Heat Treatment of Metals

Intergranular corrosion susceptibility of isothermally deformed 0Cr14Mn21NiN non-magnetic stainless steel

Qu Huapeng, Wang Liwei, Feng Hanqiu, Peng Shengtong, Chen Haitao, Luo Dingxiang, Lang Yuping

2020, 45(8): 1-6. doi:10.13251/j.issn.0254-6051.2020.08.001

Abstract ( 229 )

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The equilibrium phase regions of carbide precipitation in the 0Cr14Mn21NiN non-magnetic stainless steel (NMSS) were calculated by Thermal-Calc software, and the result was verified by aging treatment experiment. The real non-equilibrium carbide precipitation behavior was researched after isothermal deformation (ITD) experiment with different parameters. The intergranular corrosion (IGC) susceptibility of ITD specimens was tested and compared by double loop electrochemical potential-dynamic reactivation (DL-EPR) method. The results show that the Thermal-Calc calculated equilibrium Cr₂₃C₆ precipitation temperature region is from 512 ℃ to 847 ℃, while the real region obtained from the aging experiment is from 720 ℃ to 940 ℃, indicating a distinction of approximately 200 ℃. When the total deformation is less than 20%, the lower temperature limit of non-equilibrium carbide precipitation of ITD specimens is descended to 625 ℃, and both the carbide nucleation and growth are accelerated after the ITD process. The DL-EPR results show that the IGC susceptibility of the 0Cr14Mn21NiN non-magnetic stainless steel is obviously increased after the ITD process due to the accelerated carbide precipitation.

Effect of inclusions on fatigue life of engineering machinery structural steel Q345FCA

Li Mingyang, Wang Fuming, Li Ming, Gong Shuo

2020, 45(8): 7-11. doi:10.13251/j.issn.0254-6051.2020.08.002

Abstract ( 170 )

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Fatigue life of the high fatigue life Q345FCA steel for engineering machinery was determined by using QBWP-6000J rotary bending fatigue testing machine for simply supported beams. The fatigue fracture morphology was observed by scanning electron microscope (SEM) and the inclusions on fracture surface were searched by energy dispersive spectrometer (EDS). The inclusions were analyzed by inclusion automatic analysis system, and the critical size of inclusions was calculated through experimental data analysis. The results show that the fatigue strength of Q345FCA steel and Q345 steels is 273 MPa and 266 MPa, respectively. The size of the inclusions in the Q345FCA steel and Q345 steel is all small, which is smaller than the critical size of the inclusions, and the fracture morphologies show that all the fatigue fractures are not caused by inclusions, which are not the source of fatigue.

Internal friction analysis on austenitizing process of E690 steel

Du Xiaojie, He Yizhu

2020, 45(8): 12-16. doi:10.13251/j.issn.0254-6051.2020.08.003

Abstract ( 124 )

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Effect of austenitizing temperature and holding time on carbides dissolved amount of E690 steel was studied by means of measuring internal friction, hardness testing and microstructure analysis. The results show that, when the austenitizing temperature is lower than Ac₃ point, the strength of SKK peak can be significantly increased with the increase of temperature, and the dissolved amount of carbides is also increased. When the austenitizing temperature is over the Ac₃ point, the strength of SKK peak does not change obviously with the increase of temperature and holding time, the degree of carbide dissolution in the steel is relatively high. Besides, the structure of coarsened martensite can be observed by the process of an extended holding time. The carbides in the steel can completely be dissolved during austenitizing for 5 min, in which the microstructure is a martensite with a relatively small distribution of strips. Therefore, it is feasible that the efficient of quenching and tempering is improved through selecting shorter holding time when the temperature is appropriately over Ac₃ point.

Effect of thermal fatigue on microstructure and mechanical properties of B/M multiphase H13 steel

Li Yangcheng, Cheng Xiaonong, Luo Rui, Wei Jiabo, Zhang Jie, Gao Tianming

2020, 45(8): 17-21. doi:10.13251/j.issn.0254-6051.2020.08.004

Abstract ( 348 )

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In order to improve its thermal fatigue property, H13 steel was B/M multiphase heat treated to obtain a tempered sorbite+bainite multiphase microstructure. The effect of thermal fatigue on the microstructure, carbide and mechanical properties of the B/M multiphase H13 steel was investigated by means of OM, SEM, TEM, Rockwell hardness tester and universal tensile testing machine. The results show that contrast to the phenomenon of rapid decrease of hardness with the increase of fatigue circle for the traditional H13 steel, the hardness of B/M multiphase H13 steel decreases first and then rises and then decreases with the increase of fatigue circle. The tensile experiments show that the strength-plasticity cooperating of the B/M multiphase H13 steel is better than that of the traditional H13 steel, and its percentage reduction of area increases after the thermal fatigue. It is found from microscopic observation that the main reason for the change in mechanical properties is the bainite lath merging and carbide precipitation and coarsening in the B/M multiphase H13 steel during thermal fatigue.

Microstructure dynamic evolution of 17-4PH stainless steel aged at 480 ℃

Xie Wenfei, Wu Wenyun, Wang Donghong, Yang Lianjin, Wu Riming

2020, 45(8): 22-26. doi:10.13251/j.issn.0254-6051.2020.08.005

Abstract ( 124 )

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17-4PH stainless steel forged rod was solid solution treated with oil cooling, and aged at 480 ℃ for 0-5 h with air cooling. Microstructure evolution and mechanism of precipatate hardening in the aging process were observed and analyzed by means of optical microscopy (OM), ultra-depth-of-field microscopy, XRD and microhardness tester. The effect of dynamic aging precipitation process of ε-Cu phase on electric resistance was indirectly tested by resistor, and the wear resistance was tested by friction and wear tester. The results show that there are no retained austenite and reversed austenite observed in the solid solution and aging process of the 17-4PH stainless steel. After heat treatment, there are two martensite morphologies i.e. lath and block. The hardness of lath martensite is higher than that of block one. With the prolongation of aging time, the hardness of both martensite increases synchronously. Aging precipitation obviously increases the hardness of solution treated structure. Aging for 2.0-2.5 h, the hardening effect and wear resistance are the weakest in the vicinity, which may be related to the growth of ε-Cu phase and the interaction with dislocation. Meanwhile, the change trend of hardness is opposite to that of resistance.

Effect of AlMnSi dispersion particle size on recrystallization of 6061 aluminum alloy during homogenization

Qiu Chu, Guo Shijie, Ji Yanli

2020, 45(8): 27-33. doi:10.13251/j.issn.0254-6051.2020.08.006

Abstract ( 109 )

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Effect of the size of AlMnSi dispersion particles on recrystallization of 6061 aluminum alloy during homogenization was studied by using mechanical property test, OM, SEM, TEM and EDS techniques. The results show that the BBC type AlMnSi dispersion particles are precipitated and grown up along the [001] crystal axis of the aluminum matrix. With the increase of homogenization temperature, the precipitation size of dispersion particles is gradually increased. When the average size and the maximum size of AlMnSi dispersion particles respectively increasing from 0.07 μm to 0.42 μm and from 0.21 μm to 1.12 μm, the mechanical properties are increased first and then decreased correspondingly. At T6 state, the 6061 aluminum alloy is still of deformed structure. When the AlMnSi dispersion particle size is increased to 1.12 μm, coarse recrystallization microstructure appears in the alloy at T6 state, resulting that the mechanical properties are decreased sharply.

