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Measurement of CCT curves and carbide intergranular precipitation temperature of 4Cr5Mo2V hot-working die steel

Gao Xingcheng, Wu Boya, Wu Xiaochun

Heat Treatment of Metals 2024, 49 (9): 72-79. doi:10.13251/j.issn.0254-6051.2024.09.012

Abstract （221）

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Microstructure and hardness of 4Cr5Mo2V steel at different cooling rates were observed and measured by using thermal expansion meter, resistance meter and hardness tester, combined with scanning electron microscope and energy dispersive spectroscopy. CCT curves of the tested steel were plotted, and the temperature of carbide intergranular precipitation in the steel was determined. The results show that as the cooling rate decreases, the hardness decreases and the microstructure transforms into martensite, bainite and pearlite in sequence. The critical cooling rates for pearlite and bainite transformation are 0.015 ℃/s and 0.25 ℃/s, respectively. When the cooling rate is not greater than 0.25 ℃/s, carbides precipitate along the grain in the steel, and as the cooling rate decreases, the temperature of carbide precipitation along the grain increases. At a cooling rate of 0.01 ℃/s, the highest precipitation temperature is 1022 ℃.

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Element segregation and homogenization heat treatment of DIEVAR hot working die steel

Du Simin, Cheng Wenxiong, Hu Fengrong, Ren Jinqiao, Cui Xiaokang, Zhou Zhiming

Heat Treatment of Metals 2024, 49 (7): 139-145. doi:10.13251/j.issn.0254-6051.2024.07.021

Abstract （198）

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As-cast microstructure, precipitated phases and element segregation characteristics of DIEVAR hot working die steel electroslag ingot were analyzed by the means of optical microscope (OM), scanning electron microscope (SEM/EDS), electron probe (EPMA), Thermo-Calc thermodynamic calculation. Then the effect of homogenization heat treatment on the microstructure and carbide and element distribution was studied. The results show that the as-cast dendrite structure of the tested steel is well developed, and the secondary dendrites are obvious. The Cr and Mo and V elements are inter-dendritically enriched, and the segregation order is Mo>V>Cr. The interdendritic precipitated phases are M₂₃C₆, M₆C and MC carbides. With the increase of homogenization temperature and the extension of holding time, the dendrite microstructure gradually becomes fuzzy and even disappears, and the carbide redissolution forms a certain element diffusion zone and gradually diffuses until it is fully redissolved into the matrix, the element diffusion rate is Cr>Mo>V during the homogenization heat treatment. According to dendrite morphology, carbide redissolution, element diffusion and residual segregation index of Mo element after homogenization heat treatment, and considering the actual industrial production cost, the optimal homogenization heat treatment process of the DIEVAR steel is 1280 ℃×8 h.

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Research status of high-temperature mechanical properties and deformation mechanism of FeMnCoCrNi high-entropy alloy

Sheng Jian, Li Dazhao, Yan Zhijie, Bai Shaobin, Chen Yongan, Xie Ruofei

Heat Treatment of Metals 2024, 49 (10): 211-219. doi:10.13251/j.issn.0254-6051.2024.10.035

Abstract （187）

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As a class of emerging advanced metal materials in recent years and based on the unique multi-principal element alloy design concept, high-entropy alloys (HEAs) are expected to break through the performance limit of traditional single-principal element alloys. Among them, FeMnCoCrNi HEA is one of the most widely studied systems with excellent mechanical properties and broad application prospects. The unique “four core effects” of HEAs were firstly introduced, including high entropy effect, lattice distortion effect, hysteresis diffusion effect and cocktail effect. Secondly, the research status of FeMnCoCrNi high entropy alloy in high temperature tensile properties, high temperature creep properties and corresponding deformation mechanisms was reviewed.

