The wrinkling problem and preventive measures of stainless steel bends

To prevent the wrinkling problem during stainless steel cold bending, the advantages and reasonable application range of cold bending methods were first investigated and evaluated from the perspective of pipe manufacturing standardization. Then, the “thin” wall conditions and influencing factors of wrinkling during stainless steel cold bending were analyzed, and two typical cases were analyzed. The analysis points out that the essence of wrinkling waves is the instability and bending phenomenon of the “compression rod” in the plastic deformation zone of the arch web compression. Based on the “thin” wall thickness of the pipe bend determined by the t_o/d_o and R_o/d_o of the steel pipe, the “gap” determined by the core rod type and other mold processing and installation accuracy is reasonably set to enhance the compressive stability of the “compression rod” from a mechanical perspective, which is a necessary guarantee for preventing wrinkling. The to and do tolerance zones allowed by stainless steel pipe manufacturing standards are often greater than the mold gap required for “thin” wall bends. Sometimes, wrinkling may occur during small batches and multiple specifications of stainless steel bends. However, as long as the “thin” wall degree is grasped, the core rod structure, mold gap, and reasonable adjustment of bend parameters are emphasized, the wrinkling problem during stainless steel cold bending can be solved.

From 2013 to 2016, China’s crude steel production exceeded 8% for four consecutive years × 10⁸t, accounting for over 50% of global crude steel production. Although China’s stainless steel production has reached 2 × 10⁷t, accounting for over 50% of the global production of stainless steel crude steel for many years, in 2000, China’s stainless steel production was only 5% × 10⁵t, accounting for only 2.6% of global stainless steel production. In the past decade, the production and application of stainless steel pipes in China have developed rapidly, especially in the manufacturing of petrochemical, oil and gas extraction pipelines, and transport ships. The application of stainless steel pipes has developed particularly rapidly. Two famous shipyards in Shanghai are constructing 17.4 in bulk × 10⁴m³ and 8.4 × A 10⁴m³ LNG transport ship, each requiring 250-380t stainless steel pipes.
The application process of stainless steel pipes inevitably involves elbows or bends. In the manufacturing process of stainless steel elbows or bends, issues such as uneven wall thickness, wrinkling, rebound, and roundness distortion are of great concern. Occurred among users of two shipyards in 2016 Φ 88.9mm × 3.05mm specification 304 steel pipe and Φ 60mm × The problem of wrinkling waves in the 3.5mm specification 316L pipe bend is extremely typical.
Based on the investigation of the occurrence conditions, influencing factors, and prevention methods of wrinkling waves in stainless steel pipe bends, this study focuses on exploring the correlation between the manufacturing scale tolerance of stainless steel pipes and wrinkled waves of bends through two cases that have occurred. The article also discusses the advantages and disadvantages, standardization status, and reasonable application scope and precautions for different elbow manufacturing methods. I plan to work with users to promote the progress of stainless steel pipe manufacturing and application level.

1. Elbow production method, standardization, and reasonable selection

Elbows (bends or 45 °, 90 ° elbows, and 180 ° return bends or retreats) are the most commonly used pipe fittings in pipeline steel pipe applications. Standardization and intensive production are important differences between pipeline steel pipes (tubes) and other non-pipeline pipes (tubes). Elbows of various pipeline specifications can be directly purchased in domestic and foreign markets according to national pipeline standards, but elbows of tube specifications can only be self-made.
The comparison of manufacturing standards for stainless steel pipe bends between China and the United States is shown in Table 1. The standard for pipe fittings established in China based on Japanese standards is 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 and does not discuss various details related to manufacturing methods in detail. The American standard divides these details related to steel grades into various pipe product standards and corresponding general pipe material standards and provides detailed discussions. Table 1 only lists the products related to stainless steel pipe fittings and the relevant general material standards under their jurisdiction. Considering that the shape and manufacturing process of elbows and fittings are more complex than those of pipeline steel pipes, and currently, in China, it may be inappropriate to only distinguish between the two types of pipe fittings based on whether there are welds. Therefore, it is necessary to establish specialized standards for manufacturing stainless steel fittings to meet the needs of the rapid development of stainless steel pipes.
The comparison of manufacturing standards for stainless steel bend heads in Table 1 shows that the manufacturing methods for various bends in American standards mainly include cold bending or hot bending, mold pressing + welding, and forging + cutting processing. These manufacturing methods’ advantages and disadvantages, and applicability are analyzed below.
Table.1 Comparison of Manufacturing Standards for Stainless Steel Pipe Bends between China and the United States

Countries	Standard number	Standard name	Applicable scope	Bend manufacturing method
China	GB/T 12459—2009	Steel seamless butt joint fittings	All steel grades including various stainless steels	Seamless
	GB/T 13401—2014	Steel plate butt welded pipe fittings		With welds
	GB/T 10752—2005	Butt welded joints of marine steel pipes		With welds
USA	ASME B16.9—2012	Factory welded pipe fittings	Various types of steel, including stainless steel, as well as aluminum alloys and nickel	Seamless rolled, forged or steel plate
	ASME B16.11—2011	Forged fittings for lap and threaded connections	Alloy and copper alloy pipe fittings	Welding after forming
	ASTM A182/A182M	Forged stainless steel pipe fittings for high-temperature service	High temperature and high pressure applications: austenitic stainless steel (38 types); Ferritic stainless steel (14 types); Martensitic stainless steel (6 kinds);	Forging
	ASME SA182/SA182M		CrMo low alloy steel (19 types)
	ASTM A403/A403M	Rolled austenitic stainless steel pipe fittings	Medium and low temperature corrosive pressure pipelines, austenitic stainless steel	Forging, rolling, welding, bending, cutting and various methods combination
	ASME SA403/SA403M		26 types
	ASTM A774/A774M	Low and medium temperature general corrosion service welded rolled austenitic stainless steel pipe fittings	Low pressure+low and medium temperature; 5 types of low-carbon and stable austenitic stainless steel (304L, 316L, 317L, 321, 347)	Soldered and supplied as welded
	ASTM A815/A815M	Rolled ferrite, duplex and Martensite Stainless steel pipe fittings	High temperature and high pressure application ferrite (6 types), duplex (9 types),	Same as ASME SA403
	ASME SA815/SA815M		Martensitic stainless steel (3 kinds)
	ASTM A988	High temperature service Hot isostatic pressing stainless steel pipe fittings	High temperature and high pressure application austenitic stainless steel (16 kinds), Martensitic stainless steel (7 kinds), duplex stainless steel (7 kinds)	Powder metallurgy pressure forming
	ASTM A960	General requirements for rolled pipe fittings	A403/A403M, A774/A774M, A815/A815M, etc	All methods

1.1 Bend method

The bending method is a simple and commonly used method for manufacturing 45 ° -180 ° elbows and even coils, but it is not the only ideal or optimal bending method.

1.1.1 Advantages of Bend Method

(1) The process equipment is simple, from the simplest manual bending mold to a fully automatic digital program controlled hydraulic cold bending machine, which is very mature and practical.
(2) It is possible to determine the bending radius, direction, and angle according to actual needs within a certain range, which is most suitable for making three-dimensional multi-angle bending pipe structural components. Therefore, aircraft and automotive exhaust systems were the earliest applications of stainless steel bends.
(3) The length of the straight pipe section of the elbow is not limited, or the number of pipe circumferential welding joints can be minimized as much as possible.
(4) Austenitic stainless steel seamless or welded pipes with excellent plasticity can be cold bent at room temperature. They can be directly used in static load and general (uniform corrosion) environments without heat treatment. However, ferritic and Martensitic stainless steel pipes with poor plasticity need heat treatment, and only when the bending radius is large (R_o ≥ 2.5d_o) heat treatment is not required. The relevant European standards for whether heat treatment is required after cold bending of stainless steel pipes are shown in Table 2.

Table.2 Relevant European standard basis for whether heat treatment is required after cold bending of stainless steel pipes

Application conditions	Average bend radius rm/mm	Outer diameter of steel pipe do/mm	Do you need heat treatment after cold bending
General corrosive medium, static load	rm≤1.3do	All do	All stainless steel pipes require heat treatment
	1.3do＜rm＜2.5do	do≤142	All stainless steel pipes do not require heat treatment
		do＞142	All stainless steel pipes, except austenitic steel, require heat treatment
	rm≥2.5do	All do	All stainless steel pipes do not require heat treatment
Cyclic load stress corrosion medium	All rm	All do	All stainless steel pipes require heat treatment

Note: According to EN 13480-4:2012, Section 7.2 and Table 7.2.2-1 are summarized.

(5) The wall thickness of the elbow formed by the bending method is uneven. Many different formulas for calculating the wall thickness of the elbow can be found in the relevant literature. Still, they follow this variation pattern: reducing the wall thickness when the arch back (outer side of the elbow) and increasing the wall thickness when the arch belly (inner side of the elbow). The theoretical variation of wall thickness and its calculation formula for plastic deformation bending pipes without considering mold constraints are shown in Figure 1.
The calculation formula for the approximate wall thickness extreme value and the relative amount of thinning and thickening of the arch back and arch web-based on literature analysis is as follows.

Figure.1 Schematic diagram and calculation formula of theoretical changes in wall thickness of curved pipes without considering plastic deformation under mold constraints

In the formula:

• T_ex – extreme wall thickness of the arch back (outer side);
• T_in – the maximum wall thickness of the arch belly (inner side);
• T_o – original wall thickness;
• △t_ex – Extreme value of relative thinning of arch back;
• △t_in – Extreme value of relative thickening of arch soffit;
• R_o – bending radius;
• D_o – Outer diameter of the steel pipe.

For conventional pipelines subjected to internal pressure, the required wall thickness and allowable increment approximate values for the arch back and arch belly are calculated based on stress as follows.

In the formula:

• T_exr – The minimum net wall thickness required for the arch back of the pipeline under internal pressure (excluding allowances and tolerances);
• T_inr – The minimum net wall thickness required for the arch web of the pipeline under internal pressure (excluding allowances and tolerances);
• △T_exr – allowable wall thickness increment at the arch back;
• △T_inr – The allowable wall thickness increment for the soffit.

