Cold bending process of duplex stainless steel bend

This paper briefly introduces the problems encountered in the cold bending process of duplex stainless steel bend and its causes, and puts forward solutions and suggestions.

Duplex stainless steel is a kind of stainless steel with both austenite and ferrite structure. This kind of steel has the characteristics of both austenitic and ferritic stainless steel, and has good corrosion resistance. It is suitable for high corrosive medium such as seawater and some acid. Moreover, its yield strength and tensile strength are large, so it can be used for cold bending. Therefore, the application of duplex stainless steel pipe in marine pipeline system is becoming more and more popular.

However, for the commonly used duplex stainless steel pipe, its wall is usually thin and its yield strength is high, so it is difficult to bend and easy to scrap. The following is a brief introduction to the common problems in the production process of duplex stainless steel bend and its cause analysis, and puts forward some solutions and suggestions.

Characteristics of duplex stainless steel pipe

Chemical composition 

The chemical composition of the material is shown in Table 1.

Table.1 chemical composition of duplex stainless steel pipe

C Mn P S Si Cr Ni Mo N Cu
UNS S32101 0.03max 2.0max 0.03max 0.03max 1.0max 20.0~23.0 1.0~3.0 0.6max 0.05~0.17 1.0max
UNS S31803 0.03max 2.0max 0.03max 0.02max 1.0max 21.0~23.0 4.5~6.5 2.5~3.5 0.08~0.20 /
UNS S32205 0.03max 2.0max 0.03max 0.02max 1.0max 22.0~23.0 4.5~6.5 3.0~3.5

0.14~0.20

/
UNS S32750 0.03max 1.2max 0.035max 0.02max 0.8max 24.0~26.0 6.0~8.0 3.0~5.0 0.24~0.32 0.5max

Mechanical properties

The mechanical properties of the materials are shown in Table 2.
Table.2 mechanical properties of duplex stainless steel pipe

Tensile Str
(MPa) min

Yield Strength
0.2% Proof
(MPa) min

Elongation
(% in 50mm) min

Hardness

Rockwell C (HR C)

Brinell (HB)

621

448

25

31 max

293 max

Specification and wall thickness

The specification and wall thickness of common duplex stainless steel pipe are shown in Table 3.
Table.3 specification and wall thickness of duplex stainless steel pipe

