Heat Treatment of Metals

Effects of rapid heating and short time holding on microstructure and properties of 22MnB5 steel

Zhou Zeyu, Hua Fuan

2022, 47(12): 1-6. doi:10.13251/j.issn.0254-6051.2022.12.001

Abstract ( 104 )

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Rapid heat treatment method of rapid heating, short time holding and rapid cooling was applied to hot forming process of 22MnB5 steel. The simulation test of material thermal process was carried out by using continuous annealing tester to study the change rule of phase transformation point temperatures and the evolution rule of microstructure and properties of the 22MnB5 steel under different process parameters, and the corresponding mechanisms were revealed. The results show that the increase of heating rate increases the transformation point temperature, and the heating temperature needs to be higher than 950 ℃ to ensure complete austenitization. The final structure resulted by traditional hot forming process is full martensite structure, while that by the rapid heat treatment process is composed of martensite, bainite and undissolved carbide, and the grain size is refined from 11.50 μm to 7.60 μm. The mechanical properties are better when heating at 1000 ℃ for 10 s, the tensile strength is slightly improved compared with that by the traditional hot forming process, and the elongation is increased from 6.39% to 8.44%. The reason for the resulted microstructure with such excellent properties is related to the uneven composition caused by rapid heating and short time heat preservation.

Effect of high temperature aging on microstructure and hardness of P91 steel

Zhang Kai, Wang Xue, Ni Mansheng, Liu Junjian, Du Chengchao

2022, 47(12): 7-12. doi:10.13251/j.issn.0254-6051.2022.12.002

Abstract ( 109 )

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P91 steel was aged at 650 ℃ for 2935 h. The hardness change during aging was measured, and the influence of microstructure change on hardness was analyzed. The results show that in the early aging period, the hardness increases slightly due to the precipitation of M₂₃C₆. During the subsequent aging, M₂₃C₆ is matured and its quantity density decreases, at the same time, the hardness of the P91 steel decreases slowly due to the slow growth of sub crystal and slight decrease of dislocation density. According to the changes of microstructure and hardness during the aging process, it can be predicted that the P91 steel has better stability of microstructure and properties when it is in service at the temperature not exceeding 600 ℃.

Effect of quenching-long partitioning treatment on microstructure and mechanical properties of V-Ti-N microalloyed high strength steel

Sun Dingbo, Huang Xuefei

2022, 47(12): 13-18. doi:10.13251/j.issn.0254-6051.2022.12.003

Abstract ( 69 )

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Effect of quenching-long partitioning (Q-LP) treatment on microstructure and mechanical properties of V-Ti-N microalloyed steel was investigated by means of scanning electron microscope, X-ray diffractometer, universal electronic material testing machine and pendulum impact testing machine. The results show that the microstructure of the tested steel is composed of martensite, retained austenite and a little bainite after the one-step Q-LP and two-step Q-LP treatments. The content of retained austenite in all specimens reduces with the prolonging of partitioning time. However, retained austenite content is higher in the two-step Q-LP specimens than in the one-step Q-LP specimens under the same partitioning time. The yield strength of the one-step Q-LP specimens reduces with the prolonging of partitioning time, while elongation and impact property increase. The one-step Q-LP specimen exhibits the most remarkable mechanical properties with the partitioning time of 60 min. The yield strength, elongation and impact property of the two-step Q-LP specimens increase first and then become stable with the prolonging of partitioning time. The two-step Q-LP specimen exhibits the most remarkable mechanical properties with the partitioning time of 30 min. Specimens treated by the two-step Q-LP process exhibit better mechanical properties than those treated by the one-step Q-LP process under the same partitioning time. Compared with one-step Q-LP specimens, more retained austenite in two-step Q-LP specimens, as well as microstructure softening and residual stress release caused by higher partitioning temperature, are the key to improve the properties of the two-step Q-LP specimens.

Effect of heat treatment on thermal expansion behavior and mechanical properties of Fe-36Ni Invar alloy foil

Cai Chen, Gu Yu, Li Jingyuan

2022, 47(12): 19-27. doi:10.13251/j.issn.0254-6051.2022.12.004

Abstract ( 54 )

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Evolution of thermal expansion behavior and mechanical properties of Fe-36Ni Invar alloy foil with thickness of 60 μm under cold rolled, annealed and quenched conditions was investigated, respectively. The results show that the cold rolled alloy has the smallest thermal expansion coefficient, the quenched state is second, and the annealed state has the largest thermal expansion coefficient. Heat treatment can effectively increase the Curie temperature T_c of the alloy to increase the service temperature range, and the specimen quenched at 900 ℃ for 1.5 h has the best thermal expansion properties ($\bar{α}$_(20-100℃)=1.02×10^-6 K^-1, T_c=276 ℃). The rise in the number of free oriented grains is the main reason for the increase of thermal expansion coefficient. Compared with cold rolled state, complete recrystallization and annealing twinning of the alloy occur after heat treatment, accompanied by change in grain size and a rapid increase in the proportion of ∑3ⁿ grain boundaries. The specimen quenched at 800 ℃ for 1.5 h has the smallest grain size (6.6 μm) and the highest proportion of ∑3ⁿ grain boundaries, giving the highest yield strength (267 MPa) and tensile strength (414 MPa). In summary, the comprehensve properties of the quenched specimen are better than that of the annealed specimen. However, under the same heat treatment method, increasing the heat treatment temperature on the one hand reduces the coefficient of thermal expansion and increases the Curie temperature, and on the other hand reduces the strength.

Processability of AlCoCrFeNi high entropy alloy fabricated by selective laser melting

Wei Shuimiao, Ma Pan, Zhang Zhiyu, Fang Yacheng, Yang Zhilu, Zhang Nan

2022, 47(12): 28-35. doi:10.13251/j.issn.0254-6051.2022.12.005

Abstract ( 57 )

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AlCoCrFeNi high entropy alloy was fabricated by selective laser melting (SLM) technology. The effect of laser process parameters on processability, densification, microstructure and mechanical properties was studied. The results show that the relative density gradually increases with the increase of volumetric energy density. The best process parameters is the laser power of 50 W, the scanning speed of 300 mm/s, the scanning spacing of 70 μm and the layer thickness of 30 μm. The as-cast and SLMed alloy has a dual-phase body-centered cubic structure composed of disordered BCC phase (A2) and ordered BCC phase (B2). Due to the fine grain strengthening effect, the SLMed specimen has higher microhardness than that of the as-cast one, but the compressive yield strength decreases. The reason is due to the defects such as cracks and holes in the SLMed specimen.

Effect of annealing temperature on microstructure and mechanical properties of rolled wrought Mg-Li alloy

Xu Weijian, Wang Siyu, Wang Bingxing

2022, 47(12): 36-42. doi:10.13251/j.issn.0254-6051.2022.12.006

Abstract ( 52 )

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Microstructure evolution, mechanical properties and deformation mechanism of Mg-8Li-1Al-0.5Sn alloy after cold-rolling (deformation of 75%) and then isothermally annealed for 1 h at 150, 200, 250 and 300 ℃, respectively, were studied. The results show that the elongation increases first and then decreases with the increase of annealing temperature. When the annealing temperature is 200 ℃, the elongation of the alloy reaches the best, which is 40%, and compared with the cold-rolled alloy, the strength without weakening is 212 MPa, and the elongation is increased by 24.4%. The ductility of the alloy is improved mainly because that the annealing promotes the transformation of α phase from strip to bamboo-like, alleviates stress concentration and promotes static recrystallization and grain refinement of β phase. In addition, the α phase rolling texture is angular deflected during annealing, which retains the {1010} grain direction texture conducive to slip and promotes the elongation of the alloy.

