointed out later that this rough assessment is clearly of value.
Table.6 Average practical bending radius for thin-walled cold bending with spherical mandrels and wrinkle-resistant sliders
Notes.
- ① In the table do is the outside diameter, Ro is the practical average bending radius, Ro/do is the relative bending radius, to/do is the relative tube wall thickness.
- ② The table is based on the literature [24], page 469 Table 6 data preparation, where Ro/do and to/do for the analysis added by the author, the original table does not specify the number of balls, according to the textual description, should refer to a single spherical mandrel, the calculation and analysis results show that the data listed in the table ;
- ③ Original table does not give the steel species limit; the original table does not specify the position of the slider used according to the original table, but according to the original understanding should be adjusted through experiments anti-wrinkle block forward position.
The above chart data summarized from the practice of pipe bending: mandrel type or with or without mandrel on the cold bending wrinkle is extremely critical. Recent domestic experimental studies have shown that as long as the appropriate mandrel type (using a four-ball mandrel), even Φ50mm × 0.8mm 304 steel pipe or Φ123mm × 3.97mm 316L steel pipe can achieve Ro/do = 1.2 or 2.9 wrinkle-free bending [1,6]; if no mandrel, Φ20mm × 1mm 304 steel pipe Ro/ do ≤ 3.5 will always appear wrinkled waves [5].
Manufacture and installation of mandrel and bending die, etc. and their accuracy
Mold material and manufacturing, installation and other requirements
In essence, steel pipe cold bending is actually bending die, compression block and mandrel and other molds together under the constraints of plastic forming process, the manufacturing accuracy of these related molds is very important to the quality of bending forming, the higher the accuracy of the mold, the higher the quality of the bending section forming [24]. Here, the manufacturing accuracy mainly refers to the clearance size and surface quality of the bending die, compression block and mandrel, etc., and the steel pipe to be bent. Table 7 summarizes the clearance data and related constraints for determining the manufacturing accuracy of the die available in the literature.
It is important to note that.
- ① The data given in the literature [24] are mostly relative values related to the wall thickness or outside diameter of the steel pipe, some other give specific values of the actual constraints may be more narrow;
- ② If the steel pipe size beyond the range of Table 5 and Table 6, the practicality of the data in Table 7 needs further verification;
- ③ Stainless steel bending, rigid mandrel and flexible mandrel body, clamping block preferably surface Cr plating, flexible core baseball body, anti-wrinkle slider , clamping block is appropriate to use aluminum bronze. Bending mold should be used hardwood, plastic, etc., or cold bending steel pipe must be pickled after passivation. And should not be in the same bender both cold bending carbon steel pipe, and cold bending stainless steel pipe.
Installation accuracy of the mold
Rigid mandrel, flexible mandrel body endpoint location is very important to the quality of the bend formed around the bend, the endpoint referred to here is the maximum diameter endpoint. Literature [24] and others that the end point should be slightly more than the beginning of the bend, that is, the bend die cut point (see Figure 12). As can be seen from Figure 12, if the end point exceeds the cut point too much will cause the bend section “goose head” shape, but if the end point is a little far from the cut point position will cause the arch belly wrinkles, so the exact amount of mandrel front should be determined by test. The equation given in the literature [8] (see Table 7) further indicates that the front amount should be different for different mandrel clearances. The literature [4], on the other hand, argues that 0.15do should be exceeded, but the paper gives both good molding results with 0 projection and Ro=2do (see Table 1 and Figures 2 and 4 in the literature [4]), which shows that the conclusion of the paper is still worthy of speculation.
Figure.12 Effect of rigid mandrel end position on the quality of the bend
Anti-wrinkle slider installation, due to its end thinning must be as close as possible to the bending die cut point, and preferably in the bending die cut before 15 ° effective dragging steel pipe arch abdominal position (see Figure 9), otherwise it is easy to cause wrinkle wave. For this reason anti-wrinkle block is often installed at an angle, but this may cause vibration.
In addition, around the bending bender after a period of time, attention must be paid to detect the bottom of the notch of the bending die and its rotation journal wear amount. The amount of wear should be controlled within the range of values listed in Table 7, which is particularly important for thin-walled bends [24].
Operating parameters of the bending bender
Bending speed
Bending speed is the bending die rotation speed of the steel pipe to give the line speed, or the actual deformation speed of the bent section. Literature [5] study shows that the bending speed has a certain impact on the quality of the bend, especially the impact of the bend on the wrinkle wave, when to/do and Ro/do given time, reduce the bending speed can eliminate or reduce the wrinkle wave (see Figure 13). However, Table 4, Table 5 and Figure 10 are not calibrated bending speed, which shows that the results specified in the literature [5] are not absolute limits.
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Pressing block pressure
The compression block pressure is another important parameter that affects the wrinkle wave of the bend. Compression block pressure in the bending process has two important roles: one is to work with the bending die to form the necessary bending moment; the second is to produce friction on the contact surface of the compressed steel pipe, and reduce the steel pipe around the bend required to send or reduce the speed.
From the literature [1], it is known that the minimum value of the compression block pressure can be expressed as:
In the formula:
- q – pressure density on the compression block.
- Lp – length of the compression block.
- M – bending moment, the value of which depends on the yield strength of the tube, Ro/do and to/do, and a variety of expressions for M can be found in the literature [1, 5, 10].
