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Process Test of P92 Steel Intermediate Frequency Pipe Bend

The core of the P92 steel intermediate frequency pipe bend process is the measurement of the wall thickness of the pipe before bending and the determination of the bending direction. The bending temperature is 980-1020 ℃, the bending speed is controlled at 8-20 mm/min, and the bending is carried out at 1040-1080 ℃. Normalizing +750-780 ℃ tempering treatment. Tests show that the mechanical properties and microstructure of the P92 steel pipe bent according to the prescribed process have reached the performance requirements.

The steam and water pipelines of ultra-supercritical units have a lot of turning, and a large number of bends or elbows are required. However, the use of bends has disadvantages such as many pipe welds, small bending curvatures, large flow resistance, difficult welding of P92 steel, and high welding atmosphere requirements. With the increasing maturity of P92 steel intermediate frequency bend technology, more and more high-parameter power plant boilers use intermediate frequency bend instead of elbow.

Process characteristics of intermediate frequency pipe bend

P92 steel bends are currently imported domestically, with high purchase costs and long processing cycles. Compared with bends, intermediate frequency bends have the following advantages:

  • ① Pipe bends can significantly reduce pipe vibration. Reason: Enlarging the turning radius of the flowing medium reduces the impact of the fluid on the piping system due to changes in the flow direction; improves the rigidity of the pipeline.
  • ② The total resistance of the pipeline can be reduced. From DL/T5054-1996 “Technical Regulations for the Design of Steam and Water Piping System in Thermal Power Plants”, the local resistance coefficient when using a 90° bend is 0.2, while the local resistance coefficient when using a 90° bend is 0.25. It can be seen that the internal diameter is not counted. Change, the local resistance caused by only the difference of the local resistance coefficient can differ by 25%.
  • ③ The scouring of the medium flowing in the bend to the inner wall of the outer arc of the bend is smaller than that of the bend. The scouring of the inner wall of the outer arc of the curved part by the flowing medium is related to the flow velocity and the bending radius. The flow rate of the bend with 95% flow area is 5.3% higher than that of the bend with 100% flow area. The bending radius of the bend is 2 of the bend. Times more.
  • ④ Bending pipes can reduce the total number of welds in the pipeline. Generally, one bend can save one weld.
  • ⑤ The bend weld is far away from the bent part, which reduces the overlap of the heat-affected zone of the two welds and is safer.
  • ⑥ The bend with 100% flow area, the steam pipe is easy to drain, and can prevent the corrosion of the welded joint by the accumulation of water.

Put an induction coil on the part of the steel pipe to be bent and pass in an intermediate frequency current to form an annular heating area on the pipe covered with the induction coil. After a certain period of time, the steel pipe is heated to a plastic state and can be bent. When bending, the pipe head is clamped with a mechanical rotating arm fixed on the axis, and then thrust is applied to the steel pipe. The steel pipe moves in the induction loop, and then the thermoplastic tape is bent according to the required radius of curvature. The bent steel pipe is cooled, while the unbent steel pipe is continuously pushed into the induction coil for heating. The focus of this process is to control the heating temperature, the best range of push-bending speed and the coordination relationship between them.

Process flow of pipe bend

Selection of material wall thickness

Check the surface quality of the pipe and review the pipe number, material, diameter and wall thickness. Mark 4 clear longitudinal marks on the pipe. According to the specific requirements of each pipe (including front and rear straight pipe sections, angles, bending radius, etc.), mark the thickness measurement points around the pipe wall, and measure each with a thickness gauge. Point thickness, according to the test data, take the thickest side of the pipe wall as the outer arc, and mark the pipe mouth.