Effect of solution treatment on grain size of 316H stainless steel medium plate

Zhuang Ying, Yin Wei

2022, 47(12): 43-48. doi:10.13251/j.issn.0254-6051.2022.12.007

Abstract ( 81 )

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316H stainless steel with 300 mm thickness electroslag ingot forging billet rolled to steel plate with 50 mm thickness, the original grains sizes of grade 5, 6, 7 and 9 were selected. Solution tests were carried out at 1000 ℃ for 60 min and 120 min, 1050 ℃ for 30 min and 60 min, and 1100 ℃ for 20, 40 and 60 min. The results show that obtaining a fully recrystallized hot-rolled microstructure with uniform grains is a necessary condition for obtaining solution microstructure with uniform grain. For hot-rolled mixed crystals or partially recrystallized microstructure, a solution microstructure with unifor grain cannot be obtained by solution treatment. For the hot-rolled steel plate with uniform grains, by reasonably matching the solution temperature and time, the grains can grow uniformly. The reasonable solution system of the 316H stainless steel plate with thickness of 20-50 mm is solution at 1050 ℃ for 30-60 min, and at 1100 ℃ for 20-40 min , the solution time can be adjusted according to the hot-rolled microstructure and user needs to obtain the ideal grain size level.

Effect of salt bath nitriding treatment on microstructure and properties of FeCrMnNiAl_0.2Ti_0.1 high entropy alloy

Liu Yi, Gan Zhanghua, Wu Chuandong, Ni Qian, Wang Jiamin

2022, 47(12): 49-55. doi:10.13251/j.issn.0254-6051.2022.12.008

Abstract ( 52 )

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Chemical heat treatment method of salt bath nitriding was used to strengthen the surface of FeCrMnNiAl_0.2Ti_0.1high entropy alloy, and the main process was preheating + salt bath nitriding + oxidation. The effect of nitriding temperature on infiltration layer and properties of the alloy was studied. The microstructure and phases of the tested alloy at different nitriding temperatures were studied by metallographic microscope, scanning electron microscope and X-ray diffractometer. The hardness and wear resistance of the alloy were measured by microhardness tester and W-2000 friction and wear tester, respectively. The results show that after salt bath nitriding, a composite nitrided layer containing nitride and oxide is formed on the surface of the alloy. The highest depth of nitrided layer is 27.1 μm and the hardness can reach 1080.0 HV0.2. Salt bath nitriding can effectively enhance wear resistance of the high entropy alloy and improve the tribological behavior. The wear rate of the nitrided specimen at 640 ℃ is only 0.025 mm³/(N·m), which is about 76.7% lower than that of the as-cast specimen.

Effect of heat treatment on microstructure and precipitates of 5Cr steel with RE addition

Li Tao, Xu Tao, Han Qiang, Sun Hao, Bai Yansong, Liang Zhanlei

2022, 47(12): 56-61. doi:10.13251/j.issn.0254-6051.2022.12.009

Abstract ( 51 )

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Taking 5Cr steel with RE addition as the object, the effect of heat treatment (water quenching at 870, 900, 930 ℃ for 50 min , and then tempering at 670, 690, 710 ℃ for 90 min) on microstructure and second-phase precipitation behavior of the tested steel was studied. The results show that after quenching at 870 ℃, the structure is not completely austenitized. The tested steel is completely austenitized with the increase of quenching temperature, and the prior-austenite average grain size is increased from 13.49 μm at 900 ℃ to 15.01 μm at 930 ℃, while the microstructure uniformity is obviously reduced. By selecting the proper quenching temperature as 900 ℃, and after tempering at 670-710 ℃, the microstructure is tempered troostite, tempered troostite + tempered sorbite and tempered sorbite in turn. The second phases after tempering are Cr₇C₃ carbide distributed on the matrix and Cr₂₃C₆ carbide aggregated at the interface. With the increase of tempering temperature, the proportion of Cr₂₃C₆ carbide increases gradually. In order to avoid the aggregation and coarsening of M₂₃C₆ carbides during tempering, the appropriate tempering temperature is 690 ℃.

Effect of solution treatment and aging on microstructure and hardness of TC11 titanium alloy

Zhu Ningyuan, Chen Shihao, Liao Qiang, Chen Qiuming, Lai Wenkun, Zuo Shoubin

2022, 47(12): 62-66. doi:10.13251/j.issn.0254-6051.2022.12.010

Abstract ( 185 )

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Effects of solution temperature and solution time on microstructure and microhardness of TC11 titanium alloy bar under the same aging treatment processes were studied. The results show that when the TC11 titanium alloy is solution treated below the phase transition point, the content of equiaxed primary α phase increases with the increase of solution temperature. When the solution temperature approaches the phase transition point, the content of equiaxed primary α phase decreases rapidly. When the solution temperature is 950 ℃, with the increase of solution time, the grain boundary α phase begins to grow, and the lamellar α phase turns into the mixed microstructure of bulk α phase and equiaxed α phase. When the solution time is fixed, the hardness of the TC11 titanium alloy decreases first and then tends to be stable with the increase of solution temperature. The hardness reaches the maximum value when solution temperature is 950 ℃ and solution time is 120 min.

Effect of quenching temperature on microstructure and properties of ultrafine-grained Q&P steel

Guo Haodong, Tang Zhengyou, Zhao Li, Guan Guofu, You Zeyu, Ding Hua

2022, 47(12): 67-73. doi:10.13251/j.issn.0254-6051.2022.12.011

Abstract ( 72 )

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Effect of quenching temperature on microstructure, element distribution, retained austenite volume fraction and mechanical properties of ultrafine-grained Q&P steels was investigated. The results show that when the annealing temperature increases, the ferrite content gradually decreases, the martensite content increases, and the retained austenite content increases first and then decreases; and at high quenching temperatures, the diffusion rate of C atoms is accelerated, the mechanical stability of retained austenite is better. The presence of soft-phase ferrite provides good toughness for the tested steel. When the quenching temperature is 820 ℃, the comprehensive mechanical properties of the Q&P steel are the best, the tensile strength is 863 MPa, the elongation is 26.1 %, and the strength-plasticity product is 22.5 GPa·%.

Effect of cooling process on microstructure and properties of 40CrMoNbVTi steel

Sun Yongzhen, Cheng Juqiang, Li Meixuan

2022, 47(12): 74-77. doi:10.13251/j.issn.0254-6051.2022.12.012

Abstract ( 64 )

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Effect of cooling process on microstructure and properties of 40CrMoNbVTi steel was studied. The results show that the specimens quenched at 780 ℃, oil cooled and tempered at 550 ℃ have higher tensile strength and impact absorbed energy, which are 1250 MPa and 78.63 J, respectively. After cooling by 20% polyethylene glycol quenching solution, the tensile strength is 1140 MPa, and the impact absorbed energy is 80.7 J. The microstructure is sorbite and a small amount of ferrite after oil cooling and 20% polyethylene glycol quenching solution cooling. The tensile strength of the specimens quenched at 860 ℃, fog cooled/air cooled, and tempered at 550 ℃ is 1010 MPa and 945 MPa respectively, the impact absorbed energy is 35.7 J and 38.4 J, respectively, and the microstructure is tempered sorbite or granular bainite. Quenching at 780 ℃, oil cooling/commercial quenching solution cooling are more suitable quenching cooling processes. The impact fracture mechanism of quenching at 780 ℃, oil cooling/20% polyethylene glycol quenching cooling, tempering at 550 ℃ is ductile fracture, while the impact fracture mechanism of quenching at 860 ℃, fog cooling/air cooling, tempering at 550 ℃ is brittle fracture. Increasing the quenching cooling rate can improve impact fracture morphology.

