Current Issue

• MATERIALS RESEARCH

  Effect of Ni content on microstructure and mechanical properties of 40CrNi3MoV steel strengthened by NiAl-Cu

  Liang Enpu, Xu Le, Yang Yong, Wang Maoqiu

  2023, 48(8):  1-7.  doi:10.13251/j.issn.0254-6051.2023.08.001

  Abstract ( 72 )   PDF (5523KB) ( 76 )  

  For 40CrNi3MoV steel strengthened by NiAl-Cu precipitation, the effect of different Ni content on microstructure and mechanical properties of the tested steel tempered at 450-650 ℃ was studied. The microstructure of the tested steel was characterized by OM, the precipitates in the the tested steel were characterized by means of TEM and EDS, and the mechanical properties of the tested steel were measured by room temperature tensile and low temperature impact tests. The results show that the Ni content has no obvious effect on the microstructure of the tested steel. The increase of Ni content (3.0%-4.0%) promotes the precipitation of Cu-rich phase and NiAl phase in the tested steel, and increases the amount of nano particles. With the increase of Ni content, the tensile strength and yield strength of the tested steel tempered at 500 ℃ are increased by 200 MPa and 100 MPa, respectively.

  Effect of vanadium content on microstructure and properties of Cr20 high chromium cast iron for straw puffing machine screw

  Niu Xu, Liu Yue, Wang Zhuo, Zhang Yajing, Liu Pingxiang

  2023, 48(8):  8-15.  doi:10.13251/j.issn.0254-6051.2023.08.002

  Abstract ( 55 )   PDF (6962KB) ( 46 )  

  To solve the problem of serious wear of the front end of straw puffing machine screw, different contents of vanadium were added into Cr20 high chromium cast iron to study the effect of vanadium content on microstructure and properties, and then the influence of quenching and tempering temperatures on microstructure, hardness, impact property and wear resistance of the tested cast irons was studied by means of optical microscope, scanning electron microscope, hardness tester, impact tester and wear tester. The results show that the addition of vanadium makes the vanadium carbide particles precipitate from the liquid phase preferentially during the cooling process of high chromium cast iron, which plays the role of crystallization nucleation, microstructure refining and hardness improving. After quenching at 990 ℃ for 2 h, air cooling and tempering at 250 ℃ for 2 h, air cooling, the microstructure of the high chromium cast iron with 1.20%V is fine and uniform and the wear loss is less, compared with the high chromium cast iron without V addition, the wear resistance is improved. When the vanadium content is 0.50% under the same heat treatments, the impact property is the best, with an impact absorbed energy of 4.95 J.

  Effect of Sc on microstructure and hardness of 6013 aluminum alloy

  Xiao Xinrui, Sun Tao, Deng Guoyong, Peng Jie, Su Hai, Li Rudong, Zhou Jie, Wang Hebin

  2023, 48(8):  16-23.  doi:10.13251/j.issn.0254-6051.2023.08.003

  Abstract ( 32 )   PDF (5055KB) ( 32 )  

  Effect of Sc addition on microstructure and hardness of 6013 aluminum alloy as-cast, homogenized, and solution treated was studied by means of optical microscope, scanning electron microscope, energy dispersive spectroscopy, and hardness testing. The results show that the addition of 0.4%Sc has a significant refining effect on grain size of 6013 aluminum alloy. After homogenization, a large amount of second phases dissolve into the matrix, and the second phases are more uniformly distributed on the matrix. After adding Sc, it is obvious that the second phase particles on the matrix are reduced, and Al₃Sc is generated and precipitated on the grain boundaries, which plays a role of pinning grain boundaries and refining grains. The addition of 0.4%Sc can inhibit recrystallization, and the hardness of the alloy containing 0.4%Sc after solution treatment is higher than that of the alloy without the addition of Sc.

  Effect of rare earth Ce on microstructure and precipitation of second phases in BT80s-5Cr oil well pipe steel

  Han Qiang, Xu Tao, Li Tao, Jia Xin, Sun Zhaoqi, Bai Yansong

  2023, 48(8):  24-29.  doi:10.13251/j.issn.0254-6051.2023.08.004

  Abstract ( 28 )   PDF (3648KB) ( 31 )  

  Effect of rare earth Ce on microstructure and second phase precipitation of BT80s-5Cr oil well pipe steel was studied by adding different contents of rare earth Ce. The prior austenite grain size after quenching was analyzed by using metallographic microscope. The microstructure and second phase after heat treatment were observed and analyzed by using SEM, XRD and TEM. The results show that after water quenching at 900 ℃ for 50 min, when the rare earth Ce content of the tested steel increases from 0 to 0.012%, the average size of the prior austenite grains drops from 14.21 μm to 11.06 μm, and when rare earth Ce content is 0.0025%, the grain distribution is the most uniform, and the proportion of grains within the range of 6-15 μm is 60.26%. The addition of rare earth refines the prior austenite grains and makes the structure more uniform. After tempering at 690 ℃ for 90 min, the microstructure of the quenched steel is a mixed structure of tempered sorbite and tempered troostite. As the content of rare earth Ce increases from 0 to 0.012%, the amount of carbide precipitation decreases, and the addition of rare earth inhibits the precipitation of carbide. The main types of carbides are Cr₂₃C₆ and Cr₇C₃, and the addition of rare earth Ce does not change the type of carbides. When the Ce content is 0.012%, the carbides undergo a transition from continuous distribution to intermittent cluster distribution on grain boundaries.

  Effects of Al content and aging time on mechanical properties and damping capacity of Mg-Al magnesium alloys

  Wu Yang, Qian Yafeng, Zhao Yanhui

  2023, 48(8):  30-34.  doi:10.13251/j.issn.0254-6051.2023.08.005

  Abstract ( 36 )   PDF (2804KB) ( 42 )  

  Effects of aluminum content and aging time on microstructure and properties of Mg-Al magnesium alloys were studied by metallographic microscope and dynamic mechanical analyzer. The results indicate that with the increase of Al content and aging time, the volume fraction of β-Mg₁₇Al₁₂ phase continues to increase, the strength of Mg-Al magnesium alloys continues to increase. The damping capacity of AZ31 magnesium alloy is higher than that of AZ61 and AZ80 alloys due to its fewer solute atoms and larger grain size. As the aging time increases, the damping capacity of Mg-Al alloys increases with the increase of β-Mg₁₇Al₁₂ content. The damping capacity of Mg-Al alloys increases on the whole with the temperature raising. Due to the occurrence of grain boundary slip (GBS), the damping capacity is improved significantly above 250 ℃, and the damping peak of aged AZ80 alloy appears when the temperature is about 330 ℃.

  High temperature mechanical properties of medium-Mn hot stamping steel containing vanadium

  Wei Chengshi, Jia Juan, Wang Zhen, Ge Rui, Liu Jing

  2023, 48(8):  35-41.  doi:10.13251/j.issn.0254-6051.2023.08.006

  Abstract ( 40 )   PDF (5749KB) ( 23 )  

  Thermal simulation tensile testing for a kind of medium-Mn hot stamping steel containing vanadium was carried out by the Gleeble-3500 thermal simulation testing machine. The high temperature tensile fracture morphology and microstructure near the fracture were observed, and the influence rules and mechanisms of tensile temperature and soaking temperature on mechanical properties of the steel at high temperature were studied. The results show that when the steel is tensile tested after soaking at 960 ℃ and 1200 ℃, the ductility trough appears at 830 ℃ and 850 ℃, and the percentage reduction of area are 79.8% and 58.4%, respectively. After soaking at 960 ℃ and then tensile testing at 830 ℃, more and larger inclusions and precipitates are observed, however, due to the smaller prior austenite grains, the steel has good ductility, and the tensile fracture shows ductile dimples. After soaking at 1200 ℃ and then tensile testing at 850 ℃, the inclusions and precipitates are relatively fewer and smaller, the prior austenite grains are larger, and the ductility is relatively poorer.

