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  • MATERIALS RESEARCH
    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
    2023, 48(11):  1-7.  doi:10.13251/j.issn.0254-6051.2023.11.001
    Abstract ( 93 )   PDF (4216KB) ( 101 )  
    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.
    Prediction and validation of microstructure and thermophysical properties of 20Mn23AlV non-magnetic steel
    Tang Xingchang, Cheng Ganghu, Zhang Jiaqi, Zhang Zhijian, Zhou Weilian
    2023, 48(11):  8-15.  doi:10.13251/j.issn.0254-6051.2023.11.002
    Abstract ( 47 )   PDF (3129KB) ( 40 )  
    Equilibrium phase composition, precipitated phase element composition, effect of austenitizing element content on microstructure, hardenability, mechanical properties and thermophysical properties of the 20Mn23AlV non-magnetic steel were simulated and predicted using JMatPro7.0 software. The simulation parameters were compared and analyzed with the data obtained by some experiments. The calculation results show that the austenite content of the non-magnetic steel accounts for 99.89%, the rest is the dispersed phase with second phase strengthening effect, and the carbon nitride with the highest proportion in the dispersed phase is vanadium metal. The non-magnetic steel does not cause microstructure transformation at different cooling rates, and it is all austenite. The content of Mn and Al should be controlled within a certain range to maximize the proportion of austenite. There is a little difference between the predicted and the measured mechanical properties. With the decrease of temperature, the linear expansion coefficient, specific heat, Poisson's ratio and thermal conductivity decrease, while the density, shear modulus, conductivity and Young's modulus increase.
    Dynamic continuous cooling transformation behavior and transformation kinetics of 40Cr10Si2Mo martensitic heat-resistant steel
    Wang Qinren, Wang Qingjuan, Zhao Xiaofei, Han Shudong, Zhang Wei, Xu Rong, Hu Nan
    2023, 48(11):  16-21.  doi:10.13251/j.issn.0254-6051.2023.11.003
    Abstract ( 45 )   PDF (4579KB) ( 54 )  
    Phase transformation behavior of a 40Cr10Si2Mo steel during continuous cooling at different cooling rates was investigated by Gleeble-3500 thermal simulator. The dynamic continuous cooling transformation (CCT) curves and the phase transformation kinetics models of the steel were established. The effects of different cooling rates on phase transformation and alloying element precipitation were analyzed and discussed by characterizing the microstructure at different cooling rates. It is observed that the microstructure consists of ferrite and carbides at cooling rate of 0.1-0.3 ℃/s. Subsequently the precipitation of Cr and Mo in carbides decreases with the increase of cooling rate. The microstructure is ferrite and lamellar pearlite and a small amount of martensite and network carbides at cooling rate of 0.5-0.8 ℃/s. When the cooling rate is greater than 3 ℃/s, the resulted microstructure is full lath martensite. According to the established models of both the Johnson-Mehl-Avrami(JMA) diffusive type and the Koistinen-Marburger (K-M) non-diffusive type, the phase transformation kinetics curves fit well with the experimental data.
    Hot deformation behavior of heat-resistant steel 2Cr12Ni4Mo3VNbN
    Tang Hui, Tu Luhan, Li Ying, Wu Zhiwei, Zhou Zhiming, Zhang Jun
    2023, 48(11):  22-28.  doi:10.13251/j.issn.0254-6051.2023.11.004
    Abstract ( 37 )   PDF (4609KB) ( 25 )  
    Hot compression deformation behavior and microstructure evolution of heat-resistant steel 2Cr12Ni4Mo3VNbN were studied by Gleeble-3500 thermal simulation test machine under the conditions of deformation temperature of 900-1200 ℃, strain rate of 0.01-1 s-1 and deformation amount of 0.5. Based on the true stress-true strain curves, the effects of different deformation temperatures and strain rates on the hot deformation behavior of the tested steel were analyzed. The flow stress constitutive model of the tested steel was constructed by Arrhenius hyperbolic sine equation, and the hot processing map was drawn by combining with the dynamic material model (DMM). The results indicate that the peak flow stress decreases with the increase of temperature or the decrease of strain rate. As the strain rate is 0.1 s-1, the dynamic recrystallization occurs when the deformation temperature reaches 1000 ℃, and the size of dynamic recrystallized grain increases with the increase of deformation temperature. Additionally, δ ferrite is found in the microstructure at different deformation temperatures, and its content increases with the increase of temperature. Combined with the hot processing map and microstructure evolution, the optimal hot working process of the heat-resistant steel 2Cr12Ni4Mo3VNbN is determined as 1068-1172 ℃, 0.08-0.12 s-1.
    Effect of Ti microalloying on carbide precipitation behavior and high temperature strength of 25Cr3Mo3NiNbZr steel
    Lu Maoyong, Xu Le, Li Hao, Liang Enpu, Li Ruohao, Wu Run
    2023, 48(11):  29-37.  doi:10.13251/j.issn.0254-6051.2023.11.005
    Abstract ( 33 )   PDF (6073KB) ( 15 )  
    Effect of Ti microalloying on carbide precipitation behavior, microstructure and high temperature strength of the 25Cr3Mo3NiNbZr steel was investigated by means of optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), Thermo-Calc thermodynamic calculation, physicochemical phase analysis and high temperature (700 ℃) tensile test. The results show that due to the micro-addition of Ti, the prior austenite grain size of the tested steel is refined from 50 μm to 20 μm, the high temperature (700 ℃) tensile strength and yield strength are increased by 176 MPa and 54 MPa. After the addition of Ti, the number density of MC-type carbide in the steel increases from 4.36×1015 m-3 to 5.34×1019 m-3, and the proportion of MC-type carbide in the size of 5-10 nm increases from 81.8% to 90.1%, and the MC-type carbide in the size range of 1-5 nm also increases. In addition, due to the addition of Ti, the thermal stability of MC carbides is improved, and the coarsening rate is reduced from 0.301 978 nm·s-1/3 to 0.169 049 nm·s-1/3. The addition of Ti makes the MC-type carbide in the tested steel smaller in size, denser and more stable, which is the main reason for the improvement of the high temperature strength of the tested steel.
    Effect of high-temperature tempering on microstructure evolution and recrystallization mechanism of P91 steel
    Zhou Hao, Wang Yun, Zeng Yanping
    2023, 48(11):  38-44.  doi:10.13251/j.issn.0254-6051.2023.11.006
    Abstract ( 39 )   PDF (6195KB) ( 49 )  
    Phase transformation point of P91 steel was determined by thermal expansion method. The microstructure and the evolution of special grain boundaries of the P91 steel during tempering at 710, 730, 760, 770, 790, 810, 830 and 840 ℃ were studied by means of OM, TEM and EBSD. It is found that with the increase of tempering temperature, the lath martensite morphology in the P91 steel is weakened, the dislocation density is gradually reduced, and the equiaxed ferrite structure appears. By EBSD analysis, the content of high angle grain boundaries increases gradually with the increase of tempering temperature and tends to be stable, while the content of low angle grain boundaries is significantly reduced. The analysis shows that during the tempering process, recrystallization occurs and a certain amount of equiaxed ferrite is formed, which nucleates through the mechanism of subgrain polymerization migration and grain boundary expansion, so as to realize recrystallization.
    Effect of Ni content on microstructure and properties of A330M ultra-high strength steel
    Xiao Kai, Han Shun, Xu Zhongzhi, Geng Ruming, Xia Huimin, Xu Zhengwei, Li Yong, Wang Chunxu
    2023, 48(11):  45-49.  doi:10.13251/j.issn.0254-6051.2023.11.007
    Abstract ( 33 )   PDF (3839KB) ( 33 )  
    Influence of Ni content (1.85%, 3.85%, 5.00%) on microstructure and mechanical properties of A330M ultra-high strength steel was studied by means of scanning electron microscope, transmission electron microscope and mechanical properties test. The results show that the microstructure of the tested steel after quenching and tempering at low temperature is mainly composed of lath martensite, retained austenite and ε carbide. In the range of 1.85%-5.00% Ni content, the Ms point of the tested steel gradually decreases with the increase of Ni content, and the content of retained austenite in the structure gradually increases. The tensile strength and yield strength decrease with the increase of Ni content, the impact absorbed energy increases significantly, and the proportion of impact fracture dimples increases. Comparing the strength and toughness of the A330M steel with three kinds of Ni content, 3.85% Ni content can obtain the best strength and toughness matching, tensile strength is 2207 MPa, impact absorbed energy is 34 J.