The difficulty with this calculation is that the degree of deformation and hardening of each longitudinal fiber on the circumference of the circular tube during the bending process is different, and the value of the yield stress at each point on the circumference will change with the degree of hardening. Therefore, concepts or factors such as the hardening factor involved should be introduced in the calculation. That is, po will be dependent on the yield strength of the steel pipe, hardening factor, Ro/do and to/do of the dual integration calculation results, the application of the finite element method to obtain its approximate value, that is:
Figure.13 Physical samples of stainless steel bends and their corresponding finite element simulations under different bending speeds
In the formula:
- y – the bend neutral layer height of the micro-area element on the cross-section of the steel tube.
- r – the radius of the micro-area element on the cross-section of the steel pipe is taken.
- σθ – the actual stress of the micro-area element on the cross-section of the steel pipe.
- φ – the offset angle between the micro-area element on the cross-section of the steel pipe and the back of the arch.
Bending, compression block pressure will produce friction on the contact surface of the compressed steel pipe, and reduce the steel pipe around the bend required to send or reduce the speed, at this time, if not add thrust, friction will make vp < vo or vp/vo < 1 (vp for the compression block feeding speed, vo for the bending die rotation speed determined by the neutral layer of steel pipe speed), can not achieve bending; only add thrust to make vp/vo ≥ 1, to achieve bending. Obviously, the greater the compression block pressure, vp/vo value will be lower, which will have a non-negligible impact on the actual wall thickness change, wrinkle height and rebound of the bent section.
The effect of compression block pressure (p/po) on the quality of the bend at vp=1.5vo is shown in Figure 14 [1]. For Φ50mm×0.8mm specification 304 stainless steel pipe (Ro/do=1.2) cold bending, the effect of compression force and boosting speed on the wall thickness and wrinkle wave height of arch back and arch belly are shown in Table 8.
As seen in Figure 14 and Table 8: ① increase the pressure or p/po can reduce the amount of wall thickness thinning of the arch back, increase the amount of wall thickness increase of the arch belly, and make the wrinkle wave height and rebound are slightly increased; in p/po ≥ 8, the pressure has a great impact on the wrinkle wave, which is not desirable because of the special disadvantage to the wrinkle wave, although the rebound is beneficial and can reduce the amount of arch back thinning; ② when the pressure is the minimum value, the finite element Calculation and test results have proved that the amount of arch back thinning and arch belly thickening are not large, and there is no wrinkle wave, and the amount of rebound is not large and has little correlation with the presence or absence of thrust.
The above shows that the only benefit of boosting (see Figure 9) for the conventional bending bender is to reduce the amount of wall thickness thinning of the arch back with increased pressure, but it is extremely detrimental to the wrinkle wave. This is probably the reason why no boost is provided in Figure 8 and no additional boost is discussed in the literature [24].
Figure.14 Effect of compression block pressure (p/po) on the quality of the bend at vp=1.5vo
Lubrication
In the actual bending process, when p ≥ po, vp = 0.9vo, p generated by the friction force can make the compression block to follow the steel tube into. The frictional force is equivalent to reverse boost for the steel tube. Literature [5] on 304 stainless steel pipe bending wrinkling finite element study results show that: improve the friction coefficient is beneficial to reduce or eliminate wrinkling; and vp = vo that is, when applied to boost, even if the friction coefficient is smaller, increase lubrication or improve the surface finish of the compression block, will also lead to the bend arch belly wrinkling. It can be seen that the compression block and the steel pipe should not be lubricated to reduce friction, and the surface of the compression block should not be polished to excessively improve its surface finish.
Literature [5] also proved through finite element calculations, if the inner side of the steel tube (soffit) can be improved by applying reverse boost, and thus proposed to control the compression block boost distance or increase the double boost method to reduce the wrinkle wave. Double boost that the compression block for positive boost, slider or anti-wrinkle block for the reverse (asynchronous) boost. Bend using double boost, both the use of boost to reduce the arch back wall thickness of the thinning amount, but also the use of reverse boost to limit the arch belly wrinkles and wall thickness of the thickening amount, so as to ensure uniform wall thickness when cold bending. This double boost mode is a very valuable innovation if it can be achieved.
It is worth noting that the lubrication method of the pipe bender and its role in different literature there are different statements, for example: ① literature [8] pointed out that the mandrel, compression block, anti-wrinkle block, the inner and outer walls of the pipe ring should be coated with lubricant (aviation lubricant (60% to 80%) + paraffin (40% to 20%)) for lubrication. This may be the basis for the operation of many domestic winding bending machine bending, but its reasonableness is obviously worth exploring; ② literature [24] only specified that the mandrel and steel pipe must be lubricated with thick oil, the sliding block or anti-wrinkle block must be highly polished and coated with thin light lubricant, and stressed that too much oil or too thick a layer of oil will promote the arch belly wrinkle wave, the practical significance of this practice is also worth studying; ③ literature [12-13] (3) literature [12-13] indicates that only the core baseball body and the inner wall of the tube and between the slider and the outer wall of the tube need lubrication, lubrication of other parts is unnecessary; (4) literature [5] believes that “increasing the friction coefficient of the concave surface of the anti-wrinkle block work can appropriately reduce the tendency of wrinkling” (because literature [5] mainly discusses the mandrelless bend, whether the discussion is related to this (since the literature [5] mainly discusses mandrelless bends, the relevance of this statement remains to be further investigated).
Table.8 Effect of compression force and boosting speed on the wall thickness and wrinkle height of arch back and arch web, etc.
These are some disagreements, indicating that the issue of lubrication has yet to be further explored in depth, in order to unify.
Case study and anatomy
In , the user found that some of the bends appeared wrinkled when bending the stainless steel pipe produced by De Chuan Pipe Industry. The sample data of the bend was statistically analyzed, and the results are shown in Table 9.