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Effect of Zn/Mg ratio on microstructure, mechanical properties and corrosion resistance of 7075 aluminum alloy

He Zongzheng, Wu Mingdong, Yuan Shuo, Yao Shuwei, Xiao Daihong, Huang Lanping, Liu Wensheng

Heat Treatment of Metals 2024, 49 (5): 1-9. doi:10.13251/j.issn.0254-6051.2024.05.001

Abstract （185）

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Effect of Zn/Mg ratio (mass fraction) on microstructure, mechanical properties, and corrosion resistance of 7075 aluminum alloys was investigated by means of optical microscope, scanning electron microscope, transmission electron microscope, tensile test, microhardness test and corrosion test. The results show that with the increase of Zn/Mg ratio from 1.76 to 2.90, the grain size of the as-cast alloy increases, the T6 age-hardening rate of the alloy is accelerated, and the time to reach peak hardness is shortened. The increase in Zn/Mg ratio results in a decrease in the recrystallization rate of the T6 peak-aged alloy and an increase in the percentage of the low-angle grain boundaries. The tensile strength of the alloy under the T6 peak-aging treatment with the Zn/Mg ratio of 2.10 is 634 MPa, the yield strength of the alloy is 588 MPa, and elongation is 8.4%. The tensile strength of the T74 aged alloy is 613 MPa, yield strength is 542 MPa, and elongation is 8.9%, and alloys with Zn/Mg ratio of 2.10 in both aging states exhibit good strength while maintaining high elongation. The corrosion resistance of the aged alloy decreases as the Zn/Mg ratio increases from 1.76 to 2.90. The corrosion resistance of the alloy after T74 aging treatment is better than that of the alloy under the T6 peak-aging state.

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Research progress of low alloy martensitic wear-resistant steel

Li Zhongbo, Wu Zhifang, Wu Run

Heat Treatment of Metals 2024, 49 (7): 132-138. doi:10.13251/j.issn.0254-6051.2024.07.020

Abstract （138）

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Research progress of low alloy martensitic wear-resistant steel was reviewed, mainly involving composition design, heat treatment process and its on microstructure and mechanical properties, and wear resistance. Composition design mainly adopts a small number of multi-component system, heat treatment processes include reheat quenching-tempering treatment, direct quenching-tempering treatment, subtemperature quenching-tempering treatment, quenching-partitioning treatment and quenching-partitioning-tempering treatment. By controlling the structure shape and distribution, the hardness and toughness can be matched reasonably. The wear resistance is improved by adjusting chemical composition, refining grain, optimizing rolling cooling process and proper tempering.

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Effect of composite addition of trace amounts of Mn, Zn and Ti on microstructure and mechanical properties of Al-Cu-Li-Mg-Ag-Zr alloy

Xu Qing, Yin Dengfeng, Wang Hua, Xie Kaiwen, Song Zhuanzhuan

Heat Treatment of Metals 2024, 49 (11): 1-9. doi:10.13251/j.issn.0254-6051.2024.11.001

Abstract （134）

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Effect of composite addition of trace amounts of Mn, Zn and Ti on the microstructure and mechanical properties of Al-Cu-Li-Mg-Ag-Zr alloy was studied by using tensile testing machine, metallographic microscope, scanning electron microscope and transmission electron microscope. The results show that the composite addition of trace amounts of Mn, Zn and Ti elements can significantly refine the grain structure of the as-cast alloy, and at the same time, the insoluble second phase Al(CuMnFe) in the solution treated alloy is also refined. The composite addition of trace amounts of Mn, Zn and Ti elements can effectively promote the precipitation of the main strengthening phase T₁ in the T6 and T8 peak aged Al-Cu-Li-Mg-Ag-Zr alloy, improving the mechanical properties of the alloy. Compared with the T6 peak aged, the amount of T₁ phase precipitated in the T8 peak aged Al-Cu-Li-Mg-Ag-Zr alloy significantly increases, the size significantly decreases, the distribution is more uniform and dispersed, and the strength and plasticity are significantly improved.

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Microstructure of 11%Cr ferritic/martensitic steel in normally heat-treated state before and after irradiation

Wo Jianxing, Shen Yinzhong

Heat Treatment of Metals 2025, 50 (1): 6-11. doi:10.13251/j.issn.0254-6051.2025.01.001

Abstract （109）

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The microstructure of 11%Cr ferritic/martensitic steel in the normally heat-treated state before and after irradiation was experimentally studied by using transmission electron microscopy and energy dispersive spectroscopy. The results show that the primary microstructure of the 11%Cr ferritic/martensitic steel before and after irradiation consists of tempered martensitic along with a small amount of δ-ferrite. Before irradiation, a small amount of precipitates with a relatively small size are present at martensite boundaries and within martensite laths. Some irregular, blocky, large black particles are present at the interface of δ-ferrite and matrix. There are no precipitates within δ-ferrite grains. After irradiation, a large number of rod-like and blocky precipitates appear at the boundaries and interior of the martensitic lath, while the number and size of precipitates significantly increase compared to that before irradiation. The number of irregular blocky black particles at the interface of δ-ferrite and matrix increase compared to that before irradiation. Inside the δ-ferrite, a large number of blocky and needle-like precipitates are formed which may be Cr-rich nitrides, Cr-rich carbonitrides and Fe-W type precipitates. After irradiation, the precipitates inside the martensitic laths are Cr-rich M₂₃C₆ phase. Compared with the Cr-rich M₂₃C₆ phase before irradiation, the Cr and Ta contents in the M₂₃C₆ phase slightly decrease, while the Fe and Nd contents slightly increase after irradiation. However, overall, the changes in the average metal element composition of the Cr-rich M₂₃C₆ phase are relatively small.