    Precipitates of nuclear grade 316NG austenitic stainless steel under thermal aging
    Sui Kailun, Ouyang Jinfei, Gong Minghao, Hu Hongxing, Li Jiafeng, Yang Shan, Long Zhaohui, Li Xiaobo
    2023, 48(11):  50-54.  doi:10.13251/j.issn.0254-6051.2023.11.008
    Abstract ( 32 )   PDF (2122KB) ( 25 )  
    Precipitation conditions of the 316NG austenitic stainless steel in the nuclear environment were studied by means of thermal aging simulation experiment, electrolytic extraction experiment and the CALPHAD method. Then, the thermodynamic database of the 316NG austenitic stainless steel was established by the CALPHAD method. The results show that the 316NG austenitic stainless steel has no precipitated phase during annealing at 450 ℃ for 100 h, the Cr23C6 is the main precipitated phase when annealed at 450 ℃ for 500 h, and the main precipitated phases are Z-CrNbN, Cr23C6 and Laves_C14 when annealed at 450 ℃ for 1000 h. Based on the thermodynamic database, at 450 ℃, the equilibrium precipitated phases calculated by the Thermo-Calc software are Cr23C6, Z-CrNbN, Laves_C14, Ni3Si and MX, and the total atom fraction is 3.5%, among which the content of Ni3Si and MX is only 0.46%. The calculated precipitated phases by CALPHAD are good agreement with the results by thermal aging experiment of annealing at 450 ℃ for 1000 h, showing that the more complete information is obtained on the precipitated phases and their compositions of the 316NG austenitic stainless steel under long-term thermal aging.
    Effect of nickel on thermal fatigue properties of novel hot working die steel
    Cheng Xi, Gu Pengcheng, Zeng Yan, Wu Xiaochun
    2023, 48(11):  55-61.  doi:10.13251/j.issn.0254-6051.2023.11.009
    Abstract ( 29 )   PDF (4151KB) ( 23 )  
    Thermal fatigue test of SDDVA steel and a novel hot work die steel SDYZ1 was carried out by Uddeholm self-constrained thermal fatigue test method. The surface morphology, cross-section crack, microstructure and hardness of the materials after thermal fatigue were compared and analyzed. The effect of nickel element on the cold and thermal fatigue properties of the material was discussed. The results show that compared with the SDDVA steel, the number of cracks in the SDYZ1 steel is obviously less, the depth is shallower, and the degree of high temperature surface softening is smaller. This is mainly due to the fact that Ni element accumulates near grain boundaries and carbides, which has a pinning effect on carbide growth, and the SDYZ1 steel has higher high temperature strength, which can resist crack propagation. The SDYZ1 steel precipitates carbides with smaller size during thermal fatigue, and the accumulation of carbides at grain boundaries can effectively hinder grain growth, thereby improving the fatigue resistance of the material. Through transmission observation and analysis, the main forms of coarse carbide particles after thermal fatigue of the SDYZ1 steel are irregular spherical and slender rods, and the main types are M23C6 and M6C, and the size of carbides in the SDYZ1 steel is smaller than that in the SDDVA steel.
    Effect of rare earth element Ce on thermal fatigue properties of H13 steel
    Chen Xuemin, Yang Lilin, Xu Qihao, Chen Shuo, Zhao Liping
    2023, 48(11):  62-67.  doi:10.13251/j.issn.0254-6051.2023.11.010
    Abstract ( 36 )   PDF (5133KB) ( 42 )  
    Thermal fatigue cycling tests were conducted on H13 steel with different Ce contents after oil quenching at 1040 ℃ and tempering twice at 580 ℃. The microstructure, crack morphology, and hardness of the tested steel under different thermal fatigue cycles were analyzed by means of optical microscope, scanning electron microscope, and hardness tester. The results indicate that after thermal fatigue cycle, the microstructure of the tested steel is tempered sorbite. Thermal fatigue cracks first initiate at the tip of the pre-treated notch, and the oxidation pits and inclusions that occur during the thermal fatigue cycle promote crack generation. The cracks continue to extend and increase in width. Rare earth Ce has a significant refining effect on the microstructure and grain size of the tested steel, improving the resistance to softening, inhibiting the growth of thermal fatigue cracks, and the optimal rare earth Ce content is 0.026%.
    Growth law of oxygen-rich α layer of three titanium alloys
    Yan Wenxuan, Shao Zhuhao, Li Wei, Liu Jing, Wei Xiaoyan, Gu Guojun
    2023, 48(11):  68-72.  doi:10.13251/j.issn.0254-6051.2023.11.011
    Abstract ( 35 )   PDF (4658KB) ( 21 )  
    Evolution of oxygen-rich α layer of three titanium alloy (TC2, TC4 and TC6 alloys) at various heat treatment conditions was studied. The forming kinetics and thermal activation energy of oxygen-rich α layer were discussed. The results show that the thickness of oxygen-rich α layer increases with the increase of heating temperature and holding time. And heating temperature is main factor. The growth of the oxygen-rich α layer is affected by the oxygen diffusion process, and the thickness is linear with t1/2. The forming of oxygen-rich α layer belongs to thermal activation behavior, and the layer thermal activation energies of TC2, TC4 and TC6 alloys are 106.1, 110.1 and 47.7 kJ/mol, respectively. Due to the strengthening effect of oxygen, the microhardness of the oxygen-rich α layer is higher than the matrix. The thickness measured by microhardness method is higher than that by metallographic observation method.
    PROCESS RESEARCH
    Effect of annealing on microstructure and properties of Zr-Sn-Nb-Fe-Si novel zirconium alloy sheet strip
    Zhou Dianwu, Zhu Zirui, Mao Jianzhong, Gao Bo
    2023, 48(11):  73-82.  doi:10.13251/j.issn.0254-6051.2023.11.012
    Abstract ( 30 )   PDF (7744KB) ( 36 )  
    Aiming at the problem of strip cracking for zirconium alloy during the stamping forming process, the effect of recrystallization annealing on microstructure and properties of Zr-Sn-Nb-Fe-Si novel zirconium alloy sheet strip was investigated. The results show that with the increase of annealing temperature, the maximum thinning reduction rate of the novel zirconium alloy sheet strip decreases, in the grain and at the grain boundary of the novel zirconium alloy, the second phase particles are dispersed. The morphology of second phase particles is basically spherical, and the larger size second phase is Zr(NbFe)2, the smaller size second phase is β-Nb. The novel zirconium alloy sheet strip has two kinds of textures: <1010>//RD and <1120>//RD. The misorientation between the grains of <1120>//RD and the deformed matrix is 30°/<0001>. With the raising of recrystallization annealing temperature, the pinning effect generated by the second phase cannot prevent the rapid migration of ∑13 grain boundaries of the zirconium alloy superposition lattice, leading to the recrystallized grains of <1120>//RD orientation growing preferentially and annexing the deformed matrix, the proportion of small angle grain boundary decreases while the proportion of large angle grain boundary increases, resulting in the transformation of texture from <1010>//RD to <1120>//RD during recrystallization, which improves the uniformity of grain deformation, improves the uniformity of plastic deformation of the sheet strip during stamping forming. Therefore, the recrystallization annealing is in favor of improving the stamping properties of the novel zirconium alloy sheet strip.
    Effect of annealing temperature on microstructure and mechanical properties of 0.14C-7Mn hot-rolled medium manganese steel
    Wang Tao, Yang Dapeng, Yi Hongliang
    2023, 48(11):  83-89.  doi:10.13251/j.issn.0254-6051.2023.11.013
    Abstract ( 37 )   PDF (5761KB) ( 56 )  
    Annealing experiments of the hot-rolled 0.14C-7Mn medium manganese steel were carried out at 600 ℃, 620 ℃ and 640 ℃ for 10 h, respectively. The results show that after annealing, lath ferrite and austenite are obtained. The volume fraction of austenite increases with the increase of annealing temperature, the content of C and Mn in austenite gradually decreases, which leads to the decrease of mechanical stability. The tensile curves of annealed steels show continuous yield. With the increase of annealing temperature, the volume fraction of austenite increases, while the stability decreases, and the martensite produced during deformation increases, so the tensile strength increases. During the deformation process, the continuous transformation of an appropriate amount of reversed austenite with moderate stability leads to work hardening, which delays the generation of neck shrinkage and increases the uniform elongation.
