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  • MICROSTRUCTURE AND PROPERTIES
    Effects of Mo content on phase transformation behavior, microstructure and mechanical properties of high grade pipeline steel
    Duan He, Shan Yiyin, Yang Ke, Shi Xianbo, Yan Wei, Ren Yi
    2021, 46(5):  1-8.  doi:10.13251/j.issn.0254-6051.2021.05.001
    Abstract ( 102 )   PDF (654KB) ( 53 )  
    By adjusting the Mo content in a high-strength pipeline steel, the effect of Mo content on dynamic phase transformation was studied. Under the same TMCP condition, the strength and toughness of the steels with 0.2% Mo and 0.3% Mo were studied comparatively, and two different complex acicular ferrite (AF) microstructures were analyzed by optical microscope, EBSD, TEM and other means. The relationship between the microstructures and the mechanical properties was investigated. The results show that the increasce of Mo content is beneficial to refinement of the microstructure and formation of the acicular ferrite, and move the dynamic CCT curves to the lower right. Under the same rolling conditions, the strength of the steel with 0.3% Mo is higher, the increase of Mo content helps to improve the effects of precipitation strengthening and fine grain strengthening, at the same time, the volume fraction of bainitic ferrite (BF), the size and number density of MA islands are increased, and the strength is further increased. In the case of the same proportion of high angle grain boundaries, the impact property of the steel with 0.3% Mo is worse, the size and number density of MA islands as well as their degree of irregularity are the main factors to affect the impact property.
    Microstructure and brittleness analysis of FeCrAl stainless steel by hot isostatic pressing
    Bao Shun, Liu Rongpei, Feng Han, Nai Qiliang, Jia Jian, Zhu Yuliang, Song Zhigang
    2021, 46(5):  9-13.  doi:10.13251/j.issn.0254-6051.2021.05.002
    Abstract ( 86 )   PDF (578KB) ( 36 )  
    The equilibrium phase composition of the two FeCrAl stainless steel parent alloys with different aluminum content was calculated and analyzed by using Thermo-calc software. The microstructure and mechanical properties of the FeCrAl stainless steel prepared by powder atomization and hot isostatic pressing(HIP) were observed and studied by means of optical microscope, SEM and EDS. The results show that the matrix phase of the tested steel is α phase with BCC structure, and the peak precipitation temperature of MC carbide is about 700 ℃, and its ultimate precipitation amount(mass fraction) is about 0.8%. It is mainly distributed along the grain boundary of the ferrite matrix in a chain pattern, and easy to cohere in the triple line boundary and other high-energy regions. The MC carbide precipitation of the hexagonal structure with fewer slip systems and hard texture is the main reason for impacting brittle fracture of HIP FeCrAl stainless steel at room temperature. After annealing at 800 ℃ and water cooling, the MC carbide precipitation of HIP FeCrAl stainless steel can be reduced and the impact property of it can be improved.
    Research status of failure mechanisms of heat-resistant steel in thermal power units
    Yang Hao, Si Yu, Cao Pengjun, Li Kejian
    2021, 46(5):  14-24.  doi:10.13251/j.issn.0254-6051.2021.05.003
    Abstract ( 58 )   PDF (575KB) ( 38 )  
    Effectively improving the fuel combustion utilization rate of thermal power units is the main means of modern society to alleviate the energy crisis and relieve the pressure. Appropriately increasing the steam parameters of the unit can increase the thermal cycle efficiency, and greatly improve the energy utilization rate, but the performance requirements of the unit components are more restricted. Heat-resistant steel is widely used in the parts of thermal power boilers because of its excellent high temperature oxidation, thermal strength and low cost. This article introduces the microstructure of three heat-resistant steels including 25Cr2MoVA, T91 and Super 304H after different heat treatments, analyzes the high-temperature mechanical properties of the heat-resistant steels at different temperatures, and summarizes the failure mechanisms of the heat-resistant steels during service. In addition, the research status of oxide film on the surface of the heat-resistant steel is emphasized, and the application of using oxide film thickness to predict the remaining service life of metal parts is prospected.
    Evolution of austenite grain size of 42CrMo steel during heating
    Peng Ze, Li Mengnie, Bu Hengyong, Li Qi
    2021, 46(5):  25-31.  doi:10.13251/j.issn.0254-6051.2021.05.004
    Abstract ( 108 )   PDF (575KB) ( 39 )  
    Austenite grain size of the 42CrMo steel heated at 890-930 ℃ for 10-240 min was measured through optical metallography exams. The results reveal that the 42CrMo steel is fully austenitized when heated between 890-930 ℃ and the grain growth is normal during the heating process. Temperature has an important effect on grain size, but the effect of holding time is minor. Grain growth is slow as holding time prolongs, and grain size and holding time satisfy a function whose exponent is less than 1. Experimental data are fitted to determine parameters of the Beck model, Sellars model and Anelli model. The prediction accuracy of all three models is better. Anelli model is considered being the most suitable, thus is recommended for the prediction of austenite grain growth law of the 42CrMo steel.
    Phase transformation behaviors of ultra high strength cord steel LX92A in continuous cooling and isothermal cooling
    Li Zhanwei, Shen Kui, Yu Xuesen, Zhang Yu
    2021, 46(5):  32-37.  doi:10.13251/j.issn.0254-6051.2021.05.005
    Abstract ( 64 )   PDF (575KB) ( 23 )  
    The transformation behaviors of ultra high strength cord steel LX92A in continuous cooling and isothermal cooling after compressively deforming of 60% at 920 ℃ were studied by thermal simulation. The results show that the cooling rates of 0.5 K/s and below in continuous cooling produce network cementite at grain boundaries. When cooling rate reaches 1 K/s or above, the cementite becomes line-shaped and its quantity decreases with the increase of cooling rate; and when the cooling rate increases above 15 K/s, martensitic transformation occurs. For the isothermal cooling, the temperature range of pearlitic transformation is 550-700 ℃, and the nose tip temperature is about 604 ℃. With the decrease of isothermal transformation temperature, the microstructure changes from coarse lamellar pearlite to lamellar pearlite, fine lamellar pearlite, till bainite appears at 525 ℃. Predictive models of pearlite lamellar spacing based on the degree of undercooling and cooling rate are obtained by regression analysis, with their fitness factors reach above 0.99 and 0.97, respectively.
    Texture evolution of cold-rolled N18 alloy during recrystallization
    Wu Jing, Chen Jing, Jiang Yilan, Dai Xun, Zhang Ruiqian, Liu Huiqun
    2021, 46(5):  38-46.  doi:10.13251/j.issn.0254-6051.2021.05.006
    Abstract ( 62 )   PDF (579KB) ( 28 )  
    Microstructure and texture of the rolled plane(RD-TD) and cut plane(RD-ND) of 30% cold-rolled N18 alloy annealed at 530 ℃ for different time were characterized and analyzed by means of EBSD. For RD-TD plane, the initial grain orientation is <0001>//ND, the orientations of recrystallized grains are <0001>//ND and <1210>//RD texture. For the RD-ND plane, the initial grain orientation is mainly <1010>// RD and <1210>//RD. After recrystallization annealing, three different orientations of<1010>//ND, <1210>//ND and <0001> tilted to TD pole ±85° are formed. The grain size on the RD-TD plane is larger than that on the RD-ND plane, which provides theoretical support for the reprocessing of Zr alloy.