  Analysis of continuous cooling transformation characteristics of 4330-Nb steel for sucker rod

  Sun Xiaoran, Wang Chengming, Zhao Nan, Bai Lijuan, Zheng Wenyue

  2023, 48(8):  42-46.  doi:10.13251/j.issn.0254-6051.2023.08.007

  Abstract ( 34 )   PDF (3519KB) ( 36 )  

  Continuous cooling characteristics of undercooled austenite for 4330-Nb steel for sucker rod were studied by using static CCT curves, microstructure and hardness tests. The results show that the CCT curves of the steel presents three transformation zones independent and unconnected to each other during the continuous cooling process. The high temperature transformation zone is small, which is the result of the comprehensive action of Nb, Cr, Ni, Mn and other alloying elements. With the increase of cooling rate, the change of hardness shows a trend of raising segment to stable segment, which is consistent with the trend of microstructure. The important reason for the formation of banded structure when the cooling rate is less than 0.2 ℃/s is the low cooling rate and the banded segregation of C, Si, Mn, Cr, Ni and other elements. Therefore, in order to obtain a uniform microstructure for the 4330-Nb sucker rod steel, the fast cooling method should be selected during the cooling stage after hot rolling. In production, the quenching and tempering process can be used to obtain excellent performance.

  Phase transformation behavior during continuous cooling of Cr8 back-up roll steel

  Guo Yafei, Li Zhilong, Zhang Bo, Li Jie, Pang Qinghai

  2023, 48(8):  47-51.  doi:10.13251/j.issn.0254-6051.2023.08.008

  Abstract ( 55 )   PDF (4443KB) ( 35 )  

  In order to investigate the phase transformation behavior of Cr8 back-up roll steel during the continuous cooling process, the phase transformation parameters of the Cr8 back-up roll steel at different cooling rates were measured by Formastor-FⅡ automatic phase transformation analyzer. The CCT curves was established by thermal expansion method and metallographic method, and the influence of different cooling rates on microstructure and hardness of the Cr8 back-up roll steel was analyzed. The results show that the microhardness of Cr8 back-up roll steel increases gradually with the increase of cooling rate in the range of 2-60 ℃/min, and the hardness value increases from 238 HV2 to 570 HV2. The supercooled austenite transforms into a mixed structure containing pearlite when the cooling rate is less than 8 ℃/min. The pearlite transformation disappears and the supercooled austenite transforms into bainite+martensite or single martensite with higher hardness when the cooling rate is between 8 ℃/min and 60 ℃/min. Compared with Cr5 steel, the supercooled austenite of the Cr8 back-up roll steel has better stability, the steel has lower critical cooling rate and good mechanical properties, and is an ideal material for manufacturing large backup roll forgings.

  PROCESS RESEARCH

  Hot deformation behavior and hot processing map of Inconel625 alloy

  Zhang Jishun, Liao Zhenyang, Yang Gang, Tang Zhengxin, Chen Qiwei, Kong Rongzong

  2023, 48(8):  52-57.  doi:10.13251/j.issn.0254-6051.2023.08.009

  Abstract ( 45 )   PDF (4150KB) ( 44 )  

  Thermal compression test of Inconel625 alloy with deformation temperature of 950-1200 ℃ and strain rate of 0.001-10 s^-1 was carried out by Gleeble-3800 thermal simulation testing machine. The hot deformation behavior and microstructure evolution under different deformation conditions were analyzed. The hot processing map of the Inconel625 alloy was constructed. The results show that the flow stress of the Inconel625 alloy increases with the increase of strain rate and the decrease of deformation temperature. The higher the deformation temperature and the lower the compression strain rate, the more prone to dynamic recrystallization. The optimum hot processing parameters of the Inconel625 alloy are as follows: deformation temperature of 1010-1050 ℃, strain rate of 0.01-0.13 s^-1.

  Hot deformation behavior of V170 grade high-strength steel for ultra-deep wells

  Niu Ningtao, Wang Yunkai, Li Yunjie, Wang Chao, Kang Jian, Yuan Guo

  2023, 48(8):  58-63.  doi:10.13251/j.issn.0254-6051.2023.08.010

  Abstract ( 31 )   PDF (3602KB) ( 28 )  

  According to the chemical composition of traditional non-API standard oil country tubular goods(OCTG), a V170 grade ultra-deep well OCTG steel was independently designed. The thermal simulation specimens were taken from the rolled sheets of the tested steel after hot rolling, and the MMS-300 multi-functional thermal simulation test machine was used for thermal simulation compression experiments. The hot compression deformation behavior of the tested steel was studied under the conditions of strain rates of 0.01, 0.1, 1, 5, and 10 s^-1, respectively, and deformation temperatures of 800, 900, 950, 1000 and 1100 ℃. The results show that the softening mechanism of the material is mainly dynamic recovery and dynamic recrystallization, and the dynamic recrystallization is more likely to occur at high deformation temperature and low strain rate. Because the higher temperature increases atomic diffusion and increases the driving force of dislocation motion, which makes the recrystallization process easier. The peak stress constitutive equation of the tested steel is established by using the Arrhenius hyperbolic sine function, and the calculated hot deformation activation energy of the tested steel is Q_d=110.99 kJ/mol.

  Microstructure and properties of alloy steel connecting rod quenched in different quenching media

  Qi Yi, Yang Jianquan, Jiao Fei

  2023, 48(8):  64-71.  doi:10.13251/j.issn.0254-6051.2023.08.011

  Abstract ( 35 )   PDF (3715KB) ( 27 )  

  Different quenching media including water, quenching oil and AQ251 quenching liquid (concentration 4%, 8%, 12%, respectively) were selected as the research objects. Through the cooling characteristics, microstructure, hardness, tensile properties, impact properties and other tests, combined with the impact fracture morphology analysis, small batch trial testing and quality inspection, the influence of quenching medium on microstructure and properties of the alloy steel connecting rod was analyzed. The results show that after quenching and tempering, tempered sorbite can all be obtained for the five groups of the connecting rods. The quenching medium with stronger cooling capacity has higher microstructure grade, higher hardness and higher tensile strength, while the impact absorbed energy is decreased. Through the analysis of the impact fracture morphology of five groups, all of them belong to ductile fracture. Combined with the small batch testing and quality inspection, the stronger the cooling capacity of the quenching medium, the larger the crack and distortion of the tested connecting rod, the lower the qualified rate of the product. Therefore, after comprehensive consideration of the results of all aspects of data, the quenching oil is the best quenching medium with good internal structure and comprehensive mechanical properties, which includes three levels of microstructure, average diameter of the quenched Brinell hardness indentation of ø2.82 mm, average diameter of the tempered Brinell hardness indentation of ø3.83 mm, tensile strength of 836.87 MPa, yield strength of 705.23 MPa, elongation after fracture of 19.13%, the percentage reduction of area of 62.44% and impact absorbed energy of 86 J. The production practice has proved that the rejection rate is 2.0%. Compared with the other four groups, the quenching oil has more obvious comprehensive advantages and can be given priority in practical application.