    Effect of austenitizing temperature on microstructure and properties of 1.2367 hot working die steel
    Du Simin, Chen Wenxiong, Wang Hongli, Hu Fengrong, Ren Jinqiao, Zhou Zhiming
    2023, 48(11):  90-95.  doi:10.13251/j.issn.0254-6051.2023.11.014
    Abstract ( 23 )   PDF (4043KB) ( 26 )  
    Effect of austenitizing temperature on microstructure and properties 1.2367 hot working die steel was studied by means of thermodynamic calculation, optical microscope, scanning electron microscope, X-ray diffractometer, Rockwell hardness tester and impact testing machine. The results show that the room temperature equilibrium structure of the 1.2367 steel is distributed with M23C6, M6C and MC carbides on the ferrite matrix, while when quenched between 1025 ℃ and 1080 ℃, the carbide content gradually decreases and the retained austenite content gradually increases with the increase of austenitizing temperature, and the quenched microstructure is martensite+carbides+retained austenite. After twice rounds of tempering at 615 ℃ for 2 h, the microstructure is tempered martensite+carbides, and with the increase of austenitizing temperature, the hardness gradually increases, with the maximum hardness of 49.05 HRC, but the impact absorbed energy gradually reduces. After quenching at 1050 ℃ and tempering at 615 ℃, the 1.2367 hot working die steel can achieve the best strength and toughness matching.
    Effect of intercritical annealing time on microstructure and mechanical properties of cold rolled medium manganese steel
    Hu Jinpeng, Wan Decheng, Ma Shaokang, Li Jie, Feng Yunli
    2023, 48(11):  96-101.  doi:10.13251/j.issn.0254-6051.2023.11.015
    Abstract ( 34 )   PDF (3271KB) ( 29 )  
    Effect of intercritical annealing time on the microstructure and mechanical properties of 0.21C-4.1Mn-1.85Si-0.05Nb-Fe cold rolled medium manganese steel was investigated by means of tensile testing machine, scanning electron microscope, and X-ray diffraction. The results show that with the increase of annealing time, the ferrite ratio decreases, the volume fraction of retained austenite first increases and then decreases, and the martensite size gradually increases. The yield strength of the steel first increases and then gradually decreases, while the tensile strength first decreases and then increases, the elongation as well as the strength-elongation product first increase and then gradually decrease. After annealing for 10 min, the engineering stress-engineering strain curve exhibits continuous yielding, but the insufficient work hardening ability leads to the worst plasticity. With annealing time increasing, the engineering stress-engineering strain curve exhibits a yield plateau, but the continuous TRIP effect occurring over larger strain range allows the tested steel to maintain lasting work hardening, contributing to a higher plasticity. After annealing at 690 ℃ for 60 min, the comprehensive mechanical properties of the tested steel are the best, with the tensile strength of 1036.9 MPa, elongation of 25.6%, and the product of strength and elongation of 26.5 GPa·%.
    Microstructure and properties of wire arc additive manufactured 20Cr9Mo3Ni2 steel on H13 steel surface at different preheating temperatures
    Xie Jinping, Zeng Daxin, Shi Qiuyue, Yin Yijun
    2023, 48(11):  102-111.  doi:10.13251/j.issn.0254-6051.2023.11.016
    Abstract ( 26 )   PDF (12482KB) ( 24 )  
    20Cr9Mo3Ni2 steel was deposited on the H13 steel substrate using wire arc additive manufacturing at different preheating temperatures, and the macroscopic morphology, microstructure and mechanical properties of the parts were investigated. The results show that the increase of preheating temperature reduces the tendency of cracking and no crack appears when the preheating temperature is higher than 300 ℃. The preheating temperature has different effect on the microstructure of different positions in the parts. The microstructure at bottom and middle of the additive zone is mainly tempered martensite when without preheating and preheated at 150 ℃, and is composed of tempered martensite and quenched martensite when preheated at 300 ℃. When preheated at 450 ℃, the microstructure at bottom is composed of martensite and a small amount of bainite, and the middle is mainly martensite. The preheating temperature has a little effect on microstructure of top, and the microstructure is mainly martensite and retained austenite. There is a small amount of ferrite between grains at different preheating temperatures and regions. The microstructure of heat affected zone (HAZ) of the H13 steel substrate is tempered martensite when preheated below 300 ℃ and coarse martensite when preheated at 450 ℃. The tensile strength of the additive zone increases with the increase of preheating temperature, while the elongation decreases. The transverse tensile strength is slightly higher than the longitudinal one, while the elongation opposite. The tensile strength of the bonding zone is lower than the additive zone, when the preheating temperature is below 300 ℃, the fracture position is located in the substrate, and when the preheating temperature is 450 ℃, the fracture position is located in the heat affected zone of the substrate. The hardness of the additive zone is low at the bottom and middle and high at the top when without preheating and preheated at 150 ℃, while there is a high and uniform hardness from top to bottom when preheated at 450 ℃. The results indicate that the microstructure and properties change greatly when the preheating temperature changes across Ms point.
    Microstructure and properties of warm deformed 38Si7 spring steel for high-speed rail spring bars after heat treatment
    Wang Tianjiao, Ding Xiaoming, Zhuang Qianyu, Jiang Bo
    2023, 48(11):  112-119.  doi:10.13251/j.issn.0254-6051.2023.11.017
    Abstract ( 27 )   PDF (5395KB) ( 21 )  
    For tested 38Si7 spring steel, the effect of warm deformation temperature on stress and microstructure, the effect of original microstructure on the microstructure and mechanical properties after quenching and tempering, and the effect of quenching and tempering parameters on the microstructure and mechanical properties were investigated, respectively, by means of optical microscope, scanning electron microscope, hardness tester and electronic universal testing machine. The results show that a higher deformation resistance is obtained when warm deformation temperature is between 720-800 ℃,which is 225.6-242.2 MPa. The temperature of warm deformation is selected as 800 ℃, at which the homogenization of microstructure of the 38Si7 spring steel is better. The main factor to control the content and morphology of retained ferrite is the quenching temperature. When the quenching temperature is higher than or equal to 880 ℃, the amount of retained ferrite in the microstructure of the 38Si7 spring steel can be greatly reduced. The strength and hardness of the steel increase with quenching temperature raising, while, its elongation decreases first and then increases. Quenched at 900 ℃, the 38Si7 spring steel has the best comprehensive mechanical properties, with tensile strength, hardness and elongation of 1396.8 MPa, 40 HRC, 12.5%, respectively, and the yield ratio is 0.9. The microstructure of network ferrite is presented when oil quenched, which is unfavorable to mechanical properties. When the tempering temperature is between 410-430 ℃, the effect of tempering temperature on microstructure and mechanical properties of the 38Si7 spring steel is slight. However, when the tempering temperature is 450 ℃, the strength and hardness are obviously reduced while the plasticity is improved. In conclusion, the optimal heat treatment process of the 38Si7 spring steel is 900 ℃×30 min, water quenching+430 ℃×60 min, air cooling.
    Effect of solution treatment on microstructure and properties of welded joint of S32205 duplex stainless steel with multi-stranded wires
    Chen Zefang, Peng Kang, Chen Xizhang, Wang Qichen
    2023, 48(11):  120-126.  doi:10.13251/j.issn.0254-6051.2023.11.018
    Abstract ( 17 )   PDF (5115KB) ( 31 )  
    Solution treatment was performed at 1080 ℃ for S32205 duplex stainless steel welded joint welded with multi-stranded wire, and the effect of which on microstructure, mechanical properties and pitting corrosion resistance of the welded joint was analyzed by means of electronic backscatter diffraction, scanning electron microscope, electronic universal test machine and electrochemical test. The results show that after solution treatment at 1080 ℃, the main alloying elements of the welded joint are fully diffused, the microstructure is more uniform, the austenite volume fraction of the weld is increased by 11.2%, and the ratio of ferrite and austenite is more balanced. After the solution treatment, the average tensile strength of the welded joints increases by 32.7 MPa, the plasticity improves slightly, and at the same time, the pitting corrosion resistance of the welded joints is significantly improved.