Microstructure and anisotropy of mechanical properties of Ti70 alloy plates

Hao Xiaobo, Li Bobo, Liu Yinqi, Zhang Qiang, Li Yang

2020, 45(8): 34-37. doi:10.13251/j.issn.0254-6051.2020.08.007

Abstract ( 89 )

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Microstructure and anisotropy of mechanical properties of Ti70 alloy plates were studied by tensile test, low temperature impact test, optical microscopy, electron microscopy and X-ray diffraction analysis. The Results indicate that there is no obvious difference in microstructure between hot rolled and annealed Ti70 alloy plates. Recovery is the main process during annealing, with only a certain amount of recrystallized grains being found in the shear band with high density dislocations. The yield strength and low temperature impact absorbed energy of the transverse direction are higher than that of the rolling direction, whereas the tensile strength is just the opposite, indicating that the annealed Ti70 alloy plate shows significant anisotropy. After annealing the Ti70 alloy plate forms relatively strong {0002} basal texture, and its crystal plane normal direction deflects ±30° and ±41° respectively along the RD direction (longitudinal) and the TD direction (transverse). Basal texture is relatively concentrated in the rolling direction, which results in the anisotropy of mechanical properties of the Ti70 alloy plate.

Austenite grain growth behavior of 38CrMoAl steel

Liu Hao, Zhang Liwen, Li Fei, Zhang Chi, Mao Peigang, Li Chaoqun

2020, 45(8): 38-42. doi:10.13251/j.issn.0254-6051.2020.08.008

Abstract ( 113 )

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38CrMoAl steel was austenitized by heating to 1000-1200 ℃ for 0-300 s by different process configurations. The effect of austenitizing temperature and holding time on the austenite grain growth behavior was studied. The results show that the average grain size of austenite in the tested steel increases with the increase of austenitizing temperature, and the grain growth rate increases with the increase of temperature. At the same austenitizing temperature, the average grain size of austenite increases with the increase of holding time, and the grain growth rate decreases with the increase of holding time. According to the test data of austenite grain size of the tested steel, the Sellars model of the austenite grain size and austenitizing temperature and holding time of the 38CrMoAl steel was established, and the accuracy of the model was verified.

Bake hardening behavior of transformation induced plasticity steel TRIP590

Wang Wenying, Wang Hui, Qu Shiyong

2020, 45(8): 43-46. doi:10.13251/j.issn.0254-6051.2020.08.009

Abstract ( 68 )

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Influence of pre-strain and bake praess on the bake hardening(BH) behavior of TRIP590 steel was investigated by using tensile tester, SEM and TEM. The results indicate that the tested steel microstructure is composed of 75.5%ferrite+15.0%bainite+9.5%retained austenite (volume fraction). The BH value is increased with the increase of pre-strain, and the increase in the early stage is larger. The BH value is increased from 22 MPa to its maximum value 88 MPa when the strain is increased from 2% to 8%. The BH value shows a trend of increasing with the increase of both the bake time and the bake temperature. Variation of the BH value is consistent with that of internal friction, confirming the mechanism of Cottrell nailing. After 2% pre-strain+baking, the pinning phenomenon of dislocation by solution atoms can be found in the matrix.

Retained austenite evolution behavior during unidirectional tension of low carbon medium manganese steel after intercritical rolling

Zhang Rui, Cao Wenquan, Hu Jun

2020, 45(8): 47-49. doi:10.13251/j.issn.0254-6051.2020.08.010

Abstract ( 80 )

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Microstructure of a low carbon medium manganese steel after rolling under different pre-stretch amounts was analyzed, and the retained austenite evolution during unidirectional tension was investigated. The results show that transformation of retained austenite to martensite is mainly occurred in the yield stage with local plastic deformation, and is basically completed when the yield stage ends with the specimens getting into the uniform plastic deformation stage. A large number of dislocations are partially accumulated and migrated gradually during the tension, showing as the generation and migration of the Luders band macroscopically, and inducing the transformation of residual austenite to martensite in local microstructure. This phenomenon mostly occurs along the rolling direction and can't occur repeatedly.

Hydrogen embrittlement behavior of oilfield-grade Inconel 718 alloy laser welded joint

Zeng Qiang, Wu Ying, Xiao Huijin, Zhu Shaowei

2020, 45(8): 50-55. doi:10.13251/j.issn.0254-6051.2020.08.011

Abstract ( 66 )

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The oilfield-grade Inconel 718 alloy, which was respectively heat treated by 1030 ℃×2 h solution treatment+780 ℃×8 h aging and 1030 ℃×2 h solution treatment+780 ℃×16 h aging, was welded by laser beam welding process. The hydrogen embrittlement behaviors of the base metal and welded joint of the alloy were studied during in situ hydrogen charging by analyses of microstructure, tensile properties and fracture morphologies. The results show that longer time aging induces large amount of δ phase, resulting in the hydrogen embrittlement susceptibility index of the base metal increasing from 0.27 to 0.48. The hydrogen embrittlement susceptibility of the welded joint is not affected by the heat treatment before welding, which is 0.39 and 0.38, respectively. However, the precipitation of Laves phase in welded zone induces that the hydrogen embrittlement susceptibility of the welded joints is higher than that of base metal aged for 8 h.

Effect of Sc element on microstructure and properties of selective laser melted TiB₂/AlSi10Mg composite

Liu Yuxuan, Wang Richu, Cai Zhiyong, Zhou Zhaohui, Li Xiaogeng, Cao Xuanyang

2020, 45(8): 56-63. doi:10.13251/j.issn.0254-6051.2020.08.012

Abstract ( 73 )

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TiB₂/AlSi10Mg composite modified by rare earth element Sc was prepared by selective laser melting (SLM). Effect of element Sc on the microstructure, density, and properties of the composite was studied by using field emission scanning electron microscope, electron probe microanalyzer, micro-hardness tester and electronic universal testing machine. The results show that compared with TiB₂/AlSi10Mg composite, the addition of rare earth Sc can further refine the Al-Si eutectic, resulting in fine-grained strengthening and dispersion strengthening. The ultimate tensile strength and microhardness of TiB₂/AlSi10MgSc composite are increased by 56.7 MPa (14.4%) and 15.3 HV0.1 (11.3%), respectively. The hardness and the strength of the selective laser melted TiB₂/AlSi10MgSc composite are gradually decreased with the increase of solution temperature, but its elongation is significantly improved.