Norminal Pipe Size

Outside Diameter

Norminal Wall Thickness

NPS

BN

D

SCH5s

SCH10s

SCH10

SCH20

SHC30

SCH40s

STD

SCH40

SCH60

SCH80s

XS

SCH80

SCH100

SCH120

SCH104

SCH160

XXS

1/8

6

10.3

1.24

1.73

1.73

1.73

2.41

2.41

2.41

1/4

8

13.7

1.65

2.24

2.24

2.24

3.02

3.02

3.02

3/8

10

17.1

1.65

2.31

2.31

2.31

3.20

3.20

3.20

1/2

15

21.3

1.65

2.11

2.77

2.77

2.77

3.73

3.73

3.73

4.78

7.47

3/4

20

26.7

1.65

2.11

2.87

2.87

2.87

3.91

3.91

3.91

5.56

7.82

1

25

33.4

1.65

2.77

3.38

3.38

3.38

4.55

4.55

4.55

6.35

9.09

1 1/4

32

42.2

1.65

2.77

3.56

3.56

3.56

4.85

4.85

4.85

6.35

9.70

1 1/2

40

48.3

1.65

2.77

3.68

3.68

3.68

5.08

5.08

5.08

7.14

10.15

2

50

60.3

1.65

2.77

3.91

3.91

3.91

5.54

5.54

5.54

8.74

11.07

2 1/2

65

73.0

2.11

3.05

5.16

5.16

5.16

7.01

7.01

7.01

9.53

14.02

3

80

88.9

2.11

3.05

5.49

5.49

5.49

7.62

7.62

7.62

11.13

15.24

3 1/2

90

101.6

2.11

3.05

5.74

5.74

5.74

8.08

8.08

8.08

4

100

114.3

2.11

3.05

6.02

6.02

6.02

8.56

8.56

8.56

11.13

13.49

17.12

5

125

141.3

2.77

3.40

6.55

6.55

6.55

9.53

9.53

9.53

12.7

15.88

19.05

6

150

168.3

2.77

3.40

7.11

7.11

7.11

10.97

10.97

10.97

14.27

18.26

21.95

8

200

219.1

2.77

3.76

6.35

7.04

8.18

8.18

8.18

10.31

12.70

12.70

12.70

15.09

18.26

20.62

23.01

22.23

10

250

273.1

3.4

4.19

6.35

7.8

9.27

9.27

9.27

12.70

12.70

12.70

15.09

18.26

21.44

25.4

28.58

25.4

12

300

323.9

3.96

4.57

6.35

8.38

9.53

9.53

10.31

14.27

12.70

12.70

17.48

21.44

25.4

28.58

33.32

25.4

14

350

355.6

3.96

4.78

6.35

7.92

9.53

9.53

11.13

15.09

12.70

19.05

23.83

27.79

31.75

35.71

16

400

406.4

4.19

4.78

6.35

7.92

9.53

9.53

12.7

16.66

12.70

21.44

26.19

30.96

36.53

40.49

18

450

457.2

4.19

4.78

6.35

7.92

11.13

9.53

14.27

19.05

12.70

23.83

29.36

34.96

39.67

45.24

20

500

508.0

4.78

5.54

6.35

9.53

12.7

9.53

15.09

20.62

12.70

26.19

32.54

38.1

44.45

50.01

22

550

558.8

4.78

5.54

6.35

9.53

12.7

9.53

22.23

12.70

28.58

34.93

41.28

47.63

53.98

24

600

609.6

5.54

6.35

6.35

9.53

14.27

9.53

17.48

24.61

12.70

30.96

38.89

46.02

52.37

59.54

26

650

660.4

7.92

12.7

9.53

12.70

28

700

711.2

7.92

12.7

15.88

9.53

12.70

30

750

762.0

6.35

7.92

7.92

12.7

15.88

9.53

12.70

32

800

812.8

7.92

12.7

15.88

9.53

17.48

12.70

34

850

863.6

7.92

12.7

15.88

9.53

17.48

12.70

36

900

914.4

7.92

12.7

15.88

9.53

17.48

12.70

38

950

965.2

9.53

12.70

40

1000

1016.0

9.53

12.70

42

1050

1066.8

9.53

12.70

44

1100

1117.6

9.53

12.70

46

1150

1168.4

9.53

12.70

48

1200

1219.2

9.53

12.70

52

1300

1321

56

1400

1422

60

1500

1524

64

1600

1626

68

1700

1727

72

1800

1829

76

1900

1930

80

2000

2032

Note: sch5s, 10s, 40s, 80s are used for stainless steel. SCH5s、10s、40s、80s applies for stainless steel.

It can be seen from table 2 that the yield strength of duplex stainless steel pipe is 448 MPa, which is 2-3 times of that of ordinary steel.

Working principle of cold bending of bend

20200722061400 56274 - Cold bending process of duplex stainless steel bend

FIG.1 Schematic diagram of cold bending of bend

As shown in Fig.1, put the straight pipe to be bent into the bending die, align the bending starting point with the bending die center, clamp the first section of the pipe with the sliding plate close to the tail section of the pipe. Starting the pipe bender, the pipe is clamped in front of the pipe for circular motion, and the sliding plate moves in a straight line along the tangent direction driven by the pipe to exert external force on the pipe. When the external force exceeds the yield limit, plastic deformation will occur, which is the basic principle of tube bending.

Problems in the manufacturing process of bend

  • (1) Movement occurs and skid;
  • (2) Internal surface strain;
  • (3) Wrinkles appear on the surface
  • (4) Pipe wall pulling crack, etc.

Cause analysis

  • (1) Improper process method. In the process of pipe bending, the operators often do not pay enough attention to the control of the process flow of pipe bending, usually adopt the method of carbon steel pipe bending, and do not develop the relevant work instructions for the dual-phase stainless steel pipe bend.
  • (2) The bending personnel’s work responsibility and bending skill level are uneven, and their understanding of process knowledge, operation skills and equipment knowledge is insufficient.
  • (3) Dual-phase stainless steel pipe‘s yield strength, and small surface roughness, bending easy to slip.