    Surface nitriding process analysis of electromagnetic pure iron
    Wang Diangang, Sun Xiaotong, Xu Wenhua, Chen Chuanzhong, Lü Yupeng
    2023, 48(11):  127-131.  doi:10.13251/j.issn.0254-6051.2023.11.019
    Abstract ( 15 )   PDF (3706KB) ( 25 )  
    Surface and cross-sectional morphology, phase composition and hardness of domestic and imported electromagnetic workpieces with the same specifications and dimensions were tested and analyzed by using optical microscope, scanning electron microscope, X-ray diffractometer and microhardness tester, and the surface treatment processes of the two type workpieces were studied based on the Fe-N binary phase diagram. The results show that the matrix of both specimens is α pure iron, and the surface morphology and phase composition after surface treatment are basically the same. However, there is only a γ′-Fe bright compound layer above the diffusion layer of the domestic electromagnetic workpiece, with a thickness of 8-9 μm and a microhardness of 323 HV0.05. While there is a layer with a thickness of 12-13 μm between the bright compound layer and the diffusion layer of the imported electromagnetic workpieces, and the transition layer with a microhardness of 312 HV0.05 is the troostite structure formed after γ-Fe cooling. It can be inferred that domestic workpiece adopted a conventional low-temperature (below 590 ℃) nitriding process, while imported workpiece which has a deeper nitriding layer and a transition layer composed of troostite structure adopted a high-temperature ion nitriding process above 590 ℃.
    Effect of heat treatment on microstructure and mechanical properties of 15-5PH stainless steel by selective laser melting
    Wang Luyang, Sun Donghui, Huang Rui, Huang Hongwu
    2023, 48(11):  132-136.  doi:10.13251/j.issn.0254-6051.2023.11.020
    Abstract ( 19 )   PDF (2639KB) ( 34 )  
    15-5PH stainless steel specimens were prepared by selective laser melting (SLM) forming process and then were aged at 620 ℃ for 4 h, and solution treated at 1030 ℃ for 1 h plus aged at 550 ℃ for 4 h, respectively. The microstructure and mechanical properties of the 15-5PH stainless steel specimens formed by SLM under different states were compared. The results show that compared to SLMed specimen, when the tensile strength of the specimens obtained by different heat treatments is equivalent, the microstructure of the aged specimen is mainly composed of fine dendritic grains which is mix structure of martensite, lamellar pearlite and ferrite, while the microstructure of the solution treated+aged specimen is mainly composed of tempered martensite+acicular ferrite+partial reversed austenite. The dendritic structure inside the grains almost disappears and the grains grow. Compared with the aged specimen, the yield strength and yield ratio of the solution treated+aged specimens are increased by 33% and 32%, respectively, and the impact absorbed energy, elongation after fracture, and the percentage reduction of area are decreased by 72.0%, 35.5% and 18.7%, respectively.
    Effect of rolling routes on formability, microstructure and mechanical properties of Mg-Al-Ca-Mn-Zn magnesium alloy
    Fang Dejun, Sun Youping, He Jiangmei, Luo Guojian, Luo Zhang
    2023, 48(11):  137-142.  doi:10.13251/j.issn.0254-6051.2023.11.021
    Abstract ( 19 )   PDF (3496KB) ( 23 )  
    Effect of four different rolling routes on formability, microstructure and mechanical properties of the Mg-1.2Al-0.4Ca-0.3Mn-0.3Zn magnesium alloy sheet was investigated by means of metallographic microscopy, X-ray diffractometer, tensile testing machine and scanning electron microscope. The results show that the rolling route has an effect on the formability, microstructure and mechanical properties of the Mg-1.2Al-0.4Ca-0.3Mn-0.3Zn magnesium alloy sheet, and the sheet obtained by RD+TD multi-pass cross-rolling has the best formability and no edge crack. Compared with that of the unidirectional multi-pass rolling, the sheet obtains obvious grain refinement and uniform microstructure after multi-pass cross-rolling, the basal texture intensity is reduced from 9.680 to 6.111, the anisotropy is significantly weakened and the plasticity is improved, with the elongation along rolling direction of 19%.
    Effect of cryogenic treatment on microstructure and hardness of carburized gear steel
    Song Shaowei, Yu Wenchao, Shi Jie, Wang Maoqiu, He Xiaofei, Zhou Yun
    2023, 48(11):  143-148.  doi:10.13251/j.issn.0254-6051.2023.11.022
    Abstract ( 21 )   PDF (3251KB) ( 28 )  
    Microstructure and hardness of 20Cr2Ni4A and 17Cr2Ni2MoVNb carburized gear steels were characterized by means of metallographic microscope, X-ray diffractometer, EBSD and Vickers hardness tester, and the effect of cryogenic treatment on microstructure and hardness distribution of the tested steels was studied. The results show that the maximum hardness in the carburized layer of the two tested steels increases obviously after cold treatment at -75 ℃, and it increases a little further after cryogenic treatment at -196 ℃. After cold treatment at -75 ℃, the contents of retained austenite in the carburized layers of the 20Cr2Ni4A and 17Cr2Ni2MoVNb steels are decreased from 34% to 19% and 25% to 20%, respectively, and they are further decreased to 11% and 13% after cryogenic treatment at -196 ℃. There is a more amount of large-sized retained austenite in the 20Cr2Ni4A steel, which is decreased more easily after cryogenic treatment. Large-sized retained austenite has a significant effect on hardness of the carburized layer.
    Effect of austenitizing temperature on friction and wear properties of austempered ductile iron
    Du Baoshuai, Mi Chunxu, Wang Xin, Yang Chao, Yang Xinyu, Yan Zhicheng
    2023, 48(11):  149-155.  doi:10.13251/j.issn.0254-6051.2023.11.023
    Abstract ( 14 )   PDF (4729KB) ( 25 )  
    Austenitization treatment is a crucial step in obtaining high-performance austempered ductile iron (ADI). Through austenitization treatment, the cast iron matrix structure transforms into austenite, providing a precursor for the subsequent austempering process during which austenite transforms into ausferrite. The austenitization temperature was controlled, and the corresponding changes in ADI's microstructure and their effects on mechanical properties and friction-wear properties were investigated. The research reveals that the increase of austenitization temperature can enhance the carburizing effect of carbon diffusion from graphite to the matrix, resulting in higher carbon content and improved stability in austenite. On one hand, this leads to decreased nucleation driving force of ferrite in the cooling transformation process, resulting in a finer elongated ferrite morphology in the austenite-ferrite structure. On the other hand, it leads to an increase in the volume of blocky retained austenite and carbon content, resulting in a decrease in strength, hardness and elongation after fracture. Higher austenitization temperatures result in the ADI containing a significant amount of high-carbon retained austenite, making it challenging for stress-induced phase transformation to form martensite during friction-wear processes, and thus reducing wear resistance. Considering mechanical and friction-wear properties, the austenitization temperature for the ADI should not exceed 950 ℃.
    Effect of heat treatment process on mechanical properties of super pipe nozzle for nuclear power plant
    Guo Yanhui, Deng Dong, Zhang Yue, Shen Yingcai, Wang Yan
    2023, 48(11):  156-160.  doi:10.13251/j.issn.0254-6051.2023.11.024
    Abstract ( 14 )   PDF (2120KB) ( 21 )  
    Effect and mechanism of performance heat treatment (PHT) and simulated welding heat treatment (SWHT) on mechanical properties were investigated for the P280GH steel super pipe nozzles produced by top-drawing process. Effects of different heat treatments on the microstructure were studied by means of optical microscope, scanning electron microscope and energy dispersive spectroscope, and the mechanical tests such as tensile tests at room temperature and 300 ℃, impact test at 0 ℃, Brinell hardness test were carried out. The results show that after PHT or PHT+SWHT, the microstructure of the nozzles are mainly composed of two phases of ferrite and pearlite. However, after PHT, the pearlite appears mainly at the grain boundary of ferrite, while after PHT+SWHT, the pearlite has a tendency of spheroidization, and there is some pearlite inside the ferrite. The tensile properties of the PHT nozzle specimens are basically the same as that un-heat-treated main pipe, but its average impact absorbed energy is higher by about 15 J. The tensile properties of the PHT+SWHT nozzle specimens are reduced by about 18 MPa compared to that un-heat-treated main pipe, and the average impact absorbed energy is reduced by about 20 J. It is shown that fracture and spheroidization of the pearlite in the microstructure of the super pipe nozzle produced by top-drawing are the main causes of changes in the mechanical properties of the specimens with different heat treatments.