    Relationship between microstructure and tensile fracture of 6082-T6 aluminum alloy
    Zhu Gaojie, Zou Longjiang, Ren Xiaolei, Li Chunhui, Liu Shutan
    2021, 46(5):  47-54.  doi:10.13251/j.issn.0254-6051.2021.05.007
    Abstract ( 76 )   PDF (578KB) ( 40 )  
    Mechanical properties of the aged 6082-T6 aluminum alloy were studied by means of static tensile method. The microstructure of the alloy was observed by means of optical microscopy (OM) and transmission microscopy (TEM). The tensile fracture morphologies were studied by means of scanning electron microscopy (SEM), and precipitated phase distribution of the alloy was analyzed by means of electronic probe (EPMA). The relationship between microstructure and tensile fracture was comprehensively analyzed. The results show that the aged 6082-T6 aluminum alloy has high yield ratio, of which the tensile strength is 345 MPa and yield strength is 326 MPa. The spherical pit-like black Mg2Si precipitated phase is distributed in the aluminum alloy, meanwhile white iron-rich inclusion phase α-AlMnFeSi is also present. Tensile fracture of the aged 6082-T6 aluminum alloy has obvious dimple-type fracture characteristics. When the stress concentration caused by dislocation pile-up exceeds the weak interface bonding strength, local fracture of the alloy matrix will occur, and pit-like dimples will be formed from the fracture around the brittle precipitated phase particles, and finally dimple-type fracture will form, containing hard and brittle inclusions.
    Microstructure and fatigue properties of low temperature coiling hot-rolled dual phase steel
    Zhao Nan, Liu Xuewei, Sun Mingjun, Xue Jun
    2021, 46(5):  55-59.  doi:10.13251/j.issn.0254-6051.2021.05.008
    Abstract ( 53 )   PDF (579KB) ( 26 )  
    Microstructure and fatigue properties of low temperature coiling hot-rolled dual phase steel were studied by using SEM and tension-tension fatigue tests. The results show that the fatigue limit of the hot-rolled dual phase steel is approximately 530 MPa. The average size of inclusion of the hot-rolled dual phase steel produced by low temperature coiling is less than 5 μm, with small grain and tiny uniformly distributed martensite, which leads to good comprehensive mechanical propertles. The source of fatigue crack in the hot-rolled dual phase steel is located on the edge of specimen surface. There are plenty of dimples, tearing ridges, fatigue striations and secondary cracks in fatigue crack growth zone. Fatigue final rupture region mainly consists of superficial dimples. Different degree of strain occurred in ferrite and martensite finally results in the born of secondary cracks on the phase interface of ferrite and martensite. The secondary cracks are borned but don't grow. Large number of secondary cracks disperse the stress concentration at the tip of main cracks, which can effectively reduce the driving force of crack growth, slow down the fatigue crack propagation rate, and restrain fatigue crack propagation and finally improve the fatigue properties.
    Effect of different water-based quenching media on microstructure and wear resistance of ZG30CrMnSiMo low alloy steel
    Wu Zhaoyang, Ping Xianzhong, Zheng Baochao, Yi Yanliang, Li Wei
    2021, 46(5):  60-65.  doi:10.13251/j.issn.0254-6051.2021.05.009
    Abstract ( 57 )   PDF (576KB) ( 27 )  
    Microstructure, mechanical properties and wear resistance of low alloy wear resistant steel were analyzed by means of scanning electron microscope, impact testing machine and dynamic load impact abrasive wear testing machine. The results show that the as-cast microstructure consists of pearlite and carbide, of which the macroscopic hardness, the impact absorbed energy and wear loss are 41.3 HRC, 6.1 J and 1.4378 g, respectively. And after quenching in sodium silicate (Na2SiO3) and PAG, the microstructure of the is both transformed into tempered lath martensite and carbide, of which the macroscopic hardness, the impact absorbed energy and wear loss are 49.0 HRC and 51.1 HRC, 7.3 J and 9.4 J, 0.9378 g and 0.835 g, respectively. Compared with the as-cast alloy, the macroscopic hardness, impact propekly and wear resistance of the PAG quenched steel are improved by 23.7%, 54.1% and 1.7 times, respectively. Compared with quenching in sodium silicate solution, the macroscopic hardness, impact property and wear resistance of the PAG quenched steel are improved by 4.3%, 28.8% and 1.1 times, respectively.
    Formability of high strength dual phase steels
    Xiong Ziliu, Qi Jianjun, Sun Li, Dong Yikang, Zhao Yizhe, Yang Ting, Xing Chengliang
    2021, 46(5):  66-70.  doi:10.13251/j.issn.0254-6051.2021.05.010
    Abstract ( 75 )   PDF (581KB) ( 36 )  
    Microstructure, mechanical properties, FLC and cold bending property of the dual phase (DP) steels with strength of 780-1180 MPa were tested by means of optical microscope, tension and compression machine, universal forming machine and three point bending machine. The results show that with the increase of yield strength of the DP steels, martensite content is increased, grain size is refined, elongation, n and r values are decreased, plane strain forming limits are declined from 22.7% to 12.6%, the tensile limits are reduced from 15.8% to 8.6%, and minimum bending radius is increased from 1.13 to 2.86. With the increase of yield strength of the 980 MPa DP steels, microstructure homogeneity is increased, bulging deformation limit is enhanced from 22.8% to 34.5%, and minimum bending radius is decreased from 2.50 to 1.82. The Bauschinger effect constant of the CR420/780DP, CR700/980DP and CR820/1180DP steels is 0.82, 0.78 and 0.79, respectively. Instantaneous hardening rate of initial plastic deformation stage in compression process of the CR420/780DP and CR820/1180DP steels is higher than that of the reverse tension and unixial tension processes.
    Effect of retained/reversed austenite on improving -196 ℃ cryogenic impact property of high strength stainless steel
    Qiu Xuyangfan, Yang Zhuoyue, Ding Yali
    2021, 46(5):  71-74.  doi:10.13251/j.issn.0254-6051.2021.05.011
    Abstract ( 62 )   PDF (578KB) ( 28 )  
    High strength stainless steel 0Cr16Ni6 treated by quenching+low temperature tempering, and maraging stainless steel 00Cr11Ni11MoTi treated by over aging were studied by means of mechanical properties tests, SEM and XRD, and the effects of retained/reversed austenite in the tested steels on -196 ℃ cryogenic notched tensile strength and impact property were analyzed. The results show that, with the nearly same strength and toughness of the tested steels at room temperature, the notch tensile strength and impact property of the 0Cr16Ni6 steel are significantly higher than that of the 00Cr11Ni11MoTi steel at -196 ℃. According to the residual/reversed austenite contents quantitatively calculated based on the relative intensities of the martensite and austenite diffraction peaks far from the fracture and near the fracture, it is found that nearly 90% retained austenite in the 0Cr16Ni6 steel transforms into strain-induced martensite during the formation and propagation of cracks, which improves cryogenic impact property remarkably. But the reversed austenite formed by over-aging of the 00Cr11Ni11MoTi steel has higher stability, which is not suitable for strain-induced martensite transformation to occur, so the improvement of -196 ℃ cryogenic toughness is limited.