The formula (1)-equation (4) to make the R_o/d_o-Δt/t_o 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, the bend under the conditions of the bend, although it will cause uneven wall thickness, does not affect the use of the bend.

Figure.2 R_o/d_o-Δ_t/t_o curves according to Eqs. (1) – (4)

1.1.2 Disadvantages of the Bend Method

(1) Under dynamic conditions such as time-varying or alternating stress, the bending method mentioned above can cause uneven wall thickness, which is not allowed. Bends under time-varying stress conditions should have an equal thickness elbow with the required wall thickness of the arch as the minimum wall thickness of the elbow. Many studies have shown that the wall thickness reduction and roundness distortion caused by the bending method can accelerate the rate of stress and creep stress, thereby affecting dynamic stability.
(2) Unless special measures such as additional internal pressure and sand filling are taken, the pipe bend section may have additional roundness distortion. The literature indicates that the roundness distortion μ also depends mainly on R_o/d_o.

In the formula:

• D_max – maximum outer diameter of the curved section of the pipe bend;
• D_min – The minimum outer diameter of the curved section of the bend.

Under normal circumstances, curved pipes will produce roundness distortion. To μ If the limit is within 10% or lower, the R_o/d_o should be controlled above 2.
The correlation between the roundness of the pipe bend section and R_o/d_o and the allowable values in European standards are shown in Figure 3. Figure 3 is based on Figure 7.4.1.1 in EN 13480-4:2015. The original text did not specify conditions such as t_o/d_o and steel pipe material or the basis for inferring the inverse curve of 20/(R_o/d_o). The author believes that the υ curve will decrease when added.

Figure.3 Bend section roundness and R_o/d_o correlation and European standard allowable values

(3) After the completion of the bend, there will be a certain rebound in the direction of the bending angle, which is related to the material, R_o/d_o, and other factors of the bend. Therefore, in the actual bending process, a certain amount of bending angle is usually increased to eliminate the impact of rebound. The size of the bending angle is determined based on empirical values or simulation analysis.
(4) When cold bending of “thin” wall pipes with smaller R_o/d_o and lower t_o/d_o, or when the structure of the pipe bending machine is not suitable or adjusted improperly, wrinkles will appear on the arch belly. Wrinkles not only affect the appearance of the pipe bending but also affect the fluid transport performance of the pipe bending. For stainless steel pipelines, wrinkling can damage their corrosion resistance and reduce their service life. It can be seen that the study of wrinkled defects should be given special attention. The effect of cold working degree on the magnetic phase content and corrosion rate of 304L steel is shown in Table 3.
Table.3 Effect of Cold Working Degree on Magnetic Phase Content and Corrosion Rate of 304L Steel

Sample processing status①	Magnetic phase/%	Nitric acid test method corrosion rate/(mm/y) ②
		After being processed in the processing state, it is heated by 675 curses for 1h
Solid solution treatment	0.33	0.18	0.21
5%CW	0.34	0.2	0.24
10%CW	0.74	0.23	0.34
15%CW	1.6	0.185	0.43
20%CW	2	0.31	0.38
15%WW	0.35	0.23	0.38
Bend specimen with weld seam	0.82	0.15	0.25

Note: ①% CW is the degree of cold processing, and% WW is the degree of Hot-working. The bending sample with a weld is shown in Figure 4; ② The annual corrosion amount is calculated according to the test method in ASTM A262C.
Although theoretically speaking, using corrugated bends poses certain safety hazards and should be rejected. However, this is different. Since the implementation of EN 13480-4:2015, it is still stipulated that as long as the height (h) and distance (a) of the wrinkled wave are controlled within a certain range, pipe bends with wrinkled waves can still be used. The measurement method for the allowable value of bend wrinkled wave specified in the EN 13485-4:2015 standard is shown in Figure 5.

Figure.4 304L stainless steel bending specimen with weld seam in Table 3

Figure.5 Measurement Method for Allowable Wrinkle Value of Bend in EN 13485-4:2015 Standard
The specific requirements for the wave height (h) and wave distance (a) of pipe bend in the EN 13485-4:2015 standard are as follows:

In the formula:

• H – Average height of adjacent peaks, h=0.5 (d_o2+d_o4) – d_o3;
• A – Distance between adjacent peaks.

From this, it can be seen that the control wrinkled wave of bend is flexible in applying European pipelines.

1.1.3 Precautions for the application of cold-formed stainless steel elbows

(1) The allowable bending radius ratio for the direct application of different types of stainless steel pipes after cold bending differs. When the limit is exceeded, the bend must be heat treated before use.
(2) Austenitic stainless steel cold bending elbows used in environments with cyclic dynamic loads, high temperatures, and stress corrosion media must also undergo heat treatment, preferably solution heat treatment. It should be noted that although ASTM SA403/SA403M and ASTM SA815/SA815M standards allow the bending method to manufacture bends, they must be supplied in a heat-treated state. In contrast, ASTM SA182/SA182M standards do not allow the bending method to manufacture bends, as shown in Table 1.
(3) Large diameter pipelines usually do not require cold bending elbows due to the following reasons: ① high power and cost of pipe bending machines; ② The large diameter stainless steel pipe has a very small t_o/d_o ratio, making it very difficult to bend, especially to avoid wrinkling waves. Φ Cold bending method is rarely used to manufacture bends for stainless steel pipes above 219mm.

1.1.4 Local heating of steel pipes for manufacturing bends

Heating the stainless steel pipe to the Thermoforming temperature for bending can reduce the mechanical force required. This hot bending method applies to thick wall pipe bending and can also be used for bending R_o/d_o smaller steel pipes. The hot bending method for manufacturing pipe bends not only requires a heating process but also requires sand filling in the steel pipe. There are many processes and complex operations, and the formed pipe bends must also undergo solid solution or annealing treatment. Therefore, the application of using integral heating to manufacture pipe bends is limited.
Reference describes a method of hot bending U-shaped elbows that is only locally heated within 120° of the bend arch belly position. This method can reduce the thinning amount of the arch back of the elbow and is especially suitable for U-shaped bending pipes with a small curvature radius <1.5 (R_o/d_o). The hot bending method of the locally heated Φ50 mm×7 mm low carbon steel pipe is shown in Figure 6.

Figure.6 Local heating Φ 50mm × Hot bending method for 7mm low-carbon steel pipe

1.2 Molded welding method

When using the mold welding method to produce elbows, the steel plate is first cold pressed or hot molded into two symmetrical elbow parts at 45 °, 90 °, and 180 °, and then the two seams are welded to form 45 °, 90 °, and 180 ° elbows. Finally, the elbows are heat treated to produce the final product. The structural diagram of a bending machine is shown in Figure 7. The production method of molded welded elbows is the only method for manufacturing elbows in GB/T13401-2014, and it is also the method that can be used to produce elbows according to ASTM SA403/SA403M, ASTM SA815/SA815M, and ASME B16.9 standards.

1.2.1 Advantages of Mold Welding Method

The advantages of this welding method are: ① there is no significant wall thickness difference between the arch back and arch web of the elbow joint, and there is no roundness variation or wrinkling wave phenomenon in the elbow joint; ② Supplied according to different heat treatment conditions of steel grades, with a wide range of applicability; ③ Even if produced in batches according to specific standards, the quality of pipe bends is still guaranteed.

Figure.7 Structural schematic diagram of a bending machine

1.2.2 Disadvantages of mold pressing welding method

• (1) There are only two types of bend radii for each specification: long and short.
• (2) The straight pipe section on the elbow is very short. If the design is not reasonable during pipeline installation, it will increase the number of circumferential welds.
• (3) Nonprofessional production, with high costs.
• (4) High welding quality requirements. It is best to use automated GTAW or PAW without filler metal during welding, with single-sided one-time forming and double-sided welding. Only elbows welded with GTAW or PAW can be exempted from radiographic or ultrasonic inspection. Still, they must undergo strict visual inspection because the welds welded with GTAW or PAW have good corrosion resistance. If other welding methods or adding filler metal are used to weld the elbow, it must undergo X-ray or ultrasonic inspection, clearly specified in standards such as ASTM SA403/SA403M and ASTM SA815/SA815M.

1.2.3 Application of Molded Welding Method

(1) Suitable for various types of stainless steel, especially austenitic stainless steel. But it is not suitable for martensite stainless steel with poor weldability.
(2) Suitable for large-diameter thin-walled pipelines. Priority should be given to 5S and 10S wall thickness series with t_o/d_o<2% (see Table 4), and 40S and 80S wall thickness series with wall thickness below 6-7mm are also applicable.
(3) When selecting elbows, the wall thickness and steel grade of the elbow must be determined based on the working temperature, pressure, and medium environment of the pipeline, and special attention must be paid to the heat treatment status of the elbow according to the requirements of the steel grade and elbow manufacturing procedure. Suppose Grade H austenitic stainless steel elbow and other pipe fittings are Thermoforming. In that case, they must be separately solution annealed after forming, and the final solution annealing must be carried out after welding. When conducting solution annealing treatment, it is necessary to ensure that the heating temperature, holding time, and subsequent rapid cooling meet relevant requirements. The American standard also means that “the heat treatment process in the manufacturing process cannot replace the final solution annealing treatment.”
Table.4 Standard dimensions and specifications of stainless steel pipes in ANSI/ASME B36.19 and their comparison with European standards and national standards ①

