The wrinkle problem of stainless steel elbow and its preventive measures

In order to prevent wrinkling of stainless steel pipe during cold bending, firstly, the advantages of the cold-bending method and its reasonable application range were investigated and evaluated from the perspective of pipe fittings manufacturing standardization, and then the “thin” wall that wrinkled in the cold-bending stainless steel was analyzed. Conditions, influencing factors, and analyzed through two typical cases. The analysis pointed out that the essence of wrinkle is the instability and inflection of the “compression bar” in the compression plastic deformation zone of the soffit. According to the “thin” wall thickness of the bend determined by the to/do and Ro/do of the steel pipe, the mandrel type and the “gap” determined by the machining and installation accuracy of other molds should be set reasonably to enhance the compression stability of the “compression bar” mechanically Degree, this is a necessary guarantee to prevent wrinkles. The tolerance zone of to and do allowed by the stainless steel pipe manufacturing standard is often larger than the mold gap necessary for the “thin” wall elbow, and sometimes wrinkles are generated in the small batch of multi-variety stainless steel elbow, but as long as the “thin” wall is grasped, pay attention to the mandrel structure, the reasonable adjustment of the mold gap and the parameters of the elbow, the wrinkle problem of the stainless steel cold-bending pipe can be solved.

From 2013 to 2016, my country’s crude steel output exceeded 8×108t for four consecutive years, accounting for more than 50% of global crude steel output. Although my country’s stainless steel output has reached more than 2×107t, accounting for more than 50% of global stainless steel crude steel output for many years, my country’s stainless steel output in 2000 was only 5×105t, accounting for only 2.6% of global stainless steel output. In the past ten years, the production and application of stainless steel pipes in my country have developed rapidly, especially in petrochemical, oil and gas production pipelines and transportation ship manufacturing. The application of stainless steel pipes has developed rapidly. Two famous shipyards in Shanghai are building LNG carriers of 17.4×104m3 and 8.4×104m3 in batches, each of which requires 250-380 tons of stainless steel pipes.


Elbows are bound to be involved in the application of stainless steel pipes. In the manufacturing process of stainless steel elbows or pipes, there will be problems such as uneven wall thickness, wrinkling and rebound, and roundness distortion. These problems are quite interesting [1-18]. In 2016, the wrinkling problem of Φ88.9 mm×3.05 mm 304 steel pipe and Φ60 mm×3.5 mm 316L steel pipe bends that occurred among users of two shipyards was extremely typical.
On the basis of investigating the occurrence conditions, influencing factors and preventive methods of wrinkle waves in stainless steel elbows, this study focuses on exploring the correlation between stainless steel pipe manufacturing dimensional tolerances and elbow waves through two cases. The article also discusses the advantages and disadvantages of various methods of elbow manufacturing, as well as the standardization state, and clarifies the reasonable application range and precautions of different types of elbows. Hope to work with users to promote the progress of stainless steel pipe manufacturing and application level.

Elbow production method, standardization and reasonable selection

Elbows (45°, 90° and 180°) are one of the most commonly used fittings that are indispensable in the application of steel pipes. The standardized and standardized intensive production of pipes and other non-tube is an important feature difference. In domestic and foreign markets, elbows of various pipe specifications can be directly purchased according to national pipeline standards, but tube-specific elbows can only be made by themselves.
The pipe fitting standard established in my country based on the Japanese standard is actually only equivalent to the American pipe fitting standard ASME B16.9-2012. The American pipe fitting standard ASME B16.9-2012 does not distinguish between manufacturing methods, nor does it discuss various details related to manufacturing methods in detail. In the American standard, these steel-related details are divided into other various pipe fitting product standards and the corresponding general pipe fitting material standards and discussed in detail. Table 1 lists only the products related to stainless steel pipe fittings and the relevant general material standards subject to jurisdiction. Taking into account that the shape and manufacturing process of elbows and pipe fittings are not simpler than steel pipes, and my country’s current method of distinguishing two pipe fittings only by whether there is a weld seam may be a little inappropriate, so special standards for the manufacture of stainless steel pipe fittings should be established. Meet the needs of the current rapid development of stainless steel pipes. The manufacturing methods of various elbows in the American standard are mainly cold bending or hot bending, die pressing + welding, forging + cutting. The advantages and disadvantages of these manufacturing methods and their scope of application are analyzed below.