The following is a brief analysis of the force in the bending process, as shown in Figure 2.
In analyzing the bending force, the pipe can be divided into a bent pipe section (AB) and a straight pipe section (BC). The A end is clamped by the front clamp, and the AB segment is bent by the bending moment MR, as shown in FIG. 2(A). While at bending point B is affected by bending moment MR and bending die pressure P, and at point C by sliding plate supporting force P, as shown in FIG. 2(B).
During the bending process, the cross-sectional area of the pipe also changes due to the bending moment, which is related to the friction force F between the chuck and the pipe, between the pipe and the bending die, and the supporting force P between the pipe and the slide plate. However, during the bending process, the outer surface of the pipe is not in complete contact with the bending die and the slide plate.
The bending moment MR required to produce plastic deformation of the pipe is directly proportional to the yield strength of the pipe. Therefore, the torque required for the bending of duplex stainless steel pipe should be 2-3 times that of ordinary steel pipe.

  • MR – F * R

In the formula: F — static friction force (N) on the first section of the pipe;
R – Bending die radius (mm).

Due to the radius of the bending die is certain, therefore, the first phase of the bending stainless steel tube subjected to static friction force should be 2-3 times that of ordinary steel. The positive pressure N is proportional to the clamping force.

20200722063014 75146 - Cold bending process of duplex stainless steel bend

FIG.2 Simplified diagram of bending forces
According to the theory of static friction force, the maximum static friction force suffered by the object is F = µ*N.
In the formula:
µ- Coefficient of friction
N- positive pressure (N)
The friction coefficient is closely related to the roughness of the material and contact surface. The positive pressure N is proportional to the clamping force.

The solution

Through the analysis of the above reasons, the following solutions can be chosen.

Large coefficient of friction

According to tribology theory, when two surfaces with normal roughness come into contact, the friction coefficient is generally less than 1′. When the two surfaces become very rough, the friction coefficient will greatly increase. Therefore, to increase the coefficient of friction, it is necessary to increase the roughness of the collet between the pipe and the clamped pipe.
For pipe, the method of sand extraction is generally used to increase the roughness. Ordinary pressure sand, roughness increase is not obvious, the effect is not big. If a special method is used to sand, although the roughness can be significantly increased, it will damage the surface quality of the pipe, beyond the standard range, resulting in the pipe scrap. Therefore, increasing the surface roughness of the pipe is not a good solution. But the roughness of bending die can be increased to increase the friction coefficient.

Increase positive pressure

Increase the contact area, or increase the clamping force of the chuck or increase the friction to improve the effect.
In addition, the way to increase the positive pressure is to insert the mandrel at the chuck, but to control the mandrel insert length. In order to increase the positive pressure on the pipe, the pressure applied to the pipe by the pipe bender should be adjusted. In order to solve such problems, insert mandrel at the chuck and adjust the length of mandrel extending into the pipe. The distance between the mandrel and the cutting point of bending die should be neither too far forward nor too low.

Adopt special tooling

Because the wall of duplex stainless steel tube is thin, it is easy to crack or fold. In order to prevent the occurrence of such problems, it is an effective process method to adopt special tools such as bending die, mandrel, anti-wrinkle board, pipe clamp, lubricant and sandpaper.
Pipe bending may fail because of slipping, especially in stainless steel pipe bending more than 76, the pipe may slip and make the pipe deformation scrap phenomenon. The main reason is that the press die of the pipe bender cannot move forward automatically with the pipe bending deformation. Skidding can also scratch the pipe, so it should be prevented.
In order to prevent slippage, an anti-slippage strip can be welded on the edge of the pipe close to the front clip. Although this method can prevent the pipe from slippage, it is very time-consuming and laborious, and does harm to the pipe surface, so this method is not advisable.
There are many ways to prevent skidding, among which making the tooling and strengthening the lubrication inside the pipe is a feasible and simple method. First of all, an auxiliary fixture was designed. The idea of the fixture design was to mill out the anti-skid groove in the front collet and then build another active collet to clamp the front end of the collet of pipe bending equipment, which was equivalent to increasing the area of the front collet and making the front collet tighter. However, only using this method to prevent the effect of slipping may not be obvious, because the pressure plate can not move forward with the pipe bending deformation also increases the possibility of slipping, so if lubrication is added in the pipe, the result is bound to be further improved. Can be used to spray oil into the pipe, to increase its lubrication, and between the pipe and the pressure plate can be placed on emery cloth, so that the pressure plate more forward.