Case 1
First of all, take the bend of Φ88.9mm×3.05mm (serial number 1 in Table 9) stainless steel pipe as an example. According to the Nuo modal diagram (see Figure 10) and Table 9 data can be seen, the bending of this specification of steel pipe is appropriate to use a single ball flexible mandrel to bend, but the actual bending of the factory used a rigid mandrel, fundamentally speaking, there will be a greater risk of wrinkling, which is due to.
Table.9 Sample data statistics for stainless steel bends
Notes.
① The serial number with the label W for the appearance of wrinkled bends, the serial number with the label b for the use of spherical flexible mandrel cold bends, 3W serial number for the use of no mandrel cold bends, the rest are cold bends with rigid mandrel bends.
② Serial number 4 to 15 for normal cold bending, and cold bending after the bend without wrinkles.
③ “Thin” wall degree :
- (1) The bender used by the user for Figure 9 (a) structure, but not set up anti-wrinkle block. The bending machine is used to bend Φ21.3mm×2.11mm, Φ33.4mm×2.77mm, Φ42mm×2.77mm, Φ48.3mm×2.77mm, Φ60.3mm×2.77mm and Φ73mm×3.05mm stainless steel pipes without wrinkling. The relative wall thickness (to/do) of these bends is greater than that of Φ88.9mm×3.05mm bend, which is an important premise. As seen from the data analysis in Table 9, these six steel pipes have b ≥ 10.5% and can be used with rigid mandrels. And Φ141.3mm×3.4mm bend pipe with spherical flexible mandrel also has no wrinkle wave, which shows more the importance of improving mandrel setting.
- (2) The order requirement is Φ88.9mm×3.05mm, which is rounded to Φ89mm×3.0mm in the national standard (see Table 4). If the manufacturer ignores the order requirement, it is easy to produce according to Φ89mm×3.0mm, which results in the outer diameter and inner diameter of the supplied steel pipe are large, but the wall thickness is low. When bending, if the same mandrel as Φ88.9mm×3.05mm steel pipe is used, the gap between the inner wall of the steel pipe and the mandrel will increase. All of these make the wrinkling phenomenon of Φ88.9mm×3.05mm steel pipe which should be bent with flexible mandrel become more prominent under the condition of Φ89mm×3.0mm when bending with rigid mandrel. It can be seen that the rounding of such data in the national standard is not appropriate.
- (3) The outer diameter of the mandrel used in the actual bend is 80mm, and the gap between the inner wall of the steel pipe and the mandrel is (di-dm)/2=1.45, which is much larger than the recommended value in Table 7. Compared with another factory using 82mm OD mandrel bending without wrinkling, the mandrel OD is too small and the mandrel position is not adjusted to the front amount may be the key factor of wrinkling in this case. In addition, no anti-wrinkle block is used, so wrinkling is inevitable for b=8.5% of the steel pipe (Φ88.9mm×3.05mm).
- (4) If the wall thickness of the steel pipe reaches the upper limit of the specification (3.05mm×1.225=3.72mm), the clearance between the inner wall of the steel pipe and the mandrel is (di-dm)/2=0.7≈0.2to when the 80mm outer diameter mandrel is used for bending, there will be no wrinkle. It means that the use of 80mm OD mandrel is suitable for the supply state of the upper wall thickness.
Case 2
Then take Φ60mm × 3.5mm (serial number 2 in Table 9) stainless steel pipe bend as an example. According to the Nuo modal diagram (see Figure 10) and Table 9 data can be seen, the specification of the steel pipe bending is suitable for the use of rigid mandrel to bend. But the specification of the steel pipe for non-standard steel pipe, in Europe and the United States standards (see Table 4) the standard outside diameter of 60.33mm, wall thickness of only 2.77mm (2.8mm) and 3.91mm (4.0mm). 60mm for the national standard of 60.33mm rounded value. For this specification of steel pipe bending, only individual bends produced wrinkle wave, the incidence of about 10%. After careful analysis, the reasons are mainly.
(1) The bend can only be used Φ60mm × 2.8mm standard specification mandrel, when the gap is significantly larger than the recommended value in Table 7.
(2) Outside diameter of 60mm <60.33mm, making the bending die gap increased, which will also increase the tendency of wrinkling wave.
(3) When bending, if the steel pipe wall thickness is close to the lower tolerance zone, and the wall thickness is uniform, it will make part of the steel pipe wrinkle. At this time, as long as the Φ60mm × 3.5mm configuration mandrel, the occasional wrinkles can be controlled. Stainless steel pipe standard below DN200 steel pipe size tolerance summary is shown in Table 10.
Table.10 Summary of dimensional tolerances of steel pipe below DN200 in stainless steel pipe standard
(4) At the same time, the user is supplied with three non-standard specifications of Φ27mm×2.5mm, Φ89mm×3mm and Φ219mm×4.5mm 316L bends without wrinkles, which means that as long as the mandrel is reasonably set, the wrinkling problem of stainless steel pipe bending can be eliminated.
Table 9 in the two cases of comparative analysis results show that Figure 10 (a) of the Nodal diagram gives the “thin” wall degree cut-off line is basically applicable to the choice of stainless steel bending mandrel bending.
Discussion
The essence of the bend wrinkling
The essence of the bend wrinkling is the arch belly under pressure after the plastic deformation destabilization.
Normal bend
Normal bend should be the result of uniform tensile arch back and uniform compression plastic deformation of the arch belly. Theoretical and experimental research proves that: the arch back of uniform tensile plastic deformation and the arch belly of uniform compression plastic deformation of the clever combination is to protect the quality of the premise of the bend. The limit of uniform tensile plastic deformation is determined by the plastic tolerance of the steel pipe, beyond this limit, the arch back will fracture. For to / do < (4 ~ 5) % of the thin-walled tube, the arch belly compression zone will be in this limit before the lack of rigidity and wavy deformation, the essence of the material mechanics with the pressure bar instability bending is the same. Therefore, wrinkled wave is a common anomaly in the “thin” wall bending.