  Effect of intercritical quenching process on microstructure and texture of Cr-Ti-B micro-carbon steel

  Liu Xuwei, Wang Zhigang, Lin Jie, Yin Tieqi, Ye Jieyun, Zhang Yinghui

  2023, 48(8):  72-79.  doi:10.13251/j.issn.0254-6051.2023.08.012

  Abstract ( 27 )   PDF (6069KB) ( 25 )  

  Cr-Ti-B micro-carbon steel was intercritically quenched at different temperatures under heating rates of 10 ℃/s and 100 ℃/s, respectively, after reaching the temperature, held for 90 s. The effects of heating rate and quenching temperature on microstructure were analyzed, and the effect mechanism of phase transformation on matrix texture was also discussed. The results show that martensite exhibits island distribution at low intercritical quenching temperature (780 ℃) when heated at 10 ℃/s. With the increase of quenching temperature, the composite structure of martensite and bainite is observed. When heated at 100 ℃/s, martensite shows chain distribution at low intercritical quenching temperature (780 ℃), and martensite with network distribution appears with the increase of quenching temperature. The nucleation of austenite is obviously affected by heating rate. Nucleation occurs on the grain boundary of ferrite during slow heating, while nucleation occurs inside pearlite during rapid heating. The specimen heated at 10 ℃/s has strong γ texture and weak Goss texture when quenched at 830 ℃, while the specimen heated at 100 ℃/s has strong Goss texture and weak γ texture when quenched at 830 ℃. The selective phase transformation of {110} oriented grains during quenching is beneficial to weaken Goss texture.

  Effects of forging temperature and normalizing scheme on mixed crystal defects and hardness of SCr415H steel

  Liu Yang, Qi Haiquan, Han Xiangnan, Li Tianran, Xue Qihe, Wang Weimin, Liu Anqi, Guo Chuncheng

  2023, 48(8):  80-86.  doi:10.13251/j.issn.0254-6051.2023.08.013

  Abstract ( 34 )   PDF (5654KB) ( 33 )  

  During continuous normalizing treatment of circular parts produced by forged SCr415H low-alloy steel, there were low hardness and mixed crystal defects. The physical objects forged at temperatures of 1150, 1200, and 1230 ℃ were used as tested materials, and specimens were cut from the tested materials for conventionally normalizing and normalizing with two-stage cooling. The normalizing heating temperatures were 870, 900, 950, and 1000 ℃, and the holding time was 90 min. After two-stage normalizing at temperatures of 950, 1000, and 1050 ℃ for 60 min, the specimens were taken out and transferred in a box furnace preheated to 870 ℃ and 900 ℃ for further holding for 30 min then air cooled. The results show that in view of the low hardness and mixed crystal defects of the SCr415H steel in actual production, hot working specifications with forging temperature of 1200 ℃ and normalizing heating temperature of 900 ℃ can be used for normal normalizing; the two-stage normalizing process parameters are forging temperature of 1200 ℃, first heating temperature of 1000 ℃, and second heating temperature of 870-900 ℃. Taking into account the production efficiency, furnace life and other production practices, the preferred hot working process is forging temperature of 1200 ℃, normalizing temperature of 900 ℃. Thus, the results of the study have important practical significance for guiding the forging and heat treatment production of SCr415H steel.

  Austempering preparation of high silicon high vanadium high speed steel with bainite matrix and its microstructure characteristics

  Zhang Zhiyong, Shi Ruxing, Yin Litao, Pang Qinghai, Leng Wanqing, Xu Liujie

  2023, 48(8):  87-93.  doi:10.13251/j.issn.0254-6051.2023.08.014

  Abstract ( 30 )   PDF (4313KB) ( 59 )  

  After preparation by sand casting, the effect of austempering temperature on microstructure transformation of nano-bainite in a high silicon and high vanadium high speed steel was studied. The results show that by increasing the silicon content to 2.14%(mass fraction), the precipitation of cementite in bainitic ferrite can be inhibited, and the nano-bainite matrix structure can be obtained. With the decrease of austempering temperature, the bainite structure is refined. When the austempering temperature is decreased from 300 ℃ to 250 ℃, the average thickness of bainitic ferrite and thin film retained austenite decreases by 30% and 27%, respectively. While from 250 ℃ to 200 ℃, the microstructure thicknesses do not decrease. The bainite transformation amount increases and the transformation time is prolonged with the decrease of austempering temperature. The content of retained austenite decreases by 34.7% and 60.9%, respectively, when the austempering temperature decreases from 300 ℃ to 250 ℃ and 200 ℃, and the bainite transformation time increases from 84 h at 300 ℃ to more than 96 h at 200 ℃.

  Effect of isothermal temperature on microstructure and hardness of ultra-low carbon bainitic steel

  Sun Tingting, Chen Weili, Jia Zilong

  2023, 48(8):  94-98.  doi:10.13251/j.issn.0254-6051.2023.08.015

  Abstract ( 41 )   PDF (3776KB) ( 42 )  

  Effect of isothermal temperature on microstructure transformation and hardness of ultra-low carbon bainitic steel was studied by means of L78 quenching dilatometer, hardness tester, scanning electron microscope(SEM), optical microscope(OM). The results show that TTT curves of the steel includes ferrite transformation zone, bainite transformation zone, pearlite transformation zone. At isothermal temperature of 725-650 ℃, the microstructure is equiaxed ferrite with a small amount of pearlite and alloy carbides. The microstructure transforms into quasi polygonal ferrite from 625 ℃. When isothermal treated at 600-575 ℃, the ferrite appears as block and a small amount of needle-like. Under 575 ℃, the microstructure is all ultra-low carbon bainite structure. As the isothermal temperature decreases, the microstructure of the tested steel becomes finer and the hardness increases, but the hardness change is not significant. In the isothermal temperature range of 725-700 ℃, the pearlite content gradually decreases, and in the isothermal temperature range of 675-600 ℃, the bainite content shows an upward trend.

  Effect of solution treatment on microstructure and mechanical properties of turbine blade after long-term service

  Shi Feng, Yao Weimin, Shen Jian, Yin Chunhong, Cao Qi, Hong Yaguang, Xu Hui, Ding Yang

  2023, 48(8):  99-105.  doi:10.13251/j.issn.0254-6051.2023.08.016

  Abstract ( 33 )   PDF (4419KB) ( 57 )  

  A first stage rotor blade made of GTD111 alloy in heavy gas turbine after service for 27 000 h was solution treated at 1180 ℃ for 2 h and then argon blowing cooled, the microstructure and mechanical properties of different regions of the blade were compared and analyzed. The results show that dendrite morphology is maintained in the leading edge and middle region of the blade, meanwhile grain boundaries, carbides and γ′ phase are not aged obviously. In the trailing edge of the blade, however, the dendrites disappear, the grain boundaries are coarsened, the carbides degrade obviously, and the γ′ phase is coarsened and rafted. The hardness of trailing edge in the blade as a whole is higher than that of leading edge, the tensile strength of the specimens varying from blade tip to tenon increases first and then decreases on the whole at 25-871 ℃, while that of the specimens at different positions tends to be the same at 982 ℃. For the GTD111 alloy turbine blades after long-term service, the solution treatment at 1180 ℃ cannot completely eliminate the aged microstructure, which is related to the insufficient solution temperature or holding time.