Elbow method

The elbow method is a simple and commonly used method for making 45°~180° elbows and even coil pipes, but it is not the only ideal or best method for making elbows.

Advantages of the elbow method

  • (1) The process equipment is simple, from the simplest manual bending mold to the fully automatic digital program control hydraulic cold bending machine, which is very mature, practical and convenient.
  • (2) The bending radius, bending direction and bending angle can be determined according to actual needs within a certain range, and it is most suitable for spatial three-dimensional multi-angle bending pipe structural parts. Therefore, aircraft and automobile exhaust systems are the earliest application areas of stainless steel elbows.
  • (3) The length of the straight pipe section of the elbow is not limited, or the number of pipe ring welding joints can be reduced as much as possible.
  • (4) Austenitic stainless steel seamless pipes or welded pipes with excellent plasticity can be cold-formed at room temperature, and can be directly used in static load and general (uniform corrosion) environments without heat treatment. However, heat treatment is required for ferritic and martensitic stainless steel pipes with poor plasticity, and heat treatment is not required unless the bending radius is large (Ro≥2.5do).
  • (5) The wall thickness of the elbow formed by the elbow method is uneven. Many different calculation formulas for the wall thickness of the elbow can be found in the relevant literature, but they basically conform to this change law: that is, the arch back (outside the elbow) ) When the wall thickness is reduced, and when the soffit (inside the elbow) is increased. The theoretical change of the wall thickness of the plastic deformation elbow excluding the mold constraint and its calculation formula are shown in Figure 1.

According to the analysis of literature [11], the approximate wall thickness extreme value and the calculation formula of the relative amount of thinning and thickening of the arch back and soffit are as follows.
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Figure 1 Schematic diagram and calculation formula of the theoretical change of the wall thickness of the plastic deformation elbow excluding the mold constraint
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  • In the formula: tex—the extreme wall thickness of the arch back (outer side);
  • tin—the extreme wall thickness of the soffit (inside);
  • to—Original wall thickness;
  • △tex—the extreme value of relative thinning of arch back;
  • △tin—the extreme value of relative thickening of soffit;
  • Ro—bending radius;
  • do—Outer diameter of steel pipe.

For conventional pipelines that bear internal pressure, the required wall thickness and allowable increment of the arch back and soffit are calculated according to the stress as follows [19].
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  • In the formula: texr—the net value of the minimum wall thickness required for the arch back when the pipeline is under internal pressure (excluding margin and tolerance);
  • tinr—the minimum net wall thickness required by the soffit when the pipeline is under internal pressure (excluding margin and tolerance);
  • Δtexr—allowable wall thickness increment of arch back;
  • Δtinr—The allowable wall thickness increase of the soffit.

The Ro/do-Δt/to curve made according to formula (1) to formula (4) is shown in Figure 2. It can be seen from Figure 2 that the wall thickness increase and decrease caused by the bend is very close to the force requirements of the internal pressure pipe, that is, under this condition, although the bend will cause uneven wall thickness, it does not affect the use of the bend.
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Figure 2 Ro/do-Δt/to curve made according to formula (1) ~ formula (4)