Pressure rod instability
Slender rod (i.e., L / t ratio is very large) is susceptible to bending and loss of pressure function, called instability. That is to say, the cross section is very small or very thin rod, or the free length of the rod once the pressure will lose the ability to continue to withstand the pressure, increase the wall thickness or reduce the length to avoid the occurrence of instability. Therefore, L / t is actually the earliest used to judge the stability or rigidity (stiffness) of the compression rod.
There are many common instances of instability in engineering. For example: ① steel pipe hydraulic test, must be set up according to the slenderness ratio section by section pressure to hold in order to clamping conditions at both ends of the pressure, otherwise the two ends of a pressurized steel pipe will arch (bending), the pressure is less elastic (deformation) instability, the pressure is larger plastic (deformation) instability; ② I or box-shaped beam web compressive stress area must be set up ribbed plate and reinforcement plate to prevent beam bending under pressure when wavy deformation; ③ Thin plate welding when welding compressive stress area is easy to produce wavy deformation. Therefore, the welding sequence and welding parameters must be controlled to reduce the compressive stress to prevent instability. It can be seen that the use of plastic deformation of the arch belly when bending the crumpled wave, in essence, is a form of compression bar instability.
Pipeline with thin-walled stainless steel pipe
Pipeline selection of stainless steel pipe, generally choose to/do <5% of the thin-walled pipe, because: ① excellent corrosion resistance determines the design wall thickness of stainless steel pipe without adding corrosion margin; ② expensive price so that designers always choose thin-walled pipe as much as possible, 5S, 10S series pipeline with stainless steel pipe to/do for (2 ~ 4) % (see Table 4); ③ ships and other structures with limited space, pipe elbow Radius (Ro) to be the smallest possible, which leads to the production of “thin” wall bend will have wrinkled wave.
Mechanical analysis of the bend wrinkling
With the help of modern structural mechanics about the compression plastic deformation of the compression bar and its support conditions on the assessment of the impact of the compression bar stability [25] method, further analysis of the causes and conditions of stainless steel pipe bending when the arch belly wrinkling.
(1) Only the slenderness ratio (L/t) <3 to 5 compression rod can produce compression plastic deformation [25-26], otherwise the compression rod will be the first due to instability bending and impossible to produce compression plastic deformation.
(2) The stability of the compression rod depends not only on the slenderness ratio of the rod itself, but also, to a greater extent, on the constraints or degrees of freedom of motion at both ends, so the critical pressure of instability (Fc) can be expressed as:
In the formula:
- E – the modulus of elasticity of the material of the compressed rod; Iαβ – the moment of inertia of the cross-section of the compressed rod; L – the actual length of the compressed rod.
- μ – length coefficient of the compressed rod.
- ζ – stability coefficient of the compressed rod, ζ = π/μ2. The length coefficient μ and stability coefficient ζ of the compressed rod for different end support methods are shown in Table 11 [25]. As seen in Table 11, the smaller the degree of freedom of the supporting end, the smaller the value of μ and the larger the value of ζ. The mold settings and clearance conditions during bending have actually determined the supporting degrees of freedom in the pressurized area of the arch web: when to/do or Ro/do is large enough, the arch web is thick enough and its stiffness can withstand cold bending without mandrels; when to/do or Ro/do is small, the arch web itself has reduced stiffness and must limit its freedom of lateral movement by rigid mandrels at the starting bending point or use a single ball flexible mandrel to limit the actual freedom of the starting bending section To/do or Ro/do very small “thin” wall pipe, bending process to limit the freedom of each micro-bending section at both ends and the actual length of the corresponding micro-bending section free. Therefore, super “thin” wall pipe bending must be used when the multi-ball mandrel in order to prevent the generation of arch belly wrinkles.
Table.11 different end support method of the length coefficient and stability coefficient of the compression rod
Mold accuracy and clearance on the impact of bending wrinkles
The impact of mold on the bend wrinkle wave
No mold cold bending steel pipe fracture or wrinkle limit conditions shown in Figure 15. Theoretical and empirical findings show that if no mold is used to improve the bending steel pipe plastic deformation conditions, to/do = 4% or 2% of the stainless steel tube in Ro/do = 25 or 50 (that is, the nominal bending strain do/2Ro = 2% or 1% in Figure 15) may break or crumple. But if with the help of bending die, pressure block and flexible mandrel and other reasonable configuration of the mold, to/do = 2% in Ro/do = 1.5% of the stainless steel pipe bending can still avoid wrinkles. It can be seen that mold accuracy and clearance to ensure the quality of stainless steel pipe bending and the prevention of wrinkles have an important role to play.
Figure.15 no mold cold bending steel pipe fracture or wrinkle limit conditions
Gap determination
In addition to the literature [24], other literature only illustrates the importance of correct control of the gap, but not the ordinary meaning of the actual control of the gap. The literature [5] gives the critical condition for Φ20mm×1mm 304 steel pipe for Ro/do=3 without mandrel cold bending when the gap between the bending die and the outer diameter of the steel pipe is >0.2mm, but this critical condition may not be applicable when different to/do and Ro/do. The literature [2] only illustrates the larger gap between the mandrel outer diameter and the tube inner diameter in cold bending of aluminum alloy tube with Φ40mm×1mm specification. The literature [15] studied the wrinkle wave problem in cold bending of Φ38mm×1mm specification aluminum alloy tube by using energy method, and the results of the test and simulated clearance were given as follows: the clearance between mandrel and tube ID, anti-wrinkle block and tube OD, and the clearance between bending die and tube OD all produced wrinkle wave at 0.8mm, while the clearance was 0.2mm all did not produce wrinkle wave, but the clearance between pressure block and tube OD was 0.2mm or 0.8 mm did not produce wrinkle waves.
