Heat Treatment of Metals

Analysis of deformation mechanism and temperature dependence of 3D printed TiAl nano-polycrystalline alloys based on molecular dynamics

Li Pengtao, Luo Xian, Chen Jianxin

2021, 46(11): 1-8. doi:10.13251/j.issn.0254-6051.2021.11.001

Abstract ( 123 )

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By using the results from TEM observation and molecular dynamic simulation, the deformation behaviour and its temperature dependence of the 3D printed Ti-6Al-4V alloys were investigated. The results show that, the temperature plays a key role in the competition of deformation mechanisms of the TiAl nano-polycrystalline alloys. When the temperature is lower than 800 K, the single-phase TiAl nano-polycrystalline alloy with average grain size of less than 8.3 nm has dislocation movement first, and the stacking faults remain in the grains and form a staggered structure. At the same time, large grains (≥8.3 nm) provide enough space for dislocation movement, and stacking faults are rarely formed in the grains. In the dual-phase TiAl+Ti₃Al nano-polycrystalline alloy, the delivery of stacking faults is the main deformation mechanism of low strain (ε<18.0%) TiAl grains, and the Ti₃Al grains maintain their original structure. When the strain(ε) exceeds 18.0%, the dislocations in the Ti₃Al grains begin to move and form stacking faults. When the temperature is over 800 K, Ti and Al atoms are in a high-energy state, and the main deformation mechanism is related to the slip boundary with an amorphous structure. The amorphous slip boundary and recrystallization are the most important characteristics of the microstructure deformation of the dual-phase TiAl+Ti₃Al nano-polycrystalline alloy.

High temperature rheological behavior and constitutive model of 316LN austenitic stainless steel

Zhu Xiaoning, Pan Qing, Li Yibo, Jiang Xuepeng

2021, 46(11): 9-16. doi:10.13251/j.issn.0254-6051.2021.11.002

Abstract ( 175 )

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Isothermal compression test was carried out by Gleeble-3500 thermal simulation testing machine with strain rate of 0.001-1 s^-1, deformation temperature of 1223-1523 K and compression deformation of 65% to study the high temperature rheological behavior of forged 316LN stainless steel. The rheological stress constitutive model was established and applied to Deform-3D software platform. The thermal simulation test results were simulated in equal size by importing new material data and considering interface friction. The results show that the compressive stress of 316LN austenitic stainless steel decreases with the increase of deformation temperature at the same strain rate. At the same deformation temperature, the compressive stress increases with the increase of strain rate. In the true stress-true strain curves, the compressive stress gradually reaches a stable value with the increase of strain. Considering the interface friction coefficient, the deformation simulation is carried out by using Arrhenius constitutive model, which shows the effectiveness and reliability of the constitutive equation and simulation model, and can provide research basis and theoretical basis for the engineering application of 316LN stainless steel.

Austenitization kinetic characteristics and microstructure evolution of as-cast GCr15 steel during continuous heating

Zhou Denghu, Hou Qingyu, Han Weixue, Jiang Yan, Huang Zhenyi

2021, 46(11): 17-23. doi:10.13251/j.issn.0254-6051.2021.11.003

Abstract ( 105 )

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Linear expansion of the as-cast GCr15 steel with different heating rates (0.5, 3, 5, 30, 100 ℃/s) was recorded using DIL805A thermal dilatometer, and their respective thermal expansion curves and austenite volume transformation fraction curves were obtained. The effect of heating rate on austenitization of the as-cast GCr15 steel was studied. The austenite transformation process of the steel during continuous heating was observed and analyzed by means of a high temperature metallurgical microscope. The results show that austenite transformation of the GC15 steel during continuous heating can be divided into three stages: the transformation of pearlite to austenite (760-790 ℃), the dissolution of (Fe, Cr)₃C_II into austenite (790-890 ℃) and the austenite homogenization (>890 ℃). And as the heating rate increases, the critical temperature of phase transition increases, and the rate of phase transition increases. During the continuous heating process, the austenitic transformation of the as-cast GCr15 steel is an alternate process of nucleation and growth.