Disadvantages of the elbow method

(1) Under dynamic conditions such as time-varying stress, that is, alternating stress, the above-mentioned bending method will cause uneven wall thickness, which is not allowed. For elbows under time-varying stress conditions, the wall thickness required by the soffit should be equal to the minimum elbow wall thickness value [19]. Many studies have proved that the wall thickness reduction and roundness distortion caused by the elbow method will accelerate the stress and creep stress speed, thereby affecting the dynamic stability [20-22].
(2) Unless special measures such as additional internal pressure and sand filling are taken, the elbow section may have additional roundness distortion. Literature [23] pointed out that the roundness distortion degree μ also mainly depends on Ro/do.
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In the formula: dmax—the maximum outer diameter of the curved section of the elbow;
dmin—the minimum outer diameter of the curved section of the elbow.
Under normal circumstances, the elbow will produce roundness distortion. In order to limit μ within 10% or lower, Ro/dmust be controlled above 2.
The correlation between the roundness of the elbow section and Ro/do and the allowable value of the European standard are shown in Figure 3 [23]. Figure 3 is redrawn based on Figure 7.4.1.1 in EN 13480-4: 2015. The original text does not specify the conditions such as to/do and the steel pipe material, nor does it specify the basis for the inverse 20/(Ro/do) curve. The author believes that to When increasing, the υ curve will move down.
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Figure 3 Correlation between roundness of elbow section and Ro/do and allowable value of European standard
(3) After the elbow is completed, there will be a certain rebound in the direction of the bending angle. Therefore, in the actual bending process, a certain amount of cornering angle is usually increased to eliminate the effect of springback [6-10]. The size of the turning angle is determined based on empirical values or simulation analysis.
(4) Wrinkles will appear in the soffit when cold bending of “thin”-walled pipes with small Ro/do and low to/do or the pipe bending machine is not suitable or adjusted properly, which will not only affect the bending The appearance of the elbow will affect the fluid transport performance of the elbow. For stainless steel pipes, wrinkles can also damage its corrosion resistance and reduce the service life of stainless steel pipes. It can be seen that the research on wrinkle defects should be paid special 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 The effect of cold working degree on the magnetic phase content and corrosion rate of 304L steel
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Note:

  • ①%CW is the degree of cold working, %WW is the degree of hot working, and the bent sample with weld is shown in Figure 4;
  • ②The annual corrosion amount calculated according to the test method in ASTM A262 C.

Although theoretically speaking, the use of wrinkled elbows will cause certain safety hazards and should be rejected. However, this is not the case. Since the implementation of EN 13480-4:2015, it still stipulates that as long as the wave height (h) and wave distance (a) of the wrinkle wave are controlled within a certain range, bends with wrinkles can still be used. Figure 5 shows the measurement method for the allowable value of elbow wrinkles specified in the EN 13485-4:2015 standard.
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Figure 4 304L stainless steel bend specimens with welds in Table 3
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Figure 5 EN13485-4: The measurement method of the allowable value of the bending wave in the 2015 standard
EN 13485-4: The specific requirements of the wave height (h) and wave distance (a) of the bent pipe in the 2015 standard are as follows:
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  • In the formula: h—the average height of adjacent wave crests, h=0.5(do2+do4)-do3;
  • a—The distance between adjacent crests.

It can be seen that in European pipeline applications, the control of bend waves is not very strict.

Application notes for cold-formed stainless steel elbow

  • (1) Different types of stainless steel pipes allow different bending radius ratios for direct application after cold bending. When exceeding the given limit, the bend pipe must be heat treated before it can be used.
  • (2) The cold bend elbow of austenitic stainless steel pipe used in the environment of periodic dynamic load, high temperature and stress corrosion media must also be heat treated, and solution heat treatment is best. It is important to note that although the ASTM SA403/SA403M and ASTM SA815/SA815M standards allow the bending of pipes to be manufactured, they must be supplied in a heat-treated state, while the ASTM SA182/SA182M standard does not allow the bending of pipes to be manufactured. , See Table 1 for details.
  • (3) Cold-bending elbows are usually not used for large-diameter pipes. Reasons: ①The power and cost of the pipe bender are high; ②The to/do of large-diameter stainless steel pipes is very small, which makes it difficult to bend the pipe, especially to avoid wrinkling waves. For stainless steel pipes above Φ219mm, cold bending is rarely used to make bends.

