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Composition design and room temperature properties of Ni₃Al-based superalloys

Teng Zongyan, Xu Yanan, Wang Yinong, Liu Lin, Xu Zhaohui

Heat Treatment of Metals 2024, 49 (3): 209-214. doi:10.13251/j.issn.0254-6051.2024.03.035

Abstract （6）

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Chemical composition of Ni₃Al-based alloy was designed based on the “Cluster-plus-glue-atom model” to improve its room temperature properties, the microstructure, mechanical properties and corrosion resistance of the alloys were investigated. The results show that the alloys before and after annealing are composed of β phase, α phase and matrix phase γ'. After annealing at 1050 ℃ for 24 h, β phase and α phase are evenly distributed in the matrix of Ni₅Co₅Cr₂Al_2.5Ti_1.5 alloy, which improves not only the mechanical properties (maximum compression rate of 29.35%, yield strength of 1796 MPa, hardness of 546.35 HV), but also induces galvanic corrosion, enhances the passivation effect and improves the corrosion resistance (impedance of 7.74×10⁴ Ω, corrosion potential of -0.055 V(vs SCE), corrosion current density of 5.93×10^-7 A/cm²) of the alloys. The alloys designed in this paper show significant improvements in mechanical properties and corrosion resistance compared to Ni₃Al. This work provides a new avenue for research in the composition design of high-temperature alloys.

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Effect of V content on microstructure and properties of high nitrogen martensitic stainless steel

Zhang Hao, Chi Hongxiao, Wang Chengxi, Ma Dangshen, Lin Peng

Heat Treatment of Metals 2024, 49 (3): 215-221. doi:10.13251/j.issn.0254-6051.2024.03.036

Abstract （6）

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Microstructure and mechanical properties of the high nitrogen martensitic stainless steels with 0.32%V and 0.80%V (named as 0.3V and 0.8V steels) were studied by means of optical microscope, SEM, hardness test, impact test and salt spray corrosion test. The results show that carbide bands appear in both the two as-annealed steels. For the 0.8V steel, the carbide bands are more pronounced, the annealed hardness is 257.0 HBS, which is 7% higher than that of the 0.3V steel, and the increase of V content can reduce the austenite grain size and retained austenite content after quenching. After the same heat treatment (quenched at 1050 ℃ for 30 min, cryogenic treatment at -73 ℃ for 2 h, and tempered at 250 ℃ for 2 h), the hardness and impact absorbed energy of the 0.3V steel are 58.4 HRC and 9.4 J, respectively, while those of the 0.8V steel are 54.4 HRC and 12.7 J, respectively, which implies that the increase in V content reduces the hardness of the tested steel and improves the toughness. After salt spray corrosion for 120 h, the corrosion rate of 0.3V steel is 0.0235 g·m^-2·h^-1and there is no obvious corrosion pit on the steel surface, while the corrosion rate of the 0.8V steel is 0.0258 g·m^-2·h^-1, and there are obvious corrosion pits on the surface, which indicates that the corrosion resistance of the 0.3V steel is slightly better than that of the 0.8V steel, meaning that the corrosion resistance slightly decreases with the increase of V content.

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Effect of trace Ce on microstructure, mechanical properties, and thermal conductivity of hot-extruded Mg-2Zn-0.4Ca-0.2Mn alloy

Tang Bei, Wang Haili, Fu Jinlong, Xu Shanna

Heat Treatment of Metals 2024, 49 (3): 222-229. doi:10.13251/j.issn.0254-6051.2024.03.037

Abstract （8）

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Mg-2Zn-0.4Ca-0.2Mn(wt%) alloy with high thermal conductivity and elongation was prepared by near solidus temperature extrusion method, and the effect of trace Ce element on microstructure and properties was explored. The results indicate that the grain size in the extruded alloy exhibits a bimodal distribution, and the average grain size is significantly decreased compared with that of as-cast state. The addition of Ce not only transforms the second phase in the alloy from Ca₂Mg₆Zn₃ phase to (Mg, Zn)₁₁Ce phase, but also reduces the unrecrystallized area fraction from 5.4% to 1.6%, with an average grain size of 5.29 μm dropping to 3.68 μm, and the basal texture intensity of the alloy is significantly weakened. Due to the grain refinement strengthening effect and that of the second phase containing Ce, the yield strength of the alloy is significantly increased from 163.67 MPa to 237.44 MPa, while the elongation is reduced from 27.45% to 19.58%. The addition of Ce increases the volume fraction of the second phase in the microstructure, but it also reduces the solid solubility of Zn in the matrix, so the thermal conductivity of the alloy is not significantly reduced.