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Regulating microstructure and mechanical properties of 7075 aluminum alloy via microalloying

Chen Zijian, Lin Yejia, Li Chuanqiang, Deng Renxuan, Dong Yong, Zhang Zhengrong

Heat Treatment of Metals 2024, 49 (9): 58-63. doi:10.13251/j.issn.0254-6051.2024.09.010

Abstract （103）

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Microstructure, phase composition, tensile properties and hardness of a series of modified 7075 aluminum alloys were comparatively studied by co-adding trace elements of Ni, RE (La and Ce) and Mg, so as to regulate the microstructure and mechanical properties of the 7075 aluminum alloy. The results show that after Ni-RE-Mg microalloying, the type of second phases in as-cast 7075 aluminum alloy does not change, but their content increases, and also the grain size is refined to some extent. In addition, the tensile property of as-cast 7075 alloy is improved via Ni-RE-Mg microalloying, and the maximum tensile strength of the as-cast alloy is 338 MPa when the mass fraction of Ni-RE-Mg is 0.2%. After solution treatment and aging, the precipitated phases in the alloy increase and the age-hardening phenomenon is more obvious with the increase of microalloying element content. When the Ni-RE-Mg mass fraction is 1.0%, the maximum hardness of the 7075 alloy after solution treatment and aging (6 h) reaches 155 HV0.5.

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Effects of carbon content and heat treatment on microstructure and mechanical properties of TiAl alloys with high Nb

Gao Yukui, Fang Wenyi

Heat Treatment of Metals 2024, 49 (8): 9-14. doi:10.13251/j.issn.0254-6051.2024.08.002

Abstract （96）

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Taking Ti-44Al-8Nb-xC(x=0, 0.3, 0.6) alloys as the research object, the effects of three different carbon contents and two aging temperatures on the microstructure, microhardness and high-temperature tensile properties were studied. The results show that when the carbon content increases to 0.6%, Ti₂AlC precipitates in the alloy. When the aging temperature increases from 800 ℃ to 900 ℃, Ti₃AlC precipitates in the alloy containing carbon. The increase in carbon content and aging temperature are all beneficial to improving the microhardness and tensile strength at 850 ℃ of the alloy, where the highest microhardness is (477.6±9.1) HV0.3, and the highest tensile strength at 850 ℃ is (605.07±5.01) MPa, which increases by 32.7% compored to the lowest tensile strength. The elongation is related to the number and size of carbides, and all the tensile fractures show brittle fracture. The alloy with 0.3% carbon has the best plasticity when aged at 900 ℃, the highest elongation is 5.41%±0.42%, which increases by 67.5% compared to the lowest elongation.

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Effect of SiC content on microstructure and mechanical properties of SiC_p/2009Al composites

Xiao Zhitong, Wang Fangming, Yan Zehua, Zhang Yong, Cao Zhen, Hu Huaigang, Li Jiongli, Wang Xudong

Heat Treatment of Metals 2024, 49 (6): 190-197. doi:10.13251/j.issn.0254-6051.2024.06.031

Abstract （85）

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SiC_p/2009Al composites were prepared by introducing SiC particles with different mass fractions (0, 1%, 3% and 5%) into 2009Al alloy by hot isostatic pressing process, followed by hot extrusion, solution and aging treatment, and the effects of the process on microstructure and properties of the composites were investigated. The results show that the composites with uniform grain and uniform dispersion of SiC can be prepared by the hot isostatic pressing process, and the SiC particles are well bonded with the interface of Al matrix. the microstructure of the composites is obviously changed by the extrusion process, the reinforcing SiC particles are broken and refined and homogeneously distributed along the extruded flow in the matrix when the content of SiC is lower (≤3%), while when the SiC content is higher (>3%), there is a slight agglomeration of SiC particles. After solution and aging treatment, the uniformity of matrix grain size is improved, and the precipitated second phase is fine and uniformly dispersed in the matrix. By hot extrusion, solution and aging treatment, the SiC_p/2009Al composites can obtain superior comprehensive mechanical properties. When 3% SiC is introduced, the preferable mechanical properties of the composites are obtained with tensile strength of 492.7 MPa, yield strength of 400.9 MPa, elongation after fracture of 12.7%, hardness of 131 HBW, and a good wear resistance at room temperature.