Partial heating of steel pipes to produce elbows

Heating the stainless steel pipe to the hot forming temperature for bending can reduce the mechanical force required when bending the pipe. This hot bending method is suitable for thick-walled pipe bends, and can also be used for bends of smaller Ro/do steel pipes. The hot bending method not only requires a heating process, but also has to fill the steel pipe with sand. There are many processes and complicated operations, and the formed elbow must be solid solution or annealed. Therefore, the use of integral heating to manufacture elbows is not common.
Literature [24] introduced a method of hot bending U-shaped elbows that only heated locally within the 120° range of the elbow’s abdomen. This method can reduce the thinning of the arch back of the elbow, and is especially suitable for U-shaped bends with a small radius of curvature (Ro/do)<1.5. The hot bending method of local heating Φ50 mm×7 mm low carbon steel pipe is shown in Figure 6.
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Figure 6 Hot bending method of local heating Φ50 mm×7 mm low carbon steel pipe

Mold welding method

When the elbow is made by compression welding, the steel plate is first cold-pressed or hot-molded into two symmetrical elbow parts of 45°, 90°, and 180°, and then the two seams are welded to form 45°, 90°, The 180° elbow is finally heat treated to make the final product. A schematic diagram of the structure of a press brake is shown in Figure 7. The manufacturing method of press-welded elbow is the only method for manufacturing elbows in GB/T 13401-2014, and it is also the method for manufacturing elbows that can be adopted in ASTM SA403/SA403M, ASTM SA815/SA815M and ASME B16.9 standards.

Advantages of compression welding

The advantages of this welding method:

  • ① The arch back and soffit of the elbow joints have no obvious wall thickness differences, and the elbow joints have no roundness variation and wrinkling waves;
  • ② Supplied according to different heat treatment states of steel grades, and suitable for more wide;

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Figure 7 A schematic diagram of the structure of a press brake

Disadvantages of compression welding

  • (1) The elbow radius of each specification has only two types: long and short.
  • (2) The length of the straight pipe section on the elbow is very short. When the pipe is installed, if the design is not reasonable, the number of circumferential welds will increase.
  • (3) Non-professional production, high cost.
  • (4) High welding quality requirements. It is best to use automatic GTAW or PAW without filler metal when welding. Only elbows welded by GTAW or PAW can be exempted from film or ultrasonic inspection, but must undergo strict visual inspection, because the welds welded by GTAW or PAW have better corrosion resistance. If other welding methods or adding filler metal are used to weld the elbow, it must be subjected to X-ray film or ultrasonic inspection, which is clearly stipulated in ASTM SA403/SA403M and ASTM SA815/SA815M standards.

Application of compression welding method

(1) Suitable for all kinds of stainless steel, especially austenitic stainless steel. But it is not suitable for martensitic stainless steel with poor weldability.
(2) Suitable for large diameter and thin-walled pipes. For 5S and 10S wall thickness series with to/do<2% (see Table 4), it should be preferred, and 40S and 80S wall thickness series with wall thickness below 6-7mm are also suitable.
(3) When selecting an elbow, the wall thickness and steel grade of the elbow must be determined according to the pipe working temperature, pressure and medium environment, and special attention should be paid to the heat treatment state of the elbow according to the requirements of the steel type and the elbow manufacturing procedure. If H-grade austenitic stainless steel elbows and other pipe fittings are hot-formed, they must be separately solution annealed after forming, and final solution annealing must be done after welding. During the solution annealing treatment, it is necessary to ensure that the heating temperature, holding time and subsequent rapid cooling meet the relevant requirements. This is also the meaning of “the heat treatment process in the manufacturing process cannot replace the final solution annealing treatment” in the American standard.
Table 4 Standard dimensions of stainless steel pipes in ANSI/ASME B36.19 and their comparison with European standards and national standards①
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Note:

  • ① Compiled according to ANSI/ASME B36.19, deleted the outer diameter and wall thickness in the original table in units of in; to/do is the analysis data added by the author, and the data in the brackets of to is the most in the national standard or the European standard.
  • ② The corresponding outer diameter specifications can be found in GB/T 17395-2008, and the data in brackets are the data in EN ISO 1127-1997.