Nominal specification		5S wall thickness series		10S wall thickness series		40S wall thickness series		80S wall thickness series		Outside diameter do/mm
NPS/in	DN/mm	Wall thickness to/mm	(to/do)/%	t/mm	(to/do)/%	to/mm	(to/do)/%	to/mm	(to/do)/%	①	②
1/8	6			1.24(1.2)	12	1.73(1.6)	17	2.41(2.5)	23	10.29	10(10.2)
1/4	8			1.65(1.6)	12	2.24(2.2)	16	3.02(3.0)	22	13.72	13(13.6)
3/8	10			1.65	9.6	2.31(2.3)	13	3.20(3.2)	18	17.15	17(17.2)
1/2	15	1.65(1.6)	7.7	2.11(2.0)	9.8	2.77(2.8)	13	3.73(3.8)	17	21.34	21(21.3)
3/4	20	1.65	6.1	2.11	7.9	2.87(3.0)	10.8	3.91(4.0)	14.7	26.67	27(26.6)
1	25	1.65	4.9	2.77(2.8)	8.3	3.38(3.5)	10	4.55(4.5)	13.6	33.4	34(33.7)
1 1/4	32	1.65	3.9	2.77	6.1	3.56(3.6)	8.4	4.85(5.0)	11.5	42.16	42(42.4)
1 1/2	40	1.65	3.4	2.77	5.7	3.68(3.6)	7.6	5.08(5.0)	10.5	48.26	48(48.3)
2	50	1.65	2.5	2.77	4.6	3.91(4.0)	6.5	5.54(5.5)	9.2	60.33	60(60.3)
2 1/2	65	2.11(2.0)	2.8	3.05(3.0)	4.2	5.16(5.0)	7	7.01(7.0)	9.6	73.03	73(76.1)
3	80	2.11	2.3	3.05	3.4	5.49(5.0)	6.2	7.02(7.5)	7.9	88.9	89(88.9)
3 1/2	90	2.11	2	3.05	3	5.74(6.0)	5.6	8.08(8.0)	7.9	101.6	102(101.6)
4	100	2.11	1.8	3.05	2.7	6.02(6.0)	5.2	8.56(8.5)	7.5	114.3	114(114.3)
5	125	2.77(2.8)	2	3.40(3.5)	2.4	6.56(6.5)	4.6	9.52(9.5)	6.7	141.3	140(139.7)
6	150	2.77	1.6	3.40(3.6)	2	7.11(7.0)	4.2	10.97(11)	6.5	168.28	168(168.3)
8	200	2.77	1.3	3.76	1.7	8.18(8.0)	3.7	12.70(12.9)	5.8	219.08	219(219.1)
10	250	3.40(3.5)	1.2	4.19(4.0)	1.5	9.27(9.5)	3.4	12.7	4.6	273.65	273(273)
12	300	3.96(4.0)	1.2	4.57(4.5)	1.4	9.52(9.5)	2.9	12.7	3.9	323.85	325(323.9)
14	350	3.96	1.1	4.78(5.0)	1.3					355.6	356(355.6)
16	400	4.16(4.0)	0.9	4.78	1.2					406.4	406(406.4)
18	450	4.19	0.9	4.78	1					457.2	-457
20	500	4.78(5.0)	0.8	5.54(5.5)	1					508	-508
22	550	4.78	0.8	5.54	1					558.8
24	600	5.54(5.5)	0.8	6.35(6.3)	1					609.6	-610
30	750	6.35(6.3)	0.8	7.92(8.0)	1					762	(711)(813)

Note: ① According to ANSI/ASME B36.19, the outer diameter and wall thickness in inches in the original table have been deleted; To/d_o is the analysis data added by the author, and the data in parentheses to is the closest data in the national or European standards; ② The corresponding outer diameter specifications can be found in GB/T 17395-2008, and the data in parentheses is from EN ISO 1127-1997.
(4) When using the mold pressing welding method, if the welding method of adding filler wire is used, it is necessary to select appropriate welding materials according to the steel grade to ensure the welding quality.

1.3 Forged machined elbows

Forging elbows with a forging machine is the optimal solution for the high-temperature application of stainless steel elbows. It is the only manufacturing method for elbow conduit connectors specified in ASTM SA182/SA182M and is also allowed in ASTM SA403/SA403M and ASTM SA815/SA815M.
The advantages of forging machine processing elbows include: ① Seamless pipes with better material uniformity and easy control of high-temperature corrosion or creep resistance; ② Especially suitable for Martensite and H-grade austenitic stainless steel thick wall pipes with poor welding performance.
Disadvantages of forging machine processing elbows: ① Long manufacturing process flow; ② After forging, it is usually necessary to undergo machining to meet the surface finishing requirements; ③ The highest cost. Using a forging machine to process elbows is suitable for manufacturing thick-walled pipeline elbows that require high operational reliability in nuclear power plants and other areas.

2. Causes and prevention measures for wrinkling of pipe bends

The manufacturing of stainless steel pipe bends can take various forms, such as rolling bending machines, and the rotary bending machine is the most common model. Here, the analysis and discussion will be conducted on the rotating machine of the bending die.

2.1 Basic composition of a curved pipe bending machine

The basic structure of the curved pipe bending machine is shown in Figure 8. As shown in Figure 8, the curved pipe bending machine mainly consists of rotating bending modules, compression blocks, clamping blocks, and other parts. When bending, clamp the steel pipe to be bent between the clamping block and the clamping block of the bending machine so that the clamping block presses the steel pipe with appropriate pressure and rotates the bending mold. The steel pipe can be bent as the clamping block shifts from the initial position 1 to positions 2, 3, and 4.
The structures of two types of rotary bending pipe benders with boosting are shown in Figure 9. Comparing Figure 9 and Figure 8, it can be seen that the basic structure of the curved pipe bending machine with auxiliary force has two main differences from Figure 8: ① an auxiliary force is added to the end of the compression block or steel pipe (the model indirectly assisted by the compression block is relatively old-fashioned, but it is still widely used in domestic production, and its rationality is worth exploring); ② The initial position of the clamping block is close to the initial position of the clamping block. It can be seen that the clamping block on the curved pipe bending machine with a booster presses the steel pipe between the clamping block and the module from the beginning, while the clamping block in Figure 8 has a certain distance from the clamping block at the beginning. The literature indicates that the clamping block should have sufficient length to compress the bend and move with the steel pipe and states that “there should be no sliding between it and the steel pipe.” But when it helps to push, there may be sliding between the clamping block and the steel pipe. The sliding block (wiper die) that supports the other half of the steel pipe and the compression block are always in a relative sliding state with the steel pipe. For the sliding block (i.e., the anti-wrinkle block in Figure 9) to have an anti-wrinkle effect, it is necessary to thin its front end and approach the tangent point between the bending die and the pipe. This distance should be controlled between 3.2-13mm. Otherwise, it will not have an anti-wrinkle effect. Usually, the sliding block (or anti-wrinkle block) of stainless steel bends is made of aluminum bronze material. The sliding block in Figure 8 plays a certain supporting role, but the anti-wrinkle block in Figure 9 plays a very small supporting role, so some factories have removed the anti-wrinkle block on this type of pipe bending machine to reduce costs.

Figure.8 Basic structure of a curved pipe bending machine

Figure.9 Two types of rotary bending pipe benders with booster

2.2 Core rod and its type of bending pipe bending machine

The curved pipe bending machine has a core rod (see Figure 8 and Figure 9), but the core rod is only used when the steel pipe wall thickness is relatively thin. Table 5 lists the minimum bending radii of cold-formed steel pipes without needing core rods under different wall thicknesses and outer diameters.
According to the data analysis provided in Table 5, it is shown that steel pipes with t_o/d_o ≥ (3.6-4) % can be cold bent without a core rod, and the larger the t_o/d_o value, the smaller the R_o/d_o value for cold bending without a core rod. The literature does not specify the applicable steel grade, but the author believes the data for austenitic stainless steel pipes can be roughly referenced.
Table.5 Minimum bending radius of cold-formed steel pipes without mandrel under different wall thicknesses and outer diameters

The nomogram for selecting the core rod based on the relative wall thickness (t_o/d_o) and relative bending radius (R_o/d_o) of the steel pipe is shown in Figure 10. The nomogram shown in Figure 10 (a) can determine the matching conditions for different t_o/d_o and R_o/d_o during cold bending without a core rod. For example, when t_o/d_o=20%, R_o/d_o ≥ 1 can be used for coreless cold bending; But if t_o/d_o ≤ 10% and R_o/d_o ≥ 1.5, the core rod cold bending can be avoided; If t_o/d_o ≤ 5%, R_o/d_o ≥ 3.5 can still perform coreless cold bending; If t_o/d_o ≤ 4%, R_o/d_o>4 can still be used for coreless cold bending.
The core rod used in the curved pipe bending machine has two types: rigid or flexible, and can be bent. The structural type of the core rod is shown in Figure 11, where Figure 11 (a) and Figure 11 (b) are rigid core rods, Figure 11 (c) and Figure 11 (d) are flexible core rods, and Figure 11 (e) is flexible core rods for rectangular steel pipe cold bending. Figures 11 (a) and 11 (c) are widely used universal core rods in China. The rigid shaped end core rod in Figure 11 (b) is used for the cold bending of elliptical tubes. The steel cable core flexible core rod in Figure 11 (d) has greater spatial freedom, and its applicability is worth further exploration and research.
The purpose of using a core rod is to prevent wrinkling of the arch belly and to reduce the thinning of the arch back and the out of roundness of the bent section. Flexible core rods are generally used for thin-walled steel pipes with lower t/d values. The nomogram shown in Figure 10 (a) can also be used to determine the structural requirements of the core rod. For example, when t_o/d_o=3.3 and R_o/d_o=4, a rigid core rod can be used; When R_o/d_o=3, a single ball flexible core rod needs to be used, And R_o/d_o=2.0 requires the use of multi spherical flexible core rods. The nomogram shown in Figure 10 (b) can further determine the number of balls or segments of spherical or segmented flexible core rods required for bending 90 ° and above bends. It can be seen that flexible core rods are more suitable for cold bending of thin-walled pipes with t_o/d_o ≤ 3.3%. The smaller the t_o/d_o, the more balls or segments are required. Cold bending of thin-walled pipes with softer materials requires spherical flexible core rods.
It should be noted that in stainless steel pipe standards, thin-walled pipes (t_o/d_o ≤ 3%) or ultra thin-walled pipes (t_o/d_o ≤ 2%) are usually only distinguished by t_o/d_o. However, when discussing pipe bends, the value of t_o/d_o must also be related to R_o/d_o to distinguish the so-called “thin” wall concept. The average practical bending radius when using a spherical core rod and wrinkle resistant slider for thin-walled cold bending is shown in Table 6.