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Effect of particle size on microstructure and mechanical properties of high volume fraction SiC_p/Al composites

Cui Yan, Meng Lingjian, Yang Yue, Meng Yi, Liu Yuan, Cao Leigang

Heat Treatment of Metals 2024, 49 (3): 230-235. doi:10.13251/j.issn.0254-6051.2024.03.038

Abstract （6）

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In order to investigate the effect of SiC particle size on the microstructure and mechanical properties of SiC_p/Al composites, three kinds of SiC_p/2024Al composites containing 55%(volume fraction) SiC particles with size compositions being 100% 10 μm, 25% 10 μm+75% 76 μm and 100% 76 μm were fabricated by hot isostatic pressing and labelled as S, G and L, respectively. The results show that all the three SiC/2024Al composites contain SiC, Al and Al₂Cu phases. The flexural strength of the tested composites can be enhanced by increasing the proportion of fine SiC particles. The flexural strengths of S, G and L composites are 515, 489 and 422 MPa, respectively, and the corresponding fracture strains are 0.25%, 0.30% and 0.22%, respectively, indicating that the SiC particle gradation is an effective method to improve the density and the fracture strain of the composite. After annealing at 490 ℃, the flexural strengths of the three composites decrease to 494, 470 and 368 MPa, respectively, and the corresponding fracture strains increase significantly to 0.43%, 0.39% and 0.37%, respectively.

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Effect of Nb content on continuous cooling transformation rule, microstructure and properties of V-Ti-N structural steel

Tong Yang, Zhang Jing, Xin Wenbin, Luo Guoping, Peng Jun, Hou Dengyun

Heat Treatment of Metals 2024, 49 (3): 236-243. doi:10.13251/j.issn.0254-6051.2024.03.039

Abstract （6）

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Continuous cooling transformation curves of the Nb-V-Ti-N microalloyed steels were conducted on a DIL-805 dilatometer, and the effect of Nb content on the microstructure and properties was investigated. The results show that when the cooling rate increases from 0.1 ℃/s to 30 ℃/s, the undercooled austenite successively transforms into ferrite, pearlite, bainite and martensite, and the corresponding cooling rate ranges are 0.1-20, 0.1-5, 1-30 and 10-30 ℃/s, respectively. The transformation temperatures of ferrite, pearlite and bainite decrease with the increase of cooling rate, while that of martensite increases. As the Nb content increases from 0.05% to 0.10%, the solid solution of Nb in austenite increases, the CCT curves are slightly downward, and both Ac₁ and Ac₃ temperatures increase. With the increase of cooling rate, the microhardness is enhanced, and the corresponding value in the range of 1-20 ℃/s for the tested steel containing 0.10%Nb is higher than that for the steel with 0.05%Nb, which is largely related to the percentage of hardening phases, the degree of microstructure refinement and the precipitation of carbonitrides. In addition, the submicron Nb-rich carbonitrides precipitated at the high temperature region of austenite can not only pin grain boundaries but also act as heterogeneous nucleation sites to induce the formation of intragranular ferrite, which effectively refine the microstructure and consequently improve the strength and toughness.

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Effect of grain structure on stress corrosion cracking behavior and electric conductivity of Al-Cu alloy

Zhao Zhongchao, Tang Hezhuang, Cao Shanpeng, Sun Youzheng

Heat Treatment of Metals 2024, 49 (3): 244-250. doi:10.13251/j.issn.0254-6051.2024.03.040

Abstract （9）

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Though investigating and comparing the microstructure and stress corrosion cracking (SCC) behaviors at different locations of large-size Z-shaped sections of 2224 aluminum alloy in T3511 state, the effects of grain morphology on the SCC resistance and cracking modes were discussed. The results show that the specimens with fibre-like grains have lower stress corrosion sensitivity and are fractured in 52 days at stress of 250 MPa, while the specimens with nearly equiaxed grains are fractured in only 42 days. The nearly equiaxed grains have larger grain size and large-angle grain boundaries with higher interfacial energy, along which the stress corrosion crack is easy to propagate due to the lower grain boundary strength. The cracking mode of the specimens at different positions is all intergranular cracking, and the conductivity value in different positions of the Z-shaped section remains stable.