(4) When welding by compression welding, if the welding method with filler wire is used, it is necessary to select appropriate welding materials according to the steel type to ensure the welding quality.

Forging machined elbow

Machining elbows with forging machines is the best solution for high-temperature stainless steel elbows. It is the only manufacturing method for elbow pipe joints specified in ASTM SA182/SA182M, and is also a manufacturing method allowed in ASTM SA403/SA403M and ASTM SA815/SA815M .
The advantages of forging machined elbows: 

  • ① Seamless pipe, better material uniformity, easy to control high temperature corrosion resistance or creep resistance; 
  • ② Especially suitable for thick-walled pipes of martensite and H-grade austenitic stainless steel with poor welding performance.

Disadvantages of forging machined elbows: 

  • ① The manufacturing process is long;
  • ② After forging, machining is usually required to meet the surface finishing requirements;
  • ③ The cost is the highest. The method of processing elbows with a forging machine is suitable for the manufacture of thick-walled pipe elbows that require particularly high operational reliability, such as nuclear power plants.

Case analysis and anatomy

In 2016, users found that some of the bends were wrinkled when bending the stainless steel pipes produced by Dechuan Tube Industry. The sample data of this elbow was statistically analyzed, and the results are shown in Table 9.
take the elbow of Φ88.9 mm×3.05 mm (No. 1 in Table 9) stainless steel pipe as an example. According to the data in Table 9, it can be seen that a single-ball flexible mandrel should be used for the bending of the steel pipe of this specification, but the factory actually uses a rigid mandrel when the pipe is actually bent. Higher risk of wrinkling, this is because:
Table 9 Sample data statistics of stainless steel elbows

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Note:

  • ① The serial number with superscript W is the elbow with wrinkles, the serial number with superscript b is the elbow with spherical flexible mandrel cold bending, the 3W serial number is the elbow without mandrel cold bending, the rest are all The bend is cold-formed with a rigid mandrel; 
  • ② No. 4~15 are normal cold-formed, and the elbow after cold-bending has no wrinkles; 
  • ③ Thinness 1338b4957286f8d2e8ff5849d3b07e82 - The wrinkle problem of stainless steel elbow and its preventive measures

(1) From the data analysis in Table 9, it can be seen that the b ≥ 10.5% of these 6 steel pipes can be used with rigid mandrels. However, when the Φ141.3 mm×3.4 mm elbow adopts the spherical flexible mandrel, there is no wrinkle, which shows the importance of improving the setting of the mandrel.
(2) The order requirement is Φ88.9 mm×3.05 mm, which is rounded to Φ89 mm×3.0 mm in the national standard (see Table 4). If the manufacturer ignores the order requirements, it is easy to produce according to Φ89 mm×3.0 mm , Resulting in the outer diameter and inner diameter of the supplied steel pipe being too large, but the wall thickness is too low. When bending the pipe, if the same core rod as the Φ88.9 mm×3.05 mm steel pipe is used, the gap between the inner wall of the steel pipe and the core rod will increase. All these have made the wrinkling phenomenon of the steel pipe with Φ88.9 mm×3.05 mm bent by the flexible mandrel become more prominent under the condition of Φ89 mm×3.0 mm when the rigid mandrel bend is used. 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 pipe bending is 80 mm. At this time, 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 where there was no wrinkling when the 82 mm outer diameter mandrel bend was used, the mandrel’s outer diameter was too small and the mandrel position was not adjusted for the preload may be the key factors for the wrinkling in this case. In addition, no anti-wrinkle block is used, and wrinkle waves are inevitable for steel pipes with b=8.5% (Φ 88.9 mm×3.05 mm).
(4) If the wall thickness of the steel pipe reaches the upper limit of the specification (3.05 mm×1.225=3.72 mm), when the mandrel bend with an outer diameter of 80 mm is used, the gap between the inner wall of the steel pipe and the mandrel is (di-dm)/ 2=0.7≈0.2to, there will be no wrinkles at this time. It shows that the core rod with an outer diameter of 80 mm is suitable for the supply state where the wall thickness is the upper limit.