Figure.10 Nomogram for selecting the core rod based on the relative wall thickness (t_o/d_o) and relative bending radius (R_o/d_o) of the steel pipe

Figure.11 Structural Types of Core R_od_s
From the analysis of the average practical bending radius data of a single spherical core rod given in Table 6, it can be seen that for a steel pipe with to=0.89mm and do=75mm, the bending radius is 381mm, R_o/d_o=5.1, and t_o/d_o=1.2%; But when do=13mm, R_o=13mm, R_o/d_o=1.0, t_o/d_o=6.8%. This means that when the bending radius is small, t_o/d_o=6.8% should still be considered a “thin” wall bend. Analyzing the data in Tables 5 and 6,  was found that the evaluation value for selecting the “thin” wall thickness of the core rod when cold bending steel pipes can be used: when b>25%, it is advisable to use noncore rod cold bending; When b=12.6% -25%, it is advisable to use a rigid core rod for cold bending; When b<12.6%, it is advisable to use a flexible ball-shaped core rod for cold bending. According to Figure 10 (a), the corresponding b values should be b>17.5%, b=10.5% -17.5%, and b ≤ 10.5%, respectively. There are some differences between the two values, which may be due to the lack of integrity or continuity of the Data deficient in Table 5 and Table 6, as well as the different speed, pressure, structure, and other parameters of the pipe bender on which the calculation is based. It can be considered that the b value given in Figure 10 (a) is more reasonable, but the choices given only have relative significance and are not absolute boundaries. The following text will point out that this rough evaluation is valuable.
Table.6 Average practical bending radius when using spherical core rod and wrinkle resistant slider for thin-walled cold bending

The chart data summarized from the bending practice above indicates that the type of core rod or the presence or absence of a core rod is crucial for cold bending wrinkling. Recent experimental research in China has shown that as long as the appropriate core rod type (using a four-ball core rod) is used, even if Φ 50mm × 0.8mm 304 steel pipe or Φ 123mm × 3.97mm 316L steel pipes can achieve bends with R_o/d_o=1.2 or 2.9 wrinkle free; If there is no core rod, then Φ 20mm × When the R_o/d_o of 1mm 304 steel pipe is ≤ 3.5, wrinkling will always occur.

2.3 Manufacturing, installation, and accuracy of core rods, bending molds, etc.

2.3.1 Material, manufacturing, installation, and other requirements for molds

The cold bending of steel pipes is a plastic forming process under the joint constraints of bending molds, compression blocks, and core rods. The manufacturing accuracy of these related molds is very important for the quality of bend forming. The higher the molding accuracy, the higher the quality of the bend section forming. The manufacturing accuracy here mainly refers to the gap size and surface quality between the bending die, clamping block, core rod, and the steel pipe to be bent. Table 7 summarizes the gap data and related constraints for determining mold manufacturing accuracy found in the literature.
It should be noted that: ① Most of the data provided in the literature are relative values related to the wall thickness or outer diameter of the steel pipe, while other practical constraints that provide specific values may be more narrow; ② If the size of the steel pipe exceeds the range of Tables 5 and 6; ③ When bending stainless steel pipes, it is best to use Cr plating on the surface of the rigid core rod, flexible core rod body, and clamping block. For flexible core rod spheres, wrinkle resistant sliders, and clamping blocks, aluminum bronze should be used. The bending mold should be made of hardwood, plastic, etc. Otherwise, the steel pipe must be pickled and passivated after cold bending. Moreover, it is not suitable to cold bend both carbon steel pipes and stainless steel pipes on the same pipe bending machine.

2.3.2 Installation accuracy of molds

The position of the endpoints of rigid and flexible core rods is crucial for the quality of pipe bends formed by bending, and the endpoints referred to here are the maximum diameter endpoints. Literature and others believe that the endpoint should slightly exceed the starting point of the bend, i.e., the tangent point of the bending die (see Figure 12). As shown in Figure 12, if the endpoint exceeds the tangent point position too much, it will cause a “goose head” shape in the pipe bend section. However, if the endpoint is slightly far from the tangent point position, it will cause wrinkling of the arch belly. Therefore, the accurate lead amount of the core rod should be determined through experiments. The formula given in the literature (see Table 7) further indicates that the lead amount should be different for different core rod clearances. The literature suggests that it should be advanced by 0.15d_o. Still, the article also provides good forming results with an extension of 0 and R_o=2d_o (see Table 1, Figure 2, and Figure 4 in the literature), indicating that the article’s conclusion is still worthy of consideration.
When installing the anti wrinkle slider, the end must be thinned as close as possible to the cutting point of the bending mold, and it is best to effectively drag the steel pipe arch at 15 ° before the bending mold is cut (see Figure 9). Otherwise, it is easy to cause wrinkles. For this reason, anti wrinkle blocks are often installed tilted, but this may cause vibration.

Figure.12 Effect of the End Position of the Rigid Core R_od on the Quality of the Bend
In addition, after using the curved pipe bending machine for some time, it is necessary to detect the wear of the bottom of the bending die notch and its rotating journal. The wear amount should be controlled within the numerical range listed in Table 7, which is particularly important for thin-walled bends.

2.4 Operating parameters of the curved pipe bending machine

2.4.1 Bending speed

The bending speed refers to the speed of the steel pipe feeding line determined by the bending mold’s rotation speed, the bending mold, or the deformation speed of the bending section. Literature research has shown that the bending speed has a certain impact on the quality of the bend, especially on the wrinkling wave of the bend. When t_o/d_o and R_o/d_o are given, reducing the bending speed can eliminate or reduce the wrinkling wave (see Figure 13). However, Table 4, Table 5, and Figure 10 do not calibrate the bending speed, indicating that the results indicated in the literature are not absolute limits.

2.4.2 Compression block pressure

The pressure of the clamping block is another important parameter affecting the pipe bend’s wrinkling wave. The pressure of the clamping block plays two important roles in the bending process: firstly, it forms the necessary bending moment together with the bending mold; second is to generate friction on the contact surface of the compressed steel pipe and reduces the required feeding or lowering speed for steel pipe bending.
According to the literature, the minimum value of the compression block pressure can be expressed as:

In the formula:

• Q – pressure density on the compression block;
• L_p – length of compression block;
• M – bending moment, whose value depends on the steel pipe’s yield strength, R_o/d_o, and t_o/d_o. Multiple expressions of M can be found in the literature.

The difficulty in this calculation is that the degree of deformation and hardening of each longitudinal fiber on the circumference of the circular pipe during the bending process is different, and the yield stress value at each point on the circumference will change with the degree of hardening. Therefore, concepts or factors such as hardening factors should be introduced in the calculation. That is to say, p_o will depend on the yield strength, hardening factor, R_o/d_o, and t_o/d_o double integral calculation results of the steel pipe, and the application of the finite element method can obtain its approximate value, namely:

Figure.13 Physical samples of stainless steel bend at different bending speeds and their corresponding finite element simulations

In the formula:

• Y – the height of the neutral layer of the curved pipe with micro surface product elements on the cross-section of the steel pipe;
• R – The radius value of the micro surface product element on the cross-section of the steel pipe;
• σ_θ— The actual stress of the micro surface product element on the cross-section of the steel pipe;
• φ— The offset angle between the micro surface product element and the arch back on the cross-section of the steel pipe.

When bending, the pressure of the clamping block will generate friction force on the contact surface of the pressed steel pipe, reducing the required feeding or lowering the feeding speed for the steel pipe to bend. If no auxiliary thrust is added, the friction force will make v_p<v_o or v_p/v_o<1 (v_p is the feeding speed of the clamping block, v_o is the neutral layer linear speed of the steel pipe determined by the bending mold rotation speed), which cannot achieve bending; Only by adding auxiliary thrust can v_p/v_o ≥ 1 be achieved to achieve bending. The greater the pressure on the clamping block, the lower the v_p/v_o value will undeniably impact the wall thickness change, wrinkle height, and rebound of the pipe bend section.
The influence of the compression block pressure (p/p_o) on the quality of the pipe bends when v_p=1.5v_o is shown in Figure 14. right Φ 50mm × The effects of compression force and boost speed on the wall thickness and corrugation height of the arch back and arch web of 0.8mm 304 stainless steel pipe (R_o/d_o=1.2) during cold bending are shown in Table 8.
From Figure 14 and Table 8, it can be seen that ① increasing pressure or p/p_o can reduce the wall thickness reduction of the arch back, increase the wall thickness increase of the arch belly, and slightly increase the wrinkle height and rebound; When p/p_o ≥ 8, the influence of pressure on wrinkling is significant. Although it is beneficial for rebound and can reduce the thinning of the arch back, it is not advisable due to its particularly unfavorable effect on wrinkling. When the pressure is at its minimum value, both finite element calculations and experimental results prove that the arch back’s thinning and the arch web’s thickening are insignificant, and there are no wrinkles. The rebound amount is insignificant and has little correlation with the presence or absence of auxiliary thrust.
The above situation indicates that for traditional curved pipe bending machines, the only advantage of boosting (see Figure 9) is that it can reduce the wall thickness reduction of the arch back while increasing pressure, but it is extremely detrimental to wrinkling. This may be due to the absence of a boost in Figure 8 and the absence of additional boosts discussed in the literature.