    Effect of heat treatment on microstructure, properties and residual stress of TiN/AlSi10Mg alloy composites fabricated by SLM
    Huang Weidong, Wang Lu, Huang Xu, Cheng Xuheng
    2023, 48(11):  161-167.  doi:10.13251/j.issn.0254-6051.2023.11.025
    Abstract ( 14 )   PDF (4060KB) ( 27 )  
    Selective laser melting (SLM) was used to prepare TiN/AlSi10Mg alloy composites. In order to further improve the mechanical properties of the specimen and eliminate the residual stress in the forming process, different heat treatment processes (aging, stress relief annealing, T6) were selected for microstructure regulation. The results show that the average hardness of the formed specimens is 136.2 HV0.2, the tensile strength is 397.7 MPa, the elongation is 12.5%, and the microstructure is composed of primary α-Al matrix decorated with brilliant white eutectic Si networks. After T6 treatment on the SLM specimens, the continuous Al-Si eutectic structure disintegrates at 520 ℃ and 2 h, and the discrete Si particles undergo Ostwald ripening, the elongation (11.0%) is similar to that of the formed specimen, and the tensile strength decreases to 239.1 MPa. After stress relief annealing treatment at 250 ℃ for 2 h, a small number of mesh Si boundaries are spheroidized, and the tensile strength and elongation decrease (344.9 MPa, 8.8%). After aging at 160 ℃ for 6 h, the fine a-Al/Si eutectic structure formed during SLM forming is basically retained, while the residual stress is reduced by 78.9%, and at the same time, the precipitations of nanoscale Si phase and Mg2Si strengthening phase are promoted, the optimal mechanical properties are obtained with the hardness of 143.5 HV0.2, tensile strength of 408.7 MPa, and elongation of 15.3%. It is shown that an appropriate aging treatment can be used to obtain the best comprehensive mechanical properties, and also significantly reduce the residual stress.
    Effect of pulsed-magnetic field solution treatment on microstructure and properties of A356 aluminum alloy
    Huang Yuxin, Pan Hao, Xing Shuqing, Liu Yongzhen, Gong Meina, Ma Yonglin
    2023, 48(11):  168-172.  doi:10.13251/j.issn.0254-6051.2023.11.026
    Abstract ( 14 )   PDF (3017KB) ( 15 )  
    A pulsed-magnetic field with a magnetic induction intensity of 33 mT was applied to A356 aluminum alloy during solution treatment at 540 ℃ to study the effect of pulsed-magnetic field on the microstructure and mechanical properties. The tensile test was conducted using an electronic universal testing machine, and the microstructure and fracture morphology of the A356 aluminum alloy were observed using OM and SEM. The results show that compared with traditional solution treatment, the grain refinement phenomenon of the A356 aluminum alloy after pulsed-magnetic field solution treatment is obvious, and the average size, average area, and SDAS value of eutectic silicon decrease, while the quantity increases. When the pulsed-magnetic field solution treatment time of the A356 aluminum alloy is 40 min, there is no significant difference in mechanical properties compared to traditional solution treatment, and the strength is slightly improved. Compared with traditional solution treatment, the pulsed-magnetic field solution treatment can significantly shorten the solution treatment time while refining the grains. When the pulsed-magnetic field solution treatment time is 40 min, the process time is reduced by 91.11% compared with the traditional solution treatment, greatly improving production efficiency.
    Effect of hot rolling coiling temperature on microstructure and hemming performance of 6014 aluminum alloy
    Li Tao, Chi Rui, Xu Zhiyuan, Yang Limin, Ma Huilin
    2023, 48(11):  173-179.  doi:10.13251/j.issn.0254-6051.2023.11.027
    Abstract ( 17 )   PDF (5885KB) ( 18 )  
    Effect of hot rolling coiling temperature on the microstructure and hemming property of final state 6014 aluminum alloy was investigated. The causes of bending cracking of the final state 6014 aluminum alloy were analyzed by means of metallography, EBSD, XRD, SEM and TEM. The results show that the grains of the final state 6014 alloy with hot rolling coiling temperatures at 310 ℃ and 360 ℃ have been recrystallized and the ratio of grain length width is large, the grain size distribution of the specimens coiled at 310 ℃ is uneven, the grain size at the surface layer is larger than that at the center layer, which is easy to form stress concentration and deformation disharmony, and AlSiCu phase with the size of 20 nm to 40 nm is continuously precipitated on the grain boundary. The surface roughness and slip band density of the specimen with hot rolling coiling temperature at 310 ℃ are greater than that at 360 ℃, the probability of strain localization is higher. The grain size coiled at 360 ℃ is relatively uniform. The proportion of 2.5°- 5° low angle grain boundary is higher than that coiled at 310 ℃, the grain boundary is relatively clean, and the hemming performance is relatively good. When coiled at 310 ℃, there are many iron containing phases with large size in the crack of the bent specimens, which will be broken by the slip band, leading to bending cracking; However, no coarse iron containing phase is found in the crack of the bent specimens coiled at 360 ℃, besides a few dimples.
    Control of white layer porosity in gas nitrocarburizing
    Wang Jianxin, Wu Kai
    2023, 48(11):  180-184.  doi:10.13251/j.issn.0254-6051.2023.11.028
    Abstract ( 20 )   PDF (2290KB) ( 26 )  
    Porosity in white layer is an important factor which affects its performance during gas nitrocarburizing, and the most important mechanism of porosity formation is the decomposition of nitrides and the recombination of atomic nitrogen to molecular nitrogen. The formation of porosity is related to the material, gas nitrocarburizing time and temperature, and the concentrations of nitrogen and carbon in the white layer which are directly related to the nitrogen potential KN and carbon potential KC values of the atmosphere. In a typical nitrocarburizing process, the control of white layer porosity is throughout the process, including all the heating, holding and cooling stages.
    Effect of composite heating process of induction furnace and pit furnace on quenched and tempered microstructure and mechanical properties of 40Cr steel
    Wang Weimin, Liu Anqi, Guo Chuncheng, Da Zhongzhong, Qi Haiquan, Han Xiangnan, Xue Qihe, Wei Minjun
    2023, 48(11):  185-190.  doi:10.13251/j.issn.0254-6051.2023.11.029
    Abstract ( 18 )   PDF (2411KB) ( 28 )  
    Composite heating process composed of induction furnace heating to 870 ℃ or 900 ℃ and then pit furnace isothermal heating at 860 ℃ for 0.5 h was used to carry out quenching and tempering of the 40Cr steel. Effect of induction furnace and pit furnace composite heating on the microstructure and properties of the 40Cr steel was studied through microstructure observation, Rockwell hardness and impact property testing, and compared with conventional single furnace quenching heating process. The results show that the hardness obtained by the composite heating process is 29-30 HRC, which is 4±2 HRC higher than that by the conventional heating process, and the data fluctuation is minimal. The composite heating process provides excellent impact properties, and the highest impact absorbed energy of 118 J can be obtained when tempered with water-cooling. The specimen under composite heating process can obtain fine and uniform tempered sorbite. The optimal composite heating process is induction heating to 900 ℃ and then pit furnace isothermal heating at 860 ℃ for 0.5 h, water-quenching, tempering at 580 ℃ for 1.5 h and then water-cooling. It can be inferred that for the quenching and tempering treatment of slender shaft parts, the composite heating of induction furnace and pit furnace has good feasibility. While improving the comprehensive mechanical properties of the materials, the composite heating can greatly shorten the heating and holding time, which is of great practical significance for improving production efficiency, reducing energy costs and improving furnace life.
