Exploring the process of TP316LN super strong stainless steel intermediate frequency pipe bend

This paper takes the bending of AP series passive nuclear island core coolant system TP316LN super stainless steel intermediate frequency pipe bend as an example, briefly explains the working principle of intermediate frequency pipe bend, and analyzes the influence of TP316LN stainless steel intermediate frequency pipe bend on the quality of finished products. According to various factors, the targeted solutions were put forward, and the specimens that met the quality requirements were successfully bent. After inspection and testing, all the indicators reached the requirements of the specification standards, and finally obtained better process parameters.

TP316LN stainless steel bend has the advantages of high strength, good corrosion resistance, low resistance to media, and less construction welds. It is widely used in the main systems of nuclear power plants. The quality of the bend is an important factor affecting the safe and reliable operation of nuclear power plants. Because the intermediate frequency pipe bend process is complex and there are many influencing factors, it is of great significance to explore and study it to find the optimal process parameters.

Working principle of intermediate frequency bend

The working principle of the screw-driven intermediate frequency pipe bender is shown in Figure 1. The intermediate frequency current delivered to the induction coil causes the pipe surrounded by the coil to locally generate eddy current, and then the temperature rises rapidly to form an annular heating zone. When the temperature rises to the lower critical temperature of the pipe, Tc-56°C or more (TP316LN stainless steel recommended heat The bending temperature is 1050℃~1100℃), the pipe is already in a plastic state, and the drive screw is driven to rotate by the servo motor group, thereby driving the push plate assembly to move forward, exerting a thrust on the pipe end, and the shaft bends when the pipe is relative to The induction loop is fed forward slowly and uniformly, and the bend extends along the axial direction of the pipe one by one to form a smooth arc. At the same time, in order to cool the induction coil and fix the curved part, there are many small holes in the circumference of the induction coil. When the cooling water flows through the inner cavity of the induction coil, it is sprayed along the circumferential small holes to the already bent tube. The process is continuous until the entire bending is completed.
20200806123114 65443 - Exploring the process of TP316LN super strong stainless steel intermediate frequency pipe bend
Fig.1 Working principle of intermediate frequency bend

Main factors affecting the quality of intermediate frequency bends

TP316LN super-strength stainless steel is added with N element to TP316L stainless steel, which has the better corrosion resistance of TP316L stainless steel, and also improves the high temperature yield strength by about 20%. The specification of the bent specimen is φ508×40.5, bending The radius is 1500mm and the bending angle is 90°. It is a small radius and large diameter thick-walled bend. Therefore, it is more difficult to bend than ordinary stainless steel, its bending moment needs to be increased, and the control is more precise. By investigating the quality reasons of conventional stainless steel in intermediate frequency bends and the characteristics of TP316LN super-strong stainless steel, it is analyzed that the main factors affecting the quality of intermediate frequency bends are as follows:

  • (1) The quality of the raw material of the pipe mainly affects the ellipticity, the wall thickness reduction rate, and the flatness of the bend.
  • (2) The design of induction coil mainly affects heating efficiency, cooling effect, and appearance of tube forming.
  • (3) The setting of the compensation angle mainly affects the angle of the finished pipe.
  • (4) The setting of intermediate frequency power and the selection of advancing speed mainly affect the heating efficiency and the appearance of tube forming.
  • (5) The solution heat treatment process mainly affects the mechanical properties and corrosion resistance of the bend.
  • (6) The pickling passivation process mainly affects the cleanliness of the stainless steel surface and secondary pollution.