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Effect of Al and N contents on austenite grain size of gear steel

Guo Tao, Sui Yue, He Xiaofei, Xue Yanjun, Huang Feng

Heat Treatment of Metals 2024, 49 (7): 113-120. doi:10.13251/j.issn.0254-6051.2024.07.017

Abstract （81）

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Austenite grain growth behavior of two gear steels with different contents of Al and N austenitized at 980, 1050 and 1100 ℃ was experimentally studied. Equilibrium dissolution of AlN precipitates and its effect on austenite grain size were studied by means of optical microscope(OM), scanning electron microscope(SEM), transmission electron microscope(TEM) and carbon film replicas. The results show that with the increase of Al and N content, the critical temperature for abnormal growth of austenite grains in the gear steel also increases. When Al content is 0.038% and N content is 0.019%, the austenitizing temperature of the tested steel can be increased to 1050 ℃, at which after holding for 6 h, the austenite grains are still small and uniform, with grain size levels ranging from 7.5 to 8. Through the analysis of AlN precipitates, it is found that with the increase of austenitizing temperature, the AlN precipitates transforms from polygonal and nearly spherical to elongated rod-shaped, and its size continuously increases. When its equivalent radius is greater than 30 nm, the AlN precipitates cannot pin the austenite grain boundaries, resulting in mixed grains. Due to the coarsening and dissolution of the AlN precipitates, the growth rate of the ultimate grain size of austenite becomes faster and faster, and the ultimate pinning force for abnormal growth of austenite grains is between 0.045 and 0.095 J/cm³.

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Effects of cooling rate and Nb microalloying on microstructure and hardness of 25MnV non-quenched and tempered steel

Liu Yunna, Sun Lican, Dai Guanwen, Liu Xianda, Song Renbo, Zhang Chaolei

Heat Treatment of Metals 2024, 49 (8): 31-35. doi:10.13251/j.issn.0254-6051.2024.08.005

Abstract （81）

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Effect of cooling rate on microstructure and hardness of two non-quenched and tempered forged steels microalloyed with V and V-Nb was studied by single compression test carried out on a Gleeble-3800 thermal simulator. The results show that the cooling rate has a significant effect on the microstructure and hardness of the tested steels. Under the condition of continuous cooling, with the increase of cooling rate in the range of 0.1-0.5 ℃/s, the ferrite content decreases, the pearlite content increases, and the microstructure is refined. With the increase of cooling rate in the range of 1-3 ℃/s, the microstructure is further refined, and bainite transformation begins to occur, which promotes the increase of hardness. The addition of Nb element generates large-size (V, Nb)(C, N) precipitated particles, which further refines the grains by pinning the grain boundary and inducing the formation of intragranular ferrite. At the same time, the solute drag effect of Nb atom on C atom reduces the content of pearlite in the V-Nb steel. The higher hardness(218-242 HV) of the V steel in the cooling rate range of 0.1-0.5 ℃/s is due to its higher content of pearlite, and the higher hardness(256-291 HV) of the V-Nb steel within the cooling rate range of 1-3 ℃/s is due to the refinement of the microstructure and higher bainite content.

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Effect of aluminum content on microstructure and mechanical properties of high manganese high aluminum lightweight steels

Bai Rui, Du Yunfei, He Xiuli, Liang Hongyu

Heat Treatment of Metals 2024, 49 (8): 48-52. doi:10.13251/j.issn.0254-6051.2024.08.008

Abstract （81）

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Mechanical properties and the precipitation evolution of second phase of high manganese high aluminum lightweight steels as-cast and solution treated with different aluminum contents (10%, 12%, 14%, mass fraction) were investigated by using methods such as phase analysis, microstructural characterization and room temperature tensile and hardness tests. The results show that under as-cast state, with the increase of Al content, the content of ferrite in the tested steels increases and precipitating discontinuously between austenite grains, the size of κ-carbides continuously increases and mainly precipitating within austenite grains. After solution treatment at 1100 ℃, the tested steels exhibit improved strength and plasticity. The tested steel with 12% aluminum obtains excellent comprehensive properties, of which the density is 6.41 g·cm^-3, the specific strength is 157 MPa·cm³·g^-1, the elongation after fracture is 26.2%.