Figure.14 Effect of compression block pressure (p/p_o) on the quality of pipe bends at v_p=1.5v_o

2.4.3 Lubrication

In the actual bending process, when p ≥ p_o and v_p=0.9v_o, the frictional force generated by p can cause the clamping block to follow the steel pipe. This frictional force is equivalent to a reverse boost for steel pipes. The finite element research results on the wrinkling problem of 304 stainless steel pipe bend in the literature indicate that increasing the friction coefficient is beneficial for reducing or eliminating wrinkling; When v_p=v_o is applied, even if the friction coefficient is smaller, increasing lubrication or improving the surface smoothness of the compression block will still cause wrinkling of the bending arch. It can be seen that lubrication should not be applied between the compression block and the steel pipe to reduce friction, and the surface of the compression block should not be polished to improve its surface smoothness excessively.
The literature also proves through finite element calculation that applying reverse boosting on the inner side (arch belly) of the steel pipe can improve the wrinkling tendency and proposes methods to control the boosting distance of the compression block or increase double boosting to reduce wrinkling waves. The double boost means the compression block is a positive boost, and the slider or anti wrinkle block is a reverse (asynchronous) boost. When using double auxiliary push for bending, not only can the auxiliary push be used to reduce the thickness reduction of the arch back wall, but also the reverse auxiliary push can be used to limit the wrinkling of the arch belly and the increase in wall thickness, thereby ensuring uniform wall thickness during cold bending. This dual boost model would be a valuable innovation if implemented.
It is worth noting that there are different opinions on the lubrication methods and functions of the pipe bending machine in the literature. For example, ① The literature points out that the core rod, compression block, anti wrinkle block, and inner and outer walls of the pipe ring should be lubricated with lubricating oil (aviation lubricating oil (60% -80%)+paraffin (40% -20%)). This may be the operating basis for many domestic curved pipe bending machines, but its rationality is worth exploring; ② The literature only indicates that the core rod and steel pipe must be lubricated with heavy oil, and the slider or anti wrinkle block must be highly polished and coated with a thin, light lubricating oil. It is emphasized that too much oil or too thick an oil layer can promote arch belly wrinkling, and the practical significance of this approach is also worth studying; ③ The literature indicates that only lubrication is required between the core ball and the inner wall of the tube, as well as between the slider and the outer wall of the tube, while lubrication of other parts is unnecessary; ④ The literature suggests that “increasing the friction coefficient of the concave surface of the anti wrinkle block can appropriately reduce the trend of wrinkling” (as the literature mainly discusses noncore bending, whether this discussion is related to this still needs further research).
Table.7 Effects of compression force and boost speed on wall thickness and corrugation height of arch back and arch belly

Compression block pressure	Boost speed	Outer (arch back) wall thickness/mm		Inner wall (soffit) wall thickness/mm		Crease height/mm		Rebound angle/(°)
		Simulation value	Test Value	Simulation value	Test Value	Simulation value	Test Value	Simulation value	Test Value
p。	0.5vo	0.6	0.63	0.96	0.9	0	0	0.8	0.82
	0.9vo	0.67	0.63	0.97	0.93	0	0	0.83	0.78
	1.3v。	0.65	0.62	0.94	0.9	0	0	0.76	0.8
8p。	0.5vo	0.53	0.5	0.9	0.88	0	0	0.83	0.85
	0.9vo	0.68	0.64	1.02	0.95	0.18	0.15	0.86	0.91
	1.3v。	0.74	0.68	1.11	1.06	0.15	0.5	0.86	0.79

The above somewhat dissenting views indicate that the issue of lubrication needs further in-depth exploration to achieve uniformity.

3. Case analysis and dissection

In 2016, users discovered wrinkling in some of the stainless steel pipes produced by Yaang Pipe Industry during bending. Statistical analysis was conducted on the sample data of this bend, and the results are shown in Table 9.

3.1 Case 1

Firstly, take the bend of the stainless steel pipe (No. 1 in Table 9) as an example. According to the Nomogram (see Figure 10) and Table 8 data, it is advisable to use a single ball flexible core rod for bending steel pipes of this specification. However, the factory uses a rigid core rod for bending, which poses a significant risk of wrinkling. This is because:
Table.8 Sample Data Statistics of Stainless Steel Bends

Serial Number①②	Steel grade	Specifications (d°xt。)/(mmxmm)	Bend “thin” wall thickness b③/%	Ordering standards or data sources
1w	304	088.9x3.05	3.4%x2.5=8.5%	ASTM A312/A312M
2w	316L	060x3.5	5.8%x2.5=14.5%
3w	304	020x1.0	5.0%x3.5=17.5%
4	304	021.3x2.11	9.8%x2.5=24.5%	ASTM A312/A312M
5	304	033.4x2.77	8.9%x2.5=22.3%
6	304	044.0x2.77	6.1%x2.5=15.3%
7	304	048.2x2.77	5.4%x2.5=13.5%
8	304	060.3x3.05	4.6%x2.5=11.5%
9	304	073.0x3.05	4.2%x2.5=10.5%
10b	304	0141.3x3.4	2.4%x2.5=6%
11	316L	027x2.5	9.2%x2.5=23%
12	316L	089x3.0	3.4%x2.5=8.5%
13b	316L	0219x4.5	2.0%x2.5=5.1%
14b	304	050x0.8	1.6%x1.2=1.9%
15b	316L	0123x3.97	3.2%x2.9=9.3%

(1) The pipe bender the user uses is of the structure shown in FIG. 9 (a), but there is no 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 ×3.05 mm stainless steel pipes did not wrinkle. The relative wall thickness (t_o/d_o) of these boughs is larger than that of Φ88.9 mm×3.05 mm boughs, which is an important prerequisite. The data analysis in Table 9 shows that the b of these 6 kinds of steel pipes is ≥10.5%, and rigid mandrel can be used. And Φ141.3mm × 3.4mm bending pipe with a spherical flexible mandrel does not appear corrugating, which shows the importance of improving the 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 requirements, it is easy to produce according to Φ89mm × 3.0mm, resulting in the outer diameter and inner diameter of the supplied steel pipe being large while the wall thickness is low. When bending the pipe, the gap between the inner wall and the mandrel will increase if the same mandrel as the Φ88.9mm × 3.05mm steel pipe is used. All these make the wrinkling phenomenon of the steel pipe, which should be bent with a flexible mandrel Φ88.9mm × 3.05mm when bending with a rigid mandrel, becomes more prominent under the condition of Φ89mm × 3.0mm. It can be seen that the rounding of such data in the national standard is not appropriate.
(3) The outer diameter of the core rod used for actual bending is 80mm, and the gap between the inner wall of the steel pipe and the core rod is (d_i-d_m)/2=1.45, much greater than the recommended value in Table 7. Compared to another factory that did not experience wrinkling when using an outer diameter 82mm core rod bend, the key factors contributing to wrinkling may be the core rod’s small outer diameter and the core rod’s unadjusted lead position. Plus, without anti-wrinkle blocks, wrinkling is inevitable for steel pipes with b=8.5%（ Φ 88.9mm × 3.05mm).
(4) If the wall thickness of the steel pipe reaches the upper limit of this specification (3.05mm × 1.225=3.72mm) when using an outer diameter 80mm core rod bend, the gap between the inner wall of the steel pipe and the core rod is (d_i-d_m)/2=0.7 ≈ 0.2t_o, and no wrinkling will occur at this time. A core rod with an outer diameter of 80mm is suitable for supply with a wall thickness of the upper limit.

3.2 Case 2

They are taking the bend of stainless steel pipe (No. 2 in Table 9) as an example. According to the Nomogram (see Figure 10) and Table 9 data, it is recommended to use a rigid core rod for bending steel pipe bends of this specification. However, this specification of steel pipe is a non-standard steel pipe. In European and American standards (see Table 4), the standard outer diameter is 60.33mm, and the wall thickness is only 2.77mm (2.8mm) and 3.91mm (4.0mm). According to the national standard, 60mm is the rounded value of 60.33mm. When bending steel pipes of this specification, only a few bends produced wrinkled waves, with an incidence rate of about 10%. After careful analysis, the main reasons are:
(1) Only bend pipes can be used Φ 60mm × The clearance of the 2.8mm standard specification core rod is significantly greater than the recommended value in Table 7.
(2) The outer diameter of 60mm<60.33mm increases the gap between the bending molds, increasing the tendency for wrinkles.
(3) When bending, if the wall thickness of the steel pipe is close to the lower tolerance zone and the wall thickness is uniform, it will cause some steel pipes to wrinkle. At this point, press the Φ 60mm × Equipped with a 3.5mm core rod; occasional wrinkles can be controlled. The dimensional tolerances of steel pipes below DN200 in the stainless steel pipe standard are summarized in Table 9.
Table.9 Summary of Dimensional Tolerances for Steel Pipes under DN200 in Stainless Steel Pipe Standards

Region or Country	Standard Code	Pipe making method	Outside diameter do/mm	Outer diameter tolerance zone Δdo	Wall thickness tolerance zone Δto
Europe	EN 10216–5	Thermal finishing seamless pipe	<219.1	The larger of ± 1.0% do or ± 0.5	± 15% to or soil 0.6, whichever is greater
	EN 10217–7	welded pipe	>168.3		± 12% to or soil 0.4, whichever is greater
	EN 10216–5	Cold finished seamless pipe	<219.1	± 0.75% do or 0.3 soil, whichever is greater	± 10% to or soil 0.2, whichever is greater
	EN 10217–7	welded pipe	<168.3	± 0.5% do or soil 0.1, whichever is greater	The larger of ± 7.5% to or ± 0.15
USA	ASTM A999	Seamless and welded pipes	<48.3	+0.4、 –0.8	+0.2to	–0.12to
	ASME SA999
	ASTM A312	Seamless and welded pipes	48.3-114.3	±0.8	+0.225to	、–0.125to
	② Seamless and welded pipes	③ 168.3-219.1	ASME SA312①	+1.6、–0.8	+0.15to	–0.125to
China	GB/T 14976–2012	Thermal finishing seamless pipe	<219.1	±(1–1.25)%do	±(13–20)%to
		Cold finished seamless pipe	<219.1	±(0.75–0.85)%do	±(10–12.5)%to
	GB/T 12771-2012	Heat treated welded pipe	≤168.3	±0.20-±0.8	t°W 2±0.15-±0.20
		Heat treated welded pipe	>168.3	±0.75%do	t°W4±0.30

(4) Simultaneously providing the user with Φ 27mm × 2.5mm Φ 89mm × 3mm Φ 219mm × The three non-standard specifications of 316L pipes with a diameter of 4.5mm did not show any wrinkling, indicating that as long as the core rod is reasonably set, the wrinkling problem during stainless steel pipe bending can be eliminated.
The comparative analysis results of the two cases in Table 9 indicate that the “thin” wall thickness boundary given by the nomogram in Figure 10 (a) is suitable for selecting stainless steel bent-core rods.