Targeted solutions

(1) The use of high-quality pipes fundamentally guarantees the bending power of the pipe. Since the quality of the pipe is an important factor that affects the success of the pipe bending, when selecting the pipe bend, the quality must be strictly checked. The pipe must be carefully inspected from the inside and outside to ensure that there are no defects such as cracks, interlayers, uneven thickness, etc. At the same time, the outer diameter, wall thickness, ovality, straightness, etc. of the pipe should be measured, and the side with a positive tolerance and the larger wall thickness should be the back curve. The bend test piece selection
The wall thickness of the pipe used is 3~4mm greater than the nominal wall thickness, the ovality is less than 1%, and the straightness is less than 1mm, which fundamentally guarantees the bending power of the pipe.
(2) The design adjustment of the induction coil ensures the heating and cooling effect. According to the manufacturer’s recommendation, the φ508×40.5 stainless steel intermediate frequency bend adopts an induction coil with a width of 25mm, an inner diameter of 530mm, and an outer diameter of 610mm. The spray hole angle is 25° and the number of spray holes is 210. In the test bending process, the upper part is easy to return water, which causes uneven temperature of the heating zone, and the temperature is not easy to rise. After analysis, the distance between the induction coil and the pipe and the water spray angle are too small to cause the backwater phenomenon; the inner diameter of the induction coil and the spray The water angle was adjusted and tested. The width of the induction coil was adjusted to 40mm, the inner diameter was adjusted to 560mm (the gap between the coil and the tube was about 25mm), and the spray angle was adjusted to 35° for the first time. The backwater phenomenon was improved after the test, but it did not reach Ideally, adjust to 45° for the second time. After testing, there is still a slight backwater on the top. Adjust the angle of the top spray hole to 46.5°. After the test, there is no backwater phenomenon. The actual bending process has also achieved good results. The effect, the comparison before and after coil modification is shown in Figure 2.
20200806123255 75563 - Exploring the process of TP316LN super strong stainless steel intermediate frequency pipe bend
Fig.2 Comparison before and after transformation of induction coil
(3) Set an appropriate compensation angle to ensure that the angle does not exceed the tolerance.
The intermediate frequency bend is bent by the method of back-clamping and pressing. There is stress in the bending part, and springback will occur when the clamp is loosened. Therefore, a certain amount of bending needs to be considered when bending, and the allowable value of the angle deviation of the finished bend ±0.5°, the requirement is relatively high, so the compensation angle setting must be very accurate to ensure the angle of the finished pipe. TP316LN super stainless steel has high strength and small bending radius of the test piece, so the rebound amount is relatively large. After multiple tests with different compensation angles, analysis and adjustment are carried out, and it is finally determined that the bending angle of the test piece is 91.2°. See Table 1 for details.
Table.1 Comparison of bending angle and actual angle

Specification of bend


Bend angle Measured angle

















(4) Choose the appropriate intermediate frequency power and propulsion speed, carefully control, and steadily bend to ensure the quality of the appearance. The heating temperature of the stainless steel intermediate frequency bend is generally controlled at 1050℃~1100℃. If the temperature is high, it will cause over-burning. If the temperature is too low, the bending moment will increase and it will not be able to push forward steadily. Therefore, choose a suitable speed and control the heating zone. The temperature is at a relatively stable value. The high temperature yield strength of TP316LN super stainless steel is relatively high, and the diameter and wall thickness of the test piece are large, and the bending radius is small. Therefore, the required power is high and the bending speed is slow. After testing, the intermediate frequency power is set at about 460kW. The temperature is controlled at 1060℃, the torque is 95%, and the advancing speed is set at 7.5mm/min. At the same time, observe and think more during the whole bending process, and always pay attention to the temperature change of the heating belt. As the bending angle increases, adjust the gap between the induction coil and the tube in time to make the temperature on the outer arc side slightly higher than the inner arc side. , It is conducive to the outer arc stretching and inner arc extrusion forming; in addition, the power should be appropriately lowered during the bending and closing stage, the temperature of the heating belt should be slowly reduced, and when the bending is stopped, the induction coil should be moved The arc section is continuously sprayed with water for 15 minutes, which can effectively prevent the bulging phenomenon.