The case of larger or smaller gaps is not specified in these papers. Looking at Table 7, the gap control values given in the literature [24] are by far the most informative.
Effect of tube dimensional tolerances on wrinkle waves
The literature [24] gives more comprehensive and specific control values for the referred clearances, and indicates separately that: some clearances should depend on to or do; some clearances are some definite range of values; and some clearances have to be determined by tests (see Table 7 for details). These instructions have a certain reference value, but there are difficulties in the specific implementation, because the current steel manufacturing standards allow the range of dimensional tolerance zone has not yet reached the “gap” control requirements.
Comparison of Table 10 and Table 7 in the gap control requirements can be seen: many standards provide for wall thickness tolerances are in + (0.15 ~ 0.225) to and -0.125to between the maximum tolerance bandwidth of up to 0.35to. coupled with the outside diameter tolerance, if only according to the nominal specifications to determine the above gap, then it is likely that part of the standard size tolerance in line with the specifications of the pipe, but the actual clearance is difficult to control within the range required by Table 7, and thus problems such as wrinkling occur when bending the pipe.
Here we need to pay attention to.
- (1) Different manufacturing plants due to different details of the steel production process, the same specification of steel pipe tolerance zone may be different. The same factory produces the same specification of the same steel pipe may also be due to differences in the steel grade furnace number of raw materials, differences in chemical composition, resulting in tolerance bands on the upper or lower, which are normal.
- (2) The supply of the valuation method may lead to tolerance band offset. Early national standards are based on the actual quality of pricing, this pricing method is likely to lead to the manufacturer according to the upper tolerance zone supply, in order to obtain the highest sales revenue. The current national standard has allowed the length or quality of pricing, if the length of the pricing may lead to the steel pipe manufacturer to supply according to the tolerance, in order to maximize economic benefits. U.S. standards always require length-based pricing, the reasons for the literature [21] has been analyzed. For marine stainless steel pipe, increase its deadweight is equal to reduce the effective tonnage of the ship, so 17.4 × 104m3 of LNG and other marine stainless steel pipe are ordered by length. However, if the billing is still converted into unit length price according to the standard weight, the manufacturer may have failed to realize the benefits of the following tolerance band supply, so many factories will still supply according to the old tradition of production.
- (3) The differences in the mechanical properties of steel can not be ignored. Different factories produce the same specification with the mechanical properties of steel pipe, and even the same factory produces different batches of the same specification with the mechanical properties of steel pipe may be due to the raw material furnace number and the actual temperature of the final heat treatment of the steel pipe manufacturing process or holding time, cooling rate differences, and lead to differences in its mechanical properties. The literature [7] has measured the actual yield strength of three similar specifications 304 steel pipe, the difference in elastic modulus, which are normal. In fact, all stainless steel product standards specify the mechanical properties of indicators are not high, such as elongation (A) is generally only 40%, many austenitic stainless steel tube quality assurance book on the elongation of the upper limit of 55% to 60%, corresponding to the tensile strength, hardness also has corresponding fluctuations. There has been a literature to prevent the production of defects such as bending wrinkles, the hardness also put forward the corresponding requirements. Literature [24] pointed out that thin-walled bends not only strictly control the mold gap, the steel pipe should also be used in the same batch, preferably the same furnace number of the same specification steel pipe. Description of the actual production of bending wrinkles are normal, in addition to the reasons for the size of the steel tolerance, but also may involve differences in mechanical properties. Therefore, the “thin” wall pipe bending, to ensure the quality of the bend, the agreed hardness may be a reasonable way. The equivalent conversion of carbon and alloy steel hardness and strength is shown in Table 12.
Table.12 carbon steel and alloy steel hardness and strength of the equivalent conversion ①
Notes.
- ① The data in this table are taken from ASTME 140.
- ② Literature [21] does not limit the hardness, but the tensile strength of 490 ~ 690MPa corresponding to HRB 78.6 ~ 94.6; for the “thin” wall bend may vary too much hardness range.
Measures to prevent the bend from wrinkling
As seen in the above analysis, the production of stainless steel pipe bending wrinkle problem is complex, both the lack of knowledge of the bender structure, mold accuracy and gap control and mold lack of maintenance and maintenance factors, there are also factors such as performance and size tolerances generated in the manufacturing process of steel pipe. In order to effectively prevent pipe bending wrinkling, the following aspects should be started.
Strict control of steel pipe size tolerances
In order to effectively prevent the bend from wrinkling, the first from the delivery of steel pipe size tolerance control, a reasonable choice of size tolerance range.
17.4 × 104m3 LNG carrier manufacturer has proposed a clear “cryogenic stainless steel pipe order instructions” [27]. The specification was a document that came with the introduction of the shipbuilding technology in the s, and the reference standards listed in the specification were ASTM A312 and ASTM A530, and the later ASTM A999/A999M standard was separated from ASTM A530 in . Therefore, after , ASTM A312 and ASTM A999 standards should be used as the basis for the implementation of low-temperature stainless steel piping with tubes.
The “low-temperature stainless steel pipe ordering instructions” specify the dimensional tolerances of stainless steel pipes below DN300 are shown in Table 13 [21]. Comparison Table 10 can be seen, the instructions for wall thickness, inside diameter roundness and other tolerance requirements are significantly higher than the ASTM A312 and ASTM A999 standards, these requirements are very beneficial to prevent the bend wrinkling.