    PROCESS RESEARCH
    Effect of solution treatment on microstructure and properties of spray formed 1420 Al-Li alloy
    Xu Guifang, Guan Yunpeng, Guo Zhangjian, Wang Jia
    2021, 46(5):  75-80.  doi:10.13251/j.issn.0254-6051.2021.05.012
    Abstract ( 51 )   PDF (579KB) ( 25 )  
    Extruded spray formed 1420 Al-Li alloy was solution treated at different temperatures for different time. The microstructure and mechanical properties of the alloy were analyzed by optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and electronic universal testing machine. The results show that a large number of Al3Li and AlLi particles are distributed in the extruded alloy. After solid solution treatment at 450 ℃ for 0.5 h, the alloy can obtain fine microstructure and good comprehensive mechanical properties, with the average grain size being 2.91 μm, the second phase particles dissolved into the matrix, the tensile strength and the elongation being 377.47 MPa and 17.3% respectively, and the fracture mode of the alloy is ductile type. With the increase of solution treatment temperature and time, the cracks originate from the defects and propagate in transgranular mode.
    Effect of tempering temperature on microstructure, strength and toughness of EH460 grade medium and heavy ship plate steel
    Xiao Na, Xu Xiaoning, Wang Yimin, Sun Yonghui, Tian Yong
    2021, 46(5):  81-86.  doi:10.13251/j.issn.0254-6051.2021.05.013
    Abstract ( 60 )   PDF (581KB) ( 26 )  
    Effect of tempering temperature on the structure and mechanical properties of EH460 grade ship plate steel were investigated by using optical microscopy, scanning electron microscopy, electron backscattering diffraction analysis and mechanical properties testing. The results show that the tested steel tempered at 400 ℃ for 2 h has the best properties in strength and toughness, at which conditions the yield strength, tensile strength, and the impact toughness at -40 ℃ of the tested steel can reach 471 MPa, 560 MPa and 312 J, respectively. The main reasons for the improvement of low-temperature toughness of the tested steel after tempering are as follows: the decomposition of coarse M/A islands, the increase of the proportion and density of high-angle grain boundaries in the microstructure and the decrease of effective grain size.
    Influence of transverse flux induction heating excitation parameters on steel strip temperature and heater optimization
    Yan Chaohui, Wang Youhua, Liu Chengcheng, Wu Shipu
    2021, 46(5):  87-94.  doi:10.13251/j.issn.0254-6051.2021.05.014
    Abstract ( 70 )   PDF (574KB) ( 28 )  
    Under the premise of considering variation of the strip physical property parameters with temperature, the influence of different excitation current, current frequency and strip movement rate on steel strip temperature distribution was analyzed based on the iterative calculation of transverse flux induction heating coupling field. By changing the air gap distribution between the heater and the strip, the original induction heater model was optimized and other four induction heater models were designed, and the average temperature and temperature uniformity of the strips under different heater models were analyzed. The average temperature and temperature uniformity of the 45 steel and 35CrMnSiA steel under the original heater model and the improved optimal heater model were compared. The results show that the optimized induction heater model makes the surface temperature distribution of the strips more uniform and the average temperature higher.
    Effect of tempering temperature on microstructure and properties of 00Cr13Ni5Mo super martensitic stainless steel
    Li Zhaoguo, Wang Ke, Ji Xianbin, Xu Bin, Wei Haixia
    2021, 46(5):  95-98.  doi:10.13251/j.issn.0254-6051.2021.05.015
    Abstract ( 62 )   PDF (580KB) ( 43 )  
    Effect of 1050 ℃ normalizing+550-700 ℃ tempering treatment on microstructure and mechanical properties of the 00Cr13Ni5Mo super martensitic stainless steel plate was studied. The results show that after normalization at 1050 ℃ and with the increase of subsequent tempering temperature, the lath martensite gradually decomposes and produces reversed austenite. When tempered at 600 ℃, the content of reversed martensite is the highest, and then gradually decreases with the increase of tempering temperature. And the strength, hardness and yield ratio of the tested steel decrease first and then increase with the increase of tempering temperature. When tempered at 650 ℃, a complex phase microstructure containing fine tempered sorbite and reversed austenite can be obtained, and the tested steel has lower yield ratio and good impact property.
    Effect of tempering temperature on HAZ microstructure and properties of CMT welded 40CrNiMo quenched and tempered steel
    Wang Jingbo, Lian Mingyang, Fu Yadi, Liu Shengxin, Huang Zhiquan, Chen Yong
    2021, 46(5):  99-103.  doi:10.13251/j.issn.0254-6051.2021.05.016
    Abstract ( 60 )   PDF (579KB) ( 23 )  
    In order to repair the marine drilling platform gear, 40CrNiMo quenched and tempered steel was surfacing welded by cold metal transfer (CMT) technology. Effects of post-welding tempering temperature on HAZ microstructure and properties of the 40CrNiMo quenched and tempered steel were investigated via the characterization methods such as scanning electron microscope (SEM), microhardness test, impact test and tensile test. The results show that with the increase of post-welding tempering temperature, both the microhardness of the HAZ and the tensile strength of the welded specimen decrease, while the elongation increases gradually. When tempered at 400 ℃ and 500 ℃, the precipitated carbides are distributed on the lath boundary, block boundary and grain boundary of martensite, which worsens the impact property of the welded specimens. When tempered at 600 ℃, the carbides are aggregated, spheroidized, and distributed more dispersedly, so that the impact property is improved.
    Control mechanism of residual stress in 35MnB alloy steel by deep cryogenic treatment
    Wei Nasha, Yuan Ruize, Niu Xuemei, Chen Zhigang, Chen Zhi, Yan Xianguo
    2021, 46(5):  104-110.  doi:10.13251/j.issn.0254-6051.2021.05.017
    Abstract ( 64 )   PDF (608KB) ( 37 )  
    With 35MnB alloy steel as the object of study, an orthogonal test was designed with the cryogenic temperature, cryogenic time and alternating times as the independent variables and the residual stress as the dependent variable. By means of X-ray diffractometer, the distribution of residual stress on the specimen surface was measured, and the influence factors of the cryogenic process parameters on residual stress in the specimens were analyzed by range analysis method. Via scanning electron microscope, the microstructure and morphology of the specimens under different cryogenic treatment process parameters were observed, and the change of retained austenite volume fraction after cryogenic treatment was quantitatively analyzed by means of X-ray diffractometer. The mechanism of residual stress reduction was comprehensively explored from the microscopic point of view. The results show that the influencing factors of cryogenic process parameters on residual stress of the 35MnB alloy steel are as follows: cryogenic temperature>alternating times>cryogenic time, and the optimal combination is the cryogenic temperature of -160 ℃, cryogenic time of 12 h, alternating number of 1 time. After cryogenic treatment, the retained austenite is refined and transformed into new martensite. The retained austenite distributed at the martensite boundary can relax the stress concentration caused by dislocation accumulation at martensite interface, thus reducing the residual stress inside the microstructure.