Dimension effect of Ti powder size on evolution of microstructure and mechanical properties of CNT/Al composites

Wu Yaosha, Zhang Yamin, Wang Lirong, Shi Peng, Zhang Ning, Liu Xiaoqing

2020, 45(8): 64-69. doi:10.13251/j.issn.0254-6051.2020.08.013

Abstract ( 63 )

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CNT/Al composites were prepared by powder metallurgy with high density and strength, the effects of different sizes of titanium powder on microstructure and mechanical properties of the composites were systematically studied. The results show that mechanical properties of the CNT-Ti/Al composites are inversely proportional to the particle size of the Ti powders. The CNT-Ti/Al composite rod has best mechanical properties with added Ti powder with particle size of 80 nm. These results can be attributed to the Ti powder not only circumvents CNTs dispersion burden, but also plays as second phase particles that enhance the strength of matrix. Besides, owing to the heat reaction during the preparation process, a core-shell structure is formed, which improves the interface bonding and the load transfer efficiency of CNTs.

Dynamic constitutive model of 1000 MPa high-fragmentation steel

Yan Yongming, Zhang Ling, Wang Qian, Yuan Wufeng, Yu Wenchao, Wang Maoqiu

2020, 45(8): 70-75. doi:10.13251/j.issn.0254-6051.2020.08.014

Abstract ( 73 )

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Microstructure, static and dynamic mechanical properties of three different high-fragmentation steel were studied by means of SEM, Hopkinson pressure bar and tensile testing machine and other instruments. Three Johnson-Cook dynamic constitutive models for three 1000 MPa high-fragmentation steels were fitted. The results show that the three experimental steels have obvious strain hardening properties. When the strain rate is increased from 0.001 s^-1 to 8000 s^-1, the yield strength of No.1 experimental steel is increased by 546 MPa; the yield strength of No.2 experimental steel is increased by 434 MPa; the yield strength of No.3 experimental steel is increased by 667 MPa. The three experimental steels also have obvious temperature effects. The residual carbides in the microstructure of No.1 steel increase the Peirls-Nabarro stress and hinder the dislocation slip. At 500 ℃, the yield strength of the No.1 experimental steel is 450 MPa, which is significantly higher than that of the No.2 and No.3 steels, 330 MPa and 310 MPa.The Johnson-Cook equations of the three experimental high-fragmentation steels are:σe=(1008+1309.04(εp)^{0.679 97})(1+0.1498lnε·^*)(1-T^*^{1.188 53});σe=(1000+1214.321(εp)^0.6112)(1+0.480 15lnε·^*)(1-T^*^{1.263 05});σe=(1008+1334.871(εp)^{0.610 88})(1+0.116 18lnε·^*)(1-T^*^{0.992 47}).

Effect of Si content and heat treatment on microstructure and properties of as-cast Al-Si alloy

Tang Peng, Liu Yiyuan, Zhao Yanjun, Hu Zhiliu, Wang Huachun, Peng Linxin, Deng Songyun

2020, 45(8): 76-81. doi:10.13251/j.issn.0254-6051.2020.08.015

Abstract ( 87 )

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In order to improve its mechanical properties, the effect of Si content and heat treatment on the microstructure and properties of the as-cast Al-Si alloy was studied systematically. The results show that the effect of Si content, cooling medium, solution treatment temperature, aging time and aging temperature on the hardness of the alloy decreases in turn. The optimal Si content and the optimal heat treatment process are 12% Si and solid solution treating at 480 ℃ for 120 min+salt bath cooling+aging at 175 ℃ for 90 min, respectively. With the increase of Si content, the hardness and tensile strength of the alloy increase; but when the Si content is too high, the precipitation of block-shaped primary silicon tends to form stress concentration then reduces the strength of the alloy, and the fracture mode changes from ductile fracture to cleavage fracture. The eutectic silicon in as-cast Al-Si alloy is long needle like. After using the optimal heat treatment, the long needle-like silicon transforms into short rod or granular shape, and the eutectic silicon is more dispersed, the stress concentration decreases during tension, and the dislocation movement resistance increases, all of which improve the mechanical properties of the Al-Si alloy.

Effect of alloy elements Mn and Cr on microstructure and properties of SWRCH45K steel

Wang Dongchen, Wang Lijun, Guo Juncheng, Ruan Shipeng, Li Shilin

2020, 45(8): 82-85. doi:10.13251/j.issn.0254-6051.2020.08.016

Abstract ( 408 )

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Effect of Mn and Cr content on the microstructure and properties of SWRCH45K steel was studied. The steel was quenched by heating at 860 ℃+oil or water cooling, in addition, the hardness after quenching was simulated by using simulation software. The results show that with the increase of Mn and Cr content, the tensile strength of the tested steel is increased. The oil quenching microstructure of the tested steel consists of ferrite+troostite, with the ferrite fraction is slightly higher in the core of the specimen. The water quenching microstructure near the surface of the tested steel consists of martensite+a little troostite, while the troostite in the core of the specimen decreases with the increase of Mn and Cr content. The hardness of the tested steel after oil quenching changes slightly along the diameter direction, and the tested steel specimens with 0.70%Mn-0.04%Cr have the lowest hardness which is within 22-24 HRC and markedly lower than that of other specimens. The near-surface hardness difference among the water quenched specimens is relatively small, while the core hardness decreases obviously. It is found from comparison that the simulation hardness values are in good agreement with the near-surface hardness of the quenching specimens.

Effect of quenching temperature on microstructure and mechanical properties of high-boron Fe-based alloy for roll

Liang Yao, Yu Xiaohua, Chong Xiaoyu, Yuan Zhentao

2020, 45(8): 86-90. doi:10.13251/j.issn.0254-6051.2020.08.017

Abstract ( 71 )

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Effect of quenching temperature on the microstructure and mechanical properties of high-boron Fe-based alloy for roll was studied by means of optical microscope (OM), scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), pendulum impact tester and Vickers hardness tester. The results show that the hardened matrix of high-boron Fe-based alloy is martensite, while the boron carbides in the matrix, such as fishbone (M₂(B, C)), lamellar (M₂C), long (M₃(B,C)) and granular (M(B,C)), are tending to be disconnected. Compared to the as-cast, among them the change of lamellar boron carbide is the most obvious, and its morphology changes from the dense and coarse continuous to loose and fine granular, which reduces the fragmentation of matrix. The type of boron carbides in the quenched structure of high-boron Fe-based alloy does not change, but the amount of precipitation decreases with the increase of quenching temperature. The hardness and impact property of the as-quenched high-boron Fe-based alloy are obviously higher than that of the as cast. The hardness and impact property of the matrix increase with the increase of the quenching temperature, while the macro-hardness increases first and then decreases with the increase of quenching temperature. Macro-hardness of the specimen quenched at 1050 ℃ reaches the maximum value of 63.1 HRC, while the maximum value of impact absorbed energy is 10.9 J, when quenched at 1150 ℃.

Effect of rescanning on microstructure and properties of TC4 titanium alloy prepared by selective laser melting

Li Haohao, Wu Meiping, Ma Yiqing, Han Jitai

2020, 45(8): 91-96. doi:10.13251/j.issn.0254-6051.2020.08.018

Abstract ( 90 )

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TC4 (Ti-6Al-4V) alloy parts were prepared by selective laser melting (SLM) equipment, and effect of rescanning laser power on microstructure and properties was studied. The microstructure was analyzed by means of optical microscope. The dense degree, defect size and hardness of the alloy were measured by means of drainage method, white light interferometer and hardness tester. The results show that the rescanning with laser power of 85 W makes the β phase grain coarse and the grain width can be up to 150 μm, and the grain width decreases to 120 μm as the rescanning laser power further increases. The rescanning can significantly improve the dense degree of the alloy parts. After rescanning with laser power of 145 W, the dense degree of the alloy improves to 99.6%. Rescanning can reduce the defect opening size from 25 μm to about 7 μm on average, but have little effect on the defect depth. The decrease of porosity caused by rescanning can improve the hardness of the alloy parts.