Process of pipe bend

Hoist the pipe to the pipe bender, clamp the pipe, select a suitable induction coil and install it on the pipe, and connect the cooling water pipe and the air pipe. Check whether the pointer of the angle dial is at the zero position, if not, adjust the pointer of the angle dial to indicate 0°.
Because P92 steel does not have crack sensitive areas, try to bend between 980-1020℃. The bending speed is 8-20mm/min. The bending temperature must be monitored throughout the bending process to ensure the quality of the bend. When the steel pipe heating zone reaches the specified temperature, start the oil pump and turn on the oil pump forward button to steadily push the steel pipe forward, and make the pushing speed match the heating speed. When starting a bend, move the induction coil 50mm before the reference position to the reference position at the set advancing speed, and gradually slow down to ensure that the steel pipe does not wrinkle when starting the bend. Observe the angle indication of the center of rotation. When the bending angle is reached, stop pushing the steel pipe and reduce the power of the intermediate frequency power supply. The cooling water pump and air compressor must be stopped after the steel pipe is cooled to below 300°C. When the oil pump is stopped, the induction coil moves backward at the same rate as the set advancing speed, and gradually slows down, so that the bend is gradually cooled to reduce the deformation of the stop.
Adjust the bend to a horizontal state, measure the structural size and chord length of the bend, and calculate the actual bending angle of the bend according to the measured data. Stakeout method can also be used for bending pipe measurement.

Process of heat treatment

P92 steel needs to be normalized and tempered after bending. The normalizing temperature is 1040~-1080℃, and the tempering temperature is 750-780℃. Normalizing should be kept for at least 1min per 1mm wall thickness (minimum 20min), and tempering should be at least 2min per 1mm wall thickness (minimum 1h). ) To keep warm. The heat treatment process curve of ID 254×53 P92 steel bend is shown in Figure 1.
20200803030213 72114 - Process Test of P92 Steel Intermediate Frequency Pipe Bend
Fig.1 Heat treatment process of P92 pipe bend

The process of pickling passivation

After heat treatment, the intermediate frequency bend needs to be pickled and passivated to ensure that the oxide layer generated by the heat treatment is removed, and a passivation layer is formed on the surface of the pipe to ensure that the bend does not produce excessive rust layer inside the pipe before welding.

Quality Control

The bending and acceptance of the intermediate frequency bend shall comply with the DL/T515-2004 “Power Station bend” standard. Quality control measures:

  • ① Conscientiously implement the “three inspection system” in production: team self-inspection, inspector re-inspection, and final inspection by the quality department. For production-related quality standards, design drawings, and documents, implement all-round, full-process follow-up inspection and control, so that the previous process cannot be transferred to the next process for construction without inspection.
  • ② The heat treatment of the intermediate frequency bend is also a key process to ensure the quality of the bend. To ensure that the working atmosphere of the heat treatment furnace meets the process requirements, 12 thermocouple measuring points are installed around the heat treatment furnace, and measuring points are also installed on the workpiece. Ensure that the temperature of the workpiece and the furnace is consistent during heat treatment.

Quality inspection results

Various performance indicators of raw materials:

  • ① The average hardness of the steel pipe is between 181-205HBS, which conforms to the ASME SA 335/SA 335M “Seamless Ferritic Alloy Steel Pipe for High Temperature” standard, and the Brinell hardness is less than 250HBS.
  • ② The structure of the steel pipe is tempered martensite, as shown in Figure 2. The direction of the tempered martensite lath is obvious, and the structure is uniform.
  • ③ The room temperature tensile properties are shown in Table 1, which meets the requirements of ASME standards. Take samples in the horizontal and vertical directions of the steel pipes to make circular scale samples, and perform tensile tests on the WE-300A universal testing machine.

20200803030803 70021 - Process Test of P92 Steel Intermediate Frequency Pipe Bend
Fig.2 Original microstructure of P92 steel pipe
Table.1 Tensile properties of P92 steel at ambient temperature

Position

Rm /MPa

Rp0. 2 /MPa

A/%

Vertical

693. 3

530

25. 6

Horizontal

698. 3

535

24. 9

ASME Standard

≥620

≥440

≥20

The ASME standard does not specify the high temperature tensile properties of P92 steel.

EN10216-2004 “General Delivery Technical Conditions for Seamless Steel Pipes for Pressure” (2006 Supplementary Edition) has minimum requirements for the high temperature yield strength of steel pipes below 600°C , Table 2 gives the 610℃ extrapolated high temperature yield strength requirements and test values, the raw material 610℃ high temperature yield strength meets the requirements of EN10216-2004.

Table.2 High temperature tensile properties of P92 steel

Position

Rm /MPa

Rp0. 2 /MPa

A/%

Vertical

328

308. 7

33. 3

Horizontal

326

306. 3

31. 3

EN 10216—2004

237. 6

(Extrapolated value)

The ASME standard does not clearly stipulate the room temperature impact performance of P92 steel.