    Hot deformation behavior and processing maps of Maraging250 steel based on friction and temperature double correction
    Shang Limei, Wang Chunxu, Han Shun, Li Yong, Pang Xuedong, Li Jianxin, Yang Chao
    2021, 46(5):  111-117.  doi:10.13251/j.issn.0254-6051.2021.05.018
    Abstract ( 72 )   PDF (618KB) ( 31 )  
    Hot deformation behavior of Maraging250 steel at deformation temperature of 900-1200 ℃ and strain rate of 0.001-10.0 s-1 was studied by using Gleeble-3800 thermal simulation test machine. The flow stress curves were modified based on friction and temperature double correction, from which the constitutive equation and processing map of the Maraging250 steel were established. The microstructure change of the tested steel under different deformation conditions were analyzed according to the processing map at true strain of 1.2. The results show that, with the decrease of deformation temperature or the increase of strain rate under the same test conditions, the effect of friction on the flow stress of the tested steel is more significant. The deformation heat has a significant effect on the flow stress only under high strain rate and low temperature conditions, which leads to the maximum temperature rise and the maximum flow stress change, being about 80 ℃ and 20 MPa, respectively. Hot deformation activation energy of the tested steel is calculated to be 393.552 02 kJ/mol by using the double-corrected flow stress curves, and the Z parameter equation and constitutive equation are established, the processing maps at true strain ε=0.4, 0.8 and 1.2 are plotted. Combined with the microstructure analysis, it is proposed that the Maraging250 steel can obtain uniform and fine dynamic recrystallization microstructure in the range of 1000-1125 ℃, 0.001-1.0 s-1, with better hot processing performance.
    Thermal deformation behavior of a low carbon hot-rolled H-beam steel containing niobium
    Li Junliang, Xing Jun, Ge Zhangqi, Li Fan, Wang Yongqiang, Li Na, Wu Baoqiao
    2021, 46(5):  118-126.  doi:10.13251/j.issn.0254-6051.2021.05.019
    Abstract ( 58 )   PDF (611KB) ( 31 )  
    Thermal deformation behavior at temperature between 1000-1200 ℃ with strain rate of 0.1-5 s-1 of a new low-carbon hot-rolled H-beam steel containing niobium was studied by means of hot compression experiment and microstructure analysis. The microstructure of the tested steel affected by deformation parameters was analyzed. At the same time, an improved constitutive equation of coupled strain factor was established, and the critical strain model with critical ratio was adopted to predict the critical strain value for dynamic recrystallization. The results show that the original austenite structure is more uniform and the average grain is smaller at lower strain rate and deformation temperature. And the increase of strain rate is not conducive to the occurrence of dynamic recrystallization. The relationship between the critical strain for dynamic recrystallization and the peak strain is εc/εp=0.47. The constitutive equation related to the coupled strain factor and critical strain prediction model can accurately predict the flow stress and critical strain values of low-carbon niobium-containing hot-rolled H-beam steel at different deformation temperatures.
    Hot deformation behavior of 23MnNiMoCr54 steel
    Li Shuo, Fang Guangjin, Wang Qingfang, Chen Shichang, Lu Chunguang
    2021, 46(5):  127-131.  doi:10.13251/j.issn.0254-6051.2021.05.020
    Abstract ( 75 )   PDF (610KB) ( 27 )  
    Thermo dilatometric curve of 23MnNiMoCr54 steel was measured by a DIL 805A thermal dilatometer, CCT curve of the tested steel was drawn in combination with the hardness test, and the dynamic phase transformation and dynamic recrystallization laws were studied and analyzed. The results show that the critical transformation temperatures of the 23MnNiMoCr54 steel are: Ac3=806 ℃, Ac1=713 ℃, and there is no pearlite transformation zone in the CCT curve. When the cooling rate is greater than or equal to 0.5 ℃/s, the martensitic transformation begins. When the deformation amount is 10% and deformed at 850-1150 ℃, the austenite grain boundary of the tested steel is stable, the grain size does not change significantly, no dynamic recrystallization occurs, and the softening mechanism is mainly dynamic recovery. When the deformation amount is 40%, no dynamic recrystallization of the tested steel occurs at 850 ℃, and the softening mechanism is mainly recovery. Unstable austenite grain boundaries and fine grains appear when the deformation temperature is at 900-950 ℃, and dynamic recrystallization begins to occur. At 1000 ℃, complete dynamic recrystallization occurs. When the deformation amount is 50%, unstable austenite grain boundaries and fine grains appear when deformed at 850-950 ℃, and partial recrystallization occurs. At deformation temperature of 1000 ℃, complete dynamic recrystallization occurs. When the deformation amount is 60%, unstable austenite grain boundaries and fine grains of the tested steel appear when deformed at 850-950 ℃, and partial recrystallization occurs. At deformation temperature of 1000 ℃, complete dynamic recrystallization occurs.
    Influence of hot deformation parameters on bonding strength of 316L/Q370qE clad plate
    Shao Chunjuan, Zhen Fan, Qu Jinbo, Luo Guangjie
    2021, 46(5):  132-137.  doi:10.13251/j.issn.0254-6051.2021.05.021
    Abstract ( 55 )   PDF (611KB) ( 26 )  
    Rolling process of the 316L/Q370qE clad plate was simulated by Gleeble-3800 thermal compression testing. And the influence of deformation amount, deformation temperature and holding time before deformation on bonding strength of the clad plate was studied by means of tensile testing, hardness testing and microstructure analysis. The results show that the bonding strength gradually increases with the increase of deformation amount, but the increasing trend becomes slower when the deformation amount is greater than 20%. As the deformation temperature increases, the bonding strength first increases and then decreases, and the bonding strength does not change much when the deformation temperature is 1100 ℃ and 1250 ℃. The longer the holding time, the lower the bonding strength of the clad plate is.
    Effect of ferrite rolling on microstructure and properties of steel plate cold-rolled common
    Zhou Bowen, Fan Lei, Li Xian, Ye Jiang, Liu Guangquan, Feng Xinyuan
    2021, 46(5):  138-142.  doi:10.13251/j.issn.0254-6051.2021.05.022
    Abstract ( 59 )   PDF (616KB) ( 25 )  
    Effects of ferrite hot rolling process and austenite hot rolling process on hot rolled microstructure, cold rolled microstructure and properties of steel plate cold-rolled common(SPCC) were studied by optical microscope and scanning electron microscope. The results show that compared with austenite rolling process, the average grain size of the hot rolled band produced by ferrite hot rolling process increases about 17 μm, the amount of {111} texture decreases by 8.74%, and the intensity of {111} texture decreases slightly, while the amount and intensity of {001}<110> texture increases by 12.40% and 19.81, respectively. In the SPCC product produced by ferrite hot rolling process, the grains are approximately equiaxed, and compared with that produced by austenile hot raling process, it is also found that the average grain size is about 4.5 μm larger, the average yield strength is 19 MPa lower, and the average r value(plastic strain ratio) is 1.1 smaller. The lower yield strength and r value are due to the larger grain size, less {111} texture and stronger {001}<110> texture in the hot rolled band produced by ferrite hot rolling process.
    Aging process, microstructure and properties of CuCr1 alloy contact wire
    Hua Siming, Zhang Pingze, Liu Zili
    2021, 46(5):  143-149.  doi:10.13251/j.issn.0254-6051.2021.05.023
    Abstract ( 58 )   PDF (611KB) ( 20 )  
    Contact wire of CuCr1 alloy was produced by solid solution-continuous extrusion-aging-rolling-drawing process and solid solution- continuous extrusion-rolling-aging-drawing process, and the microstructure and properties of the CuCr1 alloy in different states were tested and analyzed, respectively. The results show that after aging treatment, the performance of continuous-extruded CuCr1 alloy is better than rolled. The microstructure of continuous extruded CuCr1 alloy is banded deformation structure with a small amount of recovery structure. The microstructure of rolled CuCr1 alloy is deformed grains and Cr phase distributed along the banded grain boundary, and microstructure of the aged CuCr1 alloy is mainly recrystallized structure. After solution treating, continuous extruding, aging, rolling and drawing, the tensile strength of the contact wire reaches 489 MPa, conductivity reaches 79.47%IACS, and its performance is better than that produced by the solid solution-continuous extrusion-rolling-aging-drawing process.
