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Effect of annealing temperature on microstructure and properties of small diameter thick wall HFW welded pipe

The effect of annealing treatment on the microstructure and properties of X65MO steel φ 273mm×12.7mm HFW welded pipe is studied. The results show that when the annealing temperature is lower than the austenitizing temperature, the longitudinal flexion ratio decreases slightly and the uniform elongation increases obviously. When the annealing temperature enters the two-phase region, the yield strength decreases significantly, the tensile strength increases, the yield strength ratio decreases and the uniform elongation increases with the increase of annealing temperature. The results show that moderate strength and yield ratio can be obtained by precisely controlling annealing temperature range.

Energy demand promotes the development of marginal oil and gas fields and offshore oil and gas resources, and the importance of submarine pipelines is becoming increasingly prominent [1]. Submarine pipeline is a key part of the offshore oil and gas resource exploitation and transportation system, and is also used for long-distance oil and gas transportation across the ocean, which is of great significance to energy development. In the selection of pipe type, there are mainly straight seam submerged arc welded pipe, HFW welded pipe (high frequency resistance welded pipe), seamless steel pipe, etc. Among them, HFW welded pipe is economical and has high dimensional accuracy, and has been more and more widely used in ocean pipeline engineering [2-3].

Different from the land pipeline, the harsh Marine environment puts forward more stringent quality requirements for the steel used in the submarine pipeline. Marine pipelines require not only the transverse performance, but also the longitudinal performance of steel pipes [4]. The latest edition of DNVGL-ST-F101 — 2017 Submarine Pipeline System [5] requires the longitudinal strength ratio of Marine pipelines to be less than or equal to 0.93. The yield strength of microalloyed pipeline steel produced by controlled rolling and controlled cooling technology increased significantly than that of tensile strength, so the modern pipeline steel has a higher yield strength ratio than the traditional low alloy high strength steel. In the past 10 years, the yield to strength ratio of pipeline steel has increased from 0.80 to 0.90 ~ 0.93 or above. The high flexural strength ratio limits the ultimate plastic deformation capacity of pipeline steel and affects the safe service of pipeline structure. Therefore, the research on quqiangbi has become a hot spot in the current pipeline steel and pipe research [6].

In Marine HFW pipelines, with the development of offshore oil exploitation from offshore to deep sea, steel pipes need to have a large pressure capacity. Therefore, steel pipes for submarine pipelines are developing towards the direction of large wall thickness and high steel grade [7-10]. With the increase of steel grade, the yield strength ratio increases significantly [11]. For thick-walled tubes, the yield strength of steel strips increases due to work hardening in the forming process, while the tensile strength changes very little, so the longitudinal flexural strength ratio increases greatly [12-13]. In the same pipeline, the upper limit of the longitudinal flexural strength ratio of the pipe body will increase by 2% ~ 4% compared with the upper limit of the transverse flexural strength ratio [14].
For small-diameter and thick-walled HFW welded pipes, the increase of longitudinal flexural strength ratio is more obvious than that of transverse flexural strength with the increase of thick-diameter ratio [15]. Pipe, in the process of cold forming and sizing can all result in a steel ring to the compression, the longitudinal tensile, and the longitudinal yield strength and QuQiang ratio increases significantly, and the smaller the diameter, this phenomenon is more significant, especially Φ 273 mm small diameter HFW pipe, marine pipeline longitudinal QuQiang than 0.93 or less, for small diameter thick-walled HFW pipe with great difficulty.
Table.1 X65MO test tempered chemical composition %
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In this study, the effect of annealing treatment on the microstructure and properties of X65MO steel class φ 273mm×12.7mm HFW welded pipe is studied, and the industrial practice is carried out. The tensile property and impact toughness of the steel pipe produced meet the standard requirements.

Test materials and methods

Industrial X65MO steel class φ 273mm×12.7mm HFW welded pipe was used as the test material, the chemical composition and tensile properties of which are shown in Table 1 and Table 2, and the box-type resistance furnace was used for annealing test. According to experience, the phase transformation point of test steel Ac1 is in the range of 700 ~ 750℃, so the annealing temperature is 650℃, 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 800℃, and then air cooling to room temperature. Tensile test, hardness test and microstructure analysis were performed on the annealed specimens.
Table.2 Tensile properties of X65MO test steel specimen
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Test results and discussion

Effect of annealing temperature on microstructure

FIG. 1 shows the microstructure of the samples after annealing at different temperatures and magnified 500 times. After annealing at 650℃ and 700℃, the ferrite in the original samples tended to be polygonal and the carbides decreased significantly. After annealing at 710℃, A small number of fine M-A components began to appear in the microstructure, indicating that the temperature just entered the two-phase region. With the further increase of annealing temperature, the content and size of M-A components increased, and the polygonal ferrite content increased. When the annealing temperature increased to 800℃, the original acicidal ferrite matrix was completely disappeared.
FIG. 2 shows the 1000x metallographic structure of the annealed sample in the two-phase region. It can be seen that the M-A component in the microstructure is mainly distributed at the grain boundary, and the size of m-A component gradually increases as the annealing temperature increases from 710℃ to 740℃. This is because when the annealing temperature is in the two-phase region, the proportion of austenite increases with the increase of temperature, and in the subsequent air-cooling process, the austenite soon enters the α phase region, and the austenite transforms into M-A component.
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FIG.1. 500 times of microstructure of samples annealed at different temperatures
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FIG.2 1000x microstructure of samples at different annealing temperatures