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Microstructure and properties of S30432 steel tube after long-term service at high temperature

Yang Xirui, Jiang Baoshi, Hu Fengtao, Li Yanjun, Sheng Chaojie, Wang Chen

Heat Treatment of Metals 2024, 49 (5): 17-21. doi:10.13251/j.issn.0254-6051.2024.05.003

Abstract （76）

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Microstructure and mechanical properties of the S30432 steel tube after high temperature service for 79 000 h in ultra supercritical units were studied by means of room temperature tensile and impact test, hardness test, high temperature tensile test, metallographic microscope, scanning electron microscope and energy dispersive spectrometer. The results show that the room temperature hardness and tensile strength of the 79 000 h long-term serviced S30432 steel tube increase, while the plasticity and toughness significantly decrease, the elongation is lower than the standard requirement. The tensile strength and plasticity at high temperature significantly decrease, and the tensile strength at 670 ℃ is lower than the standard extrapolation value. The microstructure of the tube material is seriously aged, and the twins disappear. Except for carbides such as NbC and M₂₃C₆, σ phases precipitate to varying degrees in the grain or at the grain boundary, which is the main reason for the significant decrease in plasticity, toughness and high temperature tensile strength.

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Research progress on C-Si-Mn partitioned steel

Zhang Hongliang, Zhou Chunbo, Hou Yan, Jiang Zhiqiang

Heat Treatment of Metals 2024, 49 (8): 22-30. doi:10.13251/j.issn.0254-6051.2024.08.004

Abstract （75）

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Quenched and partitioned(Q&P) steel has the characteristics of low cost, good properties, and relatively simple process, and becomes a key research object of the third generation of advanced high strength steel(AHSS). C, Si and Mn elements are the basic alloying elements of the Q&P steel, which have a significant impact on the microstructure and properties of the Q&P steel. Nowadays, most of the research is based on C-Si-Mn system partitioned steel. The production process of the Q&P steel is introduced, the effect of alloy elements on the microstructure and properties of the Q&P steel is clarified, as well as the relationship between the microstructure and mechanical properties of the Q&P steel. The focus is on the influence of process parameters on microstructure and properties of the Q&P steel, and prospects for the development of the Q&P steel are made.

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Effect of mixed addition of light and heavy rare earth elements on microstructure and mechanical properties of Al-Si-Mg alloy

Zeng Xiangjun, Shi Zhiming, Zhao Ge, Lian Hao, Liu Jiacheng

Heat Treatment of Metals 2024, 49 (8): 36-41. doi:10.13251/j.issn.0254-6051.2024.08.006

Abstract （74）

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Effect of mixed addition of light and heavy rare earth elements (La, Ce, Gd, Ho, Er) on the microstructure and mechanical properties of Al-6.9Si-0.36Mg alloy was studied by using optical microscope, scanning electron microscope/energy dispersive spectroscopy, and mechanical property testers. The results show that the refinement effect of α-Al grains in the as-cast alloy is significant with total rare earth addition of 0.1%. As total amount of rare earth added is 0.4%, the improvement effect on the eutectic Si phase is significant, and the eutectic Si phase changes from coarse needle-shaped to granular. After T6 heat treatment, the eutectic Si phase further transforms into small particles and uniformly distribute in the matrix, reducing the cutting effect on the matrix. When the total amount of rare earth elements added is 0.4%, the mechanical properties of the T6 heat treated alloy are the best with tensile strength of 285.4 MPa, elongation after fracture of 7.0%, and hardness of 89.2 HBW.