High temperature oxidation behavior of 309SMOD austenitic heat-resistant stainless steel

Meng Chongchong, Yang Xiaorong, Zhang Huayu, Zhang Wei, Chen Huiqin

2021, 46(11): 24-28. doi:10.13251/j.issn.0254-6051.2021.11.004

Abstract ( 171 )

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Oxidation behavior of the 309SMOD austenitic stainless steel at different temperatures was analyzed by weight gain method. The parabolic oxidation kinetics curves of the steel at high temperatures were obtained. The morphology and phases of the oxides were analyzed by means of SEM, EDS and XRD. The results show that the slab and block oxide morphologies are observed at 800 ℃, while the oxides are mainly spinel particles at 900 ℃ and 1000 ℃. The mixed three-layer oxidation films are observed on the surface of the 309SMOD austenitic stainless steel, of which the outermost layer is consisted of MnCr₂O₄ and FeCr₂O₄, the sub outer layer is Cr₂O₃, and the innermost layer is SiO₂. The oxide film structure makes the 309SMOD austenitic stainless steel with good high temperature oxidation resistance.

Thermodynamic calculation and analysis of equilibrium precipitation phases of G115 martensitic heat-resistant steel

Liu Xinyang, Chen Zhengzong, Zhou Yun, Wen Pengyu, Bao Hansheng

2021, 46(11): 29-35. doi:10.13251/j.issn.0254-6051.2021.11.005

Abstract ( 76 )

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For manufacturing large castings of G115 martensitic heat-resistant steel weighing over 10 t, TCFe9 database in Thermo-Calc software was deployed to predict the role of varied solute atoms on precipitation behavior by simulating the equilibrium parameters of varied precipitates, and study the effect of major element in the G115 steel on types, amounts and precipitation temperatures of the equilibrium precipitated phases. The calculated results show that the precipitates of G115 martensitic heat-resistant steel at 650 ℃ mainly consist of MX phase (FCC-A1#2: NbC), M₂₃C₆, Laves phase and Cu-rich phase. The precipitation temperature of NbC, M₂₃C₆, Laves phase and Cu-rich phase is 1148 ℃, 871 ℃, 811 ℃, and 734 ℃, respectively. Among them, C and Cr strongly affect the precipitation of M₂₃C₆, and Nb and C affect that of NbC, W affects that of Laves phase, while B has no obvious influence on precipitation of all the phases.

Effect of Sb on eutectic Si and tensile properties of permanent mold cast ZL114A alloy

Liu Dongyang, Yu Xiuyong, Gao Wenli, Lu Zheng, Mao Guoling

2021, 46(11): 36-41. doi:10.13251/j.issn.0254-6051.2021.11.006

Abstract ( 68 )

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ZL114A alloy is a hypoeutectic Al-Si casting alloy independently developed in China, with eutectic Si modification being one of the most important methods to improve its comprehensive tensile properties at room temperature. Effect of Sb on microstructure and properties of the as-cast and heat-treated ZL114A alloy was studied by tensile test at room temperature and SEM observation. The results show that for the as-cast ZL114A alloy, the addition of Sb modifies the eutectic Si from plate-like to fibrous or granular. For the heat-treated ZL114A alloy, the addition of Sb makes the aspect ratio of eutectic Si smaller and the degree of spheroidization higher. The Sb addition can improve the comprehensive tensile properties of the ZL114A alloy at room temperature under the condition of permanent mold casting, the tensile strength and elongation of the ZL114A alloy with Sb addition after heat treatment are much higher than those required in the aviation standard of China. Comprehensive consideration of the morphologies of eutectic Si and the tensile properties at room temperature of the as-cast and heat-treated ZL114A alloy shows that, the optimal content of Sb is 0.09% (mass fraction).