Microstructure and properties of M2052 Mn-Cu alloy prepared by selective laser melting method

Ma Conghui, Wang Changjun, Shen Tao, Hu Jiaqi, Liang Jianxiong

2020, 45(8): 97-104. doi:10.13251/j.issn.0254-6051.2020.08.019

Abstract ( 93 )

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M2052 manganese-copper powder was prepared by vacuum induction melting gas atomization (VIGA), and the alloy specimen was directly formed by the selective laser melting (SLM) method. Processed by solid solution and aging treatments and hot isostatic pressing, the manganese-copper alloy prepared by SLM method was studied by thermodynamic calculation, microstructure analysis and mechanical property tests. The results show that the tensile strength and yield strength of the manganese-copper alloy prepared by the SLM method are higher, but the impact absorbed energy is low and the plasticity is poor. After hot isostatic pressing, the comprehensive mechanical properties of the alloy are significantly improved.

Effect of quenching without holding on microstructure and mechanical properties of 40Cr steel

Kang Xueqin, Xu Zhengmao, Lin Enbo, Chen Liang

2020, 45(8): 105-107. doi:10.13251/j.issn.0254-6051.2020.08.020

Abstract ( 108 )

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Microstructure and properties of the 40Cr steel quenched at different temperatures without holding were studied. The results show that when quenched in the range of 850-910 ℃ without holding and tempered at 550 ℃, the hardness, tensile strength and impact absorbed energy of the 40Cr steel increase first and then decrease with the increase of quenching temperature. The highest hardness, tensile strength and impact absorbed energy can be obtained when quenched at 890 ℃ without holding and tempered at 550 ℃, and these properties are better than that by traditional quenching with holding. The property improvement of the 40Cr steel after quenching without holding is related to its fine lath martensite, austenite grain refinement and non-uniform distribution of carbon concentration in austenite.

Effect of annealing process on microstructure and room temperature mechanical properties of selective laser melted Ti6Al4V alloy

Xiao Meili, Zan Lin, Ke Linda, Deng Zhujun, Lai Caifang, Sun Jingli

2020, 45(8): 108-112. doi:10.13251/j.issn.0254-6051.2020.08.021

Abstract ( 86 )

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Ti6Al4V alloy specimens were prepared from Ti6Al4V spherical powder by laser selective melting (SLM) process, and effect of annealing process on the room temperature mechanical properties and microstructure of the SLM Ti6Al4V alloy were studied by means of optical microscope, scanning microscope and tensile test. The results show that the room temperature tensile strength of the alloy without heat treatment exceeds 1200 MPa, but the average fracture elongation is only 4.0%. Under the condition of vacuum annealing at 650 ℃, the tensile strength of the alloy is still around 1200 MPa, and the R_p0.2 is higher than 1150 MPa, but the percentage elongation after fracture is below 10%, the overall stvength and toughness of the material has been significemtly improved. The tensile strength of the alloy is decreased to 1100 MPa when vacuum annealed at 750 ℃ and 800 ℃, the R_p0.2(the proof strength, plastic extension) is higher than 1050 MPa and the percentage elongation after fracture is greater than 10%. Furthermore, with the increase of heating temperature and holding time of vacuum annealing, the original β grain boundary of the alloy formed by laser selective melting becomes blurred gradually and the grains tend to be equiaxed. In the meantime, the needle-like α′ martensite formed with rapid cooling transformation does not appear obviously coarsening.

Influence of remelting and annealing on residual stress of 316L stainless steel laser clad layer

Deng Dewei, Ma Yunbo, Ma Yushan, He Tao, Huang Zhiye, Sun Qi

2020, 45(8): 113-118. doi:10.13251/j.issn.0254-6051.2020.08.022

Abstract ( 91 )

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Laser cladding was carried out on Q235 steel substrate using 316L stainless steel powder, and then the clad specimens were treated by means of laser remelting and annealing. The residual stress of the clad layer and the substrate was measured through blind hole drilling. The microstructure observation and the hardness testing of the specimens were carried out respectively by optical microscope and Vickers hardness tester. The results show that the microstructure of the clad layer does not exhibit obvious changes after laser remelting. Annealed at temperature of 600 ℃ for 2 h, the grains of the clad layer grow slightly, while at 800 ℃ for 2 h recrystallization occurs and the number of dendrites decreases. The microhardness of the clad layer remains at a high level after the laser remelting and annealing. The laser remelting can reduce the clad layer residual stress by 55.9% at most, while the appropriate annealing process can reduce the residual stress by more than 70%. The annealing at 800 ℃ for 2 h can give the most significant improvement of residual stress reduction, by 83.8%.

Effect of aging treatment on microstructure and high temperature tensile deformation behavior of GH4169 alloy

Gao Tianming, Cheng Xiaonong, Luo Rui, Zheng Qi, Li Yangcheng, Wang Wei

2020, 45(8): 119-123. doi:10.13251/j.issn.0254-6051.2020.08.023

Abstract ( 243 )

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Effect of aging time on microstructure, hardness and high temperature tensile properties of GH4169 alloy under the tested conditions of over-temperature service at 900 ℃ were studied. The results show that with the extension of aging time, the δ phase is precipitated from the grain boundary with a short rod shape, and then precipitated the whole grain with long needles; the grain is grown at the initial stage of aging, and with the δ phase gradual precipitating along the grain boundary, the phenomenon of grain growth is disappeared. The strengthening phase of the alloy is dissolved and transformed when the GH4169 alloy aged at 900 ℃, which results in the microhardness of the alloy dropping sharply from 44.0 HRC to 13.6 HRC, but the microhardness is changed a little with the increase of holding time. The precipitation of δ phase has a significant effect on the high temperature mechanical properties of the alloy. The proper amount of precipitation can increase the tensile strength and high temperature plasticity of the alloy. A large amount of precipitation results in a decrease of tensile strength and a deterioration on the high-temperature plasticity. The high temperature tensile of the alloy after different aging treatments is a typical elastic-uniform plastic deformation, and the deformation fracture mechanism is a microporous aggregate fracture.