4. Discussion

4.1 The essence of wrinkling in pipe bends

The essence of bending wrinkling is that plastic deformation and instability occur after the arch web is compressed.

4.1.1 Normal bending

A normal bend should result from uniform stretching of the arch back and uniform compression plastic deformation of the arch web. Theoretical and experimental studies have shown that the clever combination of uniform tensile plastic deformation of the arch back and uniform compressive plastic deformation of the arch web is a prerequisite for ensuring the quality of pipe bends. The limit of uniform tensile plastic deformation is determined by the plastic tolerance of the steel pipe, beyond which the arch back will fracture. For thin-walled tubes with t_o/d_o<(4-5)%, the compression zone of the soffit will produce wavy deformation due to insufficient rigidity before this limit, and its essence is the same as the buckling bending of the compression bar in Strength of materials. Therefore, wrinkling is a common abnormal phenomenon in “thin” walled bends.

4.1.2 Pressure bar instability

When a slender rod (i.e., with a large L/t ratio) is subjected to compression, it is prone to bending and losing its compression function, which is called instability. This means that rods with small or thin cross-sections or rods with large free lengths will lose their ability to continue to withstand pressure once they are compressed. Increasing wall thickness or reducing length can avoid instability. Therefore, L/t is the earliest evaluation index used for the stability or stiffness of compression bars.
There are many common examples of strut instability in engineering. For example: (1) During the hydraulic test of steel pipes, pressure must be set according to the slenderness ratio step by step to be pressurized under the clamping conditions at both ends. Otherwise, the steel pipes at both ends will arch (bend) when pressurized. When the pressure is low, it is elastic (deformation) instability, and when the pressure is high, it is plastic (deformation) instability; (2) Rib plates and reinforcing rib plates must be installed in the compression stress zone of the web plate of I-shaped or box-shaped beams to prevent wave-shaped deformation when the beam is compressed and bent; (3) When welding thin plates, the welding pressure stress zone is prone to wave-shaped deformation. Therefore, it is necessary to control the welding sequence and parameters to reduce compressive stress and prevent instability. The wrinkling wave generated by the arch web when using plastic deformation bending is a form of instability of the compression rod.

4.1.3 Thin-walled stainless steel pipes for pipelines

When selecting stainless steel pipes for pipelines, thin-walled pipes are generally used because (1) excellent corrosion resistance determines the design wall thickness of stainless steel pipes without the need for additional corrosion allowance; (2) The expensive price makes designers always choose thin-walled pipes as much as possible, with stainless steel pipes for 5S and 10S series pipes having a t_o/d_o ratio of (2-4)% (see Table 4); (3) Due to the limited space of structures such as ships, the radius of pipeline bends (R_o) should be minimized as much as possible, which can lead to wrinkling during the production of “thin” wall bends.

4.2 Mechanical analysis of bending pipe wrinkling

With the help of the evaluation method in modern Structural mechanics about the compression plastic deformation of the compression bar and the influence of its supporting conditions on the stability of the compression bar, the causes, and conditions of the soffit wrinkling during stainless steel bending are further analyzed.
(1) Only compression rods with a slenderness ratio of -5 can generate compressive plastic deformation. Otherwise, the compression rods will first bend due to instability and cannot generate compressive plastic deformation.
(2) The stability of a compression rod depends not only on the slenderness ratio of the rod itself but also, to a greater extent, on the constraint conditions or degrees of freedom of motion at both ends. Therefore, the critical pressure for instability can be expressed as:

In the formula:
The elastic modulus of the compressed rod material;

• I_αβ— Cross-section moment of inertia of compression bars;
• L – The actual length of the compression member;
• μ— The length coefficient of compression bars;
• ζ— The stability coefficient of compression members, ζ= π/μ².

Length coefficient of compression rod with different end support methods μ and stability coefficient ζ See Table 11. As shown in Table 10, the smaller the degree of freedom of the support end, μ smaller the value, ζ greater the value. The mold setting and clearance conditions during bending determine the degree of freedom of support in the compression zone of the arch web: when t_o/d_o or R_o/d_o is large enough, the arch web is thick enough to withstand cold bending without a core rod; When t_o/d_o or R_o/d_o is small, the stiffness of the arch itself decreases. It is necessary to limit its lateral motion freedom through a rigid core rod at the bending point or use a single ball flexible core rod to limit the actual free length of the bending section; For “thin” walled pipes with very small t_o/d_o or R_o/d_o, the bending process needs to limit the degrees of freedom at both ends of each micro bending segment and the corresponding actual free length of the micro bending segment. Therefore, when bending ultra-thin wall pipes, it is necessary to use multiple ball core rods to prevent the generation of arch web wrinkling waves.
Table.10 Compression R_od Length Coefficient and Stability Coefficient for Different End Support Methods

Support method at both ends of the compression rod (degrees of freedom)	Schematic diagram of support degrees of freedom	Length coefficient μ	Stability coefficient ζ
Rigid fixation at both ends; Or one end is rigid, and the other end can have axial movement		0.5	39.5
One end is rigidly fixed, while the other end can move and rotate axially; Or one end can be hinged and fixed, while the other end can move axially.		0.699	20.2
One end is hinged and fixed, while the other end can move and rotate axially; Or the other end can move axially and horizontally.		1	9.87
One end is rigidly fixed and the other end is free; Or one end can be hinged and fixed, while the other end can move axially and horizontally.		2	2.47

4.3 Mold accuracy and clearance on the impact of bending wrinkle wave

4.3.1 Effect of Mold on Bend Wrinkle

The limit conditions for fracture or wrinkling of cold bending steel pipes without molds are shown in Figure 15. Theoretical and experimental research results indicate that without the use of molds to improve the plastic deformation conditions of bent steel pipes, stainless steel pipes with t_o/d_o=4% or 2% may break or wrinkle when R_o/d_o=25 or 50 (i.e., the nominal bending strain d_o/2R_o=2% or 1% in Figure 15). However, with the reasonable configuration of bending molds, compression blocks, and flexible core rods, wrinkling can still be avoided when bending stainless steel pipes with t_o/d_o=2% and R_o/d_o=1.5%. It can be seen that mold accuracy and clearance play an important role in ensuring the quality of stainless steel pipe bends and preventing the generation of wrinkles.

Figure.15 Fracture or wrinkling limit conditions for cold bending steel pipes without molds

4.3.2 Determination of Clearance

Except for the literature, other literature only explains the importance of correctly controlling gaps but needs to explain the general significance of actual gap control. The literature provides information on Φ 20mm × The critical condition for the gap between the bending die and the outer diameter of the 1mm specification 304 steel pipes during R_o/d_o=3 coreless cold bending is greater than 0.2mm, but this critical condition may not be applicable for different t_o/d_o and R_o/d_o. The literature only states that Φ 40mm × The gap between the outer diameter of the core rod and the inner diameter of the 1mm aluminum alloy pipe during cold bending is larger. The literature uses the energy method to study Φ 38mm × The wrinkling problem during cold bending of 1mm sized aluminum alloy pipes is analyzed through experimental and simulated clearance results. The results show that the gaps between the core rod and the inner diameter of the pipe, the anti wrinkling block and the outer diameter of the pipe, and the gap between the bending die and the outer diameter of the pipe all generate wrinkling at 0.8mm. In comparison, no wrinkling occurs at 0.2mm. However, no wrinkling occurs when the gap between the pressure block and the outer diameter of the pipe is 0.2mm or 0.8mm.
This literature needs to specifically address situations where gaps are larger or smaller. Looking at Table 7, the gap control values given in the literature are currently the most valuable for reference.

4.4 Impact of Steel Pipe Dimensional Tolerance on Wrinkle Waves

The literature provides comprehensive and specific control values for the referred gaps and indicates that some gaps should depend on or do; Some gaps are within a specific numerical range; some gaps still need to be determined through testing (see Table 7 for details). Although these explanations have certain reference values, there are difficulties in their specific implementation, as the current steel pipe manufacturing standards do not yet meet the control requirements of “gap” within the allowable range of dimensional tolerance zones.
Comparing the gap control requirements in Tables 9, it can be seen that many standards specify wall thickness tolerances between + (0.15 – 0.225) t_o -0.125, with a maximum tolerance bandwidth of 0.35 t_o. In addition to the outer diameter tolerance, if the above clearance is only determined according to the nominal specification, likely, some pipes that meet the standard dimensional tolerance of the specification are difficult to control the actual clearance within the range required, so wrinkling and other problems occur when bending.
It should be noted here that:

• (1) Due to the different production process details of steel pipes in different manufacturing factories, the tolerance zones of the same specification of steel pipes may be different. Steel pipes of the same specification and steel grade produced by the same factory may also have upper or lower tolerance zones due to different furnace numbers and chemical composition of raw materials, which are normal phenomena.
• (2) The pricing method of supply may result in deviation of the tolerance zone. Early national standards were based on actual quality pricing, which could easily lead manufacturers to supply within tolerance zones to achieve the highest sales revenue. The current national standard already allows pricing based on length or quality. If pricing based on length is used, it may cause steel pipe manufacturers to supply according to tolerances to achieve maximum economic benefits. The US standard always requires pricing by length, and the reasons for this have been analyzed in the literature. For marine stainless steel pipes, increasing their weight is equivalent to reducing the effective tonnage of the ship; therefore, 17.4 × 10⁴m³ of LNG and other marine stainless steel pipes are ordered according to length. But suppose the unit length price is still converted based on the standard weight during pricing. In that case, the manufacturing plant may not have realized the benefits of supplying the following tolerance zones, so many factories will still produce and supply according to the old tradition.
• (3) The difference in mechanical properties of steel pipes cannot be ignored. The mechanical properties of steel pipes of the same specification and steel grade produced by different factories, as well as the mechanical properties of steel pipes of different batches and specifications produced by the same factory, may vary due to differences in the actual temperature, insulation time, and cooling rate of the raw material furnace number and the final heat treatment during the steel pipe manufacturing process. The literature has measured the differences in actual yield strength and elastic modulus of three similar specifications of 304 steel pipes, which are all normal phenomena. The mechanical performance indicators specified in all stainless steel product standards are low. For example, the elongation (A) is generally only 40%, and many austenitic stainless steel pipes have an upper limit of 55% -60% elongation on the quality assurance certificate. The corresponding tensile strength and hardness also fluctuate accordingly. Literature indicates that to prevent defects such as wrinkling in pipe bends, corresponding requirements for hardness have also been put forward. The literature points out that thin-walled bend pipes not only strictly control the mold gap but also should use the same batch and preferably the same heat number of steel pipes of the same specification. The occurrence of wrinkling waves in the production of pipe bends is a normal situation, and the reason may also involve differences in mechanical properties in addition to the dimensional tolerance of the steel pipe. Therefore, when bending “thin” wall pipes, it may be a reasonable approach to agree on hardness to ensure the quality of the bends. The equivalent conversion of hardness and strength for carbon steel and alloy steel is shown in Table 11.