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Effect of vanadium content on microstructure and properties of stainless steel TDVS2001

Pei Mingde, Li Guoping, Qin Yuhang, Zhang Feng, Fan Xinzhi, Guo Honggang

Heat Treatment of Metals 2024, 49 (3): 251-255. doi:10.13251/j.issn.0254-6051.2024.03.041

Abstract （8）

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Effect of vanadium content on the microstructure and properties of duplex stainless steel TDVS2001 was investigated. The precipitates and microstructure were characterized by metallographic microscope, SEM, EDS and TEM. The results indicate that when the vanadium content is less than 0.11%, there is no vanadium precipitate in the metallographic structure and the mechanical properties are slightly improved. When the vanadium content is 0.27%, a large number of nano-scaled vanadium precipitates occur in the ferrite phase, the yield strength of the material increases by 68 MPa, while the impact absorbed energy has no obvious change. The vanadate is confirmed to be VN precipitation by transmission electron microscope. The smaller grain size and nano-scaled VN precipitation are the main reasons for the improvement of strength.

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Effects of alloying elements Si, V and Ti on microstructure and properties of 40Cr13 martensitic stainless steel

Liu Yu, Sun Yingjian

Heat Treatment of Metals 2024, 49 (3): 256-261. doi:10.13251/j.issn.0254-6051.2024.03.042

Abstract （8）

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Effects of Si, V and Ti alloying elements on the microstructure and mechanical properties of 40Cr13 martensitic stainless steel were studied by means of optical microscope, scanning electron microscope, room temperature tensile test and room temperature impact test. The results show that the addition of V and Ti alloy elements to the 40Cr13 martensitic stainless steel effectively inhibit the precipitation of (Fe, Cr)₂₃C₆ carbides. The hardness and impact property of the 40Cr13 martensitic stainless steel are effectively improved by increasing the content of Si element. The addition of 0.21%V improves the strength and hardness of the 40Cr13 martensitic stainless steel, but the addition of 0.21%Ti reduces the strength and hardness of the 40Cr13 martensitic stainless steel. The addition of V and Ti alloy elements at the same time reduces the tensile strength and yield strength of the 40Cr13 martensitic stainless steel slightly, but improves the impact toughness of the 40Cr13 steel effectively.

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Effect of Si addition on microstructure and properties of as-cast CoCrFeMnNi high entropy alloy

Li Rongbin, Zong Zaikang, Zhang Zhixi, Zhang Rulin

Heat Treatment of Metals 2024, 49 (2): 45-52. doi:10.13251/j.issn.0254-6051.2024.02.007

Abstract （29）

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CoCrFeMnNiSi_x(x=0,0.3, 0.6, 0.9) high entropy alloys were fabricated by cold crucible levitation melting. The changes of the microstructure of the alloy after the addition of silicon were analyzed by means of X-ray diffraction, scanning electron microscope and the attached energy spectrometer. A series of tests on the hardness, room temperature friction and wear and room temperature compression properties of CoCrFeMnNiSi_x alloy were carried out. The results show that CoCrFeMnNi alloy is a single-phase FCC structure. With the increase of silicon content, Mn, Ni and Si elements are enriched at the grain boundary of the FCC base phase, and silicide are gradually precipitated, finally resulting in fine grain strengthening and dispersion strengthening, which improves the hardness, wear resistance, compressive strength and yield strength of the alloy. Among them, CoCrFeMnNiSi_0.9 has the best comprehensive performance.

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Effect of Si on microstructure and properties of Al-1.5Cu-0.7Mg alloy

Tan Xinyu, Liu Jingjing, Liu Dongyu

Heat Treatment of Metals 2024, 49 (2): 53-59. doi:10.13251/j.issn.0254-6051.2024.02.008

Abstract （24）

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Effect of 0.3wt%Si addition on the microstructure and mechanical properties of Al-1.5Cu-0.7Mg alloy was studied by means of mechanical test, resistivity test at 20 ℃ and microstructure observation. The results show that with the 0.3wt%Si addition, the as-aged strength of the tested alloy is obviously increased. After peak aging at 473-493 K, its 20 ℃ resistivity is 3.638-3.667 μΩ·cm, and its tensile strength is 370-380 MPa, and then the relationship between the 20 ℃ resistivity and the relative volume fraction of the second phase precipitated during aging is established according to Matthiessen rule. The second phase precipitation activation energies in aging of the Al-1.5Cu-0.7Mg and Al-1.5Cu-0.7Mg-0.3Si alloys obtained by using JMA (Johnson-Mehl-Avrami) equation are 135 kJ/mol and 204 kJ/mol respectively, according to which to know that the S″ phase is dominant in the Al-1.5Cu-0.7Mg alloy and the S phase is dominant in the Al-1.5Cu-0.7Mg-0.3Si alloy. The resistivity under other aging parameters is also predicted by JMA equation, and the accuracy of the prediction model is all over 99%.