Conclusions and recommendations

  • (1) Cold bending is a general and simple method for making pipe elbows. Although there are deficiencies such as uneven wall thickness, roundness distortion, springback and soffit wrinkles, it does not affect general fluid conveying pipelines working under internal pressure conditions. Therefore, cold bending is often regarded as the main and practical method of elbow manufacturing.
  • (2) The excellent plasticity (margin) of austenitic stainless steel pipe makes the application of cold bending method more common. Domestic and international piping standards both allow austenitic stainless steel elbows with Ro/do≥1.5 to be directly used in cold-formed conditions, but the actual elbows with uneven cold working are only suitable for application under normal temperature and static load conditions. Under stress corrosion and low temperature conditions, heat treatment is still required when using austenitic stainless steel elbows, and it is best to use after solution heat treatment. Under conditions such as alternating load and high temperature creep, it is no longer suitable to use austenitic stainless steel cold-bent pipes. Instead, elbows made by die pressing + welding or forging + machining with higher wall thickness must be used.
  • (3) The bending type pipe bender with reasonable mold setting can realize the cold bending of stainless steel pipes with various relative radius (to/do) and relative bending radius (Ro/do) of Φ219 mm. Generally speaking, in order to ensure the quality of cold bending, especially to control roundness and avoid wrinkles: when the to/do is smaller or Ro/do is less than a certain value, a reasonably designed rigid mandrel or a flexible with a spherical hinge must be used Mandrel; only thick-walled elbows with large enough to/do and Ro/do can be cold-bent without mandrel; for cold-bending of thin-walled stainless steel pipes with very small to/do or small enough Ro/do, special The designed single-ball or multi-ball flexible mandrel can prevent wrinkles when the pipe is bent. b=Roto/do2≤10.5%, 10.5~17.5%, >17.5% can be used as the rough boundary evaluation value of the thinness of flexible mandrel, rigid mandrel, and non-mandrel respectively.
  • (4) After the structure of the bending machine is determined, the gap parameters determined by the mold manufacturing and installation adjustment accuracy may have a non-negligible impact on the quality of the bend, especially the tendency of wrinkles. In the production of small batches of elbows with multiple specifications, if wrinkles occur, check whether the gap between the outer diameter of the mandrel and the inner diameter of the steel pipe, the bending die, the pressing block, the anti-wrinkle block, the inner diameter of the clamping block and the outer diameter of the steel pipe one by one If it is too large, whether the mandrel end and the anti-wrinkle block are installed properly. For equipment that has been used for a long time, special attention should be paid to detecting whether the wear of the bottom of the bending die and the shaft diameter of the drive shaft is excessive.
  • (5) The operating parameters such as the bending speed and the size of the pressure block also have a certain influence on the bending wave. Appropriately reducing the speed or pressure of the bending die can sometimes significantly reduce the bending wave. However, pressure adjustment involves friction and boost methods, and requires operators to have sufficient experience accumulation.
  • (6) The to and do tolerance zones specified in the current stainless steel pipe manufacturing standards are far greater than the gap accuracy required for thin-walled elbows. A certain imported technology-built LNG “Ordering Manual for Stainless Steel Pipes for Transport Ships” stipulates that it is reasonable to use the lower tolerance zone as the wall thickness tolerance. However, the inner diameter roundness tolerance proposed therein lacks practical value because it is difficult to measure.
  • (7) Marine stainless steel pipes are delivered in length-priced manner according to American standards. This delivery method not only helps guide steel pipe manufacturers to deliver according to the tolerance zone of wall thickness, but also helps increase the effective load of the ship. The delivery method based on quality that is reserved in the national standard is still inappropriate, and this reservation should be removed as soon as possible and the delivery method measured by length should be completely removed.