    Effect of aging on tensile properties and intergranular corrosion sensitivity of 2050 Al-Li alloy
    Li Haoran, Li Zhanqi, Wang Jiayi, Ye Zhihao, Li Jinfeng, Hu Xiaoqing
    2021, 46(5):  150-155.  doi:10.13251/j.issn.0254-6051.2021.05.024
    Abstract ( 55 )   PDF (608KB) ( 23 )  
    Tensile properties, intergranular corrosion (IGC) and microstructure of 2050 Al-Li alloy sheet after T6 and T8 aging for various time were investigated through tensile property measurement, immersion corrosion and transmission electron microscope (TEM) observation. The results show that the strength and elongation of the T8 aged alloy are much higher than those of the T6 aged alloy. The IGC sensitivity decreases with the T6 and T8 aging extension from initial aging to peak-aging. Further T6 and T8 aging increases the IGC sensitivity. In addition, the IGC sensitivity under the T8 aging condition is much lower than that under the T6 aging condition. Based on the strength, elongation and IGC sensitivity, the appropriate aging for 2050 Al-Li alloy sheet is T8 peak-aging, the corresponding alloy possesses the highest strength (tensile strength 529 MPa), good elongation (10%) and the lowest IGC sensitivity.
    Conductive property of Al-Er-Cu alloy for overhead wire
    Ma Xiaojie, Liu Lining, Chen Xiangguang, Zang Wei
    2021, 46(5):  156-159.  doi:10.13251/j.issn.0254-6051.2021.05.025
    Abstract ( 49 )   PDF (608KB) ( 28 )  
    Properties change rule of the Al-Er-Cu alloy during aging was studied via electrical conductivity and hardness testing. The precipitation and growth law of the precipitated phase in the alloy were observed by means of TEM and EDS. The results show that with the increase of aging temperature, both electrical conductivity and hardness peaks of the same alloy appear earlier on the whole. When aged at 300 ℃, there are a lot of dispersed nano-precipitates in the Al-Er-0.22Cu alloy, and the precipitated phases reinforce the alloy strength and enhance the electrical conductivity. When aged for 2 h, the electrical conductivity reaches the peak value of 60.15%IACS and the hardness peak of 43.1 HV0.05 achieves when aged for 10 h, so the Al-Er-0.22Cu alloy has both high electrical conductivity and good hardness.
    TMCP process of HSLA steel produced by FTSR technology
    Zhang Xing, Song Zhigang, Hou Mingshan, Li Hongwei, Dong Jiliang, Gao Xiaoyao
    2021, 46(5):  160-165.  doi:10.13251/j.issn.0254-6051.2021.05.026
    Abstract ( 56 )   PDF (612KB) ( 28 )  
    TMCP process was carried on 700 MPa grade HSLA steel which was produced by FTSR technology and the effect of different rolling and cooling methods on mechanical properties and microstructure of the experimental steel strip was analyzed. The results show that there is significantly positive relationship between the strength of HSLA steel and accumulative deformation in finishing rolling stage. Laminar flow cooling method has little effect on mechanical properties of the experimental steel strip, but the nonuniformity of mechanical properties along width direction can be improved by interval cooling method. As ending cooling temperature increases from 600 ℃ to 670 ℃, the microstructure of the steel strip coarsens, but the precipitated phases precipitate more sufficiently, which leads to growing strength of the steel strip.
    Effect of annealing temperature on residual stress, structure and corrosion resistance of magnetron sputtering TiN/TiCN film
    Yun Lu, Hao Xin
    2021, 46(5):  166-170.  doi:10.13251/j.issn.0254-6051.2021.05.027
    Abstract ( 65 )   PDF (607KB) ( 24 )  
    TiN/TiCN film was prepared on AZ31 alloy by reactive magnetron sputtering and the deposited film was subjected to vacuum stress relief annealing. X-ray fluorescence spectrometer (XRF), scanning electron microscope (SEM), grazing incidence X-ray diffraction(GIXRD) and electrochemical workstation were used to analyze the surface chemical composition, surface morphology, residual stress and corrosion resistance of all the films before and after annealing. The results show that the films are composed of TiCN and TiN with FCC structure. After annealing, the grain size and crystallinity of the films increase, the residual stress decreases significantly, the content of Ti, C and N elements in the electrochemical corrosion area decreases, the corrosion resistance of the film annealed at 250 ℃ is equivalent to that of the deposited film, and the corrosion resistance of the film annealed at 300 ℃ decreases.
    Microstructure and properties of nitrogen-containing corrosion-resistant die steel TM420
    Chen Jianli, Zhang Xiaokun, Chu Baoshuai, Zhang Jinwen
    2021, 46(5):  171-174.  doi:10.13251/j.issn.0254-6051.2021.05.028
    Abstract ( 96 )   PDF (618KB) ( 25 )  
    Hardness, grain size, microstructure, mechanical properties and pitting corrosion resistance of the nitrogen-containing corrosion-resistant die steel TM420 under different heat treatment conditions were studied. The results show that the microstructure of the TM420 steel after annealing is mainly ferrite+carbide, and generally does not contain δ ferrite. Compared with traditional corrosion-resistant die steel, the TM420 steel has better hardenability, and the pre-hardened specimen has better strength and toughness and pitting corrosion resistance. The TM420 steel has a wide quenching and tempering temperature range and excellent tempering stability, the recommended quenching temperature range is 980-1060 ℃, and the pre-hardening tempering temperature range is 580-650 ℃.
    Effect of solution treatment on corrosion resistance of ZG40Cr25Ni20Si2 heat-resistant steel
    Yang Yajie, Chang Yuan, Guo Yin, Zhou Yuning, Lin Bingtao, Yang Sanlong, Sun Xiaoxia
    2021, 46(5):  175-179.  doi:10.13251/j.issn.0254-6051.2021.05.029
    Abstract ( 61 )   PDF (612KB) ( 24 )  
    Effect of solution treatment(solution temperature of 1080 ℃, holding time of 90 min) on corrosion resistance of the ZG40Cr25Ni20Si2 stainless steel was studied by means of potentiodynamic polarization, electrochemical impedance spectroscopy, neutral salt spray test and photoelectron spectroscopy. The results show that under this solution treatment parameter, after the solution treatment, the corrosion potential of the steel decreases, the corrosion current increases, the capacitance arc size is obviously lower than that of the as-cast one, and the corrosion resistance decreases. In neutral salt spray test, there is no obvious corrosion morphology in the as-cast and solution treated specimens within 96 h. After 144 h, the mass loss of the solution treated specimens is slightly larger than that of the as-cast one, and corrosion is more likely to occur.