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Measurement and analysis of SH-CCT curves of Q690CFD HSLA hydropower steel

Cao Jiali, Jin Hongze, Li Kangli, Xu Yapeng, Zhao Qiang, Wang Cuiping, Li Mengnan

Heat Treatment of Metals 2024, 49 (2): 60-65. doi:10.13251/j.issn.0254-6051.2024.02.009

Abstract （15）

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SH-CCT curves of Q690CFD high strength steel for hydropower were established by using Gleeble-3800 thermal simulation test machine, and the effect of cooling rate on the microstructure and hardness of the coarse grain heat affected zone of the tested steel was studied. The results show that three transformation forms as martensite, martensite and bainite and bainite occur in the heat affected zone (HAZ) of the tested steel at the measured cooling rate. When the cooling rate is 40 ℃/s, the hardness of the HAZ reaches the maximum of 344 HV10, and the microstructure is composed of lath martensite and bainite ferrite. With the decrease of cooling rate, granular bainite begins to appear, and the hardness of the matrix decreases gradually. At the same time, the shape of M-A component in bainite changes from granular and short rod to lamella. When the cooling rate is lower than 5 ℃/s, the microstructure of the HAZ is all granular bainite, and the hardness of the matrix is lower than that of the base metal.

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Determination of continuous cooling transition curves and microstructure analysis of pearlitic rail steel

Zhang Le, Jiang Hongli, Xie Benchang, Wang Dongmei, Cen Yaodong, Chen Lin

Heat Treatment of Metals 2024, 49 (2): 66-70. doi:10.13251/j.issn.0254-6051.2024.02.010

Abstract （26）

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Continuous cooling transition curves of the supercooled austenite of the pearlitic rail steel was plotted by using the L78 quenching dilatometer. Combined with the metallographic structure analysis and hardness test results, the microstructure and mechanical properties of the pearlitic rail steel during continuous cooling were studied. The results show that the critical phase transition temperature of the test steel is as follows: Ac₁=735.3 ℃,Ac_cm=812.2 ℃, Ar₁=697.9 ℃, Ar_cm=773.8 ℃. The tested steel was heated to 860 ℃ at 10 ℃/s for 10 min to complete austenitization and then cooled. When the cooling rate is 3 ℃/s or below, the microstructure is pearlite and local ferrite. When the cooling rate is 4-8 ℃/s, the microstructure is pearlite and a little martensite. When the cooling rate is above 11 ℃/s, the microstructure is martensite and retained austenite. There is no bainite transition. To comprehensively analyze the microstructure and hardness of the rail steel, in order to form the refined pearlite microstructure and a small amount of martensitic microstructure, the cooling rate between 4-8 ℃/s should be selected.

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Effect of La content on solidification process and as-cast microstructure of GH5188 alloy

Jiang Shichuan, Wang Xiaochuan, Tang Pingmei, Li Jing, Zhou Yang

Heat Treatment of Metals 2024, 49 (2): 71-76. doi:10.13251/j.issn.0254-6051.2024.02.011

Abstract （19）

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The solidification process of GH5188 alloy with different La contents (0.03%, 0.07%, 0.10% and 0.40%) at the cooling rate of 10 ℃/min was dynamic in-situ observed by using confocal laser scanning microscope (CLSM), and the effects of La content on the solidification process, as-cast microstructure and precipitated phases of the GH5188 alloy were studied. The results show that with the increase of La content, the initial solidification temperature and the solid-liquid two-phase region of the GH5188 alloy become higher and larger. The solidification process is slow-rapid-slow solidification, and the higher the La content, the smaller the peak solidification rate. With the increase of La content, the as-cast microstructure of the alloy changes from dendrite to equiaxed crystal, and the size of the precipitated phases increases. When the La content reaches 0.40%, the precipitated phases change from M₆C+M₂₃C₆ to (Ni,Co)_xLa+M₂₃C₆, and the formed La-containing intermetallic compound is easy to crack during solidification.