Effect of aging time on microstructure and properties of Zn-5.5Mg-0.4Ba-0.7Gd alloy

Liu Jingfu, Yin Kang, Lu Chaochao, Zhang Weibin

2022, 47(12): 78-83. doi:10.13251/j.issn.0254-6051.2022.12.013

Abstract ( 43 )

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Zn-5.5Mg-0.4Ba-0.7Gd alloy prepared by vacuum melting was aged at 150 ℃ for 2, 4 and 8 h respectively. The effect of aging time on microstructure and properties of the alloy was studied. The results show that aging can improve microstructure of the alloy. At the beginning of aging, MgZn₂ phase in the alloy gradually transforms into Mg₂Zn₁₁ phase. The electrochemical test results show that the alloy aged for 4 h has the best corrosion resistance, the self corrosion potential is -942.844 mV, and the self corrosion current density is 13.34 μA/cm², the micropore resistance is 1894 Ω·cm², and the charge transfer resistance is 1613 Ω·cm². The corrosion products of the alloy aged for 4 h are mainly composed of zinc phosphate (calcium), zinc carbonate (calcium) and zinc hydroxide with excellent biocompatibility.

Effect of cryogenic treatment on hardness, wear resistance and impact property of conical pick and its optimization

Zhang Haidong, Yan Xianguo, Dong Liang, Zhang Jie, Zhao Xuewen

2022, 47(12): 84-89. doi:10.13251/j.issn.0254-6051.2022.12.014

Abstract ( 47 )

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A new process of brazing-heat treatment-cryogenic treatment was proposed by introducing cryogenic treatment technology into the traditional brazing-heat treatment process of conical pick. The cryogenic treatment test scheme was designed by using the full factor method with the cryogenic temperature and time as the main process parameters, and the wear resistance of conical pick were tested by cutting rock test, along with determination of the impact toughness of pick body and the hardness of pick head and pick body, so as to study the influence of cryogenic treatment on the hardness, wear resistance and impact toughness of conical pick, finally getting the optimal cryogenic treatment process. The results show that the cryogenic treatment has no significant effect on the hardness of conical pick and a slight adverse effect on the impact toughness of conical pick, but it can significantly enhance the wear resistance of conical pick, which is beneficial to prolong the service life of conical pick. The optimal cryogenic treatment process is cryogenic temperature -196 ℃ and cryogenic time 12 h, in which the comprehensive performance is the best. At this time, the impact absorbed energy is only reduced by 1.2 J, while the wear resistance is increased by 41.6%.

Effect of aging on microstructure and properties of ADC12 die cast aluminum alloy

Ma Dongwei, Li Wenqiao, Zhang Yuanhao, Shi Qiuyue, Wu Xiaofeng, Wu Chengjie

2022, 47(12): 90-94. doi:10.13251/j.issn.0254-6051.2022.12.015

Abstract ( 104 )

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Effect of aging process on microstructure, mechanical properties and dimensional stability of ADC12 die cast aluminum alloy was studied by means of microscopy, tensile test and dimensional test. The results show that with the increase of aging time or temperature, strengthening particles are gradually dispersed and precipitated in the alloy, and the strength of the alloy tends to increase, with the further increase of aging time or temperature, the strengthened particles begin to aggregate and grow up, and the strength of the alloy decreases. In the heat preservation process after aging, there are two main factors affecting the specimen size, one is residual stress, the other is solid phase transformation. When the aging temperature is low or the aging time is short, the specimen size lengthening caused by residual stress relief is dominant, and the specimen size elongates. With the increase of aging time or temperature, the specimen length shortening caused by solid phase transformation is dominant. and the specimen size changes from positive value to negative value, that is the size shortens. Considering the tensile properties, economic benefits and the principle of minimizing size change of the alloy, the suitable aging process for the ADC12 alloy is 200-220 ℃×4 h or 220-240 ℃×2 h.

Effect of quenching specification on microstructure and mechanical properties of G50 steel

Gao Qi, Yang Zhuoyue, Wang Ao

2022, 47(12): 95-98. doi:10.13251/j.issn.0254-6051.2022.12.016

Abstract ( 44 )

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Core cooling specification of different diameter bars was formulated, based on the measured temperature change of rotor steel quenching process. The microstructure and mechanical properties of the G50 ultra-high strength steel after austenitizing and cooling at different cooling rates were studied. The results show that the cooling specimens equivalent ø430 mm and ø500 mm bars core form martensite and lower bainite complex structure, their tensile strength is higher than that of the oil quenched specimens, the yield strength is lower, but the impact property is not significantly affected. The core specimen equivalent ø650 mm rod forms more than 60% lower bainite, further reduces the yield strength, but obviously rises up the impact prooperty, showing that the steel can maintain high toughness in slower cooling rate.

Effect of tempering temperature and phosphating on mechanical properties and microstructure of 40CrNiMoA steel

Li Shijian, Pan Xin, Liu Yanmei, Yang Lixin, Hu Fuchang

2022, 47(12): 99-102. doi:10.13251/j.issn.0254-6051.2022.12.017

Abstract ( 130 )

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Aiming at the problem of unqualified shear property of standard parts which were made of 40CrNiMoA steel, the effect of tempering temperature on mechanical properties and microstructure was studied, and then the effects with and without phosphating process on shear property were compared and analyzed. The results show that with the increase of tempering temperature, both the tensile strength and shear property decrease gradually, while the plasticity is almost unchanged. The shear property of the steel with phosphating process is lower than that without. Thus, it is proposed that in order to ensure shear property of the product, the lower tempering temperature should be selected, and the effect of phosphating on reducing shear property should be simultaneously considered.

Effect of normalizing temperature on microstructure and mechanical properties of reduced activation martensitic steel

Zheng Tao, Li Yongwang, Wu Yu, Zhuo Hong, Shi Hanchao, Liu Chaohong

2022, 47(12): 103-108. doi:10.13251/j.issn.0254-6051.2022.12.018

Abstract ( 39 )

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Reduced activation martensitic steel wires were normalized at 1000-1100 ℃ for 60 min and then tempered at 790 ℃ for 90 min, and the effect of normalizing temperature on microstructure and mechanical properties of the reduced activation martensitic steel wires was investigated. The results show that after normalizing, the microstructure of the reduced activation martensitic steel is transformed from granular pearlite to lath martensite, and the carbide particles are partially dissolved in martensite matrix. With the increase of normalizing temperature, more carbide particles can be dissolved into the matrix and completely dissolved at 1100 ℃, and the grain size of prior austenitic increases (from 7.4 μm at 1000 ℃ to 34.9 μm at 1100 ℃). After tempering, the lath spacing of martensitic widens, and the high-density dislocations are rapidly recovered and disappeared, while precipitated phases are precipitated, spheroidized and grow up along grain boundary and inside, the M₂₃C₆(M is dominated by Cr) phases are short rod shape, which are distributed on the grain boundary, while the MX (M is dominated by Ta) phases are elliptic shape, which are mainly distributed in the lath martensitic. The reduced activation martensitic steel has the best comprehensive mechanical properties after normalizing at 1000 ℃ for 60 min and tempering at 790 ℃ for 90 min, with tensile strength of 745.7 MPa and elongation of 18.9%.