Effect of annealing temperature on tensile properties

Tensile properties of the samples annealed at different temperatures were tested. The tensile curves and tensile properties at different temperatures were shown in FIG. 3 and FIG. 4, respectively. The tensile curve of the original sample has no yield platform, the yield strength is high, the yield strength ratio is 0.93, and the uniform elongation is only 3.5%. After annealing at 650℃ and 700℃, the upper yield point and yield platform appear in the tensile curve. The yield strength and strength ratio decrease slightly, but the strength ratio is still higher than 0.90, and the uniform elongation increases to more than 10%, indicating that the plastic deformation in the pipe making process has basically recovered in the annealing process. The tensile curve of the specimens annealed at 710℃ showed a slight yield plateau, but no upper yield point. Due to the presence of a small amount of MA components in the microstructure, the tensile strength increased, and the flexural strength ratio decreased slightly to 0.90. As the annealing temperature further increases to 750℃, the yield strength decreases significantly, and the tensile strength increases slowly, so the yield strength ratio decreases significantly. The tensile curve is a continuous yield of vault type, and the uniform elongation is basically stable at 10% ~ 12%. When the annealing temperature continues to rise to 800℃, the yield strength and tensile strength decrease significantly, and the uniform elongation increases to 14%.
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FIG.3 Tensile curves of samples at different annealing temperatures
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FIG.4 Effect of annealing temperature on tensile properties of sample

Effect of annealing temperature on hardness

The comparison of Vickers hardness on the cross sections of the original samples and the samples annealed at 710 ~ 740℃ is shown in Figure 5, including three positions near the outer surface, the center of wall thickness and near the inner surface. It can be seen that in the direction of wall thickness, the hardness near the inner surface is the highest, and there is little difference between the hardness at the center of wall thickness and near the outer surface. In the two-phase region of 710 ~ 740℃, the hardness of the near inner surface and near outer surface increases with the increase of annealing temperature, which is consistent with the change law of tensile strength described above, while the hardness of the center of wall thickness changes relatively little.
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FIG.5 Cross section hardness of samples at different annealing temperatures

Discussion and application

The original structure of X65MO pipeline steel used in this experiment is mainly composed of acicular ferrite and carbide. Due to the large thick-diameter ratio, the cold forming and sizing of pipe making process introduce large plastic deformation, resulting in the plastic decline, the uniform elongation is only 3.5%, and the flexural strength ratio reaches 0.93. When below two phase critical temperature annealing, the change of organization mainly for the reply, carbide gradually dissolved into the alpha phase in the matrix, and the dislocation density is lower, to restore the original sheet of plastic material, at the same time, the dislocation and carbon and nitrogen atoms form his air masses, dislocation pinning, and the tensile curve appears to yield platform and the yield point, strength and QuQiang than fell slightly. During annealing in the two-phase region, with the increase of temperature, the internal substructure of acicular ferrite in the matrix gradually disappears and transforms into polygonal ferrite. At the same time, austenite nucleates and grows at the grain boundary and transforms into M-A components during the subsequent cooling, leading to A rapid decline in yield strength. The tensile strength and hardness increased with the increase of the size and content of M-A components, and the tensile curve showed A continuous yield of vault type because of the appearance of soft and hard phases in the microstructure.
It can be seen from the relationship between annealing temperature and tensile properties that the microstructure and properties of the materials annealed in the two-phase region are highly sensitive to temperature. When the annealing temperature is 720℃, the yield strength is lower than that of X65MO steel. When the annealing temperature reaches 730℃, the yield strength decreases to close to the lower limit of X65MO steel grade. Therefore, moderate strength and flexion ratio can be obtained by precisely controlling annealing temperature.

Conclusion

  • (1) When annealing below the critical temperature of the two-phase region, the microstructure recovers, the carbide decreases, and the material restores the plasticity of the original plate. At the same time, the yield platform and yield point appear in the tensile curve, and the strength and strength ratio decrease slightly.
  • (2) When annealing at the temperature range of two-phase region, with the increase of temperature, the matrix structure gradually changes from acicular ferrite to polygonal ferrite, the size and content of M-A components at grain boundary gradually increase, the tensile strength and hardness increase, and the yield strength and yield strength ratio decrease significantly.
  • (3) When the annealing temperature is 720℃, the test steel has moderate strength and low yield strength ratio, which can meet the requirements of X65MO steel grade, but when the annealing temperature reaches 730℃, the yield strength drops too much, which is close to the lower limit of X65MO steel grade.

Author: WANG Yiran, SUN Leilei

Source: Network Arrangement – China Stainless Steel Welded Pipe Manufacturer – Yaang Pipe Industry Co., Limited (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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