Effect of annealing process on properties of grooving-corrosion resistant steel strip for heating system

Lu Junhui, Wu Zhanfang, Wang Xiaochun, Qiu Shengtao, Gan Yong

2020, 45(8): 124-130. doi:10.13251/j.issn.0254-6051.2020.08.024

Abstract ( 85 )

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Through the research on the phenomenon of the resistance to grooving corrosion, and combining with the production equipment capacity, six kinds of steel strip chemical compositions and six kinds of annealing processes were designed. The effect of cold rolling annealing process on the properties of grooving-corrosion resistant steel strips for heating system was studied by means of metallography, SEM, tensile test, and salt spray test. The results show that the annealing at temperature lower than Ac₁(650 and 700 ℃) can not change the genetic structure of the original grain boundary of cold-rolled ferrite, under such conditions the recrystallization of cold rolled α phase does not grow sufficiently, and the polymerization and growth of carbides are also insufficient. When the annealing temperature reaches about Ac₁(750 ℃), the solid solving of carbon increases, leading to recrystallization and growth of α phase as well as the polymerization of precipitates, resulting in excellent ductility, which is very beneficial for deep drawing. When the annealing temperature is between Ac₁-Ac₃(850 ℃), α→γ phase transformation and recrystallization occur, and the best softening effect can be obtained. While for the Ti-bearing tested steel, the plastic strain ratio r value tends to decrease with the decrease of annealing temperature. The addition of Al is conducive to increase the resistance to grooving corrosion, but the Cr addition has the opposite effect. Using two-stage annealing process for industrial production can get grooving-corrosion resistant products for heating systems with excellent appearance and properties.

Effect of homogenization process on microstructure and properties of as-extruded Al-4Si(-0.2Er) alloy

Zhao Juan, Cui Xiaoli, Cui Hongwei, Ye Hui

2020, 45(8): 131-136. doi:10.13251/j.issn.0254-6051.2020.08.025

Abstract ( 61 )

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In order to change the size, morphology and distribution of the secondary phase and improve the electrical and mechanical properties of the Ai-Si alloy, Al-4Si and Al-4Si-0.2Er alloys were prepared by selecting rare earth element Er as the alloying element, and the effect of rare earth Er and homogenization on the microstructure, electrical and mechanical properties of the as-extruded Al-4Si and Al-4Si-0.2Er alloys was studied. The results show that rare earth Er is beneficial to promote the precipitation of Si from solid solution and the formation of new phase ErSi₂, which increases the number of the secondary phase in the alloy and results in obvious dispersion strengthening effect, so that the mechanical properties of the alloy are improved, with the conductivity keeps stable. When homogenized at 540 ℃ for 1 h then furnace-cooled, the Al-4Si-0.2Er alloy has better comprehensive performance, where the homogenization treatment promotes the secondary phase to be precipitated with finer size and dispersed in a granular form. Meanwhile, the conductivity of the Al-4Si-0.2Er alloy is increased from 51.61%IACS in the extruded state to 56.44%IACS, which is 9.4% higher. The furnace cooling process results in a slight decrease of mechanical properties of the Al-4Si-0.2Er alloy, with tensile strength of 87.80 MPa, elongation of 35.9%, and hardness of 36.00 HV0.3.

Effect of annealing on microstructure and properties of TC4 alloy rolled sheet made from sponge titanium/electrolytic titanium

Tan Cong, Fan Yongbin, Yu Kun, Xiao Han, Zhang Hongyu, Huang Haiguang, Zhou Rongfeng

2020, 45(8): 137-141. doi:10.13251/j.issn.0254-6051.2020.08.026

Abstract ( 86 )

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Sponge titanium and electrolytic titanium were used as raw materials for making TC4 titanium alloy. The smelted ingot was hot rolled and annealed to study the microstructure and mechanical properties after different raw material ingot rolling. The results show that the stress relief annealing has little effect on the microstructure of electrolytic titanium and titanium sponge TC4 alloy sheet. After recrystallization annealing, both the electrolytic titanium and the sponge titanium TC4 alloy plate have recrystallized, producing equiaxed α phase, while the electrolytic titanium TC4 alloy has a higher degree of equiaxification and a more uniform internal structure. The stress relief effect of the sponge titanium TC4 alloy sheet after annealing at 550 ℃ is better than that of the electrolytic titanium TC4 alloy, and its strength is slightly reduced, but the improvement of plasticity is more obvious. The recrystallization effect of electrolytic titanium TC4 alloy plate after annealing at 800 ℃ is better than that of sponge titanium TC4 alloy, its strength is slightly reduced, but the plasticity is greatly improved. The fracture mode of the two titanium alloy sheets after annealing is all ductile fracture. The hardness of the sponge titanium TC4 alloy plate is reduced after annealing, but the hardness of the electrolytic titanium TC4 alloy plate is increased after annealing.

Effect of annealing process on mechanical properties and microstructure of additive manufactured TC18 titanium alloy

Chen Suming, Hu Shengshuang, Zhang Ying, Zheng Chao, Wang Xiaoguang, Gao Tingting

2020, 45(8): 142-146. doi:10.13251/j.issn.0254-6051.2020.08.027

Abstract ( 83 )

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Effect of annealing parameters on mechanical properties and microstructure of TC18 titanium alloy was studied by means of microstructure observation and mechanical properties test. The results show that the tested block of TC18 titanium alloy made by additive manufacturing has no cracks observed, and possesses flat, smooth and uniformly silver-white surface. The mechanical properties of specimens annealed at 600 ℃ for 2 h are both satisfied with standard requirement of the GJB 2744A—2007, and its R_p0.2, R_m, A and Z are 1036 MPa, 1084 MPa, 9.8% and 30%, respectively. The microstructure of TC18 titanium alloy made by additive manufacturing is the typical structure of columnar crystal, and in the coarse columnar grains of β phase, there is elongated needle-like α phase and fine woven α+β lath structure. With the increase of annealing temperature, the needle-like α phase in the columnar grain is coarsened gradually.

Finite element simulation of temperature field for AP1000 main pipeline during quenching under different flow rates

Liu Huaying, Peng Xinyuan, Tang Longshu, Li Chao

2020, 45(8): 147-151. doi:10.13251/j.issn.0254-6051.2020.08.028

Abstract ( 66 )

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A three-dimensional finite element model of AP1000 main pipeline was established by ANSYS finite element analysis software. The heat transfer coefficients of 316LN stainless steel at different water flow rates of 0.3, 0.5, 0.7 and 1.0 m/s were calculated according to the inverse heat conduction method. The temperature field of AP1000 nuclear power main pipeline was simulated by ANSYS. The possibility of cooling to below 427 ℃ within 180 s proposed by Westinghouse company was preliminarily discussed. The results show that when water flow rate increases from 0.3 m/s to 1.0 m/s, the surface heat transfer coefficient of 316LN stainless steel only increases from 3013 W/(m²·℃) to 3560 W/(m²·℃). At different flow rates, the temperature field of main pipeline surface and center decrease with the extension of quenching time. When the water flow rate is 1.0 m/s, the temperature of inner and outer surface of the main pipeline drops very quickly. It can be quenched to below 200 ℃ for 180 s and to room temperature for 600 s. However, the temperatures of main pipeline center (thickness is about 83 mm) and filler neck convex center decrease slowly, only reach about 580 ℃ and 860 ℃ respectively after quenching for 180 s, which failed to meet Westinghouse's requirement of cooling below 427 ℃ within 180 s. All parts of the main pipeline can be cooled to below 427 ℃ after quenching for about 530 s.