    Effect of nitrocarburing temperature on tribological properties of AerMet100 steel after low temperature ion nitrocarburing
    Zhang Hao, Li Changsheng, Gou Zhou, Ji Linlin
    2021, 46(5):  180-185.  doi:10.13251/j.issn.0254-6051.2021.05.030
    Abstract ( 66 )   PDF (608KB) ( 26 )  
    Effect of temperature on microstructure, surface hardness, cross-section hardness gradient and wear resistance of the nitrided layer and nitrocarburized layer of the AerMet100 steel by low-temperature ion nitriding and low-temperature ion nitrocarburizing was studied, and the wear mechanism of the infiltrated layer was also discussed. The results show that the compound formed on the nitrocarburized layer is smaller compared to the nitrided layer and its surface is more even and smoother. Both ion nitriding and ion nitrocarburizing treatments can significantly increase the surface hardness of AerMet100 steel. It is found that the thickness of the nitrocarburized layer also increases with the increase of temperature. The nitrocarburized layer has a lower friction coefficient than the nitrided layer. The specimen exhibits the lowest friction coefficient and wear rate when the nitrocarburizing temperature is 480 ℃, and shows excellent wear resistance. The wear mechanism of the nitrided layer is oxidative wear and surface fatigue wear, and that of the nitrocarburized layer is oxidative wear, abrasive wear and surface fatigue wear.
    Effect of pre-oxidation and rare earth cerium on plasma nitriding of 42CrMo steel
    Li Shuangxi, Chen Lin, Wang Meitao, Yan Zhiqiang
    2021, 46(5):  186-189.  doi:10.13251/j.issn.0254-6051.2021.05.031
    Abstract ( 63 )   PDF (609KB) ( 22 )  
    To solve the technical problems such as shallow nitrided layer and long production cycle, the commonly used structural steel 42CrMo was plasma nitrided by pre-oxidation and rare earth complex activated nitriding. The nitriding rate, the microstructure of the nitrided layer and surface morphologies were studied systematically by using microhardness tester, optical microscope (OM), and scanning electron microscope (SEM). The results show that the best nitriding effect can be realized by pre-oxidation at 400 ℃ for 1 h+complex activated nitriding process with 0.6 cm2/kg(cerium surface area/charging weight). Compared with that without activated nitriding, the optimized complex activated nitriding not only improves the nitriding efficiency but also eliminates vein-like nitride, reduces the hardness gradient of the nitrided layer.
    Effect of vacuum pressure on ion nitriding properties of small modulus spline
    Ma Xiaofeng, Ma Ning
    2021, 46(5):  190-192.  doi:10.13251/j.issn.0254-6051.2021.05.032
    Abstract ( 55 )   PDF (693KB) ( 25 )  
    In the process of ion nitriding of small modulus spline parts, the tooth root or node circle position is often difficult to be nitrided or their nitrided layer is too shallow. Aiming at this problem, ion nitriding test was carried out on the internal spline of typical rotor parts under the vacuum pressure of 300, 400, 500, 600 and 700 Pa. The results show that when argon gas is injected into the furnace and the vacuum pressure is 600 and 700 Pa, the nitrided layer deviation of tooth top, joint circle and tooth root of the internal spline can be controlled within 0.02 mm and meet the technical design requirement, at the same time, the microstructure and surface hardness also meet the process requirements.
    Spheroidizing annealing process of medium carbon non-alloyed structural steel S55C
    Shi Xiaofeng, Liu Ping, Li Bo, Zheng Wenchao, Li Bopeng, Pei Dandan, Lu Qiang
    2021, 46(5):  193-195.  doi:10.13251/j.issn.0254-6051.2021.05.033
    Abstract ( 83 )   PDF (609KB) ( 48 )  
    Different spheroidizing annealing processes of the S55C steel were carried out and the microstructure and hardness were studied. The results show that after holding at 740 ℃ for 5 h, slowly cooling to 700 ℃ for 5 h at a cooling rate of ≤20 ℃/h, and then slowly cooling to 680 ℃ for 5 h at a cooling rate of ≤40 ℃/h, and then air cooling, the microstructure of the S55C steel is spherical pearlite, the spheroidization rate is ≥90%, and the hardness value is 165 HBW, which meet the requirements of the customer.
    SURFACE ENGINEERING
    Microstructure and wear resistance of laser clad Fe/NiCr-Cr3C2 composite coating
    Xia Tongchuan, Liu Ting, Zhang Lin, Fang Zhao, Li Mingxi
    2021, 46(5):  196-201.  doi:10.13251/j.issn.0254-6051.2021.05.034
    Abstract ( 55 )   PDF (613KB) ( 21 )  
    Fe/NiCr-Cr3C2 composite coating was prepared on GCr15 steel surface by laser cladding, and the effect of NiCr-Cr3C2 on microstructure and wear resistance of the composite coating was investigated. The results indicate that the microstructure of the composite coating refines gradually with the increase of NiCr-Cr3C2 content from 0 to 20%. With addition of NiCr-Cr3C2 into the Fe-base alloy, the residual austenite content increases in the composite coating, while the α-Fe phase content decreases, resulting in the decrease of cross-section microhardness. The composite coating added with 10% NiCr-Cr3C2 exhibits optimal wear resistance, and reduces the wear loss and friction resistance, which can be attributed to the formation of many hard carbide phases (Cr3C2 and Cr23C6) and oxide lubricating phase during wear process.
    Microstructure and properties of laser clad Al-Si coating on AZ33M magnesium alloy
    Wang Xin, Pan Xide, Niu Qiang, He Xiangqian
    2021, 46(5):  202-206.  doi:10.13251/j.issn.0254-6051.2021.05.035
    Abstract ( 61 )   PDF (617KB) ( 31 )  
    Al-Si coating was prepared on the surface of AZ33M magnesium alloy by laser cladding technology. The microstructure and properties of the clad layer were characterized through corrosion electrochemical method test combined with X-ray diffractometer (XRD), scanning electron microscope (SEM), microhardness tester, etc.. The results show that the clad layer is mainly composed of Mg matrix, Mg17Al12, Mg2Si, and Mg2Al3 phases. The microstructure is composed of columnar dendrites and dendrites with different directions. Due to second phase strengthening and fine-grain strengthening, the prepared Al-Si coating has higher hardness compared with magnesium alloy matrix. Compared with the matrix, the self-corrosion potential of the clad layer is increased by 400 mV, the self-corrosion current is reduced by an order of magnitude, and the corrosion resistance of the clad layer is significantly better than that of the magnesium alloy matrix.
    Path selection and quality analysis of laser clad CoCrMo strengthened coating on titanium alloy surface
    Shang Peng, Li Jingman, Zhang Yihai, Zhang Jie, Xia Lei, Zhang Dawei
    2021, 46(5):  207-212.  doi:10.13251/j.issn.0254-6051.2021.05.036
    Abstract ( 57 )   PDF (614KB) ( 23 )  
    In view of the poor surface hardness and wear resistance of titanium alloy, commonly used as artificial bone material in biomedicine, a new type of high hardness wear-resistant clad layer was prepared on pure titanium plate by laser cladding with CoCrMo powder. In order to ensure the clad layer quality, three different scanning paths were designed, including unidirectional overlapping, rectangular nested overlapping and circular spiral overlapping. The results show that through the micro and macro structure comparison of laser clad specimens under three different scanning paths, it is found that the surface quality of laser clad specimens obtained by unidirectional overlapping is the best, and the microstructure of which is the best. Therefore, the best specimens prepared by unidirectional overlapping were selected to further test and analyze the hardness and wear resistance. It is found that compared with the substrate, the hardness of the clad layer is significantly improved by 5-6 times of that of the substrate, and the wear resistance is also improved significantly.