Effect of multi-stage heat treatment on microstructure and properties of GH4742 alloy

Jia Zhiming, Qin Heyong, Li Zhentuan, Liu Huimin, Yu Ping

2022, 47(12): 109-114. doi:10.13251/j.issn.0254-6051.2022.12.019

Abstract ( 41 )

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Effects of different solution temperature and high aging temperature on microstructure and mechanical properties of GH4742 superalloy were studied by multi-stage heat treatment. The results show that the grain size and primary γ′ phase morphology can be significantly influenced by adjusting solution temperature. When the solution temperature is 1090 ℃, there are a large amount of undissolved primary γ′ phases in the matrix, which can effectively prevent the growth of grains, and the coexistence of those coarse primary γ′ phase and fine γ′ phase formed in the subsequent aging process makes the alloy achieves an excellent comprehensive mechanical properties. High temperature aging treatment has little effect on microstructure, but it can change the size of the γ′ phase and obviously affects the morphology of carbides on the grain boundary. By replacing Nb and Ti in the primary carbide MC with Mo, the carbides continuously distributed on the grain boundaries are dissolved to form granular carbides, which significantly reduces the high temperature durability of the GH4742 superalloy.

Effect of sensitization temperature on precipitation behavior and mechanical properties of high carbon austenitic stainless steel

Xing Mengnan, Zhang Haiming, Sui Songyan, Hu Xinming, Ouyang Xin, Wang Chu, Jia Chuntang, Liu Chenxi

2022, 47(12): 115-119. doi:10.13251/j.issn.0254-6051.2022.12.020

Abstract ( 54 )

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Effect of sensitization temperature on quantity, morphology and distribution of precipitates in a high carbon austenitic stainless steel was studied by means of metallographic microscope, scanning electron microscope, energy dispersive spectrometer and transmission electron microscope. The effect of precipitate on mechanical properties of the tested steel at different sensitization temperatures was analyzed by means of tensile testing machine, impact testing machine and Brinell hardness tester. The results show that microstructure of the specimens sensitized at different temperatures is austenite, while with the increase of sensitization temperature, the precipitation of Cr-rich carbides on the austenite grain boundaries increases and exists in granular and strip forms, and the size of precipitates on the grain boundaries is in the range of 100-400 nm. After sensitizing treatment at 650 ℃ for 2 h, the strength and toughness of the specimen match well, and the comprehensive mechanical properties are excellent.

Quenching process of Q1300E ultra high strength steel for construction machinery

Zhang Qingxue, Zhou Wenhao, Liu Dan, Zhang Yongwei, Zheng Dongsheng, Li Hongying

2022, 47(12): 120-125. doi:10.13251/j.issn.0254-6051.2022.12.021

Abstract ( 61 )

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Mechanical properties and microstructure of Q1300E steel plate under different quenching processes were studied by means of automatic tensile testing machine, automatic impact testing machine, metallographic microscope and scanning electron microscope. The results show that when the quenching heating time is 60 min and the quenching temperature is 840 ℃, the strength and low-temperature impact property are the best, and the mechanical properties in tempered state meet the GB/T 28909—2012 standard requirements. The yield strength is 1302 MPa, the tensile strength is 1505 MPa, and the longitudinal and transverse impact absorbed energy at -40 ℃ are 74 J and 61 J, respectively. When the quenching temperature is 870, 900 and 930 ℃, the tensile strength and low-temperature impact absorbed energy meet the GB/T 28909—2012 standard requirements, but the yield strength is lower than 1300 MPa. The change of quenching temperature has obvious influence on grain size. When the quenching temperature is 840 ℃, the average grain size is the smallest,which is 5.7 μm. When quenching temperature is 930 ℃, the average grain size is 15.9 μm. The influence of quenching heating time on mechanical properties and grain size is relatively small. When the quenching temperature is 840 ℃ and the quenching heating time is 40-80 min, the mechanical properties in the tempered state meet the GB/T 28909—2012 standard requirements, and the grain size is 4.5-6.5 μm.

Modulation of structure and properties of Fe_3.85Co_66.18Si_14.5Ni_0.97B_14.5 cobalt-based amorphous alloy by stepwise longitudinal magnetic heat treatment

Geng Junzhao, Liu Tiancheng, Li Lijun, Pan Yun, Zhang Wei

2022, 47(12): 126-131. doi:10.13251/j.issn.0254-6051.2022.12.022

Abstract ( 44 )

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In order to prepare cobalt-based amorphous alloys with low permeability and high rectangular ratio and low coercivity, Fe_3.85Co_66.18Si_14.5Ni_0.97B_14.5 amorphous alloy strips were subjected to different heat treatments and then their microstructure and properties were tested and analyzed. The results show that the relaxation enthalpy of the cores decreases after stepwise longitudinal magnetic heat treatment, while the densities and orderliness increase, and the alloys annealed below the crystallization temperature remain amorphous. The stepwise longitudinal magnetic heat treatment significantly improves the magnetic properties of the cores, and the best properties are achieved when the traditional heat treatment temperature is 480 ℃, with a coercivity of 0.17 A/m, a rectangular ratio of 96.81%, and an effective magnetic permeability of 5675.05 at 1 kHz. At this time the magnetic domains are in the form of regular slats and oriented at an angle of 12.8° to the external magnetic field. Based on Bertitto model, the loss separation is found to satisfy the equation:P_v=11.05×f×B^0.9814_m+0.7495×f²×B²_m.

Hardenability of B containing spring steel 60Si2Mn and its CCT curves

Lu Xinchun, Luo Yizheng, Wang Yanlin

2022, 47(12): 132-137. doi:10.13251/j.issn.0254-6051.2022.12.023

Abstract ( 40 )

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End quenching test of spring steel 60Si2Mn with different B content (0.0005%-0.0047%) was carried out to study the effect of B on hardenability of the spring steel 60Si2Mn, and the theoretical calculation and experimental measurement of the CCT curves of 0.0026%B spring steel 60Si2Mn were carried out. The results show that the hardenability of the spring steel 60Si2Mn can be improved by adding B. When the B content is increased from 0.0005% to 0.0026%, the hardenability of the spring steel 60Si2Mn is significantly improved, but when the B content reaches 0.0047%, the hardenability is reduced compared with 0.0026%B content. The static CCT curves obtained by theoretical calculation show that the bainite transformation will occur during the cooling process after rolling at 300-400 ℃, which leads to higher hardness of the steel and affects the quality of rolled final products. The results of dynamic CCT curves show that when the cooling rate is less than 9 ℃/s, there are ferrite precipitation and pearlite transformation in the steel. When the cooling rate is greater than 10 ℃/s, there is only martensite transformation.