  Effects of rolling method and heat treatment process on microstructure and properties of medium-Mn steel

  Peng Longsheng, Liu Chunquan, Xiong Fen, Zhang Ying, Huang Wei, Lin Yinghua

  2023, 48(8):  106-112.  doi:10.13251/j.issn.0254-6051.2023.08.017

  Abstract ( 38 )   PDF (3912KB) ( 37 )  

  Medium-Mn steel Fe-0.11C-5.23Mn-1.11Al-0.10Si was heat treated with different processes (austenite reverse transformation(ART) and quenching and tempering(Q&T)) after different rolling methods (hot rolling(HR), warm rolling(WR) and cold rolling(CR)). The effects of different rolling and heat treatment processes on microstructure and properties of the steel were investigated. The results show that the microstructure of the steel treated by HR+WR+ART is mainly composed of lath austenite and ferrite, with a small amount of equiaxed massive austenite. The grain size of the steel treated by HR+WR+ART is significantly reduced, with a high volume fraction of retained austenite, which can generate a wider TRIP effect. After treated by HR+WR+ART and HR+WR+Q&T, the steel shows good comprehensive properties, but the former is superior, with the tensile strength of 1001 MPa, yield strength of 745.7 MPa, elongation after fracture of 49.2% and the product of strength and elongation of 49.25 GPa·%.

  Effect of solution temperature on microstructure and properties of lightweight high manganese steel

  Yang Yi, Liu Rangxian, Yang Haokun, Long Jun, Han Peixian, Zheng Zhibin

  2023, 48(8):  113-117.  doi:10.13251/j.issn.0254-6051.2023.08.018

  Abstract ( 24 )   PDF (2181KB) ( 47 )  

  Microstructure and mechanical properties of ZGMn18Cr2-4Al lightweight high manganese steel solution treated at different temperatures were studied by means of optical microscope, X-ray diffractometer, scanning electron microscope, hardness testing, impact testing and tensile testing. The results show that when solution treated at 980-1150 ℃, with the increase of solution temperature, the coarse carbides on the grain boundary dissolve, the grain size first decreases and then increases, the hardness decreases slightly, and then fluctuates in a relatively stable range, the impact absorbed energy increases firstly and then tends to be stable, and the yield strength, tensile strength and elongation increase first and then decrease. The optimal solution treatment temperature of the lightweight steel is 1100 ℃. At the solution treatment temperature of 1100 ℃, the comprehensive properties are the best, with hardness of 269 HV0.2, impact absorbed energy of 160 J, the product of strength and elongation of 39.4 GPa·%, and the tensile fracture is typical ductile fracture.

  Growing rule and mathematical model of austenite grain in T90 high carbon cord steel

  Zhang Lianyou, Zhao Zhuo, Che An, Li Lingmin, Zhou Yanwen, Chen Xuejiao, Xie Shimian, Lü Zhe

  2023, 48(8):  118-123.  doi:10.13251/j.issn.0254-6051.2023.08.019

  Abstract ( 29 )   PDF (4098KB) ( 41 )  

  Growth behavior of austenite grains in high carbon wire steel T90 heated at different temperatures (950-1200 ℃) and time (0-600 s) was investigated by high temperature laser confocal microscope, and the effect of different holding temperatures on the microstructure and mechanical properties of the T90 steel was studied. The results show that, at the same heating temperature, the average grain size increases with the prolongation of holding time, but the growth rate is relatively faster between 1150 ℃ and 1200 ℃ and relatively slower between 1050 ℃ and 1100 ℃. Therefore, the heating temperature should be preferably controlled between 1050 ℃ and 1100 ℃. At the same holding time, with the increase of heating temperature, the austenite grains gradually grow, and grain coarsening begins at 1000 ℃. With the increase of heating temperature, the grains merge and grow. Finally, the kinetic equation for the growth of austenite in the T90 steel is obtained.

  Effects of heating and cooling conditions on surface oxidation and decarbonization of alloy tool steel S2

  Tu Xingkuang, Zuo Jinzhong, Zhao Yun, Su Zhenwei

  2023, 48(8):  124-131.  doi:10.13251/j.issn.0254-6051.2023.08.020

  Abstract ( 33 )   PDF (6098KB) ( 26 )  

  Effects of heating temperature, holding time and cooling method on surface oxidation and decarbonization of the alloy tool steel S2 were investigated by means of metallographic method, microhardness and mass loss tests. The results show that the oxidation mass loss rate increases with the increase of heating temperature and holding time. Under the same holding time, with the increase of heating temperature, the depth of complete decarbonization layer first increases and then gradually decreases, and reaches the maximum at 800 ℃. When the heating temperature is below 1050 ℃, the depth of complete decarbonization layer increases slowly with the increase of heating temperature. When the heating temperature is above 1050 ℃, the depth of complete decarbonization layer increases obviously. The complete decarbonization layer almost disappears when heated at 1200 ℃, and the burning damage is serious at this temperature. Under the same heating condition, the smaller the cooling rate, the more serious the decarbonization.

  Hot working process of GH4169 superalloy

  Lü Da, Han Yanguang, Cui Yi, Zhao Yingli, Chen Wen, Zhang Zefeng, Ji Shuang

  2023, 48(8):  132-137.  doi:10.13251/j.issn.0254-6051.2023.08.021

  Abstract ( 40 )   PDF (3174KB) ( 57 )  

  By adjusting the process parameters, combined with analysis of mechanical properties and microstructure, the hot working process of GH4169 superalloy was studied. The results show that the segregation between dendrites can be improved by adjusting the homogenization treatment process. The size-mixed grains can be improved when the forging ratio is greater than or equal to 3. The mechanical properties of the GH4169 superalloy can be significantly improved by medium temperature deformation heat treatment process. Billet forging at(1130-1160) ℃+medium temperature rolling heat treatment at(980-1020) ℃ (water cooling) +direct aging can improve the mechanical properties of the alloy, the tensile strength and hardness can reach 1350 MPa and 50 HRC, respectively.

  Mechanism of α lamellae spheroidization and strengthening plasticization of TC21 titanium alloy after post forging heat treatment

  Tao Cheng, Cui Xia, Ouyang Delai, Lan Xixin, Wan Xingcai

  2023, 48(8):  138-143.  doi:10.13251/j.issn.0254-6051.2023.08.022

  Abstract ( 69 )   PDF (4261KB) ( 21 )  

  TC21 titanium alloy was subjected to spheroidization heat treatment after multi-directional forging under optimal parameters(920 ℃-0.92 single pass true strain-3 cycles-water cooling), and the tensile properties of the specimens before and after forging and after spheroidization heat treatment were tested. The evolution law of α lamellae spheroidization (equiaxation) under different heat treatment parameters (temperature of 830-920 ℃, holding time of 1-4 h) of the alloy was studied and the strengthening plasticization mechanism of the alloy was revealed. The results show that after optimal multi-directional forging, the preferred subsequent spheroidizing heat treatment parameters for the TC21 titanium alloy are 890 ℃-2 h-air cooling, which can promote α lamellae spheroidization, and make that the yield strength and elongation of the TC21 titanium alloy are significantly improved, reaching 894.0 MPa and 13.2%, respectively. The strengthening plasticization mechanism of the TC21 titanium alloy after spheroidization heat treatment is mainly fine grain strengthening caused by α phase grain refinement and dislocation strengthening induced by needle-like secondary α phase in β phase.