Laser quenching process of 40Cr steel rack

Zhen Yanbo, Qin Jun, Fan Chao, Ma Leipeng, Li Xuchen, Shen Guoqing

2020, 45(8): 152-155. doi:10.13251/j.issn.0254-6051.2020.08.029

Abstract ( 131 )

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The quenching treatment was researched on the tooth surface of the 40Cr steel rack by using LDF 4000-100 semiconductor lasers. The results show that under the rectangular facular sized 17 mm×5 mm, the distance of 375 mm from the laser quenching head to the center of the workpiece surface, 75° of laser beams incident tooth surface,the uniform depth of the hardened layer on the tooth surface can reach 1.2 mm, the microstructure of hardened layer is the fine and uniform distributed needle-like martensite and its hardness can reach up to 750 HV0.2 under the selected optimal parameter with laser heating temperature of 1400 ℃ and scanning speed of 10 mm/s. Finally, the excellent application effect has been acquired in industrial pilot production.

Effect of cooling media temperature for solid solution treatment on as-aged microstructure and hardness of Mg-13Gd-4Y-2Zn-0.5Zr alloy

Shi Zheng, Meng Mu, Yang Xue, Zhang Zhiming, Gao Zhi

2020, 45(8): 156-160. doi:10.13251/j.issn.0254-6051.2020.08.030

Abstract ( 66 )

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Effect of cooling media temperature for solid solution treatment under different parameters on as-aged microstructure and hardness of high rare earth Mg-13Gd-4Y-2Zn-0.5Zr magnesium alloy was studied by SEM, EBSD and Vickers hardness testing. The results show that when solution treated at 520 ℃×24 h, the cooling medium temperature has the most significant effect on the hardness and microstructure, and the hardness value increases gradually with the decrease of cooling media temperature. Moreover, the lower the cooling medium temperature, the more the dot-like Mg₅(Gd,Y) phase and lamellar phase are precipitated during aging. However, when the cooling media temperature is high, needle-shaped Mg₅(Gd,Y) phase is mainly precipitated, while the phases with stacking faults (SFs) are less. In addition, because the grain size of the alloy is not changed obviously under low temperature or short time solid solution treatment, the effect of refinement strengthening is higher than that of the precipitation strengthening in subsequent aging process, thus the cooling media temperature for solid solution treatment has a weak effect on the hardness and microstructure.

Effect of heat treatment on microstructure and properties of as-extruded Mg-6Gd-5Y-1Zn alloy

Xu Hengyuan, Zhen Rui, Mao Zhongfeng, Wu Zhen, Dai Shengpeng

2020, 45(8): 161-165. doi:10.13251/j.issn.0254-6051.2020.08.031

Abstract ( 69 )

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Microstructure and mechanical properties of as-extruded Mg-6Gd-5Y-1Zn (mass fraction, %) alloy after solid solution and aging treatment were investigated. The results show that the as-extruded alloy is composed of α-Mg matrix, strip shaped 18R-LPSO phase arranged parallel to extrusion direction, Mg₂₄(GdYZn)₅phase and fine lamellar 14H-LPSO phase. Solid solution (T4) treatment of the as-extruded alloy at high temperature results in the dissolution of 18R-LPSO phase into the matrix and the lengthening and coarsening of the 14H-LPSO phase. A large number of β′ precipitates exist in the microstructure of the T6 treated specimens. The T6 treated specimen shows the best tensile properties at room temperature with tensile strength of 406 MPa, yield strength of 272 MPa and elongation of 6.1%. T5 treatment also results in the β′ precipitation but its volume fraction, as well as the volume fraction of the 14H-LPSO phase are much lower than that of the T6 treated specimens.

Continuous annealing process of cold rolled high strength low alloy HC420LA

Yang Yuanyuan, Dong Lili, Liang Shiyin

2020, 45(8): 166-169. doi:10.13251/j.issn.0254-6051.2020.08.032

Abstract ( 64 )

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Continuous annealing process of cold rolled high strength low alloy (HSLA) HC420LA steel was studied according to the production requirements. The composition of HC420LA steel product adopted Nb and Ti microalloying composition system. The hot rolled and cold rolled continuous annealing process were selected as the production process. The recrystallization temperature of HC420LA steel product was determined by laboratory recrystallization temperature test. The continuous annealing process was studied in the industrial trial production stage. It is concluded that the recrystallization temperature of the cold-rolled HSLA HC420LA steel is 700 ℃ and the optimum heating soaking temperature in industrial production is 760 ℃.

Effect of solution and aging treatment on impact properties of 7005 aluminum alloy

Zhang Jianyu, Wang Zheng, Wang Jinlu, Ke Wei, Lu Qigao

2020, 45(8): 170-172. doi:10.13251/j.issn.0254-6051.2020.08.033

Abstract ( 67 )

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Impact properties of the 7005 aluminum alloy after solution and aging treatment was studied by means of OM, SEM and impact test machine. The results show that coarse grain boundaries are observed in the as-cast 7005 aluminum alloy, and there are a large number of non-equilibrium precipitated phases in the grain, while after solution and aging treatment, the number of precipitated phases in the grain decreases greatly. The precipitate phases on the as-cast alloy grain boundary contain more Zn, Mg and Mn elements. After solution and aging treatment the precipitated phases existed in grain are η (MgZn₂) equilibrium phase mainly, and there are still some residual precipitated phases AlZnMg(Mn) on grain boundary because of the good thermal stability, which leads to dissolution incomplete. After solution and aging treatment, the solid solubility increases because most precipitated phase particles in the grain dissolve into the matrix, which causes the strength of the alloy to increase and improves the impact properties of the 7005 aluminum alloy.

Effect of solution treatment on microstructure of GH99 alloy

Qin Shengxue, Wang Yan, Zhang Hongbin, Zhou Haiping, Liu Jie

2020, 45(8): 173-178. doi:10.13251/j.issn.0254-6051.2020.08.034

Abstract ( 58 )

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Grain size and grain boundary characteristic distribution of the GH99 alloy were analyzed by electron backscattered diffraction (EBSD) at different solution treatment temperatures and time. The results show that the grains of the GH99 alloy gradually become equiaxed after the solution treatment. There are a large number of twin boundaries in the grains, and the most of them are flatter Σ3 grain boundary. The grains grow gradually as the temperature and the time of solution treatment increase, while the grain growth rate is different at different temperatures and time, and the annealing twins grow with the growth of the grains. With the increase of solution treatment temperature and time, the fraction of the Σ3 grain boundary increases firstly and then decreases, while the fraction of Σ9 and Σ27 grain boundaries shows a gradual decreasing trend.

Heat treatment process of new air-cooling bainitic steel for shovel teeth

Zheng Lili, An Shengli, Peng Jun, Zhang Fang, Gao Zhanyong, Jiao Haidong

2020, 45(8): 179-184. doi:10.13251/j.issn.0254-6051.2020.08.035

Abstract ( 53 )

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In order to solve the problem of short service life of shovel teeth, the new bainitic steel and its heat treatment process were studied. The chemical composition and phase transition points of the bainitic steel were tested by spark direct reading spectrometer and thermal dilatometer. Effect of normalizing temperature, tempering temperature and tempering time on the toughness of the bainitic steel was studied by orthogonal test method, and the optimal heat treatment process was determined. Microstructure, phase composition ratio and hardness of bainitic steel for shovel teeth were measured by means of scanning electron microscopy, Image-J software, X-ray diffractometer and digital microhardness tester. The results show that the most important factor affecting the impact toughness of the bainitic steel is the normalizing temperature, then the tempering temperature and the tempering time in order. The optimal heat treatment process is normalizing at 1080 ℃+tempering at 250 ℃ for 90 min, by which the bainitic steel obtains a good combination of toughness (18.45 J) and hardness (46.85 HRC), and the content of martensite and retained austenite in the microstructure is 23.985% and 9.850%, respectively.