Effect of Al on microstructure and oxidation resistance of Fe-20Cr-35Ni-0.6Nb alloy

Xu Wanjian, Jia Guodong, Yang Chunli, Wang Zixie, Pan Jie, Xiao Xueshan

2022, 47(12): 138-145. doi:10.13251/j.issn.0254-6051.2022.12.024

Abstract ( 63 )

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Oxidation resistance behaviors of Fe-25Cr-35Ni-0.6Nb-xAl (x=0.5%, 1.5%, 2.5%, mass fraction) Nb-containing alloys at 1000 ℃ in air were investigated by mass gain method. The microstructure and oxide scales of the alloys were observed and analyzed by SEM, EDS, TEM and Raman spectroscopy. The results show that the microstructure of the three Nb-containing alloys is single-phase austenite, and a small amount of NbC precipitated phases are dispersed in the matrix. The content of precipitated phase and the grain size remain unchanged before and after oxidation. With the addition of 0.5% and 1.5% Al, a multilayer oxide film forms on the surface of the Nb-containing alloys, of which the first and third layers are Cr₂O₃, the subsurface layer is mainly composed of NiCr₂O₄, NiFe₂O₄ and Fe₂O₃, and the innermost layer is Al₂O₃ internal oxide. In addition, the porosity of the oxide film is aggravated by NbC precipitates in the matrix and a small amount of Nb oxides (Nb₂O₅) in the oxide film. When the Al content increases to 2.5%, a continuous and dense Al₂O₃ oxide film forms on the surface of Fe-25Cr-35Ni-0.6Nb alloy, which reduces the oxidation rate and improves the oxidation resistance of the alloy.

Effect of Ni content on microstructure and properties of Cu-xNi-3Ti-0.1Zr alloys

Jing Qingxiu, Wei Miao, Xiao Xiangpeng, Sun Yuqing, Peng Yong, Huang Xiaodong

2022, 47(12): 146-151. doi:10.13251/j.issn.0254-6051.2022.12.025

Abstract ( 44 )

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Cu-xNi-3Ti-0.1Zr (x=2, 4, 6) alloy was prepared by vacuum melting and homogenizing annealing, hot rolling, solution treatment, cold rolling and aging treatment. The precipitates of the alloy were characterized and analyzed by means of X-ray diffractometer, optical microscope and scanning electron microscope. The results show that the addition of Ni can significantly increase the electrical conductivity of the alloy, and the effect on the hardness is also obvious. Ni exists mainly in CuNiTi phase in the Cu-xNi-3Ti-0.1Zr alloys; The addition of Ni leads to the precipitation of a large amount of CuNiTi phase in the alloy, which decreases the dissolving degree of Ti in the solid solution matrix and reduces the degree of lattice distortion of the alloy, thus increasing the electrical conductivity. However, with the precipitation of Ti element, the strengthening effect of Ti on the alloy is weakened, which leads to the decrease of the hardness of the alloy. The peak hardness of the Cu-xNi-3Ti-0.1Zr (x=2, 4, 6) alloys aged at 500 ℃ is 295, 231 and 201 HV0.5, respectively.

Effect of carbon and nitrogen content on quenching temperature and corrosion resistance of 4Cr16NiMo steel

Chu Baoshuai, Chen Jianli, Li Zhenjiang, Li Yongtang, Yang Wen

2022, 47(12): 152-157. doi:10.13251/j.issn.0254-6051.2022.12.026

Abstract ( 69 )

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Effect of carbon and nitrogen content on microstructure, quenching temperature, hardness and pitting resistance of 4Cr16NiMo steel was analyzed by means of optical microscope and Rockwell hardness tester. The results show that with the increase of N content to 0.1%, the optimum quenching temperature of the 4Cr16NiMo steel decreases from 1070 ℃ without nitrogen to 1010 ℃, the microstructure is obviously refined and the maximum hardness is improved. At the same time, adding nitrogen can improve pitting resistance of the 4Cr16NiMo steel, and in a certain range, the higher the nitrogen content, the stronger the pitting resistance. On the basis of the same nitrogen content, the carbon content decreases by 0.1%, the hardness decreases, the quenched microstructure changes from cryptocrystalline martensite to lath martensite, and the pitting resistance is improved. Therefore, by controlling the carbon and nitrogen content, the quenching temperature of the 4Cr16NiMo steel can be effectively reduced, and its corrosion resistance can be improved.

Effect of Al on austenite grain growth of SWRH62A steel

Wang Jintao, Li Zhanwei, Yu Xuesen, Zhang Yu

2022, 47(12): 158-161. doi:10.13251/j.issn.0254-6051.2022.12.027

Abstract ( 42 )

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Effect of Al content on the austenite grain growth of SWRH62A steel during heating was investigated by using high temperature optical microscopy. The results show that addition of 0.015%-0.045% Al increases the grain boundary activation energy of the SWRH62A steel from 474.14 kJ/mol to 1175.73 kJ/mol, and decreases the austenite grain growth rate from 0.18 μm/℃ to 0.12 μm/℃ with a soaking temperature below 1050 ℃, which can significantly refine grains.

Effect of rare earth element Ce and heat treatment on microstructure and mechanical properties of hypereutectoid rail steel

Song Ran, Zhao Wenqian, Bao Xirong, Chen Lin, Wang Xiaodong

2022, 47(12): 162-167. doi:10.13251/j.issn.0254-6051.2022.12.028

Abstract ( 33 )

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Effect of heat treatment and rare earth element Ce on inclusions, microstructure and mechanical properties of hypereutectoid rail steel was studied. The results show that heat treatment promotes the segregation of rare earth Ce at the interface, giving full play to the microalloying role of Ce in refining pearlite lamellar spacing, purifying and strengthening grain boundaries, modifying and refining brittle inclusions, and making the phase transformation sufficiently completed, so as to finally obtain a uniform and continuous fine pearlite lamellar structure. The Ce refines the pearlite lamellar spacing in the heat treated hypereutectoid rail to 87 nm, with a refining rate of 43.8%. At the same time, it refines the long MnS-MgO inclusions and turns them into near spherical rare earth inclusions, so that the hypereutectoid rail steel can obtain the optimal mechanical properties. The tensile strength reaches 1378 MPa, the hardness reaches 380 HBW, the percentage reduction of area after fracture increases to 23.95%, and the tensile fracture shows the characteristics of ductile fracture.

Effect of Mo on thermal stability of 4Cr5Mo2V hot-work die steel

Shen Yutao, Zuo Pengpeng, Wu Xiaochun

2022, 47(12): 168-174. doi:10.13251/j.issn.0254-6051.2022.12.029

Abstract ( 43 )

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Effect of Mo on the thermal stability of 4Cr5Mo2V hot-work die steel was investigated, and the microstructure evolution was characterized by means of scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the difference of thermal stability between the tested steels containing 1.9%Mo and 2.4%Mo at 600 ℃ is small. After holding at 650 ℃ for 48 h, the hardness of the 1.9%Mo steel is lower by 2.3 HRC than that of the 2.4%Mo steel, the recovery in the martensite matrix is more obvious, and there are more Cr-rich large-sized spherical M₂₃C₆ carbides with the size between 150-200 nm. While in the 2.4%Mo steel, more fine M₂C type secondary carbides are precipitated which hinder the Cr-rich carbides growth, the average particle size of carbides is smaller, so that the 2.4% Mo steel has better thermal stability.