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Effect of Er on microstructure and properties of multi-directional forged 7050 aluminum alloy

Wang Huaxing, Rong Li, Huang Hui, Wei Wu, Wang Zezhong, Zhou Li, Wang Meng

Heat Treatment of Metals 2023, 48 (12): 230-235. doi:10.13251/j.issn.0254-6051.2023.12.038

Abstract （12）

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7050 aluminum alloy and Er-containing 7050 aluminum alloy (7E50) were selected as the research object. After solution and aging treatment of free forgings of both aluminum alloys, the microstructure and mechanical properties were studied by means of SEM, TEM and room temperature tensile test. The results show that after 470 ℃×1 h solution treatment, the recrystallized fraction in the 7050 aluminum alloy is 69.45%, while that in the 7E50 alloy is only 62.08%, which shows that the addition of Er can inhibit the recrystallization behavior of the alloy. After the optimal single-stage peak aging process, that is 120 ℃×24 h, the strength, hardness and elongation of the 7E50 alloy are higher than that of the 7050 alloy, it can be seen that the Er addition can effectively improve the mechanical properties of the alloy. The precipitated phases of the 7E50 aluminum alloy are mainly η′ phase, GP region and Al₃(Er ,Zr) particles. The grain boundary precipitated phases of the two alloys show a continuous chain distribution, but the size of grain boundary precipitated phases in the 7E50 alloy is significantly finer than that in the 7050 alloy, which may be one of the main reasons why the elongation of the 7E50 alloy is higher than that of the 7050 alloy.

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Research progress of NiTi-based bearing alloys

Guan Lei, Liu Jiaxing, Sun Shibo, Tong Yunxiang

Heat Treatment of Metals 2023, 48 (12): 236-243. doi:10.13251/j.issn.0254-6051.2023.12.039

Abstract （9）

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NiTi-based alloys with higher Ni content have become an important bearing alloy because of their high hardness, low elastic modulus, good corrosion resistance and nonmagnetic, etc. In present work, recent progress of NiTi-based bearing alloys is comprehensively overviewed, mainly including microstructure, mechanical properties, frictional wear properties and corrosion resistance. The emphasis is laid on the effect of heat treatment and alloying element on mechanical properties of alloys. Some important issues on NiTi-based bearing alloys are discussed.

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Effect of Mo content on microstructure and corrosion resistance of 9Cr18Mo steel

Zhou Jian, Pian Liping, Fang Feng, Chi Hongxiao, Ma Dangshen

Heat Treatment of Metals 2023, 48 (12): 244-249. doi:10.13251/j.issn.0254-6051.2023.12.040

Abstract （10）

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Microstructure and corrosion resistance of the 9Cr18Mo steel with four different Mo contents were studied by microstructure analysis, electrochemical test and neutral salt spray test, then the influence mechanism of Mo on the corrosion resistance was discussed. The results show that the 9Cr18Mo steel with different Mo contents has a large number of chromium-rich carbides, and the size, quantity and hardness of carbides increase with the increase of Mo content. When the Mo content is less than 0.5%, the corrosion potential increases with the increase of Mo content, but decreases when the Mo content reaches 0.7%. After 120 h neutral salt spray test, the corrosion rate of 0.5Mo tested steel is low. The decrease of pitting corrosion resistance of the specimen containing 0.7% Mo is related to the formation of chromium-rich carbides and the formation of chromium-poor zone near the interface between the carbide and matrix.

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Effect of bionic hexagonal texture on tribological properties of 40Cr steel

Yang Yang, Long Weimin, Cheng Zhan, Wu Lifeng, Shi Lubing, Wu Yuanke

Heat Treatment of Metals 2023, 48 (12): 250-254. doi:10.13251/j.issn.0254-6051.2023.12.041

Abstract （7）

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Bionic hexagonal textures with different area densities were processed on the surface of 40Cr steel by laser processing technology. Friction and wear tests were carried out on high temperature tribometer under oil lubrication conditions. The influence in variation of rotational speed and load on the tribological properties of bionic hexagonal texture with different area density was studied. The wear morphology of the specimen surface was analyzed by means of metallurgical microscope. The results show that the friction coefficient of surface textured specimens is lower than that of un-textured specimens, decreases first and then increases with the increase of texture density. The friction coefficient of the specimen with texture density of 10% is 9.8% lower than that of the untextured surface. Wear resistance is improved. The friction coefficient of low density textured specimens increases first and then decreases with the increase of load, and the friction coefficient of high density decreases with the increase of load. The friction coefficient of the specimen decreases with the increase of speed. Under the condition of oil lubrication, the bionic hexagonal texture can effectively improve the friction and wear properties of the friction pair surface, and the reasonable texture area density can maximize the friction reduction effect of the bionic texture.