    Effect of laser power on microstructure and mechanical properties of high-speed laser clad Ni/316L layer
    Liu Delai, Wang Bo, Zhou Panhu, Dong Hui, Bao Zhengyu
    2021, 46(5):  213-217.  doi:10.13251/j.issn.0254-6051.2021.05.037
    Abstract ( 55 )   PDF (617KB) ( 28 )  
    In order to evaluate the effect of laser power on microstructure and mechanical properties of the Ni/316L layer deposited via high-speed laser cladding, the Ni/316L layer was clad on the Q235 steel surface under laser power of 1.1, 1.3, and 1.5 kW respectively. The microstructure, Vickers hardness, impact property and elastic modulus of the laser clad layers were examined by means of scanning electron microscope (SEM), energy dispersive spectrometer (EDS), Vickers hardness tester, impact testing machine and Knoop indentation method. The results show that when the laser power is 1.1 kW, the grain growth in the laser clad layer contains perpendicular-to-interface growth mode and slant growth mode, the clad layer also has a small amount of pores and other defects, and the elements at the clad layer/substrate interface are distributed in a cliff-like manner, which means that the element diffusion between the substrate and the clad layer is limited. As the laser power increases, the perpendicular-to-interface growth mode of grains becomes the main mode, and the porosity of the clad layer is lower and the defect is less, the interdiffusion degree of elements at the clad layer/substrate interface is improved. The laser power has a significant effect on hardness and impact property of the clad layer, but has little effect on elastic modulus. The maximum hardness of the clad layer with different laser power is always about 100 μm away from the interface. When the laser power increases from 1.1 kW to 1.5 kW, the maximum hardness of the clad layer fusion zone is increased by about 50%, and the impact property of the clad layer specimen is about 20% higher than that of the substrate when the laser power is 1.5 kW.
    Effect of CO2 atmosphere and spraying distance on microstructure and corrosion resistance of arc sprayed Fe-based amorphous coating
    Tian Fang, Ji Xiulin, Zhao Lijuan, Zhao Jianhua
    2021, 46(5):  218-223.  doi:10.13251/j.issn.0254-6051.2021.05.038
    Abstract ( 60 )   PDF (624KB) ( 26 )  
    To increase the amorphous content and corrosion resistance of arc sprayed Fe-based amorphous coating, CO2 atmosphere was used instead of air for arc spraying and the effect of spraying distance on microstructure and corrosion resistance of the coatings was studied. The results show that the thickness of the coating is about 200 μm and the coating consists of crystalline phase and amorphous phase. Compared with air, the CO2 inhibits the crystallization, which leads to the amorphous content increase in the coating. Meanwhile, the spraying distance has a great influence on microstructure and corrosion resistance of the coating. With the increase of spraying distance, the amorphous content decreases. When the spraying distance is 100 mm, the coating has the highest open circuit potential of -0.498 V(vs SCE) and the smallest self-corrosion current density of 4.281 μA/cm2, the best corrosion resistance can be obtained. The corrosion resistance of the coating is the worst when the spraying distance is 150 mm. The increase of amorphous content and the uniformity of the microstructure can enhance the corrosion resistance of the coating.
    Silica sol sealing process and properties of micro-arc oxidation film on 7075 aluminum alloy
    Li Jun, Jiang Bo, Wang Chao, Song Renguo
    2021, 46(5):  224-228.  doi:10.13251/j.issn.0254-6051.2021.05.039
    Abstract ( 64 )   PDF (614KB) ( 19 )  
    Silica sol sealing process was carried out for the micro-arc oxidation film on 7075 aluminum alloy. The microscopic surface morphology, cross-sectional morphology, phase composition, wear resistance and corrosion resistance of the micro-arc oxide film before and after the silica sol sealing were analyzed by scanning electron microscope, X-ray diffractometer, high temperature friction and wear test machine and electrochemical workstation. And the influence of different silica sol-gel sealing processes on the sealing effect was investigated. The results show that compared with the unsealed micro-arc oxidation film, the micro-holes in the micro-arc oxidation film after sealing are obviously reduced, the roughness of the film is reduced, and the wear resistance is improved. The corrosion potential of the micro-arc oxidation film after sealing is positively shifted from -0.62 V(vs SCE) to -0.36 V(vs SCE), the corrosion current density drops from 2.44×10-3 A·cm-2 to 2.03×10-4 A·cm-2, and the corrosion resistance of the micro-arc oxidation film is significantly improved after sealing. However, the corrosion resistance of the micro-arc oxidation film sealed by the dipping and pulling method is better than that sealed by the spin coating method.
    Preparation and properties of laser remelted Ni60/50%WC composite coating
    Zhang Leitao, Li Haitao, Jia Runnan, Wang Zhe, Fan Shuaiqi, Wang Xuesong, Dai Jiaoyan, Xu Jinfu
    2021, 46(5):  229-234.  doi:10.13251/j.issn.0254-6051.2021.05.040
    Abstract ( 63 )   PDF (618KB) ( 35 )  
    To eliminate defects such as cracks and holes in the Ni60/50%WC composite coating, laser cladding+remelting processing were carried out on the 45 steel surface by using CO2 laser device. The coating cracks were visualized by using commercial colored flaw detection agent, and the microstructure of coating was analyzed by means of X-ray diffraction (XRD) and scanning electron microscope (SEM/EDS). The microhardness and wear resistance of the coating were examined by micro hardness tester and friction and wear tester. The results show that the laser remelting can achieve the goal of secondary slag discharging, crack healing and surface roughness improvement. The crack-free coating can be obtained by laser cladding with laser power of 3.0 kW, scanning rate of 350 mm/min, overlap rate of 50%, powder feeding rate of 5 g/min, combined with laser remelting with remelting power of 1.5 kW, scanning rate of 300 mm /min, and remelting overlap rate of 50%. Laser remelting can improve the non-uniformity of coating microstructure and enhance the crystallinity and density. Before and after laser remelting, the average microhardness of the composite coating is 740.07 and 700.02 HV0.2, the average friction coefficient is 0.475 and 0.462, and the wear rate is 4.223×10-15 and 4.874×10-15 m3/(N·m), in each respectively.
    TEST AND ANALYSIS
    Influence mechanisms of instantaneous overheating on fracture and precipitate distortion of high chromium nickel austenitic steel
    Che Pengcheng, Shao Wenzhu, Cheng Yi, Wang Shuo, Xu Hang, Li Xianshuang, Tan Shuping
    2021, 46(5):  235-241.  doi:10.13251/j.issn.0254-6051.2021.05.041
    Abstract ( 54 )   PDF (616KB) ( 27 )  
    Cracks were found in the sealing weld between the reheater tube (22Cr-15Ni steel) and the sleeve (18Cr-8Ni-Nb steel) during the test tube screen pressure test in a power plant. The cracking cause was studied through macro and micro methods, including macro-analysis of the sample tube, metallographic analysis of the crack, and SEM/EDS analysis of the fracture and precipitates. The results indicate that near the failure site there are obvious traces of mechanical stress input and weld stress concentration, meanwhile the welding heat input causes local microstructure overheating, and the abnormal growth of M23C6 globular precipitate causes microscopic tensile stress. Finally, the combination of the above reasons causes that the mechanical stress is input from the stress concentration to produce microcracks that propagate along the overheating-weakened grain boundary. Such cracks coalesce with the microcracks generated by the matrix precipitates and eventually propagate into a circumferential fracture. The wind with pressure erupts and leads to the distortion and fragmentation of the globular precipitates, finally pits on surface and internal dendritic crystal of M23C6 globular precipitates come to appear.