Design and use of tooling

For different pipe diameters, anti-wrinkle board and pipe clamping tooling are respectively set up, as shown in Figure 3 and Figure 4. The tool, combined with snow oil, graphite oil and oil, carefully prepared lubricant to ensure the good lubrication of the inner wall and core of the pipe, and sandpaper covering the pipe clamp, so as to solve various problems in the pipe bend.
20200722063710 98638 - Cold bending process of duplex stainless steel bend

FIG.3 Production diagram of anti-wrinkle board

20200722063745 50838 - Cold bending process of duplex stainless steel bend

FIG.4 Front clip drawing

Some Suggestions

In order to improve the quality of bend products and reduce the scrap rate, in addition to adopting feasible process methods, attention should also be paid to the relevant preconditions. Therefore, the following Suggestions are put forward:

Enhance operator skills

The operator’s responsibility and bending skill is one of the important factors for the success of pipe bending. However, in the current situation, the skill level of bending personnel is often uneven, and the quality awareness is not strong. Therefore, before the bending control, the process technicians should tell the operators about the process, analyze its importance and difficulties in detail, and train them with process knowledge, operation skills and equipment knowledge theoretically, so as to improve their theoretical and technical level. At the same time, it is required to observe more in the process of operation and adjust the problems in time.

Ensure the integrity of the production equipment

In order to ensure the quality of pipe bending, the good working condition of pipe bending machine should not be neglected. Before the bending work, the moving parts of the pipe bender (such as bending die, sliding block, mandrel, etc.) should be overhauled and adjusted comprehensively to create conditions for successful thin-wall pipe bending.

Make pipe bend operation instruction

Before bending work, appropriate operating instructions should be prepared to clarify relevant matters needing attention, operating procedures and preconditions, etc.

Select high quality pipes

As the quality of pipe is a very important factor for the success of bending, when selecting the pipe to be bent, the quality should be strictly controlled, the pipe should be carefully checked from inside to outside, to ensure that there are no cracks, interlayers, scars, burns and other defects, and the pipe diameter, ellipticity, straightness and other measurements.

Strictly follow the process procedure

Usually follow the following process:

  • (1) According to the SPECIFICATION of DSS pipe, the bending die and core head are selected and adjusted properly;
  • (2) Select the appropriate anti-wrinkle board according to the DSS tube specifications and install it. During installation, the front end should be as close as possible to the cutting point of bending die;
  • (3) After installing the DSS tube anti-wrinkle board, the clamping distance of the pipe bender should be adjusted first. Proper adjustment directly affects the bending quality star. Therefore, after repeated practice and exploration, feasible positions should be marked on the pipe bender or the working pressure value of the equipment should be recorded.
  • (4) Check whether the inner and core of the stainless steel tube are clean and free from sundries. If there are sundries, please clean them.
  • (5) Lubricants made by mixing prepared snow oil, graphite oil and motor oil are evenly applied on the core head to ensure lubrication of the bending contact friction parts inside the stainless steel tube;
  • (6) Pay attention to bending. Since the pipe produced by different manufacturers has different hardness characteristics, it is necessary to slowly control the equipment during bending so as to adjust it at any time. When bending to about 25°, it can be completed without any problems.

Quality evaluation of bend finished products

Appearance quality

There should be no obvious dents caused by scratches and collisions on the pipe wall, and no cracks, scars, burns, delamination and other defects in the pipe bend. The thinning of the wall thickness of the eliminated part shall be within the allowable range of the wall thickness. Otherwise, it shall be scrapped.

The thinning rate of wall thickness

Steel tube :≤25%(cold bending) ≤10%(hot bending)
Copper pipe :≤30%(cold bending) ≤15% (hot bending)
Thinning rate of wall thickness :(t-tl)/t x 100%
In the formula:

t — the actual average thickness of the pipe wall before bending (mm);
tl — Thickness (mm) at the thinnest point of the pipe wall after bending.