Effect of solution treatment on microstructure and hardness of Al-Zn-Mg-Cu super-strength aluminum alloy

Li Zhiyong, Shi Qingnan

2020, 45(8): 185-188. doi:10.13251/j.issn.0254-6051.2020.08.036

Abstract ( 60 )

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Effects of different solid solution treatments on the microstructure after solution and the hardness after solution and aging of Al-Zn-Mg-Cu super-strength aluminum alloy 7050 were studied by means of optical microscopy (OM), scanning electron microscopy (SEM) and Vickers hardness tester. The results show that there is coarse bulk primary phase, needle-like and point-like η(MgZn₂) phase in the microstructure of as-forged 7050 aluminum alloy. With the increase of solution temperature and solution time, the undissolved phase in the alloy decreases, and the volume fraction of recrystallized microstructure increases. The hardness of the 7050 aluminium alloy solution treated at 470 ℃ for 4 h and then aged at 120 ℃ for 24 h reaches the maximum value of 189 HV0.2.

Effect of gas nitriding process on microstructure and properties of NbVTi alloyed gray cast iron

Meng Chao, Guo Caiping

2020, 45(8): 189-193. doi:10.13251/j.issn.0254-6051.2020.08.037

Abstract ( 54 )

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In view of the development trend of high speed and heavy load of engine, a new type of NbVTi alloyed gray cast iron cylinder liner material was gas nitrided under different temperature. The effect of gas nitriding process on microstructure and properties of NbVTi alloyed gray cast iron cylinder liner was studied by means of metallographic microscope, Vickers hardness tester and friction wear testing machine. The results show that with the increase of nitriding temperature, the nitrided depth of NbVTi alloyed gray cast iron cylinder liner is increased, and the microhardness at the depth of 100 μm and 130 μm from the surface is increased first and then decreased. After nitriding at the optimum nitriding temperature of 570 ℃, the cylinder liner size expands in the standard range of 0.010-0.055 mm, the average thickness of nitrided layer is about 140 μm, and the average hardness of the diffusion zone at the depth of 100 μm from the surface is above 500 HV0.1, which can meet the requirements of high speed and heavy load cylinder liner. After nitriding, the friction and wear performance of cylinder liner is greatly improved. With the increase of load and speed, the wear rate increases, and the wear mechanism changes from abrasive wear to adhesive wear.

Effect of tempering on microstructure and properties of Cr alloyed layer prepared by high current pulsed electron beam on Cr4Mo4V steel

Liu Zhaobing, Zhou Lina, Luo Dian, Li Xuewen

2020, 45(8): 194-198. doi:10.13251/j.issn.0254-6051.2020.08.038

Abstract ( 49 )

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Microstructure evolution law of Cr alloyed layer prepared by electron beam on Cr4Mo4V steel with different tempering time and temperatures was studied. Effect of tempering process on the phase transformation of Cr alloyed layer was analyzed by means of XRD and TEM, and the hardness and corrosion resistance of tempered Cr alloyed layer were tested by nanoindenter and electrochemical workstation, respectively. The results show that nano-scale carbides precipitate in the alloyed layer during tempering process, results in the increase of the hardness. However, the carbides gradually coarsen and the hardness decreases as the tempering time and temperature increase. The reduction of Cr content caused by carbides precipitation leads to a decrease of corrosion resistance, however, which is still superior to the Cr4Mo4V steel matrix.

Effect of cold rolling pretreatment on microstructure of TC4 alloy nitrided at 540 ℃

Leng Ke, Feng Xiaoxue, Yan Feng, Fu Yudong

2020, 45(8): 199-202. doi:10.13251/j.issn.0254-6051.2020.08.039

Abstract ( 61 )

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TC4 titanium alloy was solution treated at 820 ℃ and cold rolled at room temperature to obtain specimens with different deformation of 10%, 20%, 30%, 40%, 50%, and then nitrided and aged simultaneously at 540 ℃ for 8 h. The change of nitrided microstructure and aged microstructure with different deformation amounts and the change of phase structure and morphology of the nitrided specimen surface were analyzed by means of metallographic microscope, X-ray diffractometer, scanning electron microscope (SEM). The results show that after nitriding treatment, high-hardness nitride particles such as TiN and Ti₂N are formed on the surface of TC4 alloy, and the larger the amount of deformation, the more nitride particles with larger size are formed on the specimen surface. After solution treatment, the microstructure of the TC4 alloy is α phase+metastable β phase. After nitriding, the metastable β phase decomposes into α phase, while α phase transforms into β phase, and the larger the amount of deformation, the more β phase precipitates, but this phenomenon is not as obvious as that only aged.

Heat treatment of pin shaft for large engine chain

Zhang Mengjiu, Shang Zhaohua, Liu Yi, Li Cunzhi, Jiang Depeng

2020, 45(8): 203-206. doi:10.13251/j.issn.0254-6051.2020.08.040

Abstract ( 71 )

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Aiming at the defects of the original heat treatment process of the pin shaft for the large engine chain, the heat treatment process of the pin shaft was improved. The results show that the distortion of the pin shaft after quenching is reduced effectively, and the retained austenite content on the surface of the part is adjusted reasonably. By improving the heat treatment process, the quality of chain pin shaft is improved remarkably.

Rolling contact fatigue property of laser dispersed quenched steel rail

Yang Zhixiang, Ye Zufu, Wang Aihua, Ye Bing, Xiong Dahui

2020, 45(8): 207-215. doi:10.13251/j.issn.0254-6051.2020.08.041

Abstract ( 50 )

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Laser dispersed quenched process was carried out on steel rail surface in order to improve the service life of the rail. Three different distribution patterns for the laser dispersed quenched were used, and rolling contact fatigue wear test machine was used to measure the fatigue wear property of the rail, and the wear surface morphology, cross-sectional morphology and wear mechanism were analyzed. The results show that the wear loss of original state specimen is 1.102 g, and fatigue spall and plastic deformation with severe corrugation occur on the surface of the original specimen. After laser dispersed quenched process, the corrugation is eliminated, and the wear loss of rail specimen reduces significantly, but cracks occur at the edge of contact zone and the interface of laser hardened region. The cracks at the center of contact zone can be effectively removed when the hardened regions are distributed with 1 mm spacing and 60° inclination, and the lowest wear loss is acquired at this condition, which decreases by 63% compared with that of the original state specimen. It can effectively improve the cracking resistance and wear property of the laser dispersed hardened layer by combination of reducing the spacing and increasing the tilt angle of laser hardened region.