    Determination of decarburization depth of steels by EPMA mapping analysis method
    Yan Chunlian, Ju Xinhua, Yao Wugang, Jia Huiping, Wen Juan
    2021, 46(5):  242-247.  doi:10.13251/j.issn.0254-6051.2021.05.042
    Abstract ( 63 )   PDF (620KB) ( 40 )  
    EPMA mapping analysis method was proposed to determine the depth of decarburization of steels. The determination of the depth of decarburization was completed on three typical steels such as hot rolled 75Cr1 high carbon tool steel, 20CrMnTi quenched gear steel and cold rolled and annealed SH1100 low-carbon carriage plate steel respectively. The results show that for the medium and high carbon steel specimen with pearlite plus ferrite microstructures, the value of depth of decarburization provided by micrographic method is about 77% of that by EPMA mapping analysis method. For the determination of decarburization depth of quenched and tempered steels, the accuracy degree of EPMA mapping analysis is less than 10 μm, which is one tenth of the micro-hardness testing. For low carbon steels which are difficult to determine the depth of decarburization generally, the measurement values of EPMA mapping analysis are accurate considering that the depth of decarburization is determined based on the transition curve of average carbon content over a large zone of the steel specimen. EPMA mapping analysis method deserves to be applied extensively with apparent advantages such as unlimited steel microstructure, loose requirement for sample shape, good data stability, high accuracy and easy to operate, etc.
    Cracking failure analysis of 20Cr1Mo1VNbTiB steel steam turbine valve bolts in a power plant
    Li Ge, Guo Yanjun, Wang Lu, Zhu Haibao, Tian Linan
    2021, 46(5):  248-252.  doi:10.13251/j.issn.0254-6051.2021.05.043
    Abstract ( 52 )   PDF (616KB) ( 37 )  
    Reasons of the cracking failure were analyzed for 20Cr1Mo1VNbTiB steel high pressure speed regulating valve bolt and medium pressure combination valve bolt in the steam turbine of a power plant. The results show that the chemical composition and hardness of the two bolt specimens are in accordance with the relevant standards. In the metallographic structure of the two bolt specimens, there are localized mixed grains and a large number of hard and brittle MC-type second phase particles, the latter of which can act as the crack nucleation sites, thus increasing the crack initiation probability, accelerating the crack propagation of the material, and increasing the crack failure tendency of the material. Such precipitated phases have high stability and are difficult to be eliminated by heat treatment, and are usually formed in the smelting process. Thus the smelting process control should be strengthened to avoid the occurrence of a large number of such hard and brittle MC phases.
    Fracture cause analysis of 30CrMnSiA steel bolt for motor fixing
    Huang Xin, Zhou Jiguang, Fang Zhixiao, Wu Wei, Wen Yonghong
    2021, 46(5):  253-258.  doi:10.13251/j.issn.0254-6051.2021.05.044
    Abstract ( 56 )   PDF (620KB) ( 35 )  
    A 30CrMnSiA steel bolt for fixing a motor was fractured in service. The fracture cause was analyzed through load analysis, bolt appearance and fracture morphology observation, microstructure examination, chemical composition analysis and hardness measurement. The results show that the fracture is fatigue type, the machining damage nearby the thread root is the source of fatigue crack, and the decarburization on the bolt surface accelerates the fatigue fracture.
    Failure analysis of main shaft fracture of wind power
    Yan Youzeng, Meng Gaoqiang, Cao Qi, Zhang Hao, Xia Guofeng
    2021, 46(5):  259-261.  doi:10.13251/j.issn.0254-6051.2021.05.045
    Abstract ( 54 )   PDF (916KB) ( 47 )  
    Failure analysis of main shaft fracture of wind power was studied. Through the analysis of designing, manufacturing and applying of the main shaft, it is found that there are multiple spot and dense over-standard defects(maximum defects of ϕ(7.0-14.2) mm), dendritic segregation, and the microstructure is not quenched and tempered structure. The results show that these defects are produced during steelmaking, forging and heat treatment, which cause the brittle fracture of the main shaft.
    Cause analysis and prevention of edge cracks in cold-rolled 65Mn wide steel strip
    Jin Yajun, Wang Chuang, Dou Xiaoyao, Huang Aijun, Huang Yongheng
    2021, 46(5):  262-266.  doi:10.13251/j.issn.0254-6051.2021.05.046
    Abstract ( 106 )   PDF (615KB) ( 25 )  
    Causes of edge cracks in cold-rolled 65Mn wide steel strip were studied by metallographic observation and hardness test. The results show that the microstructure of the hot-rolled steel strip is ferrite and pearlite, the pearlitic structure in the edge region is relatively thinner than that in the middle region due to faster water-cooling rate. During the intercritical spheroidizing annealing process, these finer pearlites become brittle coarse pearlite more easily, and under the condition of relatively large cold rolling reduction, the edge cracks appear. By substituting the intercritical spheroidizing annealing to subcritical spheroidizing annealing, the microstructure spheroidization rate at the edge of the 65Mn steel strip is significantly increased, and basically avoiding the occurrence of edge cracks, so that providing positive guidance for the industrial production.
    COOLING TECHNIQUE
    Effect of water atomization quenching on thin steel sheet 35CrMnSiA
    Fan Zhiyang, Li Xianjun, Liu Junjie, Hu Bo, Zhou Tong, Hao Yuan
    2021, 46(5):  267-271.  doi:10.13251/j.issn.0254-6051.2021.05.047
    Abstract ( 51 )   PDF (615KB) ( 23 )  
    A set of experimental equipment for atomized water spray quenching was designed to study change of the temperature drop curves of thin steel sheet 35CrMnSiA under the vertical state during spray quenching, and by using Inverse Heat Transfer module in the DEFORM software to complete the solution of the comprehensive heat transfer coefficient on the tested steel sheet. The results show that, when the tested steel sheet meets as-follow conditions, with the nozzle size of 5 mm, distance from nozzle to the sheet surface is 300 mm, and the nozzle pressure is 0.1, 0.2 and 0.3 MPa respectively, the surface heat transfer coefficient of that is correspondingly lower in the high temperature zone above 450 ℃ and the low temperature zone below 200 ℃. But the tested steel sheet can obtain a higher surface heat transfer coefficient in the middle temperature zone. Compared with the nozzle pressure of 0.1 MPa, the tested steel sheet of that has a significant improvement under the nozzle pressure of 0.2 MPa. While increasing the nozzle pressure to 0.3 MPa, the cooling capacity has little improvement.
    Influence of oil pollution on cooling characteristics of PAG water-based quenchant
    Liu Chengmiao, Dong Lihua, Li Qinghong, Zhu Hongling, Ren Lin
    2021, 46(5):  272-275.  doi:10.13251/j.issn.0254-6051.2021.05.048
    Abstract ( 48 )   PDF (613KB) ( 24 )  
    In order to explore the influence of oil pollution on cooling performance of water-based quenchant, the cooling rate curve and hardening capacity of quenchant with different oil contents were tested. The mechanism of oil pollution influencing the quenchant cooling performance was discussed by analyzing the specific quenching process and characteristic value function. The results show that when the oil content in the quenchant exceeds 0.3%, the cooling performance of the quenchant is significantly worse, specifically, the boiling stage is delayed, the cooling rate decreases obviously at high temperature stage and the hardening capacity of the quenchant is also reduced. But the content of oil has little effect on the quenching distortion of workpiece.