Although the “low-temperature stainless steel pipe order instructions” on the stainless steel pipe dimensional tolerances put forward high requirements, but to achieve such tolerances are not easy. First, in the seamless tube instead of welded tube, to wall thickness to reach (0, -12.5%to) under the tolerance zone is very difficult, the literature [27] specified that the ordered steel pipe priority welded tube, just available seamless tube instead, indicating that the welded tube is easier to achieve the requirements. Second, the specification of ID roundness tolerance IDmax-IDmin ≤ 1% (do-2t) requirements, compared to the ASTM A312 standard OD roundness tolerance OPmax-ODmin ≤ 1.5% do requirements for to/do ≤ 3% of the thin-walled tube Compared with the requirements of ASTM A312 OD tolerance OPmax-ODmin≤1.5%do, for thin-walled tubes with to/do≤3%, the roundness requirement measured by OD is slightly improved, but for thick-walled tubes is more demanding.
Table.13 “Low-temperature stainless steel pipe ordering instructions” provides for the size tolerance of stainless steel pipe below DN300 mm
The “low-temperature stainless steel pipe order instructions” proposed to control the inner diameter roundness is also very difficult: ① If you can not accurately measure the inner diameter roundness, there is no control of the inner diameter roundness; ② Although the control of the inner diameter roundness of the bend when the mandrel is pushed into the beneficial, but as long as reasonable control of wall thickness, outer diameter and perimeter tolerance, thin-walled pipe roundness tolerance is easy to “correct” Correct, no impact on the bend when the mandrel is pushed in; ③ thick-walled tube cold bending without a mandrel, the outside diameter and bending die, the gap between the pressure block is more important to the quality of the bend, only control the inner diameter roundness without specifying the concentricity or wall thickness of the unevenness of the bend is not beneficial to improve the quality of the bend; ④ from Table 13 on the wrong side of the requirements can be seen, which is clearly for the additional requirements of the welded pipe.
If seamless pipe bending is used, attention must be paid to the problem of large tolerances on the wall thickness of seamless pipe. The above-mentioned Φ88.9mm×3.05mm specification steel pipe with 80 mandrel outside diameter fully illustrates this point.
Reasonable selection of configuration parameters
According to to/do, Ro/do value size, reasonable selection of cold bending mold, optimize the gap configuration, adjust the necessary parameters, can improve the quality of pipe bending.
- (1) b = Roto/do2 ≤ 10.5 thin-walled tube is best to use a single ball or multi-ball flexible mandrel.
- (2) Reasonable configuration of the mandrel, pressure block, bending die clearance. For b = 8.5% of the stainless steel thin-walled tube, you must also detect the bottom of the bending die and the amount of wear on the drive shaft, but also in the bend with a rigid mandrel to prevent the generation of wrinkle waves.
- (3) According to the actual tolerance of the steel pipe reasonable adjustment of bending speed, block pressure and other parameters. Sometimes reduce the bending speed can effectively eliminate wrinkles, but the premise is that the operator must have sufficient experience and judgment, and a better understanding of the structure and performance of the bender operated. The data in Table 8 and Figure 14 illustrate that the appearance of wrinkle waves may also be related to the setting of pressure and boost conditions. The literature [24] specifies that as long as the bender parameters are adjusted reasonably, the pressure of the required pressure block is small when bending thin-walled tubes, but this is not the case in practice for various reasons.
- (4) Improve the structure of the bender. For example, the use of steel pipe tail directly boosted instead of the original pressure block boost (see Figure 9 (b)), or to the mandrel set before and after the shock type push into the device. The literature [24] pointed out that this oscillating push-in device helps to push the steel pipe with high precision multi-sphere mandrel, and specifies the oscillation frequency of 1 to 500 weeks / min, the amplitude of 3.2 to 25mm adjustable. This oscillating push-in device is a thin-walled pipe bender worthy of additional auxiliary institutions.
A shipyard in Shanghai in the manufacture of 8.4 × 104m3 LNG carrier, according to ASTM A312 standard order the required stainless steel pipe, bending according to the above-mentioned method to take the appropriate measures to effectively prevent the generation of wrinkled waves when bending. As long as the reasonable choice of cold bending tooling, optimize the gap configuration, adjust the necessary parameters, bending wrinkling problem can be solved.
Mandrel length and stiffness selection
The outer diameter of the front part of the rigid mandrel is determined by the inner diameter of the pipe. The literature [24] points out that the mandrel used in thin-walled bending should be thick enough, which refers to the mandrel’s stiffness or resistance to deformation should be good enough. This requires: ① mandrel length can not be very long, so the choice of long mandrel to meet the end of the long tube for the bending program is not desirable, and the long tube on the clamping side (requiring a large radius of gyration program) is more reasonable; ② even for short mandrel, in addition to the end of the largest outside diameter to finish, the rest of the outside diameter can only be slightly reduced. Many literature diagrams (such as Figure 9 (b) [8, 14]) will be the diameter of the second half of the mandrel section is drawn very thin, which is not desirable.