Table.11 Equivalent Conversion of Hardness and Strength for Carbon Steel and Alloy Steel (1)

HB	HV	HRB	Tensile strength/MPa
241	253	100	800
217	228	96.4	728
197	207	92.8	655②
179	188	89	600
159	167	83.9	538
143	150	78.6	490②
131	137	74.2	448
110	122	67.2	400

Note: (1) The data in this table is taken from ASTM E140; (2) The literature does not limit hardness but specifies a tensile strength of 490-690MPa corresponding to HRB of 78.6-94.6; For ‘thin’ wall bends, the range of hardness variation may be too large.

4.5 Measures to prevent wrinkling of pipe bends

From the above analysis, it can be seen that the causes of wrinkling problems in the production of stainless steel pipe bend are complex, including factors such as insufficient understanding of the structure of the pipe bending machine, improper control of mold accuracy and clearance, and lack of maintenance and upkeep of the mold, as well as factors such as performance and dimensional tolerances generated during the steel pipe manufacturing process. To effectively prevent bending wrinkling, the following aspects should be considered.

4.5.1 Strictly control the dimensional tolerance of steel pipes

To effectively prevent bending wrinkling, the first step is to control the dimensional tolerance of the delivered steel pipe and select a reasonable range of dimensional tolerances.
The manufacturing factory of the 17.4×10⁴m³ LNG transport ship has proposed a clear “low-temperature stainless steel pipe ordering manual.” This manual is a document accompanying the introduction of ship manufacturing technology in the 1990s. The reference standards listed in the manual include ASTM A312 and ASTM A530. The later ASTM A999/A999M standard was separated from ASTM A530 in 1996. Therefore, after 1996, the ASTM A312 and ASTM A999 standards should be used as the basis for the execution of pipes for low-temperature stainless steel pipelines.
The “Low-Temperature Stainless Steel Pipe Ordering Manual” stipulates that the dimensional tolerances for stainless steel pipes below DN300 are shown in Table 12. Compared to Table 9, it can be seen that the tolerance requirements for wall thickness, inner diameter roundness, etc., in this manual are significantly higher than those of ASTM A312 and ASTM A999 standards, which are very beneficial for preventing bending wrinkling.
Although the “Low-Temperature Stainless Steel Pipe Ordering Manual” places high requirements on the dimensional tolerance of stainless steel pipes, it is not easy to achieve such tolerance requirements. Firstly, when replacing welded pipes with seamless pipes, it isn’t easy to achieve a tolerance zone of (0, -12.5%t_o) wall thickness. The literature indicates that welded pipes are preferred for ordered steel pipes, but seamless pipes can be used instead, indicating that welded pipes are more likely to meet this requirement. Secondly, the requirement of inner diameter roundness tolerance ID_max – ID_min ≤ 1% (d_o – 2t) in the manual is slightly higher for thin-walled pipes with t_o/d_o ≤ 3% compared to the requirement of outer diameter roundness tolerance OP_max – OD_min ≤ 1.5%d_o in the ASTM A312 standard. However, the roundness requirement measured by outer diameter is more stringent for thick walled pipes.
Table.12 “Low-temperature stainless steel pipe ordering manual,” specifies the dimensional tolerance/mm for stainless steel pipes below DN300.

Straightness	Branch length	Wall thickness (at any point)	Perimeter	Inner diameter roundness	Misalignment	Outside  diameter
						≤DN40	DN50-DN100	DN125-DN300
W 3.2/3 000	–0,+6	–12.5%to	±0.5e	IDmax–IDminW 1%(d°–2t°)	±0.1e,0.4 Max	+0.4,–0.8	±0.8	+1.6,–0.8

The control of inner diameter roundness proposed in the “low-temperature stainless steel pipe ordering manual” is also difficult: (1) if the inner diameter roundness cannot be accurately measured, there is no control of inner diameter roundness; (2) Although controlling the roundness of the inner diameter is beneficial for the insertion of the core rod during bending, as long as the wall thickness, outer diameter, and perimeter tolerances are reasonably controlled, the roundness tolerance of thin-walled pipes can be easily “corrected” and has no effect on the insertion of the core rod during bending; (3) When cold bending thick walled pipes, there is no need for a core rod, and the gap between the outer diameter and the bending die and pressure block is more important for the quality of the bend. Only controlling the inner diameter roundness without specifying concentricity or uneven wall thickness is not beneficial for improving the quality of the bend; (4) From the requirements for misalignment in Table 13, it can be seen that this is an additional requirement specifically for welded pipes.
If the use of seamless pipe bending, we must pay attention to the problem of large tolerances on the wall thickness of the seamless pipe. The above Φ88.9 mm × 3.05 mm specification steel pipe using 80 mandrels outside diameter of the situation fully illustrates this point.

4.5.2 Reasonable Selection of Configuration Parameters

According to the size of the t_o/d_o, R_o/d_o value, the reasonable selection of cold bending tooling, optimizing the gap configuration, adjust the necessary parameters can improve the quality of the pipe bend.

• (1) A single or multi-ball flexible mandrel is preferable for thin-walled pipes of R_ot_o/d_o²≤10.5.
• (2) Reasonably configure the gaps between core rods, pressing blocks, bending molds, etc. For stainless steel thin-walled pipes with b=8.5%, it is also necessary to detect the wear on the bottom of the bending die and the driving shaft. A rigid core rod can also be used during bending to prevent wrinkling.
• (3) Reasonably adjust parameters such as bending speed and block pressure based on the actual tolerance of the steel pipe. Sometimes reducing the bending speed can effectively eliminate wrinkles. Still, the prerequisite is that the operator must have sufficient experience and judgment ability and understand the structure and performance of the bending machine being operated. The data in Table 8 and Figure 14 indicate that the appearance of wrinkles may also be related to the setting of pressure and boost conditions. The literature indicates that as long as the parameters of the pipe bending machine are adjusted reasonably, the pressure required for the pressure block when bending thin-walled pipes is very small, but this is not the case in practical operation for various reasons.
• (4) Improve the structure of the pipe bending machine. For example, using a steel pipe tail to directly assist instead of the original pressure block to assist (see Figure 9 (b)) or setting a front and rear oscillation type pushing device for the core rod. The literature points out that this oscillation-type pushing device helps to push high-precision multi-spherical core rods into steel pipes and indicates that the oscillation frequency is 1-500 cycles/min, and the amplitude is adjustable from 3.2-25mm. This oscillating pushing device is an auxiliary mechanism worth adding to the thin-walled pipe bending machine.

A shipyard in Shanghai manufactures 8.4×10⁴m³ LNG transport ships, and the required stainless steel pipes were ordered according to the ASTM A312 standard. Corresponding measures were taken during the bending process according to the above methods, effectively preventing the generation of wrinkles during the bending process. As long as the cold bending mold is reasonably selected, the gap configuration is optimized, and the necessary parameters are adjusted, the problem of bending wrinkling can be solved.

4.6 Selection of core rod length and stiffness

The outer diameter of the front end of the rigid core rod is determined by the inner diameter of the steel pipe. The literature points out that the core rod used in thin-walled bending should be thick enough, which means that the stiffness or deformation resistance of the core rod should be good enough. This requires that: (1) the length of the mandrel cannot be very long, so it is not advisable to select a long mandrel to meet the requirement of bending at the end of the long tube, but it is reasonable to place the long tube on the clamping side (the scheme requiring a large Radius of gyration); (2) Even for short core rods, the outer diameter of the other parts can only be slightly reduced except for the maximum outer diameter at the end that needs to be precision machined. It is not advisable to draw the diameter of the second half of the core rod very finely in many literature figures (such as Figure 9 (b)).

4.7 Characteristics of Marine Bend Production

Many literatures have pointed out that anti wrinkle blocks are an effective means to prevent wrinkling waves in thin-walled bends, but in producing marine bends, anti wrinkle blocks still need to be installed. Based on production practice, the reasons for not setting anti wrinkle blocks when bending pipes are analyzed as follows: (1) The anti wrinkle blocks must achieve sufficient installation accuracy. Otherwise, their effect is not significant, and it will increase the risk of scratches on the outer surface of stainless steel pipes; (2) As the diameter of the bend increases and the pressure of the pressure block increases, the wear of the anti wrinkle block will intensify. After wear, the installation position of the anti wrinkle block must be adjusted promptly. Otherwise, it will not have its intended effect; (3) Small batch and multi-specification pipe bending machines are often mixed with low-carbon steel pipes and stainless steel pipes, with the former generally having a large t_o/d_o ratio and therefore no need for anti wrinkle blocks; (4) The wrinkle resistant block in Figure 8 requires lubrication with light oil. Otherwise, it may be counterproductive; The installation method shown in Figure 9 may cause a vibration of the anti wrinkle block and affect the anti wrinkle effect.
The above analysis indicates that the reasonable application of wrinkle resistant blocks may only (more) be suitable for the intensive large-scale production of thin-walled pipe bends of a single variety and specification.