  Effect of hot working process on microstructure and properties of TC6 titanium alloy bars

  Wang Zhe, Wang Fei, Cao Heng, Wang Shujun, Hao Xiaobo, Zhang Qiang

  2023, 48(8):  144-148.  doi:10.13251/j.issn.0254-6051.2023.08.023

  Abstract ( 33 )   PDF (2893KB) ( 53 )  

  Effect of hot working process on microstructure and mechanical properties of TC6 titanium alloy bars was studied. The results indicate that the ø160 and ø60 mm bars are composed of equiaxed α phase+transformed β including platelet α phase when the TC6 titanium alloy is hot worked within 50 ℃ below the β phase transformation point. Compared with the core, more equiaxed α grains are found and the number and size of the platelet α phase are smaller in the surface. Furthermore, the continuous and discontinuous grain boundary α along prior β grain boundary occurs in core of the ø60 mm bar produced by the first hot rolling while that isn't found in the ø25 mm bar deformed fully due to the secondary hot rolling. When annealed at 900 ℃, more primary α and most of equiaxed α grains occur, but the grain size distribution is uneven in the ø25 mm bar. The strength of the ø60 mm bar in the surface is greater than in the core, and the strength decreases greatly and elongation varies slightly with the annealing temperature increasing. The strength of the ø25 mm bar decreases and the elongation increases gradually when the annealing temperature increases from as-rolled to 800 ℃, while the strength increases and the elongation decreases gradually with the continuous increase of annealing temperature to 900 ℃.

  Effect of annealing treatment on microstructure and mechanical properties of Ti6321 alloy

  Kang Cong, Jiao Zhen, Mu Botao, Ren Chiqiang, Gao Wenchao, Li Wei, Yun Pengfei, Ouyang Wenbo

  2023, 48(8):  149-153.  doi:10.13251/j.issn.0254-6051.2023.08.024

  Abstract ( 19 )   PDF (3295KB) ( 31 )  

  Microstructure and room temperature tensile and impact properties of Ti6321 alloy bars under different annealing processes were systematically analyzed. The results show that when the single annealing process is adopted, with the increase of annealing temperature, the content of primary α phase shows a trend of first decreasing and then increasing and finally decreasing, the content of original secondary α phase gradually decreases, and the strength shows the trend of first decreasing and then rising, and the impact absorbed energy first decreases and then increases. When the multiple annealing process is adopted, with the increase of annealing temperature in the first stage, the content of the primary α phase decreases, the strength decreases, but the impact absorbed energy increases. With the increase of annealing temperature in the second stage, the width of the secondary α phase layer increases, the strength decreases, while the impact absorbed energy increases. When the triple annealing system of 960 ℃×80 min/WQ+920 ℃×80 min/AC+550 ℃×240 min/AC is adopted, the alloy has the best strength and toughness matching.

  Effect of annealing temperature on microstructure and mechanical properties of silver layer in silver/steel composite plate fabricated by explosive welding

  Zhu Lei, Wei Yibin

  2023, 48(8):  154-160.  doi:10.13251/j.issn.0254-6051.2023.08.025

  Abstract ( 18 )   PDF (6368KB) ( 13 )  

  Silver/steel composite plate with typical explosive wavy bonding interface was fabricated by explosive welding. Under the same holding time, the effect of different annealing temperatures on structure and microhardness of the silver layer and the interface shear strength of the composite plate interface was studied. The results show that there is a mixture of Ag and Fe in the wavy vortex zone generated by explosive welding, and a temperature gradient is generated in the vortex zone during the heat treatment cooling process, which makes the silver layer present a columnar grain growth mode during recrystallization. As the annealing temperature increases, the diffusion of Ag and Fe atoms intensifies within the vortex zone, leading to the disappearance of temperature gradients and the uniform growth of silver layer grains towards the surrounding areas. The hardness of the silver layer reaches its maximum around 90 HV0.1 in the explosive state. When the annealing temperature exceeds 200 ℃, the hardness of the silver layer decreases by 36% and its sensitivity to the annealing temperature decreases, with the average hardness remaining around 60 HV0.1. The interface shear strength of the composite plate reaches the maximum 238 MPa under the explosive state. With the increase of annealing temperature, the shear strength decreases. When the temperature reaches 500 ℃, the shear strength remains stable at about 150 MPa, which is 37% lower than that of the explosive state.

  Effect of annealing process on microstructure and natural aged properties of 7X50 aluminum alloy wire

  Zhou Zeyu, Han Mingming, Xiao Xiang, Fu Jinlai

  2023, 48(8):  161-165.  doi:10.13251/j.issn.0254-6051.2023.08.026

  Abstract ( 20 )   PDF (3764KB) ( 22 )  

  In the preparation process of 7X50 aluminum alloy wire, intermediate annealing-cold drawing-cold heading and other intermediate processes are required. After single-stage annealing, the wire often occurs natural aging and strength increases, which seriously affects the re-drawing forming. The microstructure and properties of the single-stage and double-stage intermediate annealed 7X50 aluminum alloy wires were analyzed by means of optical microscope, transmission electron microscope and electronic tensile testing machine. The results show that partial recrystallization occurs after single-stage annealing at 404 ℃ for 2 h, and the strength decreases from 235 MPa(before annealing) to 188 MPa, and increases to 244 MPa after placing for 270 d because of obvious natural aging effect. After double-stage annealing at 404 ℃ for 2 h and 233 ℃ for 4 h, partial recrystallization occurs and the strength decreases to 173 MPa, and no obvious natural aging hardening effect occurs during the 270 d placing. The second stage 233 ℃×4 h annealing holding process promotes the precipitation and growth of AlZnMgCu phases and Mg, Zn phases, which consumes a large amount of Zn, Mg elements, effectively eliminates the natural aging hardening effect during the process of placing.

  Effect of heat treatment on stress corrosion sensitivity of 7005 aluminum alloy welded joint for high-speed train

  Zhang Mingkun, Yang Min, Wang Yu, Zhao Xiang, Jing Deqiang

  2023, 48(8):  166-171.  doi:10.13251/j.issn.0254-6051.2023.08.027

  Abstract ( 22 )   PDF (3291KB) ( 18 )  

  Combined with microstructure analysis and fracture characteristics analysis, the effect of post-weld heat treatment of T6 and RRA on stress corrosion susceptibility of 7005 aluminum alloy welded joint was studied by means of slow strain rate tensile test. The results show that after T6 and RRA heat treatment, stress corrosion cracking resistance of the joint are improved, and the stress corrosion sensitivity index of RRA heat treated joints is significantly decreased from 0.37 as welded to 0.05. The microstructure of RRA heat treated joints is uniform, and the tensile strength and elongation after fracture of the joints increase from 329 MPa and 6.8% as welded to 341 MPa and 8.8%, respectively. Thus, RRA heat treatment can significantly improve the mechanical properties and stress corrosion cracking resistance of the joints.

  Heat treatment and strengthening mechanisms of laser additive manufactured 6061 aluminum alloy

  Chen Wentao, Yin Jing, Wang Chunyan, Hu Guiling

  2023, 48(8):  172-178.  doi:10.13251/j.issn.0254-6051.2023.08.028

  Abstract ( 51 )   PDF (3502KB) ( 19 )  

  High strength 6061 aluminum alloy was printed by the laser powder bed fusion laser additive manufacturing technology. Firstly, the crack free 6061 aluminum alloy was obtained with the substrate preheating. And then, the printed alloys were heat treated, and the corresponding effect of heat treatments on the microstructure and mechanical properties was compared and analyzed. The results show that under the as printed (AP) condition, the alloy elements exist in the matrix in the form of supersaturated solution, and the tensile strength of the alloy is only 249 MPa. In the solution treatment (ST)+aging treatment (AT) and direct manual AT process, the mechanical properties of the aluminum alloy are significantly improved. After direct AT treatment, the alloy has high tensile strength and fraction elongation, 359 MPa and 12.5%, respectively. While after ST+AT, the alloy has a higher strength of 377 MPa and a lower elongation of 10.1%. The AT treatment is conducive to promoting the diffusion and precipitation of fine-needle Mg₂Si phase in the Al matrix, and the tensile strength of the alloy is significantly improved by the precipitate strengthening effect.