    Effect of holding time on carbide dissolution and bainite transformation of bearing steel
    Wu Bingbing, Wang Qingxiang, Liu Xiaoming, Xing Yunxiang, Xu Rongchang, Li Hui
    2023, 48(11):  191-195.  doi:10.13251/j.issn.0254-6051.2023.11.030
    Abstract ( 20 )   PDF (3482KB) ( 40 )  
    Size and volume fraction of carbides in the GCr15 bearing steel at 860 ℃ for different holding time in two-phase region were studied by means of scanning electron microscope, Nano Measure 1.2 and Image J image software. The hardness was measured using a Vickers microhardness tester. And the bainitic transformation in bearing steel treated under different two-phase region holding time and different bainitic isothermal time was analyzed using DIL thermal expansion instrument and scanning electron microscope. The results show that at 860 ℃, the longer the holding time in two-phase region, the more carbides are dissolved, the greater the increase in hardness, and the size and volume fraction of carbides are significantly reduced. As the holding time in the two-phase region increases, the more carbides dissolve, the more stable and uniform C/Cr content in austenite becomes, and the bainite transformation incubation period extends. The optimal holding time in two-phase region is 30 min. The longer the bainitic isothermal treatment time, the greater the proportion of bainite transformation.
    Effect of bake hardening on trace element segregation at grain boundary of Nb-Ti micro-alloyed steel
    Song Qing, Wang Lihui, Pan Yingjun, Zhang Caidong, Huang Feng, Dong Yikang
    2023, 48(11):  196-202.  doi:10.13251/j.issn.0254-6051.2023.11.031
    Abstract ( 10 )   PDF (4572KB) ( 16 )  
    Effect of precipitation behavior during bake hardening on the mechanical properties of the bake hardening steel (HC180B) was investigated. The segregation behavior of dislocations, precipitates and atoms such as C and N at grain boundaries was studied by means of optical microscope (OM), transmission electron microscope (TEM) and three-dimensional atom probe (3DAP). The results show that the yield strength of 2% pre-tensile deformed specimen and that of the baked one is increased by 2 MPa and 73 MPa compared with that of the annealed cold-rolled plates, respectively. 3DAP and TEM results show that only C atoms in the annealed cold-rolled sheet are slightly segregated at the grain boundary, and N, Ti and Nb are evenly distributed in the grain. After 2% pre-stretching deformation, C, Ti and Nb are different degrees of segregation at the grain boundary. According to the difference between the Gibbs free energy and the atomic concentration at the grain boundary, it can be concluded that TiC is mostly rectangular and preferentially precipitates at the grain boundary, and NbC precipitates in a small amount. After 2% pre-stretching deformation and baking, the segregation of C, Ti and Nb at the grain boundary is more obvious, NbC precipitates in a large amount at the grain boundary in the form of an ellipsoid, TiC precipitates in a small amount, and the distribution of N atoms in different states is relatively uniform, while Ti and Nb carbides appear in the form of compound precipitation.
    Laser quenching process for small area convex dots and large area smooth surface of Cr12MoV steel
    Chen Yangsheng
    2023, 48(11):  203-209.  doi:10.13251/j.issn.0254-6051.2023.11.032
    Abstract ( 12 )   PDF (3551KB) ( 20 )  
    Aiming to solve some process pain points encountered during the heat treatment process of Cr12MoV steel molds with different shapes, differences in laser quenching processes were investigated for the large area smooth surface and small area convex dots on such molds by using the Cr12MoV steel tested blocks and fiber laser equipment. The results show that the two processes have similarities and differences. The common point is that the depth and hardness of the hardened layer are directly proportional to the laser heating temperature and inversely proportional to the scanning speed, while it is possible to cause overheating or even micro melting of the surface layer if the temperature is too high or the scanning speed is too slow. The difference is that, caused by the differences in heated areas and shape, their process parameters may vary significantly. The most suitable process parameters for the small area convex dots are laser heating temperature of 960 ℃ and scanning speed of 250 mm/min, while for the large area smooth surface are 1180 ℃ and 250 mm/min, and the occurrence of soft band phenomenon is inevitable for the latter case. Such research results are helpful for process personnel to choose appropriate process parameters according to actual needs when laser surface quenching is applied to molds with different shapes.
    Effect of pearlite morphology on microstructure and hardness after spheroidizing annealing of SK85 steel
    Deng Suhuai, Liang Jiangtao, Liu Kun, Li Yue, Xu Haiwei
    2023, 48(11):  210-215.  doi:10.13251/j.issn.0254-6051.2023.11.033
    Abstract ( 13 )  
    Simulation of spheroidizing annealing process of the SK85 steel cold rolled sheet was carried on by simultaneous thermal analyzer. The effect of morphology of deformed pearlite and granular pearlite on the spheroidized structure and properties was studied by means of optical microscope, scanning electron microscope and hardness tester. The results show that as the annealing temperature increases, the spheroidizing hardness of two morphic pearlites specimens first decreases and then increases, and reaches the minimum values when the annealing temperature is 740-750 ℃. When the annealing temperature is lower than 740 ℃, the spheroidized microstructure is granular pearlite, the spheroidizing speed of deformed pearlite is greater than that of granular pearlite, and the annealing hardness is greater than that of granular pearlite specimen. When the annealing temperature is higher than 750 ℃, the spheroidized microstructure is lamellar pearlite and granular pearlite, and both hardness changes tend to be consistent. The carbide particle size obtained after granular pearlite spheroidizing is larger than that of deformed pearlite, which is the reason why the spheroidization hardness of granular pearlite is lower than that of deformed pearlite. Adjusting the carbide dispersion through softening annealing process design can further reduce the hardness of SK85 steel cold rolled sheet and meet the industry requirements of cold stamping.
    Effect of isothermal time on microstructure and properties of low carbon micro-alloyed steel
    Sun Tingting, Liu Pengcheng, Wu Zhongwang, Tian Fangcheng, Xing Lei
    2023, 48(11):  216-220.  doi:10.13251/j.issn.0254-6051.2023.11.034
    Abstract ( 19 )   PDF (2738KB) ( 43 )  
    Effect of different isothermal time on microstructure and properties of a low carbon micro-alloyed steel (0.058%C) was studied by means of metallographic microscope, field emission scanning electron microscope, transmission electron microscope, tensile testing and X-ray diffractometer. The results show that when the isothermal time increases from 1800 s to 86 400 s at 725 ℃, the volume fraction of lath bainite decreases while the volume fraction of ferrite increases. Almost all the retained austenite in the specimen under isothermal time of 86 400 s decomposes. The tensile strength of the tested steel decreases with the prolongation of isothermal time, the yield strength decreases, while the elongation after fracture increases. When the isothermal time is 1800 s, the lower bainite structure with good match of strength and toughness can be obtained.
    Effect of post weld heat treatment on microstructure and properties of TA2 titanium plate argon arc welded joint
    Chen Yuanyuan, Feng Qing, Zhang Le, Wang Chao, Li Mengyuan, Shen Chu, Wang Siqi, Han Kunyan
    2023, 48(11):  221-224.  doi:10.13251/j.issn.0254-6051.2023.11.035
    Abstract ( 12 )   PDF (2991KB) ( 17 )  
    TA2 titanium plate was welded by argon arc welding, and the microstructure and mechanical properties of the as-welded and post weld heat treated joint were compared and analyzed. The results show that the weld zone width of the TA2 titanium plate argon arc welded joint is about 6 mm, and the heat affected zone width is about 4 mm, the microstructure of the weld zone is mainly composed of β equiaxed crystal, internal acicular α martensite and small amount of α′ martensite, and the microstructure of the heat affected zone is mainly composed of α lath grains. The maximum hardness of the weld zone is about 195 HV0.5, the resistivity is higher than that of base metal, and the residual stress is the largest, such as the transverse and longitudinal residual stresses reach 500 MPa and 225 MPa, respectively. When post weld heat treated at 550 ℃, the grain size of the weld zone is basically the same as that of the as-welded ones, the grain size of heat affected zone is significantly smaller, the hardness of welded joint is significantly increased, and the resistivity and residual stress are reduced. When post weld heat treated at 600 ℃, the grain size, hardness and resistivity of the welded joint are basically the same as that of the as-welded ones, and the residual stress is significantly reduced. When post weld heat treated at 650 ℃, the grain size of welded joint grows significantly, while the hardness and resistivity decrease, and the residual stress decreases significantly.