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Stress corrosion behavior of TP347H stainless steel and BT700-l alloy

Li Dongsheng, Liu Rencai, Tu Yuhang, Lan Keyu

Heat Treatment of Metals 2024, 49 (8): 1-8. doi:10.13251/j.issn.0254-6051.2024.08.001

Abstract （70）

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Stress corrosion cracking (SCC) behavior of TP347H stainless steel and BT700-l alloy in high concentration salt corrosion environment was studied by using slow strain rate tensile (SSRT) stress corrosion test. The results show that there is no SCC behavior between the TP347H stainless steel and BT700-l alloy in high concentration salt solution with different pH values. The TP347H stainless steel and BT700-l alloy show a certain SCC tendency under the condition of Cl^- sensitization and H⁺ catalysis in acidic corrosion environment, and the fractures are tough-brittle mixed cracking. The stress corrosion resistance of the BT700-l alloy in acid corrosion environment is significantly higher than that of the TP347H stainless steel.

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Hot deformation behavior and hot processing map of 2Ni6Cr5Al2MoV bearing steel

Han Weiran, Gao Yongliang, Li Meng, Zhu Chengxi, Wang Lin, Zhao Yan, Chen Dayu

Heat Treatment of Metals 2024, 49 (11): 10-16. doi:10.13251/j.issn.0254-6051.2024.11.002

Abstract （70）

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Hot deformation behavior of 2Ni6Cr5Al2MoV bearing steel under deformation temperature of 900-1200 ℃ and strain rate of 0.01-10 s^-1 was studied by Gleeble-3800 simulation machine. The high temperature constitutive equation and recrystallization critical strain model were established and verified. Based on the theory of dynamic material model, the hot processing maps of the steel at true strain of 0.6 and 0.7 were drawn. The results show that the flow stress of the steel increases with the increase of strain rate and the decrease of deformation temperature. Increasing deformation temperature and decreasing strain rate are beneficial to the dynamic recrystallization. The maximum energy dissipation rate η of the steel is about 0.37, and the strain variable has little influence on the dissipation factor. The suitable hot working conditions for the steel are deformation temperature of 1100-1200 ℃ and strain rate of 0.01-0.1 s^-1.

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Research progress of κ-carbide in Fe-Mn-Al-C low density steels

Li Changhao, Liu Zhi, Zhang Xiaofeng, Yang Yong, Zhou Huasheng, Zhao Xinlei

Heat Treatment of Metals 2025, 50 (1): 22-30. doi:10.13251/j.issn.0254-6051.2025.01.003

Abstract （69）

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As a new generation of automotive steel, the Fe-Mn-Al-C system low-density steel is a potential material for future automotive light-weighting research due to its advantages of high strength, low density, and good plasticity and toughness. As a result of κ-carbide affect the comprehensive mechanical properties of the steel, the characteristics and formation mechanism of κ-carbides in Fe-Mn-Al-C low-density steels and the effect of differences in the content of alloying elements on κ-carbides were expounded, and the effect of solution and aging treatments on κ-carbides and the effect of κ-carbides on the toughening mechanism were described in detail. It is concluded that the mass fraction of Al and Mn in Fe-Mn-Al-C steel should be reasonably controlled to provide driving force for κ-carbide precipitation, and the solution treatment temperature should be 900-1100 ℃, after which the aging should be carried out in the range of 450-600 ℃, and the aging time should be 1-2 h, to avoid the precipitation of coarse κ-carbides at the grain boundaries to deteriorate the material properties.

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Comparison on microstructure and mechanical properties of TP347HFG steel tubes served for different terms

Bao Zheng, Cheng Xiang, Wang Ruomin, Miao Chunhui, Chen Guohong, Tang Wenming

Heat Treatment of Metals 2024, 49 (5): 10-16. doi:10.13251/j.issn.0254-6051.2024.05.002

Abstract （69）

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Microstructure and mechanical properties of the TP347HFG high-temperature reheater steel tubes were studied comparatively in supplied state and that serviced for about 50 000 h and 110 000 h at 600 ℃, respectively. The results show that for the TP347HFG steel tube serviced for 50 000 h, normal austenite grain growth takes place, and the accompanying precipitation of the tiny M₂₃C₆ particles leads to the increase of dispersion strengthening effect, resulting in the yield strength higher than that of the steel tube in supplied state. However, the microstructure of the TP347HFG steel tube serviced for 110 000 h is severely degenerated with some abnormally growing austenite grains. Moreover, the coarse M₂₃C₆ particles have an increased separation effect on grain boundaries, resulting in a significant decrease of mechanical properties at room temperature and high temperature. Moreover, with the service time increasing, the impact properties and plastic deformation characteristics at fracture of the TP347HFG steel tube are continuously degraded, and the effect of microstructure aging on mechanical properties of the TP347HFG steel tubes in service for different terms turns to be more pronounced.

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