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Effect of C content on microstructure and mechanical properties of HT9 steel

Xie Pu, Zhao Jiqing, Yan Tingting

Heat Treatment of Metals 2023, 48 (12): 255-259. doi:10.13251/j.issn.0254-6051.2023.12.042

Abstract （16）

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Effect of carbon content change on the microstructure and mechanical properties of HT9 steel was studied by means of optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and tensile testing machines. The results show that the change of C content has a significant impact on the content of carbides and properties. With the increase of C content, the Ni_eq increases, which inhibits the precipitation of δ-Fe; the C also promotes the precipitation of M₂₃C₆ carbides, hinders the migration of martensitic laths, and leads to lath refinement, precipitation strengthening and interface strengthening, which increases the strength and hardness of the steel and reduces the plasticity and toughness of the material. When the C content is in the range of 0.17%-0.20%, this change is the most significant, which is mainly related to the precipitation of solute atoms in the steel and the decline of solution strengthening effect.

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Effect of Cr on microstructure evolution and corrosion resistance of pipeline steel

Peng Haoping, Su Wei, Zhao Yonggang, Li Zhiwei, Su Xuping, Wu Changjun

Heat Treatment of Metals 2023, 48 (12): 260-268. doi:10.13251/j.issn.0254-6051.2023.12.043

Abstract （7）

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Three specimens of X80 steel with different Cr contents (mass fraction) of 1%, 2% and 3% were prepared by means of magnetron arc furnace, and the effects of Cr on microstructure and corrosion resistance were tested by means of SEM, DSC, electrochemical test, EDS and Raman characterization. The results show that the ferrite grain size decreases with the increase of Cr content, and the number of pearlite increases, and the change from the X80 steel to 2Cr steel is obvious. The value of impedance modulus |Z|_0.01Hz at low frequency 0.01 Hz increases with the increase of Cr content and decreases with the prolorgate of soaking time. When soaked in 3.5%NaCl solution for 1 day, the |Z|_0.01Hzvalue of the tested 3Cr steel is about 1315 Ω·cm², and that of the X80 steel is about 910 Ω·cm². When soaked for 14 days, the |Z|_0.01Hzvalue of the tested 3Cr steel is about 800 Ω·cm², and that of the X80 steel is about 470 Ω·cm². The corrosive medium and the corrosive reaction breaks down the steel surface and pitting pits can be seen clearly. Corrosion products are mainly composed of β-FeOOH, γ-FeOOH, α-FeOOH, Fe₃O₄ and Fe₂O₃, etc. Elevated Cr content, more and more stable phases of Fe₃O₄ and α-FeOOH appear on the steel surface, helping to enhance the stability of the corrosion product film.

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Carbide evolution law of high chromium and high vanadium powder metallurgy tool steel M390 during austenitizing

Deng Jinqiang, Zhang Qinyi, Chen Jun, Wu Dong, Liu Wei, Zhou Hongfeng

Heat Treatment of Metals 2023, 48 (11): 1-7. doi:10.13251/j.issn.0254-6051.2023.11.001

Abstract （93）

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Evolution of carbides in the M390 steel during different austenitizing processes was studied using analysis methods such as SEM-EDS, TEM, and Image-Pro Plus. The results show that under the austenitizing conditions of heating at 1075-1150 ℃ and holding for 15-120 min, the volume fraction and average size of carbides in the M390 steel show a decreasing trend as the quenching temperature increases. At the same quenching temperature, with the prolongation of holding time, the amount of carbides decreases as a whole, the average size of carbides decreases first and then increases, while the amount of small carbides and hardness increase first and then decrease. The types of undissolved carbides in high temperature oil quenched steel are mainly M₇C₃ and M₈C₇. During austenitizing, the change of carbides in the M390 steel is mainly the dissolution, growth and dissolution of M₇C₃.

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