The EN 10216-2004 (2006 supplementary edition) standard requires the 20℃ impact absorption energy of ferritic steel seamless steel pipes for high temperature use as longitudinal KV240J and transverse KV227J. Compared with the standard sample of V-notch test and KV2 notch, there are 9 samples in total. The JB-300B semi-automatic impact tester is used for impact test. The result of the raw material room temperature impact energy absorption test is: the average longitudinal KV2 is 135.5J, The average value of horizontal KV2 is 97J, which meets the requirements of EN 10216-2004.

The position of the arc surface of the bend is shown in Figure 3, and the performance indicators after normalizing + tempering heat treatment are shown in Table 3.

After inspection, the hardness of the bend is 217~-243HBS, which meets the requirement of P92 steel Brinell hardness ≤250HBS in ASME SA335.

20200803031347 31951 - Process Test of P92 Steel Intermediate Frequency Pipe Bend
Fig.3 Arc surface scheme of pipe bend
Table.3 Hardness of bending pipe after normalizing and tempering treatment

The structure of the bend is tempered martensite, the direction of the tempered martensite lath is obvious, the structure is uniform, and no intergranular cracks are found, as shown in Figure 4.

20200803031705 27859 - Process Test of P92 Steel Intermediate Frequency Pipe Bend 20200803031713 34090 - Process Test of P92 Steel Intermediate Frequency Pipe Bend 20200803031723 10297 - Process Test of P92 Steel Intermediate Frequency Pipe Bend
(a) 0° (b) 22.5° (c) 45°

Fig.4 Arc surface microstructure at different positions of P92 steel pipe bend

Take the impact samples in both the horizontal and vertical directions to make KV2 Charpy V-notch test standard samples, and conduct the impact test.

The room temperature tensile and impact properties of the bend are shown in Table 4. The results meet the requirements of ASME and EN10216-2004 standards.

Table.4 Room temperature tensile and impact properties of pipe bend

Position Rm /MPa Rp0. 2 /MPa

A /%

Longitudinal impact

absorption energy /J

Lateral impact

absorption energy /J

0°(starting bending)

Vertical

Horizontal

745

755

620

625

25

23

138 84

22. 5° (middle)

Vertical

Horizontal

755

760

625

630

27

22

94 55.3

45°(final bending)

Vertical

Horizontal

775

780

655

655

20

22

56 65
ASME ≥620 ≥440 ≥20
EN 10216—2004 40 27

The high temperature tensile properties of the bend are shown in Table 5.

The 610℃ high temperature yield strength meets the requirements of EN10216-2004.

Table.5 High temperature tensile properties of pipe bend(610℃)

Position

 

Rm /MPa

Rp0. 2 /MPa

A/%

0°(starting bending)

Vertical

Horizontal

359

371

339

351

28

34

22. 5° (middle)

Vertical

Horizontal

366

364

351

347

26

30

45°(final bending)

Vertical

Horizontal

337

390

360

384

33

34

Application effect

The use of one intermediate frequency bend can reduce the number of welds by one compared with the use of bends. The two 1000MW ultra-supercritical units in the third phase of Shanghai Waigaoqiao Power Plant use a total of 120 bends. The cost of each weld is 20,000 yuan. , A total of about 2.4 million yuan can be saved.
Based on the advanced experience of using bends in the four major pipelines of Waigaoqiao Power Plant to significantly improve boiler efficiency, China Huadian Group held a seminar on the use of bend technology for the four major pipelines in October 2008 and decided to use bends in the design as much as possible. In addition, the four major pipelines of the 7 projects, including Taishan Power Plant, Laizhou Power Plant, Gongxian Power Plant, and Xisaishan Power Plant, which will be started soon, are designed with bends instead of bends.

Conclusion

The hardness, microstructure, room temperature tensile properties, room temperature impact properties and 610℃ high temperature tensile properties of the intermediate frequency bends bent according to this process meet the requirements of the corresponding material standards. According to the construction of this process, a qualified bend can be bent and has good economic benefits.

Source: China Pipe Bend Manufacturer – Yaang Pipe Industry (www.epowermetals.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@epowermetals.com

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