Analysis of the reasons for the low yield strength and tensile strength of induction heating bend

The yield strength and tensile strength of the pipe bend body outside the arc side of the transition zone produced by a pipe factory are analyzed. The mechanical, chemical and metallographic analysis and test of the pipe bend sampling were carried out. The results show that the low yield strength and tensile strength of the bend body on the outer arc side of the transition zone of the bend are due to the low initial heating temperature (lower than 950 ℃), and the ferrite is the main structure after quenching and tempering, which reduces the tensile strength and makes it lower than the standard requirements. It is suggested that when the bend is pushed, the heating temperature should be increased properly to meet the requirements of the mechanical properties of the transition zone.

Bend is an important connecting piece in oil and gas transmission pipeline, whose function is to adapt to the pipeline design requirements and change the direction of the pipeline [1]. Tensile property is one of the main performance indexes of bend, and it is an important basis for bend design and safety evaluation [2-3]. Trial production of a steel pipe plant Φ 813mm × During the tensile test of x70m induction heating bend with 22.23mm specification, it is found that the yield strength and tensile strength of the pipe body on the outer arc side of the transition zone are lower than the standard requirements. The main pipe of the bend is longitudinal submerged arc welded pipe, which is made by local induction heating process. Local induction heating method [4] is a process that uses induction heating coil to form a narrow annular heating belt around the bend main pipe, and make it continuously bend while the main pipe moves. The straight section of the bend is not quenched, the bending part is quenched, and then the bend is heat treated as a whole. The pushing temperature is 960 ± 25 ℃, the pushing speed is 20 mm/min, and the forced single side water cooling. Adopting tempering heat treatment, tempering temperature 530 ℃, tempering heating rate 100 ℃/h, holding for 1h, air cooling process. In this study, a series of physical and chemical properties tests were carried out on the bend, and the reasons why the strength index did not meet the requirements were analyzed systematically.

Test analysis

Chemical composition analysis

According to the requirements of GB/T 4336-2016 [5], ARL-4460 direct reading spectrometer was used to analyze the chemical composition of the pipe body. The results are shown in Table 1. The content of each element meets the requirements of GB/T 29168.1-2012 [6], induction heating bends, pipe fittings and flanges for pipeline transportation system in petroleum and natural gas industry Part 1: induction heating bends.
Table.1 Φ 813mm × Chemical composition of 22.23mm x70m induction heating bend

Note: ① cepcm = C + Si/30 + (Mn + Cu + CR)/20 + Ni/60 + Mo/15 + V/10 + 5B.

Mechanical property test

The sampling position and number of mechanical property test are shown in Figure 1. As shown in Figure 1, samples are taken at positions 1, 2, 3a, 3b, 4, 5, 6 and 7. According to the requirements of GB/T 228.1-2010 [7], sht4106 material testing machine is used to conduct tensile test on pipe body and welded joint. The test results are shown in table 2.
It can be seen from table 2 that except the yield strength and tensile strength of the tube at 3A position are lower than the standard requirements, the tensile property test results of the other 1, 4, 5 and 7 parts of the tube meet the standard requirements. The tensile properties of 3A part of the pipe body were re inspected, and the re inspection results are shown in Table 3. The re inspection results show that the yield strength and tensile strength of 3A pipe are still lower than the standard requirements.

Fig.1 sampling position and number of bend samples for tensile test
Samples were taken at positions 2 and 6 for weld guided bending test. The test was conducted in accordance with GB/T 2653-2008 [8], and the sample size was 400mm × 38mm × 22.23mm (long) × wide × The test results meet the requirements of GB/T 29168.1-2012.
Table.2 tensile property test results of induction heating bend

Table.3 re inspection results of tensile properties at 3A position of induction heating bend