  Effect of temperature uniformity of natural gas-fired heat treatment furnaces on mechanical properties of aluminum alloy wheel hub

  Zhang Yanxin, Ala Teng, Zhao Lei, Li Jiaxiao, Zhou Zhe, Lü Jinqi

  2023, 48(8):  179-184.  doi:10.13251/j.issn.0254-6051.2023.08.029

  Abstract ( 24 )   PDF (2663KB) ( 16 )  

  For the natural gas heat treatment furnace of the newly built automobile aluminum wheel hub plant overseas, considering the influence of furnace temperature uniformity, the process verification of heat treatment of aluminum alloy wheel hub was carried out. Before and after the optimization of the furnace temperature uniformity, the mechanical properties and microstructure of the aluminum alloy wheel hub were tested respectively. The results show that the elongation after fracture of the wheel hub on A layer closing to the nozzle is relatively lower, and coarsening of Al-Si eutectic particles appears in the microstructure. The wheel hub on the bottom layer C, compared with that on the top A layer and the B layer in the middle, has the worst mechanical properties and insufficient spheroidization of the Al-Si eutectic. After optimizing the temperature uniformity of the furnace, there is no obvious difference in the specimens on each layer according to the mechanical properties and microstructure test, which can meet the technical requirements of the product.

  Properties of modified layer treated by ion nitriding and laser quenching on 30CrMnSi steel

  Li Ling, Liu Xuke, Meng Qingyu, Liu Shaojun, Li Taotao, Gao Yang

  2023, 48(8):  185-189.  doi:10.13251/j.issn.0254-6051.2023.08.030

  Abstract ( 25 )   PDF (3587KB) ( 27 )  

  In order to obtain a surface modification process with both corrosion resistance and wear resistance, the composite treatment of ion nitriding and laser quenching of 30CrMnSi steel was researched. Firstly, the specimen was ion nitrided, and then it was quenched by using laser scanning surface. The standard test specimens were processed using the wire cutting method, the microstructure observation, hardness test, friction wear and corrosion resistance tests were carried out. The results show that after ion nitriding and laser quenching, the microstructure is cryptocrystalline martensite on the surface and gradually transforms into tempered sorbite in the core, the thickness of the composite modified layer is two times that of the ion nitrided, and the surface hardness is increased by 50 HV. The composite treatment can effectively improve the hardness and thickness of the subsurface structure, and improve the wear resistance of the material. Compared with the single ion nitrided specimen, the corrosion resistance of the composite modified layer is poorer.

  Spheroidizing process of SWRCH35K cold heading steel

  Tian Wei, Wang Xing, Zhi Xubo, Chen Zongle

  2023, 48(8):  190-193.  doi:10.13251/j.issn.0254-6051.2023.08.031

  Abstract ( 67 )   PDF (2579KB) ( 29 )  

  In order to improve the spheroidized structure and cold heading quality of SWRCH35K steel, and to realize on-line annealing control after hot rolling, the phase transformation point of the SWRCH35K cold heading steel was obtained by thermodynamic simulation, and the hot rolled steel was heat treated by isothermal spheroidizing, high temperature tempering and high temperature tempering+isothermal spheroidizing. The results show that there are some long lamellar carbides in the granular pearlite formed after isothermal spheroidizing, and the uneven distribution of carbides is due to the re-precipitation of proeutectoid ferrite heated at the high temperature of the two-phase region, and lamellar pearlite is formed at 790 ℃; the carbides in the lamellar pearlite can become granular form by high temperature tempering, but the distribution of granular carbides is not uniform. The granular pearlite obtained by high temperature tempering and isothermal spheroidizing has good microstructure, uniform distribution of carbides and no obvious long lamellar carbides, so its best heat treatment system is 685 ℃×1.5 h, air cooling+740 ℃×0.5 h, furnace cooling to 685 ℃×3 h, then air cooling.

  Effect of quenching and partitioning on microstructure and mechanical properties of 30CrNi3MoV steel

  Zhao Qian, Qiao Zhixia, Zhang Yaran, Dong Ji, Xue Dongyang

  2023, 48(8):  194-198.  doi:10.13251/j.issn.0254-6051.2023.08.032

  Abstract ( 32 )   PDF (3675KB) ( 43 )  

  Microstructure control is a commonly used method to optimize the mechanical properties of low alloy high strength steels. The microstructure of the 30CrNi3MoV steel was investigated by TEM, and the mechanical properties were tested by uniaxial tensile test. The results indicate that the quenching and partitioning treatment leads to the formation of a multiphase structure composed of martensite, bainite, and a minute quantity of retained austenite. The differences in carbide size, the relative content of different phases, and the finial mechanical properties can be attributed by the different partitioning temperatures.

  Effect of quenching holding time on microstructure and properties of FB2 heat-resistant steel

  Zhao Jiqing, Yang Gang, Yin Huifang

  2023, 48(8):  199-204.  doi:10.13251/j.issn.0254-6051.2023.08.033

  Abstract ( 24 )   PDF (3256KB) ( 61 )  

  Microstructure of FB2 martensitic heat-resistant steel after quenching and tempering under different quenching holding time were observed by means of optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The tensile properties, impact absorbed energy and hardness of the heat treated FB2 steel were tested at room temperature. The results show that when quenching at 1100 ℃, the strength, plasticity and hardness of the FB2 steel after quenching and tempering have no apparently change with the quenching holding time increasing from 1 h to 8 h, while the impact absorbed energy decreases. When the quenching holding time is increasing from 8 h to 16 h, the strength and plasticity change little, the hardness decreases and the impact absorbed energy increases. With the increase of quenching holding time in the range of 1-8 h, the prior austenite grain size of the FB2 steel is increased, and the number of undissolved carbides is decreased, meanwhile, the alloying elements diffuse unevenly and the grain boundary carbide coarsening increases after tempering. However, there are still carbides undissolved when the quenching holding time is extended to 16 h, and no significant difference in the carbide sizes at grain boundaries and the intragranular boundaries.

  Hardness profile and control in thickness direction of thick wear-resistant steel

  Shen Binbin, Fang Jinlin

  2023, 48(8):  205-208.  doi:10.13251/j.issn.0254-6051.2023.08.034

  Abstract ( 34 )   PDF (1949KB) ( 31 )  

  Cooling capacity of high-pressure section of quenching machine was improved by developing the cooling control program. Hardness profile in the thickness direction of thick wear-resistant steel plates were systematically studied through temperature measurement during quenching under different processes, combined with the variation of microstructure and thickness direction hardness. The results show that increasing the cooling water flow rate in the high-pressure section of the quenching machine can significantly improve the cooling speed. The martensite bainite duplex structure appears in the center of the steel plate thickness, effectively improving the hardness of the wear-resistant steel in the thickness direction.