    Effect of cooling rate on microstructure and properties of LY225 low yield point steel
    Wu Fengjuan, Yang Hao, Qu Jinbo
    2023, 48(11):  225-229.  doi:10.13251/j.issn.0254-6051.2023.11.036
    Abstract ( 13 )   PDF (3486KB) ( 15 )  
    Effect of different cooling rate on microstructure, tensile property, impact property and fatigue performance of the LY225 low yield point steel was studied by hot rolling trials. The results show that, at the low cooling rate (1 ℃/s), the grain size is coarse and the pearlite lamellar spacing is larger with a large amount of free cementite continuously distributed along the grain boundary, leading to poor toughness and fatigue performance. At the medium cooling rate (10 ℃/s) and high cooling rate (20 ℃/s), the grains are refined and the pearlite lamellar spacing is decreased with much less dispersedly distributed free cementite, showing much improved strength, toughness and fatigue performance. Compared to the medium cooling rate (10 ℃/s), the high cooling rate (20 ℃/s) offers a more refined microstructure, but with lower plasticity and fatigue performance.
    Effect of annealing on microstructure and properties of a rare earth magnesium alloy
    Xu Xiangmei, Wang Qichen, Ma Shengbin
    2023, 48(11):  230-234.  doi:10.13251/j.issn.0254-6051.2023.11.037
    Abstract ( 14 )   PDF (3654KB) ( 19 )  
    Effect of annealing process on microstructure and mechanical properties of a rare earth magnesium was investigated. The microstructure was characterized by using optical microscope and scanning electron microscope. The phase constitution and texture were analyzed by using X-ray diffractometer. The mechanical properties were tested by means of universal tensile testing machine. The results indicate that as the annealing temperature increases, the deformation twins gradually disappear, and the deformed grains undergo recovery and recrystallization. The average grain size gradually grows from 6.35 μm at 200 ℃ to 12.45 μm at 350 ℃, increased by 96.1%. Annealing time has a little effect on the grain size and texture constitution of the rare earth magnesium alloy. The Al2Y phase is distributed within and at the grain boundaries of the matrix of the rare earth magnesium, which crystallizes in the liquid phase and precipitates in the solid phase, to inhibite the growth of crystalline grains during solidification, refine the initial grain size of the solidified state and pin the recrystallized grain boundaries during deformation and heat treatment processes to refine the grains. As the annealing temperature increases, the yield strength and tensile strength first decrease and then increase, and the elongation first increases and then decreases. After annealing at 300 ℃ for 60 min, the lowest yield strength and tensile strength of a rare earth magnesium alloy specimen can be obtained, with 158 MPa and 215 MPa, respectively, and the highest elongation is 16.0%.
    Effect of single-stage solution treatment on microstructure and properties of Al-Zn-Mg-Cu alloy thick plate
    Sun Ning, Wang Zhidong, Wang Jingtao, Chi Rui, Guo Fengjia
    2023, 48(11):  235-240.  doi:10.13251/j.issn.0254-6051.2023.11.038
    Abstract ( 10 )   PDF (2855KB) ( 20 )  
    Microstructure and properties of Al-Zn-Mg-Cu aluminum alloy after different single-stage solid solution processes were characterized by means of OM, SEM, DSC, XRD, tensile test and conductivity test. The effects of different solution processes on the microstructure and properties of the alloy were studied. The results show that the second phase in the 7055 aluminum alloy plate dissolves and the retained content of the second phase in the matrix gradually decreases when the alloy solution treated within the range of 475-482 ℃. Meanwhile, the conductivity of the alloy significantly decreases. The strength shows a trend of first increasing and then decreasing. Moreover, the alloy exhibits excellent comprehensive mechanical properties, when solution treated at 479 ℃ for 1 h, the yield strength and the tensile strength of the alloy reach 377 MPa and 544 MPa, respectively.
    Carburization simulation analysis on different materials and diffusion coefficient models
    Wang Xin, Li Zhenjie, Zhang Yanhui, Li Hui
    2023, 48(11):  241-244.  doi:10.13251/j.issn.0254-6051.2023.11.039
    Abstract ( 15 )   PDF (1855KB) ( 16 )  
    Taking alloy steels 17Cr2Ni2Mo and 20CrNi2Mo as examples, a new diffusion coefficient calculation model D(T,C,Q) was proposed by adding an alloy factor Q on the basis of previous diffusion coefficient calculation model D(T,C). By comparison with another diffusion coefficient calculation model D(T,C,M), the carburizing process of the two alloy steels was simulated and analyzed by using DEFORM software, and the simulation results were verified by stripping test. The results show that the simulation accuracy of D(T,C,Q) and D(T,C,M) models affected by alloy elements is higher than that of D(T,C) model. The simulation accuracy of D(T,C,Q) model of the 17Cr2Ni2Mo steel is higher than that of D(T,C,M), and that of D(T,C,M) model of the 20CrNi2Mo steel is slightly higher than that of D(T,C,Q) model, indicating that the D(T,C,Q) model has higher accuracy in calculating the diffusion coefficient.
    Heat treatment processes of 42CrMo steel gear ring after forging
    Chen Yongxiang, Li Yong, Wang Tao, Yang Gang, Yang Min, Zhong Xiaomei
    2023, 48(11):  245-249.  doi:10.13251/j.issn.0254-6051.2023.11.040
    Abstract ( 20 )   PDF (3444KB) ( 30 )  
    Post-forging heat treatments (directly air-cooling, reheating annealing and forging waste-heat annealing after forging) were carried out for the 42CrMo steel forged gear rings, and then the gears under three different post-forging heat treatment conditions were finally quenched and tempered. The microstructure and hardness of the post-forging heat treated gear rings were characterized, and the body properties and microstructure of the quenched and tempered gear rings were tested and analyzed. The results show that the 42CrMo steel forging gear rings after three different post-forging heat treatments can meet the final property requirements after quenching and tempering. However, the microstructure of the body after the forging waste-heat annealing+quenching and tempering is more uniform, the comprehensive property matching of the impact property and strength is better, and the waste-heat annealing after forging is also a green manufacturing process for energy saving and consumption reduction, and coincides with the goals of carbon peaking and carbon neutrality.
    Effect of cryogenic treatment on friction and wear properties of DC53 cold work die steel
    Zhao Dongmei, Ni Lei, Jiang Dongxu, Ding Hongxing
    2023, 48(11):  250-253.  doi:10.13251/j.issn.0254-6051.2023.11.041
    Abstract ( 15 )   PDF (3218KB) ( 22 )  
    Hardness and friction and wear properties of DC53 cold work die steel with different soaking time were tested by Rockwell hardness tester, friction and wear testing machine to investigate the effect of cryogenic treatment on its friction and wear properties. The results show that, after cryogenic treatment, the hardness of the DC53 cold work die steel changes a little, and the wear resistance is greatly improved. The friction and wear properties of the DC53 cold work die steel cryogenic treated for different time are better than those without cryogenic treatment. Increasing the cryogenic treatment time cannot further improve the properties of the material. The friction and wear properties of the material are better when the cryogenic treatment time is 2 h and 6 h, thereby, compared with the specimen without cryogenic treatment, the wear rate is reduced by 48.3% and 47.0%, respectively.
    Effect of baking time on microstructure and properties of AA7075 high strength aluminum alloy
    Jiang Yifu, Xu Shengnan
    2023, 48(11):  254-257.  doi:10.13251/j.issn.0254-6051.2023.11.042
    Abstract ( 11 )   PDF (1786KB) ( 17 )  
    Effect of baking time on microstructure and properties of AA7075 high strength aluminum alloy was studied by means of transmission electron microscope, universal testing machine and electrochemical testing. The results show that, with the extension of baking time, the quantity of η′ precipitates increases first and then decreases. The main phase of matrix is coarsened η phase, which is baked for 3 h. At the same time, η′ is the main strengthening phase of the alloy. The increase of η′ phases and the transformation of η′ to η promote the alloy to exhibite the trend that the strength increases first and then decreases. The alloy has the highest strength baked for 2 h, and the yield strength and ultimate tensile strength reach 392 MPa and 458 MPa, respectively. The coarse grain boundary precipitates of the specimen baked for 3 h enables the test material to reduce the grain boundary sensitivity, which hinders the corrosion propagation along the grain boundary, and improves the corrosion resistance of the alloy significantly.