Effect of microstructure on hydrogen diffusion behavior of GCr15 bearing steel

Fu Siyu, Du Yu, Huang Zhiyuan, Lu Jianing, Du Linxiu

2022, 47(12): 175-180. doi:10.13251/j.issn.0254-6051.2022.12.030

Abstract ( 41 )

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Electrochemical hydrogen permeation test of spheroidizing annealed, spheroidizing annealed+quenched and tempered (Q&T) GCr15 bearing steel was carried out to describe hydrogen diffusion behavior. The effects of microstructure and heat treatment status on hydrogen diffusion behavior were analyzed. The results show that the distribution of carbides in the spheroidizing annealed specimens has a greater effect on hydrogen diffusion behavior. Hydrogen diffuses fastest in banded carbides, while the network carbides along grain boundaries acts as hydrogen traps, trapping more hydrogen atoms. In the Q&T specimens, hydrogen diffuses fastest in banded undissolved carbides. The effective hydrogen diffusion coefficient decreases with the increase of the volume fraction of retained austenite. The hydrogen diffusion coefficient of the spheroidizing annealed tested steel is much higher than that of the Q&T steel. The number of hydrogen traps in Q&T specimens increases due to the presence of retained austenite, making hydrogen permeation more difficult.

Microstructure and wear resistance of GCr15 bearing steel with low-temperature plasma sulfurizing coatings

Li De, Shan Qiongfei, Wang Xin, Liu Yong, Zhou Meng, Zhang Yi

2022, 47(12): 181-187. doi:10.13251/j.issn.0254-6051.2022.12.031

Abstract ( 44 )

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Microstructure and wear resistance of GCr15 bearing steel after different surface treatments were studied by means of OM, XRD, SEM and UMT friction and wear tester. The results show that compared with that carbonitrided, a FeS-based sulfurized layer with uniform and fine microstructure is formed on the surface of the GCr15 bearing steel after carbonitriding and low-temperature plasma sulfurizing, though the hardness decreases slightly, the friction coefficient and volume wear rate are significantly reduced, so that the ability of the bearing steel surface to resist scratching and to anti-biting is significantly improved, the service life of the bearing is also prolonged. During the wear process, the FeS layer falls off and produces a smearing effect, which significantly reduces the degree of abrasive wear. The analysis of the wear morphology confirms that the wear mechanisms are abrasive wear and adhesive wear.

Characteristics of direct current field enhanced pack boron-aluminizing of 45 steel

Ren Zhonghua, Xie Fei, Pan Jianwei

2022, 47(12): 188-195. doi:10.13251/j.issn.0254-6051.2022.12.032

Abstract ( 40 )

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Direct current field enhanced pack boron-aluminizing (DCFEPBA) was carried out on 45 steel at 750 ℃ by using pack agent with content of aluminum powder varied from 0 to 1.5%, and compared with the corresponding conventional diffusion and direct current field enhanced boronizing. The phase structure, microstructure and hardness distribution of the specimen case at different positions in the direct current field were observed and analyzed by means of X-ray diffraction, optical microscopy and microhardness test. The results show that the direct current field has a significant enhancing effect on both pack boriding and pack boron-aluminizing. The formation characteristics of the direct current electric field enhanced case are not only related to the position of the specimen and the orientation of the diffusion surface, but also to the content of aluminum powder in the penetrant. When the content of aluminum powder in the penetrant increases from 0 to 1.5%, the thickness of the case of the cathode specimen facing the anode, and that of the intermediate specimen facing the anode increase first and then decrease; While the thickness of the case of the intermediate specimen facing the cathode increases slowly first and then increases significantly, and that of the anode specimen facing the cathode increases first, then decreases, and then increases. When the aluminum content is 0, the case of the anode specimen facing the cathode side and that of the intermediate specimen facing the cathode side are both serrated single-phase Fe₂B, and that of the cathode specimen facing the anode side and that of the intermediate specimen facing the anode side are composed of “serrated” FeB+Fe₂B. When the aluminum content is 0.3%, each case is composed of two-phase “serrated” (Fe, Al)B and (Fe, Al)₂B; and when the case is serrated, the peak hardness is 1400-1900 HV0.025. When the aluminum powder content is ≥ 0.5%, the “serrated” feature of the case gradually disappears from the cathode to the anode, the KAlF₄, AlF₃, and Fe₃Si phases gradually appear on the surface of the case of each specimen, and the hardness of the case decreases significantly, the peak hardness is 450-1400 HV0.025.

Formation mechanism of dispersed carbide in carburized case of 20MnCr5 gear steel

Li Yang, Hou Shengwen, He Erkang, Zhang Xiaotian

2022, 47(12): 196-200. doi:10.13251/j.issn.0254-6051.2022.12.033

Abstract ( 34 )

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20MnCr5 gear steel was carburized and quenched by the newly developed diffusion carburizing process, and the microstructure of precipitates was studied by means of metallographic microscope. The results show that within a certain depth range of the carburized case, two kinds of carbide with different dispersion distribution are precipitated, which is mainly large particles with a size of 1-5 μm at the grain boundary, while at the intragranular the carbide is mainly small in size, which is less than 1 μm in size and accounts for more than 80%. The precipitation mechanism of two different types of carbides is also revealed.

Evolution of microstructure and properties of nickel-based N06022 alloy pipe during solution treatment

Qin Xingwen, Ding Wenyan, Tu Zhengping, Shen Chengfei

2022, 47(12): 201-209. doi:10.13251/j.issn.0254-6051.2022.12.034

Abstract ( 38 )

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Influence of solution treatment on microstructure evolution and properties of nickel-based N06022 alloy pipe was investigated by metallographic microscopy, scanning electron microscopy, energy spectrum analyzer, mechanical and corrosion performance test. The results show that the cold-rolled N06022 alloy pipe contains a large amount of precipitated phases, which have a pinning effect on grain boundaries, and the average grain size of the steel is hardly affected by the holding time, when the solution temperature is below 1100 ℃. When solution temperature increases to 1150 ℃, the grain size and growth rate increase sharply, which is mainly related to the dissolution of the precipitated phase. In the solution temperature range of 1000-1200 ℃, the activation energy of grain growth for 15 min and 30 min is Q_B=646.56 kJ/mol, Q_C=566.45 kJ/mol, respectively, and the relationship between the properties of the alloy and the grain size satisfies the Hall-Petch relationship. With the increase of solution temperature, the intergranular corrosion rate of the alloy decreases first and then stabilizes, the strength and hardness show slow-fast downward trend, and the elongation shows slow-fast upward trend. When the holding time is 15 min and the solution temperature is about 1150 ℃, the intersection of hardness curve and elongation curve appears, and the corrosion rate is stable, which is the optimal solution treatment process.

Microstructure and tribological properties of amorphous coatings by supersonic atmospheric plasma spraying

He Tao, Wang Xuepeng, Liu Haibo, Liu Qi, Jia Hua, Liu Na, Ding Fei

2022, 47(12): 210-215. doi:10.13251/j.issn.0254-6051.2022.12.035

Abstract ( 34 )

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Fe-based and Mo-based amorphous coatings are fabricated by the new generation of supersonic atmospheric plasma spraying (SAPS), and microstructure and tribological properties of the coatings were comparatively analyzed. The results show that a small number of pores in the SAPS sprayed Fe-based and Mo-based amorphous coatings lead to high structural density. Compared with Fe-based coating, the Mo-based coating with lower porosity has a lower wear rate (1.1×10^-4μm·N^-1·s^-1) during friction, resulting in a better wear resistance. The wear mechanism of the coatings is mainly abrasive wear and fatigue wear, accompanied by delamination and debris oxidation.