  OVERVIEW

  Research status of pearlitic steel and dual phase steel for commercial vehicle wheel

  Zhou Linjun, Li Linze, Tian Yaqiang, Dai Xin, Chen Liansheng

  2023, 48(8):  209-218.  doi:10.13251/j.issn.0254-6051.2023.08.035

  Abstract ( 23 )   PDF (2065KB) ( 45 )  

  Pearlitic steel and dual phase steel are widely used in automobile wheel. Pearlitic steel has good toughness but insufficient strength, while dual phase steel has high strength but insufficient toughness. The research status of pearlite steel and dual phase steel for commercial vehicle wheels were systematically summarized. The effect of alloy elements on mechanical properties was introduced. In addition, the influence mechanism of rolling process, heat treatment process and welding process on the performance of wheel steel were also revealed. Finally, the future research directions of pearlite steel and dual phase steel are predicted, so as to provide reference for the development and safe service of commercial vehicle wheel steel in the future.

  Research status of bainitic non-quenched and tempered cold heading steels for grade 10.9 fasteners

  Huang Zhiguo, Cao Yongzheng, Li Yanyan, Jiang Bo

  2023, 48(8):  219-224.  doi:10.13251/j.issn.0254-6051.2023.08.036

  Abstract ( 27 )   PDF (1978KB) ( 51 )  

  Research status of non-quenched and tempered cold heading steels for fasteners in China was summarized. The strengthening and toughening mechanisms were discussed from the aspects such as grain refinement and dislocation strengthening. In addition, the key to control the microstructure and performance of cold heading steel rolled materials and deep processing fasteners was clarified. Finally, the development trend and research focus of non-quenched and tempered cold heading steels were summarized, aiming to provide a theoretical basis for the development and production of 10.9 grade high-strength non-quenched and tempered cold heading steels.

  Effect of stacking fault energy on deformation mechanism in face centered cubic high-entropy alloy

  Zhang Bo, Li Jie, Wu Kaidi, Niu Lichong, Wan Decheng, Feng Yunli

  2023, 48(8):  225-234.  doi:10.13251/j.issn.0254-6051.2023.08.037

  Abstract ( 41 )   PDF (4336KB) ( 61 )  

  Properties of high-entropy alloys can be tuned by changing the types and ratios of alloying elements, which affect the stacking fault energy and phase stability of the alloy system, then the plastic deformation mechanism of the alloy can be changed, thus the optimal comprehensive mechanical properties can be obtained. The factors that affect the stacking fault energy of face centered cubic high-entropy alloys, and the influence of stacking fault energy on deformation mechanism, are reviewed. And the methods that can affect the mechanical properties by changing the activation order of deformation mechanisms by adjusting the stacking fault energy are prospected.

  COMPUTER APPLICATION

  Design of fuzzy PID heat treatment temperature control system based on Kalman filter

  Li Guangbao, Gao Dong, Lu Yong, Ping Hao, Zhou Yuanyuan

  2023, 48(8):  235-241.  doi:10.13251/j.issn.0254-6051.2023.08.038

  Abstract ( 23 )   PDF (4184KB) ( 14 )  

  Aiming at the problems of hysteresis, overshoot, easy interference and slow response of temperature control in the heat treatment process of aerospace complex casting parts, a fuzzy PID(proportional-integral-derivative) temperature control method based on Kalman filter was proposed. Particle swarm optimization (PSO) was used to optimize the parameters of PID controller, and Kalman filter was also used to filter the measurement noise. Finally, the fuzzy strategy was used to realize the rapid response and overshoot control of temperature control. Through the research of fuzzy PID control, fuzzy rules were analyzed and fuzzy reasoning was carried out. According to the requirements of heat treatment process temperature control and its reasoning rules, selecting the most suitable control factor, through the simulation comparison with MATLAB software, it is proved that the accuracy of the designed heat treatment temperature control system is greatly improved, the robustness of the system is better, the response speed is obviously improved, the overshoot is reduced, the adjustment time is shortened, and the interference is also strongly inhibited.

  SURFACE ENGINEERING

  Effect of heat treatment on microstructure and properties of coatings on GCr15 bearing steel prepared by flame spraying+induction remelting technology

  Zhang Xi, Zhang Haoran, Xie Fang, Yan Jisen, Wu Bingbing, Kong Fanxiao

  2023, 48(8):  242-247.  doi:10.13251/j.issn.0254-6051.2023.08.039

  Abstract ( 37 )   PDF (3647KB) ( 52 )  

  Using Ni-Cr borosilicon powder (HG-201) as coating material, coatings on surface of GCr15 bearing steel were prepared by flame spraying+induction remelting composite melt coating technology, and then heat treated by different processes. Effects of different heat treatments on the microstructure, interface morphology, microhardness, impact property and impact fracture morphology of the coatings were studied by means of SEM, OM, microhardness test and impact test. The results show that the composite melt coating can refine the grain and improve the microhardness and impact property of GCr15 steel. The average impact absorbed energy of composite melt coated GCr15 steel is improved by 335.85% compared with that of conventional quenched GCr15 steel. After austempering or austempering+tempering treatment, the retained austenite and internal stress in the composite melt coated GCr15 steel are further eliminated, but the microhardness and impact property are slightly decreased, and the impact fracture of coatings shows the fracture mode of facet quasi-cleavage.

  Effect of scanning speed on microstructure and properties of 316L stainless steel coatings by high-speed laser cladding

  He Wei, Wang Yanyan, Shu Linsen

  2023, 48(8):  248-253.  doi:10.13251/j.issn.0254-6051.2023.08.040

  Abstract ( 52 )   PDF (3820KB) ( 60 )  

  316L stainless steel coating was prepared on 27SiMn steel substrate by high-speed laser cladding technology, the surface morphology, microstructure, microhardness and friction and wear properties of the clad coating were tested and analyzed. The effect of scanning speed on microstructure and properties of high-speed laser clad coating was studied. The results show that, when the scanning speed is 75 mm/s, the coating quality and metallurgical combination effect is the best. The coating microstructure is uniform, the joint is dominated by cellular crystals, and the middle and upper part is dominated by columnar crystals and equiaxed dendrites without secondary crystal arms, with the increase of scanning speed, the grain is obviously refined. When the scanning speed is 105 mm/s, compared with the coating at scanning speed of 45 mm/s, the microhardness increases from 210.4 HV0.5 to 391.5 HV0.5, which increases by 1.87 times. The friction coefficient of the coating is 0.521, and the wear loss is 2 mg. Therefore, the macroscopic properties of the coating can be significantly improved with the increase of scanning speed, and the microhardness and wear resistance of the coating can be improved by the fine grain strengthening at high scanning speed.

  Microstructure and properties of magnetic field assisted hot dip aluminized coatings on TA15 titanium alloy surface

  Li Yong, Wang Qiulin, Men Zhengxing, Zhao Yulin, Zhang Hong

  2023, 48(8):  254-259.  doi:10.13251/j.issn.0254-6051.2023.08.041

  Abstract ( 24 )   PDF (2827KB) ( 18 )  

  Microstructure and properties of hot dip aluminized coatings on the surface of TA15 titanium alloy under the action of alternating electromagnetic field were studied, and the thermal diffusion treatment was carried out at 800 ℃. The morphology, microstructure and element distribution of the coatings were analyzed by means of energy dispersive spectroscopy, scanning electron microscope and X-ray diffractometer. The microhardness of the coatings was tested by microhardness tester. The results show that after applying alternating electromagnetic field with current intensity of 15 A and 20 A during the process of hot dip aluminizing and thermal diffusion, the coatings bind well with the alloy substrate, no obvious crack is found, the formation of pores and cavities is significantly reduced, the dipping time is shortened, and thus the production cycle is accelerated. When the current intensity is 15 A and 20 A, the maximum hardness of the bonding zones can reach 518.7 HV0.2. When the current intensity increases to 25 A, the electromagnetic stirring effect sharply increases, and no high-quality coating can be obtained.