  • (8) Although the to and do tolerance zones of stainless steel pipes may cause some trouble for thin-walled stainless steel elbows, as long as the mandrel type is selected reasonably and attention is paid to the adjustment accuracy of mold manufacturing and installation, the impact will not be significant. This has been confirmed by the production practice of a large number of stainless steel bends in shipyards in recent years.
  • (9) The use of a single rigid mandrel with multiple specifications (one Ro/do or similar Ro/do but multiple to/do) adjustable bending machine structure and steel type bending pipe production method, although it can save production investment costs , But obviously not suitable for bends of thin-walled stainless steel pipes, and it is necessary to set up a special flexible mandrel with single or multiple spherical hinges. The length of the mandrel of this type of pipe bending machine should not be too long. The end of the mandrel should be chrome-plated, and the sphere should be aluminum bronze with w(Al)为5.0%~6.5%, and w(Ze)>7%、 w(Al)<2.5% aluminum brass.
  • (10) Determine the thinness of the elbow according to to/do and Ro/do. Use the internationally popular nomogram or b=Roto/do2 to quickly check the thinness of the elbow. Suitable core rod structure. However, because this map selection does not fully consider the influence of parameters such as gap and bending speed, the query result can only be used as a relative reference, especially the query result near the nomogram boundary. For example, for an elbow with Φ88.9 mm×3.05 mm and Ro/do=2.5, it is a reasonable query result to use a single-ball flexible mandrel when the pipe is bent. However, if a rigid mandrel is used when bending the pipe, as long as the gap and the installation position of the end of the mandrel are strictly controlled, the bend pipe is still qualified.
  • (11) Increasing the pressure of the briquette and the boosting speed will obviously reduce the thickness reduction of the arch back when the pipe is bent, but at the same time it will inevitably increase the thickness of the soffit wall. It may also increase when the thin-walled stainless steel pipe is bent. Wrinkle. Therefore, when bending stainless steel thin-walled elbow pipes, lower pressure block pressure and boost speed should be selected as much as possible.
  • (12) The wrinkle phenomenon that occurs in the soffit during the bending of the thin-walled elbow is essentially the instability bending (buckling) of the compression bar caused by insufficient compressive rigidity in the plastic deformation zone under compression. From a mechanical point of view, reasonable selection of mold parameters, especially the structure, size and placement of the mandrel, can improve the compression stability (coefficient) of the soffit area or microbend section, and is an effective measure to prevent wrinkles in the bend. .
  • (13) Stainless steel welded pipes with a wall thickness of 3.0~3.5 mm and below DN100 are difficult to form with a single piece, and it is difficult to obtain ideal weld forming quality in GTAW welding; the use of continuous roll forming and PAW welding methods to manufacture welded pipes has a significant impact on the width of the steel strip. The requirements for straightness of the incision are extremely high, otherwise it is difficult to achieve the ideal interface gap, realize the PAW welding of the low-error-edge interface, and obtain a stable weld quality. How to break through this bottleneck deserves the attention of domestic stainless steel welded pipe manufacturers.

Author: HE Defu1,SU Yongqiang2,RONG Songru1,LUO Jian2

Source: China Stainless Steel Elbows Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@steeljrv.com

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Summary
the wrinkle problem of stainless steel elbow and its preventive measures - The wrinkle problem of stainless steel elbow and its preventive measures
Article Name
The wrinkle problem of stainless steel elbow and its preventive measures
Description
In order to prevent the problem of wrinkles during cold bending of stainless steel pipes, the advantages of the cold bending method and its reasonable application range are first investigated and evaluated from the perspective of pipe fittings manufacturing standardization, and then the "thinness" of wrinkles occurring during cold bending is analyzed through typical cases. Wall conditions and influencing factors.
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www.steeljrv.com
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