Microstructure and mechanical properties of MgAlSnZnCu lightweight high entropy alloys

Yang Tuoyu, Wang Kehong, Zhang Deku, Chen Feng, Guo Chun, Zhang Yajing

2022, 47(12): 216-221. doi:10.13251/j.issn.0254-6051.2022.12.036

Abstract ( 36 )

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In order to improve the stability of the barrier layer, MgAlSnZnCu series of lightweight high entropy alloys were prepared by electromagnetic induction heating method under the protection of a covering agent and argon gas. The as-cast microstructure, annealed microstructure and phases of the alloys were characterized and analyzed by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy spectrum analysis (EDS), and microhardness test was used to study its hardness. The results show that the microstructure of as-cast MgAlSnZnCu alloy is composed of (AlMgZnCu)-FCC phase and Mg₂Sn phase. The morphology of Mg₂Sn phase is 2-6 μm granular and irregular block. When the content of Mg decreases from 22% to 12%, the hardness of the alloy increases from 277 HV0.2 to 326 HV0.2, and the compressive strength increases from 223 MPa to 237 MPa. Needle-like and flake-like impurity phases appear in the microstructure after the decrease of Sn and Zn contents, and the dispersion of hardness values is large, after annealing at 450 ℃ for 6 h, the impurity phases in the microstructure tend to accumulate and grow obviously. The mechanical test results show that the annealing have little effect on compressive strength and hardness. The MgAlSnZnCu series alloys can maintain good thermal stability at 450 ℃.

Effect of matrix microstructure on mechanism of strength and ductility for 800 MPa grade dual phase steel

Hou Xiaoying, Wang Jun, Ding Mingkai, Liu Wanchun, Sun Weihua, Kang Huawei

2022, 47(12): 222-227. doi:10.13251/j.issn.0254-6051.2022.12.037

Abstract ( 30 )

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Two heat treatment processes were adopted to obtain 800 MPa grade dual phase steels with different matrix microstructure, and the microstructure morphology characteristics of the matrix and its effect on the mechanism of strength and ductility were studied systematically. The results show that the matrix microstructure has a significant indigenous effect on the plastic deformation mechanism of 800 MPa grade dual phase steels, resulting in differences in the properties. The (F+M) dual phase steel is composed of polygonal ferrite and about 28% second-phase martensite, with yield ratio of 0.540 and elongation of 23.3%. The (BF+γ) dual phase steel is composed of bainite ferrite matrix structure and about 24% second-phase retained austenite, with yield ratio of 0.702 and hole expansion ratio of 56%. During the plastic deformation process of (BF+γ) dual phase steel, the stress concentration at the crack tip can be effectively decomposed by the γ phase with a thickness of about 60-150 nm, and the crack propagation energy is also consumed. At the same time, the cracks generated by microcracks can be bridged by the volume expansion caused by the induced martensite transformation of residual austenite. Under the coordinated deformation mechanism of α phase BF and residual austenite phase, it is beneficial to improve the strength, plasticity and stretch-flange property. In addition, the proportion of large angle grain boundaries in (BF+γ) dual phase steel increases to 63.1%, and higher dislocation density exists in the matrix, the driving energy required for micro-crack propagation is weakened effectively, and the energy required for further propagation is also increased, the stress concentration generated during deformation or hole expansion is alleviated.

Phase transformation behavior of non-quenched and tempered NM400 multiphase wear-resistant steel

Li Guannan, Lei Minggang, Chen Haotian, Lü Dewen

2022, 47(12): 228-233. doi:10.13251/j.issn.0254-6051.2022.12.038

Abstract ( 36 )

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In order to further optimize the different microstructure ratios of non-quenched and tempered NM400 multiphase wear-resistant steel, the Gleeble-3800 thermal simulation testing machine was used to explore the microstructure transformation law of the tested steel under continuous cooling conditions, and the metallographic method and the hardness method were combined to draw the dynamic continuous cooling transformation (CCT) curves. The results show that when the cooling rate is lower than 1 ℃/s, the microstructure of the steel is composed of ferrite+granular bainite+pearlite, there are some coarse austenite grains transform into granular bainite and pearlite either. When the cooling rate is 5-40 ℃/s, the pearlite transformation no longer occurs, and the microstructure is composed of ferrite+bainite+martensite. With the increase of cooling rate, the martensite content increases and the hardness increases. In addition, under different step cooling schemes, the lower middle cooling temperature and longer air cooling time are beneficial to the transformation of ferrite and bainite, and the retained austenite content increases with the increase of ferrite content. Due to higher Ms point, the martensite lath of the steel is wide, and self-tempering phenomenon occurs in the steel.

Tempering brittleness of 2Cr13 stainless steel

Zhu Hongwei, Wang Ping, Hu Wanting, Chen Xin, Huang Yanjun, Yu Ruixing, Zhuang Yuanhong, Chen Yongshuan

2022, 47(12): 234-236. doi:10.13251/j.issn.0254-6051.2022.12.039

Abstract ( 226 )

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Changes of hardness, impact property and fracture morphology evolution of the 2Cr13 stainless steel after tempering near the tempering brittle temperature interval were studied. The results show that hardness of the 2Cr13 stainless steel after tempering at 490-550 ℃ is 40-50 HRC, the corresponding impact property shows a downward trend; After tempering at 570-620 ℃, the hardness of the 2Cr13 stainless steel is 30-38 HRC, the impact property gradually increases. Through the observation of the impact fracture morphology, after tempering at 490-560 ℃, the fracture mode first changes from cleavage fracture to intergranular fracture. With the increase of the tempering temperature, it eventually changes to a fracture mode along the grain+cleavage. Microscopically, the size of the fracture surface along the grain gradually decreases, and the number of cleavage gradually increases, which is manifested by the improvement of the impact performance on the macroscopic level.

End-quenching experimental curves and simulation calculation of 18CrNiMo7-6 steel for high-speed railway gears

Zhu Yuefeng, Gu Jialin, Zhou Huihua, Wang Zhixiang, Zhang Jinwen

2022, 47(12): 237-243. doi:10.13251/j.issn.0254-6051.2022.12.040

Abstract ( 74 )

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Three prediction models were used to calculate and predict the end-quenching curves of two batches of 18CrNiMo7-6 steel for high-speed railway gears, and then the prediction errors of different models were analyzed by comparing with the end-quenching test results. The results show that the Jin model obtains small error compared with the end-quenching test results, and can be used to predict the end-quenching curves of the tested steels. According to the comparative analysis of the predicted and the measured results of the end-quenching test, and combing with the hardness specification for technical conditions of the high-speed railway gear steels, the controlling method of the smelting composition of 18CrNiMo7-6 steel for high-speed railway gears is proposed.

Process design of high-pressure gas quenching for a gear hub based on multi-field coupling simulation

Wang Hao, Wang Jing, Xu Jun, Gu Jianfeng, Guo Zhenghong

2022, 47(12): 244-251. doi:10.13251/j.issn.0254-6051.2022.12.041

Abstract ( 36 )

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According to the requirements of process design of high-pressure gas quenching for 20CrMnTi gear hub, a coupling simulation system of the flow field, temperature field, phase field and stress-strain field was established, which can realize the synchronous coupling calculation of the quenching gas flow and workpiece. Using this system, the high-pressure gas quenching process of gear hub under different quenching gas pressures and gas blowing modes was simulated. By analyzing the microstructure, hardness and distortion of the gear hub under different process schemes, an optimum process scheme of 2.0 MPa and lower unidirectional blowing is proposed, and the simulation results meet the performance requirements of the gear hub.