    OVERVIEW
    Research progress on heat treatment of β titanium alloy
    Xiao Hao, Sun Yang, Fan Juanjuan, Wei Haitao, Yi Junying, Yao Liu
    2023, 48(11):  258-265.  doi:10.13251/j.issn.0254-6051.2023.11.043
    Abstract ( 60 )   PDF (3348KB) ( 22 )  
    β titanium alloy is widely used in the fields of high performance and advanced engineering such as aerospace, marine, military and biomedical, but the current process application puts forward higher requirements for its properties. β titanium alloy can be strengthened by heat treatment. The heat treatment process of β titanium alloy in recent years was reviewed. Firstly, the main phase composition and phase transformation of β titanium alloy were systematically overviewed. Then, the effects of typical heat treatment process, plastic deformation mechanism and newly developed pre-deformation aging process on microstructure evolution and properties were described and analyzed. Finally, the development trend of heat treatment process of β titanium alloy was prospected.
    SURFACE ENGINEERING
    Optimization of process parameters for laser clad Ni-based coatings on gear steel surface
    Shi Lubing, Du Jiajun, Zhang Zhihong, Ding Haohao, Wang Wenjian, Liu Zhongming
    2023, 48(11):  266-275.  doi:10.13251/j.issn.0254-6051.2023.11.044
    Abstract ( 16 )   PDF (9786KB) ( 24 )  
    Ni-based coating was fabricated on the surface of 18CrNiMo7-6 gear steel by laser cladding technique, and the effects of laser power, scanning speed, and powder feeding rate on the cladding quality and microstructure of Ni-based coating were investigated. The relationship between laser cladding process parameters and the macroscopic size, microstructure and microhardness of the clad coating was established. The results show that, with the increase of laser power, the melted height, width and depth of the coating increase, the microstructure is refined, and the microhardness increases. With the increase of scanning speed, the melted height and width of the coating increase, the melted depth decreases, the microstructure is coarsened, and the microhardness decreases. With the increase of powder feeding rate, the melted height of the coating increases, the melted width increases first and then decreases, the melted depth decreases, the microstructure is refined, and the microhardness increases. Based on the macroscopic morphology, microstructure and microhardness of the clad coating, the optimal process parameters are selected as the laser power of 700 W, scanning speed of 2 mm/s, and powder feeding rate of 11.1 g/min.
    Effect of Al addition on microstructure and wear resistance of Co-free high-entropy alloy coatings
    Wei Shiyong, Wang Chaomin, Peng Wenyi, Chen Yun, Luo Rukai, Liu Wenzheng
    2023, 48(11):  276-281.  doi:10.13251/j.issn.0254-6051.2023.11.045
    Abstract ( 14 )   PDF (2265KB) ( 17 )  
    AlxCrFeMnNi high-entropy alloy coatings (x=0.2,0.4,0.6,0.8,1) were successfully prepared on a Q235 steel substrate by plasma transfer arc. The influence of Al addition on the phase constituent, microstructure, and mechanical properties of the HEA coatings was investigated. The results show that, due to the high-entropy effect, the AlxCrFeMnNi HEA coatings are mainly composed of simple BCC and FCC phases and a small amount of carbide phases. Moreover, the Al addition inhibits the formation of FCC phases, and the inter-dendritic areas are significantly increased with the increase of Al content. When the Al addition is 0.8, the average Vickers hardness of the HEA coating is 386.5 HV0.2, average friction coefficient is 0.595, and the most stable wear resistance is obtained.
    Effect of laser scanning rate on microstructure and wear resistance of NiCr/Cr3C2 coating
    Dong Hui, Gan Shaoming, Du Yongqi, Bai Jiaxin, Cheng Yuchen, Li Ming, Zhu Chaoxuan
    2023, 48(11):  282-287.  doi:10.13251/j.issn.0254-6051.2023.11.046
    Abstract ( 12 )   PDF (3359KB) ( 17 )  
    NiCr/Cr3C2 composite coating was prepared by laser cladding at three laser scanning rates. The microstructure, hardness and friction and wear properties of the coatings were characterized using scanning electron microscope (SEM), X-ray diffractometer (XRD), micro-hardness tester and friction and wear testing machine. The results show that the structure in clad layer transfer from dendrite to equiaxed as the laser scanning rate increases from 2 mm/s to 4 mm/s, and the defects transfer from pores to large gaps and microcracks. When the scanning rate is below 3 mm/s, the melting and decomposition of Cr3C2 results in the clad layer mainly containing Cr7C3. As the laser scanning rate increases, the melting degree of Cr3C2 decreases, and the clad layer is mainly composed of Cr7C3 and Cr3C2. Therefore, the hardness of the clad layer increases from 400 HV0.3 to 780 HV0.3 when the laser scanning rate rises from 2 mm/s to 4 mm/s. The wear mechanism of the clad layer at different laser scanning rates is mainly abrasive wear, but due to its dense structure and higher hardness, the wear loss of the clad layer at scanning rate of 3 mm/s is the smallest and the wear resistance is the best.
    Properties of WC-25Co/NiCrAlY coating and its application in titanium alloy blade shoulder
    Zhang Mourui, Liu Lixiang, Yu Ting, Li Changyin, Cui Yongjing
    2023, 48(11):  288-292.  doi:10.13251/j.issn.0254-6051.2023.11.047
    Abstract ( 13 )   PDF (2728KB) ( 12 )  
    NiCrAlY bottom layer and WC-25Co surface layer were prepared by spraying on TC6 titanium alloy specimens. The microstructure, microhardness, flexural behavior, bonding strength, thermal shock and wear behavior of the coating were studied. Based on the test results, a TC6 rotor blade of an aeroengine was taken as an example to verify the applicability of spraying coating on the blade shoulder, in particular, a long-term test evaluation of the coating was carried out. The results show that the coating has good comprehensive performance and can effectively improve the wear resistance and vibration of the shoulder of the TC6 rotor blades, meeting the requirements of aeroengines.
    TEST AND ANALYSIS
    Fracture analysis of 20Cr1Mo1VTiB steel fastening bolts used for intermediate pressure main stop valve
    Zheng Jianjun, Fan Ziming, Cheng Xu, Zhang Yanfei, Shi Quanqiang
    2023, 48(11):  293-299.  doi:10.13251/j.issn.0254-6051.2023.11.048
    Abstract ( 19 )   PDF (4252KB) ( 31 )  
    Two 20Cr1Mo1VTiB high temperature fastening bolts were found to be broken during the disassembly and maintenance of the intermediate pressure main stop valve used for a steam turbine unit. The reasons for the premature failure of bolts were analyzed by means of macro and micro morphology observation, chemical composition analysis, impact fracture analysis as well as mechanical property test, and the fracture failure mechanism was also clarified. The results show that the fracture behavior of the two 20Cr1Mo1VTiB steel high temperature fastening bolts is mainly belong to brittle fracture triggered at the bottom of the first thread, which is mainly related to the long-term high temperature operation condition. The carbide particles in the matrix of the 20Cr1Mo1VTiB high temperature fastening bolt are gradually Ostwald ripening with the combined action of long-term high temperature and alternating cyclic stress, resulting in a decrease of toughness. Besides, creep cavities are also formed at the nut end of the first thread bottom with high stress concentration, leading to premature fracture failure. In addition, the linearly distributed Ti(N, C) inclusions with average size of 5 μm further lead to the reduction of impact property of the No.1 bolt, causing the fracture of No.1 bolt prior to bolt No.2. Then the overall stress condition of the intermediate pressure main stop valve is changed, which leads to brittle fracture of the No.2 bolt with a small impact property margin.
    Analysis of unqualified mechanical properties of 1Cr12MoV steel forgings and process improvement
    Li Wei, Liu Jing, Shao Zhuhao, Hao Hongjun, Tang Lin, Jiang Tao
    2023, 48(11):  300-304.  doi:10.13251/j.issn.0254-6051.2023.11.049
    Abstract ( 23 )   PDF (5207KB) ( 30 )  
    In order to study the cause of unqualified mechanical testing for 1Cr12MoV steel forgings, chemical composition analysis, optical microscopy and scanning electron microscopy were used to analyze the chemical composition, microstructure and fracture morphology of the forgings. The results show that large-sized and aggregated eutectic carbides are existed in the matrix which reaches level 5 and doesn't meet the requirement of below level 3, which is the main reason for the failure of mechanical performance testing. Through elongating the forging stock by free forging, increasing the deformation during the forging process, and adding a preliminary heat treatment process, the distribution of eutectic carbides is greatly improved, and mechanical properties meet the delivery requirements.

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