As shown in Figure 1, samples are taken at positions 1, 2, 3a, 3b, 4, 5, 6 and 7 for Charpy impact test. The impact test is conducted in accordance with GB/T 229-2007, and the results are shown in Table 4. The test results meet the requirements of GB/T 29168.1-2012.
It can be seen from table 4 that the absorbed energy and average value of single sample at 3A position are very high, which are higher than the test values at 1, 3b, 4, 5, 7 and other positions, and also far higher than the standard requirements. The single and average values of shear section rate at 3a are 100%, which are much higher than those of 3b, 4, 5 and 6. The energy absorbed by Charpy impact and shear section rate reflect the toughness of the material. The higher Charpy impact energy and shear section rate, the better toughness of the material. The above test results show that the toughness of 3A position is better than that of base metal tube body, 4, 5 and 7 positions.
Table.4 Charpy impact test results of induction heating bend

Note: the sample size is 10 mm × 10mm × 55mm at – 5 ℃.

Fig.2 schematic diagram of Charpy impact energy at position 4/5/7

Fig.3 Schematic diagram of shear section rate at 4/5/7
In addition, it can be seen from table 4 that the impact energy and shear section rate fluctuate greatly at bend 4, 5 and 7 (as shown in Fig. 2 and Fig. 3). The impact energy and shear section rate of position 7 fluctuate the most, the minimum impact energy is 32j, the maximum impact energy is 348j, the difference is 316J. The minimum shear section ratio (percentage) is 20, the maximum is 90, and the difference is 70. The fluctuation of impact toughness is generally related to the inhomogeneity of material structure.
As shown in Figure 1, samples are taken at positions 1, 2, 3a, 3b, 4, 5, 6 and 7, and hardness test is carried out according to standard ASTM E384-11e1 [9]. The location of Vickers hardness indentation of pipe body and welded joint is shown in Fig. 4 and Fig. 5. The hardness test results are shown in Table 5 and Figure 6. It can be seen that the hardness test results meet the standard requirements.

Fig.4 Schematic diagram of indentation position in Vickers hardness test of pipe body

Fig.5 Schematic diagram of indentation position of Vickers hardness test for welded joint
It can be seen from table 5 and Figure 6 that the hardness value of 3a is generally lower than that of 1, 3b, 4, 5 and 7. The hardness value can reflect the strength to a certain extent. The low hardness value indicates the low strength of the material.
Table.5 results of Vickers hardness test

Fig. 6 distribution of Vickers hardness values at different positions of pipe body

Metallographic analysis

As shown in Figure 1, samples were taken at positions 1, 2, 3a, 3b, 4, 5, 6 and 7. The metallographic analysis was carried out by using mef3a metallographic microscope, mef4m metallographic microscope and image analysis system according to ASTM E3-11, ASTM E45-13, ASTM E112-13, GB/T 4335-2015 and other standards. The results are shown in Table 6. The microstructure of 1-tube body is shown in Figure 7, and that of 3A tube body is shown in Figure 8. The results show that the metallographic structure of the base metal meets the requirements of the standard.
Table.6 metallographic analysis results of pipe body

Note: ① grain B is granular bainite, PF is polygonal ferrite, P is pearlite, Ma is martensitic austenite island; ② 3a and 3b samples are longitudinal samples, and the grain size results are only for reference.

Figure 7.1 – metallographic structure of the core of pipe body

Fig.8 3A – metallographic structure of tube body core
It can be seen from the test results that the microstructure of 3a is mainly polygonal ferrite with a small amount of granular bainite, while the microstructure of base metal and other parts of tube body is mainly granular bainite. Polygonal ferrite has low strength and good toughness, granular bainite has good matching of strength and toughness, and the reflection of microstructure and mechanical properties is consistent.