Analysis on temperature and fluid fields at vacuum solid solution treatment furnace based on Fluent

He Ruijun, Feng Runhua, Han Xiao, Kong Lingli

2022, 47(12): 252-257. doi:10.13251/j.issn.0254-6051.2022.12.042

Abstract ( 42 )

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Based on finite element simulation method, the temperature and fluid fields of the vacuum heat treatment furnace hot zone were simulated precisely. The temperature and fluid fields involving three different actual product situations which are empty loading, only loading frame and full loading blade were simulated. The temperature tendency in qualified working zone is monitored by nine points temperature testing method. Meanwhile, the fluid field at several different parts were monitored. Compared to the measuring data, the variation of the temperature and the coupling relationship between furnace temperature and its fluid field were summarized. The foundation was laid to summarize the temperature-velocity interaction and distribution pattern and achieve the precisely controlling of the vacuum heating treatment in the future.

Research progress on surface modification of zirconium alloys

Liu Zhanghui, Hu Haiyang, Wang Jiangang, Zhu Xiaoye, Ma Jing, Li Jianhui, Yu Huichao, Feng Zhihao

2022, 47(12): 258-268. doi:10.13251/j.issn.0254-6051.2022.12.043

Abstract ( 51 )

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Taking the surface modification technology of zirconium alloy as the main line, the types and research status of current modification methods were briefly described, and the principle, research progress, advantages and disadvantages of common surface modification preparation technologies such as physical vapor deposition, laser surface treatment, anodic oxidation, microarc oxidation and ion implantation were emphatically introduced. The theoretical problems to be solved and the technical bottlenecks to be overcome of the surface modification of zirconium alloys were discussed. Finally, the future research work and engineering application prospect of zirconium alloy surface modification were prospected based on the related research progress.

Low cycle fatigue behavior and fracture characteristics of tap changer main drive shaft

Zhou Xuan, Huang Yao, Yang Fan, Zhao Dingxuan, Li Keer, Yu Guoxiang, Chen Wei, Cai Heping

2022, 47(12): 269-274. doi:10.13251/j.issn.0254-6051.2022.12.044

Abstract ( 30 )

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Low cycle fatigue properties and fracture characteristics of the main drive shaft of tap changer were systematically studied. The results show that the fatigue life of the EN-GJS-700-2 ductile iron strongly depends on the applied cyclic strain amplitude. It is further found that the fatigue life mainly depends on the plastic component in the cyclic strain amplitude, so that the fatigue life and the total cyclic strain amplitude satisfy the Coffin-Manson equation. SEM fracture morphology shows that the spheroidal graphite/pearlite interface on the surface of ductile iron is the preferred initiation position of fatigue cracks under low or high strain amplitude, and presents multi-source characteristics. In the subsequent crack propagation process, the high strain amplitude fracture is flat, the cleavage fracture characteristics are obvious, and it is difficult to observe the fatigue stripes. However, at low strain amplitude, the spherical graphite and pearlite significantly participate in plastic deformation, and the fatigue stripes become obvious.

Cracking failure analysis of 18CrNiMo7-6 steel gear after heat treatment

Zheng Kai, Zhong Zhenqian, Cao Wenquan, Lin Shuangping, Liu Ming

2022, 47(12): 275-280. doi:10.13251/j.issn.0254-6051.2022.12.045

Abstract ( 61 )

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A batch of gears after production and processing, quenching and low temperature tempering cracked during storage in the warehouse. The causes of gear cracking were analyzed by means of ultrasonic flaw detection, metallographic examination, chemical composition analysis, hardness test, fracture morphology observation, end quenching test and supplementary heat treatment. The results show that the high strength of the cracked gear leads to high hydrogen embrittlement sensitivity, and the hydrogen induced delayed crack at the sub-surface is the direct cause of rapid brittle cracking of the gear. The root cause of the gear cracking is that the hardening capacity and hardenability of the cracked gear are too high, which exceed the hardenability requirements of BS EN 10084-2008 for 18CrNiMo7-6+HH steel. Thus, the cracking can be effectively avoided by reducing the strength and hardness of the quenched and tempered gear through adjusting the quenching medium and reducing the cooling rate of the high temperature zone during quenching.

Fracture analysis and process improvement of 65Mn steel spring leaf

Xu Feng

2022, 47(12): 281-283. doi:10.13251/j.issn.0254-6051.2022.12.046

Abstract ( 82 )

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A 65Mn steel spring leaf used in a certain type of product was found to break at the peak position when testing elastic force. Failure cause of the spring leaf was analyzed through the chemical composition, microstructure and microhardness test and the processing process analysis, and the then the process was improved. The results show that due to the high quenching temperature, high quenched hardness, and lower temperature tempering after quenching, the elimination of quenching stress is not sufficient, causing the spring leaf to produce microcracks under the combined effect of axial compressive stress and quenching residual stress during the installation of the fixture when the spring leaf is tempered at high temperature. The microcracks expand and transform during subsequent strong pressure treatment, resulting in brittle fracture. After adopting the improved process of 815 ℃ quenching+280 ℃ low temperature tempering+390 ℃ high temperature tempering, the hardness of the spring leaf can meet the technical requirement, the fracture is 100% ductile fracture, and no brittle fracture occurs again.

Typical fracture and cause analysis of 35CrMnSiA ultra-high strength steel

Wang Chunyan, Zhang Fa, Jie Ruihua, Wang Chunmei

2022, 47(12): 284-288. doi:10.13251/j.issn.0254-6051.2022.12.047

Abstract ( 86 )

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In order to investigate the fracture mechanism and fracture causes of the low percentage reduction of area of the 35CrMnSiA ultra-high strength steel tensile specimens, four common types of fractures were observed and analyzed by means of scanning electron microscope and energy dispersive spectrometer. The results show that the fish-eye defect caused by the combined action of a certain amount of hydrogen and tensile stress is the main reason for the low percentage reduction of area of conventional plastic fracture. Brittle-plastic mixed fracture is caused by hydrogen embrittlement around large inclusions, which leads to the decrease of the percentage reduction of area of the steel. The pencil-tip shaped fracture is mainly caused by large particle inclusions or hydrogen damage defects in steel. Brittle fracture is caused by mechanical damage to the material or the formation of micro-cracks caused by hydrogen accumulation on the damaged surface.

Analysis of size out-of-tolerance of 55SiCr steel suspension spring

Sun Guocai, Zhang Yu, Yuan Changsheng

2022, 47(12): 289-292. doi:10.13251/j.issn.0254-6051.2022.12.048

Abstract ( 35 )

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Sizes of 55SiCr steel suspension springs were found out-of-tolerance in the rolling process. The reason of the size out-of-tolerance was analyzed by means of OM, EMPA and XRD. The results show that excessive retained austenite is found in the 55SiCr steel wires to produce the suspension springs due to insufficient cooling during heat treatment, which is the main reason of size out-of-tolerance. Content of retained austenite in the steel wire can be effectively reduced by reducing the temperature of cooling water.

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