  FAILURE ANALYSIS

  Failure analysis on copper bars of traction motor key component

  Lu Jiping, Yang Chuan, Cui Guodong, Ma Ji, Zhang Chengsong, Guo Qiyang, Chen Dazhi

  2023, 48(8):  260-264.  doi:10.13251/j.issn.0254-6051.2023.08.042

  Abstract ( 26 )   PDF (2795KB) ( 46 )  

  Many accidents of the motor rotor copper guide bar fracture occurred in service for the 5GEB32B1 type traction motor of HXN5 locomotive. The fracture causes were analyzed, and the corresponding improvement measures were proposed by means of chemical composition test, metallographic examination, tensile test and fracture morphology analysis. The results show that the chemical composition, microstructure and properties of the copper all meet the requirements of relevant standards, and the fracture mechanisms are fatigue fracture and creep fracture. Obvious fatigue striations are observed on the broken copper bars, and the temperature around the copper bar and throughout the whole motor raises caused by the copper bars rubbing violently with the spindle after fracture, which further causes the guide bars to creep under various loads at high temperature and for a long time.

  Analysis of cracking cause of acid gas post-cooler head

  Liu Min, Lu Meipeng, Liu Jiachen, Gu Baolan, Guo Jing, Sun Yuejun, Song Ming

  2023, 48(8):  265-271.  doi:10.13251/j.issn.0254-6051.2023.08.043

  Abstract ( 29 )   PDF (6313KB) ( 19 )  

  Bulges and cracks appeared in the inner wall of floating head of acid gas post-cooler in a natural gas purification plant. The causes of bulges and cracks were analyzed by means of macroscopic examination, chemical composition analysis, hardness test, metallographic examination, energy spectrum analysis, fracture analysis and EBSD test. The results show that the bulge and cracking of the inner wall of the head are hydrogen bulge (HB) and hydrogen-induced cracking (HIC). Since the microstructure of the head material (20g steel) is banded ferrite+pearlite along the rolling direction, with bainite and more MnS inclusions appearing at the position of 1/2 plate thickness caused by element segregation, and the service environment of the head contains wet H₂S medium which generates hydrogen after electrochemical reaction with the steel, then the hydrogen accumulates at the defects of 1/2 plate thickness, leading to bulge and cracking of the inner wall of the head.

  Cause analysis on cold drawing fracture of ER70S-6 steel coil and process optimization measures

  Fan Zhenxia, Zhang Yuming, Dong Qing, Zhao Haoqian, Li Yashuo , Wei Qiaoling

  2023, 48(8):  272-275.  doi:10.13251/j.issn.0254-6051.2023.08.044

  Abstract ( 45 )   PDF (2513KB) ( 33 )  

  ER70S-6 steel ø6.5 mm hot-rolled coil fractured during cold drawing, and the batch of coils was tested by means of chemical composition, mechanical properties, and microstructure inspection. The results show that the martensite in the core and bainite at the edge of the coil are the causes of drawing fracture. The production of martensite in the core and bainite at the edge can be effectively reduced by reducing the entering temperature of finishing rolling from 960 ℃ to 920 ℃ and increasing the length of insulation cover by 5 m.

  Low-temperature fracture failure analysis of 30CrMnSiA steel structural part

  Guo Qiang, Chi Kegang

  2023, 48(8):  276-279.  doi:10.13251/j.issn.0254-6051.2023.08.045

  Abstract ( 33 )   PDF (2169KB) ( 35 )  

  Fracture of 30CrMnSiA steel structural part occurred under impact load in a low-temperature environment. The fracture morphology, microstructure and inclusions, chemical composition, hardness, tensile properties and impact properties were tested and analyzed by means of scanning electron microscope, metallurgical microscope, inductively coupled plasma spectrometer (ICP), hardness test, tensile test, and impact test. The results indicate that during quenching and tempering, the actual tempering temperature is lower than the required temperature, which results in the transformation of the metallographic structure into tempered martensite. The tempered martensite has low impact absorbed energy at -40 ℃, which is the main reason for brittle fracture under impact at -40 ℃. The grade of non-metallic inclusions such as sulfides and silicates exceeds the design requirements, and a large number of inclusions promote rapid crack initiation and propagation.

  Cause analysis and improvement measures for defects of high strength 4145 steel used in oil drilling tool

  Ling Wendan, Wang Jinyong, Gao Jie, Qi Xilun, Chen Mingyi, Chen Xinhe

  2023, 48(8):  280-285.  doi:10.13251/j.issn.0254-6051.2023.08.046

  Abstract ( 26 )   PDF (3171KB) ( 24 )  

  Forming and heat treatment process of high strength 4145 steel was developed, but two types of defects were resulted during the producing and research processes. The macro and micro characteristics of the defects were compared, the causes were analyzed, and countermeasures were proposed. The results show that the first type defect is the folding caused by the poor surface quality of rolling tools in the rolling forming process, which forms a certain angle with the axis direction of the steel pipe, the depth is about 0.9 mm, and with the decarburized layer thickness of 0.090-0.157 mm. The second type defect is perpendicular to the axis of the steel pipe, cracking through or the depth exceeding 2 mm, and the thickness of the decarburized layer is not more than 0.1 mm, which is determined as the quenching crack caused by rapid quenching cooling rate. Then the rolling defects are avoided by controlling the quality of rolling forming tools, and quenching crack defects are solved by optimizing the chemical composition and quenching process. After testing and verification, the quality of the steel pipe produced by the improved measures is qualified.

  Failure analysis of high temperature fastening bolt of 20Cr1Mo1VNbTiB steel

  Huang Jiang, Mou Shenzhou, Wang Jing, Tao Liang, Huang Qiaosheng, Wang Xue

  2023, 48(8):  286-290.  doi:10.13251/j.issn.0254-6051.2023.08.047

  Abstract ( 28 )   PDF (3872KB) ( 26 )  

  A high-temperature bolt made of 20Cr1Mo1VNbTiB steel for a certain supercritical unit was early fractured. The macroscopic and microscopic characteristics of the fracture were observed, the aging condition of the bolt was analyzed, and the failure reasons were discussed. The results show that after running at high temperature, the aggregate growth of carbides in the bolt is not obvious, the decrease of tensile strength and hardness is not significant, but the decrease of impact absorbed energy is obvious, which means that the bolt fracture failure is not related to the aging. Excessive preload during the bolt assembly accelerates its creep damage. According to the fracture morphology, it is determined that the fracture belongs to high temperature creep rupture.

  Failure analysis of superalloy bolts for reducer cover plate

  Jing Dong

  2023, 48(8):  291-294.  doi:10.13251/j.issn.0254-6051.2023.08.048

  Abstract ( 35 )   PDF (3071KB) ( 30 )  

  During the abnormal life cycle of the butterfly valve equipped in the dehydrogenation unit of a company, the nickel-base alloy bolts of the blind flange were broken. Through dimension measurement and stress deformation analysis, the fracture sequence of bolts was confirmed. Further physical and chemical analysis on the first broken piece confirms that the bolt fracture is brittle type, which is related to improper heat treatment in the manufacturing process, thus a large number of fibre structures without complete recovery and recrystallization are resulted.