Defect analysis and mechanical properties of supersonic plasma sprayed ZrO₂ coating

Cui Jing, Guo Yuzhu, Pang Ming, Yang Guangfeng

2020, 45(8): 216-221. doi:10.13251/j.issn.0254-6051.2020.08.042

Abstract ( 52 )

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ZrO₂ coating was prepared on the surface of Q235 steel substrate by supersonic plasma spraying technology (SAPS). Microstructure, phase composition and element distribution of the ZrO₂ coating were detected and analyzed by scanning electron microscope (SEM), energy spectrometer (EDS) and X-ray diffractometer (XRD), respectively, and the hardness of the ZrO₂coating was measured by Vickers hardness tester, and the hardness of the coating, the coating-substrate junction and the substrate were obtained respectively. The results show that there are a large number of grain bulges and holes in the surface of the coating prepared by SAPS. The longitudinal section of the coating has transverse cracks and pores in the shape of horseshoe, meniscus, ellipsoid and irregular polygon, and the porosity is 13%. The Zr element diffuses in the coating at high temperature and the content of Zr increases from the top to the bottom of the coating, and a small amount of Zr element appears on the surface of the substrate. The coating material undergoes a phase change from a monoclinic phase to a tetragonal phase during the spraying process. The microhardness of the coating, the coating-substrate junction and the substrate is 740.51, 205.79 and 189.33 HV0.2, respectively, and the microhardness of the coating-substrate junction is increased compared with that of the substrate.

Laser surface strengthening of 42CrMo steel for bending die

Wang Huizhen, Zhai Yuewen, Zhou Leyu

2020, 45(8): 222-227. doi:10.13251/j.issn.0254-6051.2020.08.043

Abstract ( 73 )

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Laser surface quenching characteristics of bending die made of 42CrMo steel were studied by means of SEM and EBSD. The results show that the laser scanning speed, power and workpiece thickness have significant influence on the depth and hardness of the hardened layer. Under the condition of 2200 W laser power, 1800 mm/min scanning speed, 2 mm of spot diameter, auxiliary water cooling and one pass scanning, the hardness and hardened layer thickness of the bending die blade reach 734 HV0.2 and 1.05 mm respectively, and the hardness distributions are uniform on both sides of the blade. The microstructure of the laser hardened layer was fine martensite, especially near the substrate.

Effect of electrical contact strengthening on microstructure and properties of Ni/Cu composite coatings

Li Zhihong, Ding Hao, Zhu Shigen

2020, 45(8): 228-232. doi:10.13251/j.issn.0254-6051.2020.08.044

Abstract ( 62 )

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Ni/Cu composite coating with a thickness of 1 mm was prepared by brush plating on the surface of the cast iron, and the coating was subjected to electrical contact strengthening test. The effect of electrical contact strengthening on the microstructure and property of the coating was analyzed by means of field emission electron microscopy (FESEM) and Vickers hardness tester. The test results show that the electrical contact strengthening improves the surface and internal quality of the coating and reduces defects such as cracks in the coating. At the same time, electrical contact strengthening results in partial mosaic melting between the coating and the substrate. The electrical contact strengthening changes the microstructure of the Ni layer and the Cu layer, making the coating structure denser and tighter, and plastic deformation occurs between the Cu layer and Ni layer, resulting in a certain inlay. After strengthening, the surface and cross-section hardness of the coating are improved. The hardness of the hardened layer formed between the plating layer and the substrate is much greater than that of the substrate, and the electrical contact strengthening improves the bonding strength between the plating layer and the substrate.

Research progress on causes and control of cracks in laser clad composite coatings

Zhang Leitao, Zhang Hongxing, Liu Dexin, Zhang Weiqiang, Wang Xuesong, Dai Jiaoyan, Xu Jinfu

2020, 45(8): 233-239. doi:10.13251/j.issn.0254-6051.2020.08.045

Abstract ( 79 )

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Causes of cracks in laser clad composite coatings and the affecting factors were reviewed. The methods for controlling cracks were summarized from the aspects of preheating of the substrate, overall tempering and remelting of the coating after cladding, gradient structure, process parameter optimization, the composition control of cladding materials and addition and assistive technologies.

Cause analysis of inner folding of 34CrMo4 seamless steel tube

Wang Xiao, Peng Haoping, Lei Yun, Su Xuping, Wu Changjun, Deng Song

2020, 45(8): 240-244. doi:10.13251/j.issn.0254-6051.2020.08.046

Abstract ( 94 )

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Aiming at the inner folding defects of a domestic 34CrMo4 seamless steel tube in the production process, optical microscope (OM), scanning electron microscope (SEM) and energy spectrometer (EDS) were used to carry out systematic inspection and analysis on microstructure and component of the defects. And the cause of formation of inner folding defects in 34CrMo4 seamless steel tube were discussed. The results show that there are a large number of obvious cracks in the inner surface of the steel tube, and Cu enrichment is found in the inner folding area, when the tube blank after hot processing in the production line and the tube blank after heat treatment under simulated production line. During the perforation and biting process of the tube blank in the production line, high Cu-containing inclusions are crushed, squeezed to the grain boundary, and infiltrate along the grain boundary, finally, destroy the continuity of the grain boundary. Due to the axial tensile stress generated by the perforation and the presence of high Cu content inclusions, the matrix discontinuity is destroyed during the axial deformation of the tube blank. Therefore, micro-cracks appear during the continuous extension of the tube blank, and the inner folding defects are formed on the inner wall of the seamless steel tube during the continuous rolling and sizing process.

Causes analysis of cracking of boiler water wall tube

Xu Weixian, Qiao Lijie, Liang Zhigang

2020, 45(8): 245-248. doi:10.13251/j.issn.0254-6051.2020.08.047

Abstract ( 117 )

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Aiming at the cracking of the water wall tube of a thermal power plant boiler, the failed water wall tube was sampled and analyzed by means of macroscopic morphology, material analysis, hardness test, metallographic structure inspection, scanning electron microscopy, energy spectrum analysis and XRD. The results show that the cause of the cracking of the water wall tube is hydrogen corrosion. There is a large amount of copper observed inside the main crack and oxide gaps. Owing to the poor thermal conductivity of the oxide layer, the local heat transfer is deteriorated. When the local heat load exceeds a certain limit, copper element is greatly enriched at the main crack, and the enriched copper accelerates the propagation of crack.

Design and application of high precision temperature control system for vacuum low-pressure carburizing furnace

He Longxiang, Yin Chengkun, Xu Yueming, Cong Peiwu, Liu Junxiang

2020, 45(8): 249-253. doi:10.13251/j.issn.0254-6051.2020.08.048

Abstract ( 216 )

PDF (529KB) ( 148 )

A monitoring system of vacuum low-pressure carburizing upper computer was designed based on Kingview 6.55 software. Through modbus communication between Kingview software and Yamatake instrument DCP31 and using the 2-DOF PID function of the instrument, real-time PID correction was carried out to ensure the accuracy of temperature control. Variables in carburizing process, such as flow control of C₂H₂and NH₃ and N₂, carburizing diffusion time, pulse number, holding time and quenching mode, were solidified into formulation form for users to call and modify. The design and application of this software platform reduce the mis-operation and complexity of the operators, and greatly improve the heat treatment quality of low pressure carburizing.

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