Discussion and analysis

The yield strength and tensile strength of the tube (3a) on the outer arc side of the transition zone are lower than the standard requirements, and the tensile properties of other parts meet the standard requirements. Charpy impact test shows that the energy absorption and shear section ratio of 3A position are higher, and the toughness of the material is better. The results of hardness test show that the hardness of 3A position is lower than that of other parts, which is confirmed by tensile property and Charpy impact property. The results of metallographic analysis show that the microstructure of 3a is mainly polygonal ferrite with low strength and a small amount of pearlite and granular bainite. The results of microstructure analysis show that the strength of 3A position is lower than the standard requirement, which is determined by the microstructure characteristics of the material in this position.
In order to find out the reason why there is a big difference between the metallographic structure of 3A position and other parts of the pipe body, the following aspects are analyzed, such as chemical composition, pushing temperature, cooling mode, pushing speed and heat treatment mode.

Chemical composition of bend header

The chemical composition analysis of the main pipe shows that the element content meets the requirements of the standard. X70m bend mother pipe adopts the composition design of “low carbon + high Mn + Mo + a small amount of Nb, V and Ti”. Controlled rolling adopts on-line watering and rapid cooling after rolling. This process makes the steel pipe not only have high strength and toughness, but also have good weldability and corrosion resistance. Its structure is a mixed structure dominated by granular bainite and has excellent mechanical properties.

Bending temperature

The pushing process of bend pipe is that the induction heating coil is connected with current, and the steel pipe body is gradually heated by electromagnetic induction. When it is heated to the specified temperature, the main pipe is pushed forward and water is cooled at the same time. The pushing temperature of the bend is 960 ± 25 ℃, the pushing speed is 20 mm/min, and the water cooling is forced on one side. The microstructure of bend pipe has experienced austenitizing high temperature heating [11], and the non-equilibrium structure formed has a tendency to transform to steady state. The subsequent tempering heat treatment process provides the thermodynamic conditions for the transformation.
After heating to 530 ℃ for 1 h and air cooling, the microstructure at 3A position is composed of a large amount of polygonal ferrite, a small amount of pearlite and granular bainite. The microstructure at 3B site is composed of partial ferrite and partial martensite austenite Island, and the rest at 4, 5 and 7 sites are composed of granular bainite. The tissue in 3A was significantly different from that in 3b, 4, 5 and 7. It is pointed out in reference [12] that the metallographic structure of x70m bend is basically not bainite when it is quenched below 950 ℃. The reason is that the heating temperature is roughly between AC1 and AC3, and the heating state is composed of ferrite and austenite. After quenching, the ferrite is retained, and a small amount of bainite and pearlite are found. After quenching, the microstructure and hardness are uneven, and the strength and hardness are reduced, After tempering, the elimination of part of internal stress and higher grain size are beneficial to the impact toughness, but a small proportion of bainite structure is unfavorable to the strength.
When heated to 950-1100 ℃, the heating temperature is above AC3 and the heating state is in austenite single phase region. The transformation kinetics shows that austenite grain size has an important influence on the transformation products after cooling. The larger the austenite grain size is, the higher the stability is, and the more non diffusible substances are formed after cooling. At the same time, the full dissolution of alloy elements at high temperature is conducive to improving the stability of undercooled austenite. Under the condition of rapid cooling, the mixed structure with granular bainite is obtained. Bainite has higher strength, which is beneficial to the strength. Therefore, to a certain extent, the higher the heating temperature, the higher the strength of the material. The microstructure of bend main pipe is a large amount of granular bainite, a small amount of polygonal ferrite and pearlite, granular bainite is the majority, and the strength and toughness of the material are high. 4. The microstructure of 5 and 7 position is granular bainite, while that of 3A position is composed of a large number of polygonal ferrite, a small amount of pearlite and granular bainite, and polygonal ferrite is the majority. The pushing temperature of the bend is controlled at 935 ~ 985 ℃. According to the factory investigation, the starting pushing temperature is lower than 950 ℃, because higher than 950 ℃ will cause the waviness of the bending position to exceed the standard requirements. It can be concluded that when the heating temperature of 3A transition zone is below 950 ℃, the microstructure after quenching and tempering may be composed of a large number of polygonal ferrite, a small amount of pearlite and granular bainite, otherwise, the microstructure should be basically granular bainite. Therefore, the temperature did not reach 950 ℃ or above at the beginning of pushing. In the process of backward pushing, the temperature gradually increased to above 950 ℃. The microstructure of 4, 5 and 7 positions was mainly granular bainite, and the yield strength and tensile strength of 4, 5 and 7 position tube material reached the requirements.
In addition, in the pushing process, the pushing temperature is within a control range, which can not ensure that every point is uniform, so it will cause the uneven temperature of each part in the pushing process. Due to the low content of Mn, Mo, Nb and other alloy elements in the bend, the strength of the bend is unfavorable, the tolerance of bending conditions in the bending process is reduced, and the requirements of bending conditions are more stringent. In order to meet the strength requirements of the standard, it is necessary to increase the heating temperature (the actual heating control temperature is far more than 960 ℃) and the degree of supercooling in the cooling process, and it is necessary to control the heating temperature accurately. However, with the increase of heating temperature, the bending pipe will be heated for a long time at high temperature, and the grains of the material will grow up continuously. When the grains grow to a certain extent, the bonding force between grains will be weakened, and the shape and toughness of the material will be deteriorated, resulting in overheating. If the temperature increases again, the grain boundary of the steel will start to melt, the crystal structure will be destroyed, and the phenomenon of overburning will appear. Overheating or overburning will reduce the plasticity and toughness of the bend. Considering the strength, the higher the heating temperature, the better. But considering the microstructure and impact toughness, the maximum heating temperature should be reduced as far as possible. Therefore, the heating temperature of the bend should not be too high. In order to maintain the better plasticity and toughness of the bend, the lower pushing temperature should be selected as far as possible under the condition of satisfying the strength. The test results show that the grain size of the bend at positions 4, 5 and 7 is grade 6.0, which is the lower limit required by the standard GB/T 29168.1-2012, and the grain size is relatively coarse, which also confirms the influence of raising the pushing temperature. During the pushing process, the temperature fluctuates continuously (controlled at ± 25 ℃), and it is difficult to accurately control the pushing temperature and increase the pushing temperature at 4, 5 and 7 positions of the bend, resulting in the uneven local structure and coarse grain of the material at 4, 5 and 7 positions, resulting in the fluctuation of impact toughness at 4, 5 and 7 positions of the bend.