Characteristics of marine bend production
Many literatures specify that the anti-wrinkle block is an effective means to prevent wrinkling of thin-walled bends [1-20, 22-23], but most of the marine bends are produced without setting the anti-wrinkle block. Combined with the actual production, the analysis of the bend does not set the anti-wrinkle block for the following reasons:
- ① Anti-wrinkle block must be installed with sufficient precision, otherwise the role is not great, but will increase the risk of scratching the outer surface of the stainless steel tube;
- ② With the bend diameter increases, the pressure of the pressure block increases, anti-wrinkle block wear will intensify, and wear must be timely adjustment of the installation position of the anti-wrinkle block, otherwise it will not play its role;
- ③ Small batch multi-specification bender Often mild steel tubes, stainless steel tubes mixed, the former is generally to/do larger, so no anti-wrinkle block;
- ④ Figure 8 of the anti-wrinkle block requires light oil lubrication, otherwise it may be counterproductive; Figure 9 of the installation may bring anti-wrinkle block vibration affect the anti-wrinkle effect.
The above analysis shows that the reasonable application of anti-wrinkle block is perhaps only (more) suitable for the production of a single variety of specifications of intensive high-volume thin-walled bends.
4.8 Cold elbow with low temperature stainless steel shall be solid solution annealing treatment
The literature [27] provides that the procurement of low-temperature stainless steel tubes to meet the -196 ℃ low-temperature impact test (41J). Therefore, the stainless steel tube must be supplied after solution annealing. After cold bending the elbow arch back has different degrees of cold-working tensile plastic deformation, the arch belly has different degrees of compression plastic deformation, which means that the steel pipe material has different degrees of embrittlement (see Figure 16). Therefore, such elbows working in low-temperature environments should be solid solution annealed before use. American standard ASTM SA403 and ASTM SA815 also provides: cold bending method can be used to produce elbows, but must be in the solution annealing treatment after delivery.
Figure.16 304 stainless steel σ-ε curve under different degrees of cold working
Thin-walled stainless steel bend pressure and boosting speed
Table 8 data show that: the use of four spherical flexible mandrel for thin-walled austenitic stainless steel bend, should not use a larger pressure block and larger pressure and boosting speed. Otherwise, although it can reduce the amount of wall thickness thinning of the arch back, but will certainly increase the amount of wall thickness thickening of the arch belly, and produce serious wrinkle wave. The reason is: the excessive pressure of the pressure block makes the bend starting point of the arch belly withstand a high compressive stress, plus the pressure brought about by the boost, so that the thickened arch belly still does not have a high enough compressive stability, and therefore produces a crumpled wave.
It is important to note that.
- ① In the boost model shown in Figure 9 (a), the boost is only to minimize the sliding between the pressure block and the outer surface of the steel pipe, thus making the vpÌvo. usually, the boost is also not an independent adjustment parameter, but in the steel pipe tail boost model shown in Figure 9 (b), the boost is a completely independent adjustment parameter from the pressure of the pressure block.
- ② The surface roughness of the pressure block and lubrication conditions determine the interfacial friction coefficient has a decisive impact on the pressure regulation, the more polished the surface, the more adequate lubrication, the greater the required pressure, but the rough pressure block and will scratch the surface of the steel pipe. Therefore, accurate grasp of the degree of lubrication is very important for pressure regulation.
Φ88.9mm × 3.05mm stainless steel welded pipe difficult to meet the marine supply requirements
Literature [27] specified that the ordered steel pipe according to ASTM A312/A312M standard can be preferred to welded pipe, but the current domestic production of welded pipe so far difficult to meet the corresponding supply requirements. The main reasons may be.
- (1) Wall thickness of 3.0 ~ 3.5mm, DN100 stainless steel welded pipe is difficult to use single-arc GTAW method to achieve single-sided welding double-sided forming, and to ensure stable weld forming quality.
- (2) Using PAW or multi-cathode GTAW welding method, although it is possible to obtain stable weld forming quality, but at the same time must meet certain additional conditions: ① strict strip width tolerance and notch perpendicularity; ② the best working condition of the forming unit; ③ strict control of the welding arc parameters and welding conditions.
Stainless steel welded pipe welding, strict control of the width tolerance of the steel strip and notch verticality, is to ensure the uniformity of the weld interface gap, to reduce the left and right drift of the weld, it is necessary to increase the steel strip milling device. In order to keep the forming unit in the best working condition, timely maintenance and servicing of the forming machine with experienced shaping staff is also a must. Strict control of welding parameters and welding conditions and maintaining a stable welding speed also help to obtain a stable weld forming quality. Although tri-cathode GTAW welding can achieve high welding speed, but the stability is very poor. The double cathode or PAW + GTAW double arc welding is a more effective and stable welding method, especially PAW + GTAW welding can ensure the back and front side of the forming quality, but also to achieve a certain welding speed, this welding method is recommended. The impact of the frontal weld is also very low tendency to bite the edge [28-32].
Conclusions and recommendations
- (1) Cold bending is a common and simple method for making pipe elbows, although there are uneven wall thickness, roundness distortion, rebound and arch belly wrinkle wave and other deficiencies, but does not affect the normal operation of the general fluid transfer pipeline working under internal pressure conditions, therefore, cold bending is often seen as the main and practical method of elbow manufacturing.
- (2) Austenitic stainless steel pipe excellent plasticity (margin) makes the application of cold bending method more common. Domestic and foreign pipeline standards allow Ro/do ≥ 1.5 austenitic stainless steel bend can be applied directly in the cold bending state, but the actual extent of this retains the uneven cold working elbow is only suitable for application under normal temperature static load conditions. In stress corrosion and low temperature conditions, the use of austenitic stainless steel bends still have to be heat treated, preferably after solution heat treatment before use. In the alternating load, high temperature creep and other conditions, it is no longer suitable for the use of austenitic stainless steel cold bend, and must be used for higher wall thickness uniformity of the mold + welding method or forging + machining method of manufacturing elbow.