(5) Adopt solution treatment process to prevent stainless steel from producing intergranular corrosion tendency. During the bending process, the stainless steel tube has undergone processes such as medium-frequency high-temperature heating, stretching and extrusion deformation, and water spray cooling. It will experience the sensitization temperature range (400℃~850℃), and it is easy to precipitate high chromium carbides and produce intercrystalline Corrosion tendency, so solution heat treatment is required. The test piece bend solid solution treatment adopts a rapid quenching furnace. The entire stainless steel bend is put into a gas heat treatment furnace, heated to about 1065℃, kept at a constant temperature for 1h, and quickly immersed in water to cool to room temperature after being released from the furnace. All bends are immersed in water for only 45s, which quickly crosses the stainless steel sensitization temperature range, avoids the occurrence of intergranular corrosion tendency, and ensures the mechanical strength and corrosion resistance of stainless steel bends.
(6) The pickling passivation process ensures the cleanliness of the bend surface. In the process of bending and solution treatment of stainless steel bend, the surface is inevitably stained with harmful pollutants such as dust, oil, oxide scale, so it needs to be pickled and passivated. TP316LN super stainless steel has good corrosion resistance. The concentration of the pickling passivation solution is relatively high, and the soaking time should be longer. The entire bend is in the pickling passivation solution (8% hydrofluoric acid and 20% nitric acid mixture) Soak for 2h until all pollutants such as the gray oxide layer on the surface are removed, and a silver passivation protective film is formed. Lift out and rinse with demineralized water until the pH value of the residual liquid on the surface is neutral (6~8) is qualified, and then dry and oil-free The compressed air is dried and wrapped with a special plastic film to prevent secondary pollution, protect the surface of the bend from corrosion, and effectively ensure the performance and life of stainless steel.
(7) The quality inspection indexes meet the requirements. Dimensional measurement inspection and physical and chemical tests are carried out on the bent pipe specimens. All the data meet the standard requirements, as shown in Table 2.

Table.2 Inspection test results of pipe bend

Project name

Test items

Standard value

Measured value


Form inspection

Visual inspection

No defects such as cracks, folds, heavy skins, dents, sharp scratches, etc., no over-burned structure, no intercrystalline cracks

Meet the requirements


Bending angle




Bending radius




Thinning rate

14.25%, and not less than the minimum wall thickness







Wave degree

Wave height/outer wave ≤2%
Wave length/wave height >12

Wave height/outer wave: 0.31%
Wave length/wave height: 17.67






Surface roughness




Physical and chemical test

Room temperature stretch

Tensile strength ≥515MPa

Yield strength ≥205MPa
Transverse elongation ≥25%

Tensile strength: 621MPa
Yield strength: 312MPa
Transverse elongation: 53.5%


High temperature stretching

Tensile strength ≥432MPa

Yield strength ≥122MPa

Tensile strength: 490MPa
Yield strength: 221MPa


Grain size

Level 5

Level 5


Intergranular corrosion

No intercrystalline corrosion

No intercrystalline corrosion



Through this test piece production activity, we obtained the small radius large diameter thick wall TP316LN super-strong stainless steel intermediate frequency bend process and solution heat treatment process. See Table 3 for details. At the same time, I was familiar with the equipment operation method and mastered the fine intermediate frequency bend process. At the same time, there are some shortcomings. The main reason is that thick-walled large-diameter pipes are used for intermediate frequency bends in this test. The process parameters are of little significance for small-caliber and thin-walled pipes. In the future, intermediate frequency bends will be carried out for pipes of different specifications. Process research, categorized into a summary table of process parameters of intermediate frequency bends with operability.

Table.3 Process parameters of TP316LN stainless steel intermediate frequency bend

Specification of bend


Set angle

Bending radius


IF power

Bending temperature

Advancing speed

Solution temperature

Constant temperature time











Author: Zhang Chenming, Qiujian Yu, Xu Renyao <Process and Equipment>

Source: Network Arrangement – 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.)

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Exploring the process of TP316LN super strong stainless steel intermediate frequency pipe bend
Briefly explains the working principle of intermediate frequency pipe bend, and analyzes the influence of TP316LN stainless steel intermediate frequency pipe bend on the quality of finished products.
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