Cooling mode and pushing speed

According to the manufacturer’s data investigation, the bend adopts single-sided forced water-cooling mode, the steel pipe propulsion speed is 20 mm/min, and the uniform propulsion speed does not have abnormal cooling or uneven propulsion speed, so the problems of cooling mode and propulsion speed are excluded.

Heat treatment process after bending

After being pushed, the bend was treated by high temperature tempering, tempering temperature 530 ℃, holding time 1H and air cooling. The main function of heat treatment is to reduce stress and refine grain. After heat treatment, the mechanical properties of other parts of the pipe meet the standard requirements, so the heat treatment process should meet the standard requirements.

Conclusions and suggestions

• (1) The lower yield strength and tensile strength of the outer arc side of the bend transition zone is due to the lower heating temperature of this part, which is mainly composed of ferrite, which reduces the tensile strength and makes it lower than the standard requirements.
• (2) The content of Mn, Mo, Nb and other alloy elements in the bend is low. In order to improve the strength of the material, the pushing temperature is increased, so that the grain size of the material after quenching and tempering is larger. The fluctuation of pushing temperature results in the uneven structure, resulting in the fluctuation of impact toughness at positions 4, 5 and 7.
• (3) It is suggested that the heating temperature should be controlled reasonably to make the mechanical properties of the transition zone meet the requirements. Increasing the content of Mn, Mo, Nb and other alloy elements properly makes the composition design of the material more reasonable.

Authors: Luo HuaQuan, Zhang Lina, Tong Ke, Yang Linang, he Xiaodong, Zhang Xueqin

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.)

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