- (3) Reasonable mold settings around the bending bender can achieve a variety of relative radius (to/do), relative bending radius (Ro/do) of the Φ219mm stainless steel pipe below the cold bend. In general, in order to ensure the quality of cold bending, especially to control roundness and avoid wrinkles: to/do the smaller or Ro/do less than a certain value, you must use a reasonably designed rigid mandrel or a flexible mandrel with a ball hinge; only to/do and Ro/do are large enough thick-walled bent pipe can be coreless cold bending; for to/do particularly small or Ro/do small enough thin-walled stainless steel pipe The cold bending, to use a specially designed single ball or multi-ball flexible mandrel in order to prevent the bending of the wrinkle wave. b = Roto/do2 ≤ 10.5%, 10.5 ~ 17.5%, > 17.5% can be used as a flexible mandrel, rigid mandrel, without the mandrel of the thin-walled degree of rough dividing assessment value.
- (4) After the structure of the bending machine to determine the mold manufacturing and installation adjustment accuracy of the gap parameters may have a non-negligible impact on the quality of the bend, especially the tendency to wrinkle. In the production of small quantities of multi-species bends, if wrinkles, should be checked item by item mandrel outside diameter and steel pipe inside diameter, bending die, pressure block, anti-wrinkle block, clip block inside diameter and steel pipe outside diameter between the gap is too large, the mandrel end and anti-wrinkle block installation position is appropriate. For the use of longer equipment, but also pay special attention to detect the bottom of the bending die and its drive shaft shaft diameter wear is excessive.
- (5) Bending speed and pressure size of the pressure block and other operating parameters also have a certain impact on the bend wrinkle wave. Properly reduce the bending die speed or pressure can sometimes significantly reduce the bend wrinkles. But the pressure adjustment involves friction and boosting mode, requiring the operation of the operator has sufficient experience accumulated.
- (6) The current stainless steel pipe manufacturing standards specified in the to, do tolerance zone is much larger than the thin-walled bend required clearance accuracy. An introduction of technology to build the LNG “transport vessel with stainless steel pipe order instructions” provides for to under the tolerance zone for wall thickness tolerance has a certain degree of reasonableness. But the proposed tolerance of the inner diameter roundness, due to the difficulty of actual measurement and lack of practical value.
- (7) Marine stainless steel tubes are delivered in accordance with the U.S. standard to the length of the way, this delivery method helps guide the steel pipe manufacturer to the wall thickness under the tolerance band delivery, but also conducive to increasing the ship’s payload. The national standard retained by the quality of measurement of delivery is still inappropriate, should be removed early this reservation and thoroughly measured by length of delivery.
- (8) Stainless steel tube to, do tolerance zone may cause some trouble to the thin-walled stainless steel bend, but as long as you follow a reasonable choice of mandrel type, pay attention to the mold manufacturing and installation of the adjustment accuracy, its impact is not significant. This has been a large number of shipyards in recent years the application of stainless steel bend production practice confirmed.
- (9) The use of a single rigid mandrel multiple specifications (a Ro/do or similar Ro/do but more to/do) adjustable around the bender structure and does not distinguish between steel bending production method, although it can save investment costs of production operations, but obviously not for thin-walled stainless steel pipe bending, and set up a dedicated with a single or multiple ball hinge flexible mandrel is very necessary. This type of bender mandrel length should not be too long, the mandrel end should be chrome-plated surface, the sphere is suitable for w (Al) 5.0% to 6.5% of aluminum bronze, should not use w (Ze) > 7%, w (Al) <2.5% of aluminum brass.
- (10) Based on to/do and Ro/do to determine the thin wall of the bend, with the internationally popular bend no mode chart or b = Roto/do2 bend thin wall can quickly query the appropriate mandrel structure needed when bending around the bender. However, because the chart selection does not fully consider the impact of parameters such as clearance and bending speed, the query results can only be used as a relative reference, especially in the No-mode diagram near the dividing line of the query results. For example, for the bend of Φ88.9mm×3.05mm and Ro/do=2.5, the use of a single ball flexible mandrel in bending is a more reasonable query result. However, if a rigid mandrel is used when bending, as long as the gap and the installation position of the mandrel end are strictly controlled, the bent pipe made is still qualified.
- (11) Increase the pressure of the pressure block and boost speed will significantly reduce the amount of wall thickness thinning when bending the arch back, but at the same time will certainly increase the amount of thickening of the arch belly wall thickness, in thin-walled stainless steel bending may also increase the wrinkle wave. Therefore, in the bending stainless steel thin-walled bend should be as low as possible to choose the pressure of the pressure block and boost speed.
- (12) Thin-walled bending process occurs in the arch belly of the crumpled wave phenomenon, the essence of the compressed plastic deformation zone compressive stiffness is not enough to cause the pressure bar destabilization bending (buckling). From a mechanical point of view, the reasonable choice of mold parameters, especially the structural form of the mandrel, size and placement position, etc., can improve the arch belly area or micro-bending section according to the pressure stability (coefficient), is an effective measure to prevent the bend to produce wrinkles.
- (13) Wall thickness of 3.0 ~ 3.5mm, DN100 stainless steel welded pipe is difficult to use a single forming, and GTAW welding is difficult to obtain the ideal weld forming quality; using continuous roll forming and PAW welding method of manufacturing welded pipe and the strip width and notch straightness requirements are extremely high, otherwise it is difficult to achieve the ideal interface gap, to achieve a low misalignment interface PAW welding, to get the formation Stable quality of the weld seam. How to break through this bottleneck, it is worth the attention of the domestic stainless steel welded pipe manufacturing plant.
Author:HE Defu, SU Yongqiang, RONG Songru, LUO Jian
Source: China Pipe Elbow 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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