4.8 Low temperature stainless steel cold bending elbows must undergo solution annealing treatment

According to the literature, the purchased stainless steel pipes for low-temperature use must meet the requirements of the -196 ℃ low-temperature impact test (41J). Therefore, stainless steel pipes must be supplied after solution annealing. After cold bending, the arch back of the elbow undergoes varying degrees of cold-worked tensile plastic deformation. In contrast, the arch belly undergoes varying degrees of compressive plastic deformation, indicating that the steel pipe material has undergone varying degrees of embrittlement (see Figure 16). Therefore, elbows working in low-temperature environments should undergo solution annealing treatment before use. The American standards ASTM SA403 and ASTM SA815 also stipulate that bends can be made using the cold bending method but must be supplied after solution annealing treatment.

Figure.16 304 stainless steel under different degrees of cold working σ-ε curve

4.9 Pressure and Boosting Speed of Thin-walled Stainless Steel Bend

The data in Table 8 indicates that when using a four-ball flexible core rod as a thin-walled austenitic stainless steel bend, it is not advisable to use larger pressure blocks, as well as larger pressure and boost speeds. Otherwise, although the wall thickness reduction of the arch back can be reduced, it will inevitably increase the wall thickness increase of the arch belly and generate serious wrinkles. The reason is that the excessive pressure of the compression block causes the arch web at the starting point of the bend to bear high compressive stress, and the pressure brought by the auxiliary force still makes the thickened arch web not having sufficient compressive stability, resulting in wrinkling.
It should be noted that (1) in the booster model shown in Figure 9 (a), the booster is only used to minimize the sliding between the pressure block and the outer surface of the steel pipe to make v_p≌v_o. In general, boosting is not an independent adjustment parameter, but in the steel pipe tail boosting model shown in Figure 9 (b), boosting is a completely independent adjustment parameter of the pressure block; (2) The interface friction coefficient determined by the surface roughness and lubrication conditions of the pressure block has a decisive impact on pressure regulation. The smoother the surface and the more sufficient the lubrication, the greater the required pressure. However, rough pressure blocks can scratch the surface of the steel pipe. Therefore, accurately grasping the degree of lubrication is crucial for pressure regulation.
4.10 Φ 88.9mm × 3.05mm stainless steel welded pipe is difficult to meet the supply requirements for ships
The literature indicates that the ordered steel pipes can be preferentially welded according to the ASTM A312/A312M standard. However, the welded pipes currently produced in China still need to meet the corresponding supply requirements. The main reason may be:
(1) Stainless steel welded pipes with a wall thickness of 3.0-3.5mm below DN100 are difficult to achieve single-sided welding and double-sided forming using the single arc GTAW method while ensuring stable weld forming quality.
(2) By using PAW or multi-cathode GTAW welding methods, although it is possible to achieve stable weld forming quality, certain additional conditions must be met: (1) Strict steel strip width tolerance and notch perpendicularity; (2) The optimal working state of the molding unit; (3) Strictly control welding arc parameters and welding conditions.
When welding stainless steel welded pipes, it is necessary to strictly control the width tolerance of the steel strip and the perpendicularity of the incision to ensure the uniformity of the weld interface gap and reduce the left and right drift during welding. At this time, it is necessary to add a steel strip milling device. To maintain the molding unit in its optimal working condition, timely maintenance and upkeep of the molding machine and the provision of experienced molding personnel are also necessary. Strictly controlling welding parameters and conditions, and maintaining stable welding speed, also helps to achieve stable weld forming quality. Although three-cathode GTAW welding can achieve high welding speed, its stability is poor. The double cathode or PAW + GTAW, double arc welding, is a relatively effective and stable welding method. PAW + GTAW welding can ensure the forming quality of the back and front sides and achieve a certain welding speed. This welding method is worth recommending. The biggest advantage of PAW + GTAW welding is that it can reduce the stability of the weld quality of the same steel due to the difference in furnace number and Surface states fluctuation, and the undercut tendency of the front weld is also very low.

5. Conclusion and Suggestions

• (1) Cold bending is a common and simple method for making pipeline elbows. Although it has shortcomings such as uneven wall thickness, roundness distortion, rebound, and arch web wrinkling, it does not affect general fluid conveying pipelines under internal pressure conditions. Therefore, cold bending is often regarded as the main and practical method for elbow manufacturing.
• (2) The excellent plasticity (margin) of austenitic stainless steel pipes makes applying the cold bending method more common. Both domestic and international pipeline standards allow austenitic stainless steel bends with R_o/d_o ≥ 1.5 to be directly applied in the cold bending state. Still, such bends that retain uneven cold working degrees should only be applied under static load conditions at room temperature. Under stress corrosion and low-temperature conditions, austenitic stainless steel bends still require heat treatment, preferably after solid solution heat treatment before use. Under conditions such as alternating load and high-temperature creep, austenitic stainless steel cold bending pipes are no longer suitable, and elbows manufactured by die pressing + welding method or forging + machining method with higher wall thickness uniformity must be used.
• (3) The mold set reasonable bending pipe bender can achieve a variety of relative radius (t_o/d_o) and relative bending radius (R_o/d_o) of Φ219mm below the cold bending of stainless steel pipe. Generally speaking, to ensure the quality of cold bending, especially to control roundness and avoid corrugation: when the to/do is smaller or R_o/d_o is less than a certain value, a reasonably designed rigid mandrel or a flexible mandrel with a ball hinge must be used; Only to/do and R_o/d_o are large enough for thick wall bending can be performed without mandrel cold bending. A specially designed single or multi-ball flexible mandrel prevents corrugation during bending for cold bending of thin-walled stainless steel pipes with a particularly small to/do or a sufficiently small Ro/do. b=R_ot_o/d_o²≤10.5%, 10.5-17.5%, >17.5% can be used as the rough boundary assessment values of the thin wall of the flexible mandrel, rigid mandrel, and non-mandrel, respectively.
• (4) After the structure of the bending machine is determined, the gap parameters determined by the precision of mold manufacturing and installation adjustment may undeniably impact the quality of the pipe bend, especially the tendency for wrinkles to occur. In the production of small batches and multiple specifications of pipe bends, if wrinkling occurs, the gap between the outer diameter of the core rod and the inner diameter of the steel pipe, bending mold, pressing block, anti-wrinkling block, clamping block inner diameter and the outer diameter of the steel pipe should be checked item by item to see if it is too large, and if the installation position of the core rod end and the anti wrinkling block is appropriate. For equipment that has been used for a long time, special attention should be paid to detecting excessive wear on the bottom of the bending mold and its transmission shaft diameter.
• (5) The operating parameters, such as bending speed and block pressure, also impact bending wrinkling. Reducing the bending die speed or pressure appropriately can sometimes significantly reduce bending wrinkling. But pressure regulation involves friction and boosting methods, requiring operators to have sufficient experience and accumulation.
• (6) The current stainless steel tube manufacturing standards provide for a tolerance zone 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 specification” provides under the tolerance zone for wall thickness tolerance has a certain degree of rationality. But the proposed tolerance of the inner diameter roundness is because of the difficulty of actual measurement and lack of practical value.
• (7) Marine stainless steel pipes are delivered by U.S. standards by length; this delivery method will help guide the steel pipe manufacturer according to the wall thickness under the tolerance zone delivery but is also conducive to increasing the ship’s payload. The national standard retained by the quality of delivery measurement is still inappropriate and should be removed early this reservation and thoroughly measured by the delivery length.
• (8) Although the to and do tolerance zones of stainless steel pipes may cause some trouble for thin-walled stainless steel bends, as long as the core rod type is selected reasonably and attention is paid to the adjustment accuracy of mold manufacturing and installation, its impact is not significant. In recent years, the production practice of large-scale application of stainless steel bends in shipyards has confirmed this.
• (9) Adopting a single rigid core rod with multiple specifications (one or similar R_o/d_o but multiple t_o/d_o) adjustable bending machine structure and a bending production method without distinguishing steel grades can save production investment costs. Still, it is not suitable for thin-walled stainless steel pipe bending. It is necessary to set up a dedicated, flexible core rod with single or multiple ball hinges. The length of the core rod of this type of pipe bending machine should not be too long. The end of the core rod should use a chrome-plated surface, while the ball should use aluminum bronze with a w (Al) of 5.0% -6.5% and aluminum brass with a w (Ze)>7% and w (Al)<2.5% should not be used.
• (10) Based on and determining the thin-wall of the bend, the internationally popular bend nomogram or b=R_ot_o/d_o² bend thin-wall can quickly search for the appropriate core rod structure required for bending by the bending machine. However, since the selection of this graph did not fully consider the influence of parameters such as gap and bending speed, its query results can only be used as a relative reference, especially near the boundary of the nomogram. For example, for a bend of Φ88.9 mm×3.05 mm with R_o/d_o=2.5, using a single ball flexible mandrel is a more reasonable result of the inquiry. However, if a rigid mandrel is used in the bend, as long as the clearance and the installation position of the mandrel end are strictly controlled, the bend is still qualified.
• (11) Increasing the pressure and boosting the speed of the pressure block will significantly reduce the wall thickness reduction of the arch back during bending. Still, at the same time, it will inevitably increase the wall thickness increase of the arch belly and may also increase wrinkling during thin-walled stainless steel bending. Therefore, when bending stainless steel thin-walled bends, lower block pressure and boost speed should be chosen as much as possible.
• (12) The wrinkling phenomenon at the arch belly during the bending process of thin-walled pipe bends is essentially caused by the insufficient compressive stiffness in the plastic deformation zone under compression, resulting in the buckling of the compression bar (bending). From a mechanical perspective, a reasonable selection of mold parameters, especially the core rod’s structural form, size, and placement position, can improve the arch or micro-bending section’s compressive stability (coefficient) and is an effective measure to prevent wrinkling waves in pipe bends.
• (13) Stainless steel welded pipes with a wall thickness of 3.0-3.5mm and DN100 or less are difficult to form with a single support, and GTAW welding is difficult to achieve ideal weld seam forming quality; The use of continuous rolling forming and PAW welding methods to manufacture welded pipes also requires extremely high requirements for steel strip width and notch straightness. Otherwise, achieving ideal interface clearance, achieving PAW welding of low misalignment interfaces, and obtaining stable weld quality in forming is difficult. How to break through this bottleneck is worth the attention of domestic stainless steel welded pipe manufacturers.

Author: He Defu