0vality

  • Cold bending: ≤10 %
  • Ovality: (A-B)/DW x 100%

In the formula:

DW — actual outside diameter (mm) of pipe;
A — Maximum outside diameter of section at bending point (mm);
B — Minimum outside diameter (mm) of section at bending point.

The fold of the wall

Uniform creasing is allowed after bending the pipe, but the creasing height should not exceed 3% of the pipe diameter, and the maximum limit should not exceed 5%.

Shrinkage of the pipe

Shrinkage: 1-(A+B)/(2DW) x 100%≤5%

The process method is validated

For the above process methods and assumptions, the maximum test has been carried out in the course of bending control of a special ship, and the demonstration has been obtained.

The validation process

On a special ship, through repeated tests, the correct mandrel insertion length and bending die cutting distance parameters of stainless steel pipe bending of each specification were finally summarized, as shown in Table 3.
Table.3 Insert length parameters of stainless steel tube mandrel

Specifications (mm) Processing equipment Tangent distance (mm) Mandrel insertion length (mm)
Duplex stainless steel seamless pipe 22x2 Shanghai awards new machines -(The device pressure is adjusted properly, without mandrel bending) -(The device pressure is adjusted properly, without mandrel bending)
Duplex stainless steel seamless pipe 27x2 Shanghai awards new machines
Duplex stainless steel seamless pipe 34x2 Shanghai awards new machines
Duplex stainless steel seamless pipe 42x2 Shanghai awards new machines
Duplex stainless steel seamless pipe 48x2 Shanghai awards new machines
Duplex stainless steel seamless pipe 60x3 Shanghai awards new machines
Duplex stainless steel seamless pipe 76x3 Shanghai awards new machines 1 8
Duplex stainless steel seamless pipe 76x3.5 Changzhi 4 machine 1.2 8
Duplex stainless steel seamless pipe 88.9x3.5 Changzhi 4 machine 1.2 8
Duplex stainless steel seamless pipe 114.3x3.5 Changzhi 4 machine 1.5 10
Duplex stainless steel seamless pipe 140x4.5 8″ hydraulic pipe bender 1.5 10

The verification results

Pipe bending is carried out according to the aforementioned process method, and all the tested parameters are within the standard tolerance range, meeting the requirements of quality evaluation of finished products.

How to get high quality pipe bend?

If we want to get high quality bend processing products, then we must grasp the different process conditions and operation skills. And if choose the word of cold state processing mode, so we must choose the shape of mandrel reasonably, grasp correct application method even together. We know that after the mandrel shape and size is confirmed, the orientation at the beginning of the bend is also an important factor affecting the quality of the bend.
Therefore, we must do everything possible to ensure its reasonableness. If the orientation inserted into the pipe is too far forward, then the outer wall of the bending workpiece may be squeezed; And if stab too small word, so cannot rise corresponding protection action again, even still can make circular arc inside appear corrugate phenomenon.
So, how on earth should we grasp this suitable position? In fact, we can combine this with a formula: E ≤(1/4 ~ 1/2)D. The letter E represents the interval at which the mandrel plunges into the pipe at the beginning of the bend, while the letter D represents the nominal size of the inside diameter of the bent pipe. And then by doing the calculation you can get the exact number.
After confirming the bearing size, we should try the bend according to this value first. During the bending test, we can adjust the mandrel properly according to the specific conditions, so that the mandrel can reach the proper orientation. In this way, the obtained bend processing products can meet the requirements of the use.

Conclusion

In this paper, through the analysis of various factors affecting the cold production process of duplex stainless steel pipe bend, the mandrel and control of its insertion length, the use of special tooling and combined with tube lubrication, and other technological methods, verified by the actual ship, show that this technological method is feasible.
Author: Zhan Ren

Source: China Pipe Bend 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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cold bending process of duplex stainless steel bend - Cold bending process of duplex stainless steel bend
Article Name
Cold bending process of duplex stainless steel bend
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This paper briefly introduces the problems encountered in the cold bending process of duplex stainless steel bend and its causes, and puts forward solutions and suggestions.
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