Effect of cerium additive on low temperature impact properties of S32750 super duplex stainless steel

Bao Shun, Liu Rongpei, Wang Baoshun, Wu Minghua, Luo Youxin, Yao Liang, Feng Han, Song Zhigang

2021, 46(11): 42-47. doi:10.13251/j.issn.0254-6051.2021.11.007

Abstract ( 72 )

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The differences of impact absorption energy and energy composition between the two super duplex stainless steels with different Ce additives in the range of 20 ℃ to -100 ℃ was analyzed by means of Charpy impact test and oscilloscope shock method. Effects of Ce additive on modification of inclusion and impact fracture behavior of the steel were studied by means of Aspex automatic scanning electron microscope, SEM and EDS. The results show that the low temperature impact fracture resistance of the high Ce tested steel is obviously better than that of the low Ce tested steel, and the ductile-brittle transition temperature of the former is 16 ℃ lower than that of the latter. The addition of cerium increases the impact absorbed energy at －80 ℃ by 45 J, which is mainly due to the increase of crack growth energy W_P (76%). The results of impact fracture morphology observation and inclusion analysis show that the impact fracture of low Ce tested steel at －80 ℃ shows complete cleavage fracture. Compared with the low Ce tested steel, the inclusion of Al₂O₃ in the high Ce tested steel is significantly reduced, and most of the inclusion becomes the modified composite Ce aluminum oxygen inclusion. The decrease of the content of hard and brittle Al₂O₃ inclusions effectively improves the impact properties of the tested steel.

Effect of Co on microstructure and strength and toughness of 4Cr5Mo2V steel

Deng Junjie, Zhou Jian, Liu Jianxiong, Chi Hongxiao, Fan Yi

2021, 46(11): 48-53. doi:10.13251/j.issn.0254-6051.2021.11.008

Abstract ( 84 )

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Effect of Co on microstructure, strength and toughness of the 4Cr5Mo2V steel was studied by means of OM, SEM, EDS, hardness test, room temperature impact and high temperature tensile tests. The results show that the 4Co5Mo2V-Co steel has more undissolved carbides and the martensite laths are thinner after quenching at 1010 ℃ for 30 min. When tempered at 510-600 ℃, the hardness of the 4Co5Mo2V-Co steel is 1-2 HRC higher than that of the 4Co5Mo2V steel, while the two steels have similar impact properties. The 4Cr5Mo2V-Co steel has higher high-temperature strength under the same initial hardness, which is because the addition of Co promotes the nucleation rate of secondary hardened carbides in the 4Cr5Mo2V-Co steel, and reduces the coarsening rate of carbides, thereby improving the strength of the 4Cr5Mo2V-Co steel.

Effect of aging temperature on microstructure and mechanical properties of direct quenched Ni-Cr-Mo-V-Cu steel

Zhu Fei, Luo Xiaobing, Yang Caifu, Chai Feng, Zhang Zhengyan

2021, 46(11): 54-63. doi:10.13251/j.issn.0254-6051.2021.11.009

Abstract ( 109 )

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Effect of aging temperature on microstructure and mechanical properties of the direct quenched Ni-Cr-Mo-V-Cu low alloy steel was investigated by means of OM, SEM, TEM, physicochemical phase analysis and so on. At the same time, the yield strength model was established in the aged specimen with the best strength toughness matching. Microstructure of direct quenched Ni-Cr-Mo-V-Cu steel consists of martensite and a small amount of bainite. When the direct quenched Ni-Cr-Mo-V-Cu steel is aged in the range of 400 ℃ to 600 ℃ respectively, strength and Vickers hardness of the tested steel show typical under-aged state, peaking-aged state and over-aged state. The change of dislocation recovery, precipitation of MC and Cu-rich particles, desolvation of solid solution elements in bcc iron matrix and so on with aging temperature is responsible for the aged specimens showing the above three states. Elongation after fracture of the aged specimen generally improves with the increase of ageing temperature. Impact property at -20 ℃ of the over-aged specimen increases with the increase of ageing temperature. The direct quenched specimen aged at 600 ℃ shows superior combination of strength and toughness, and precipitating strengthening increment caused by MC and Cu-rich particles is about 240 MPa.

Heat treatment process of Q1100 steel for construction machinery

Liu Hongyuan, Wu Guangliang, Zhang Yongji

2021, 46(11): 64-70. doi:10.13251/j.issn.0254-6051.2021.11.010

Abstract ( 99 )

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Effects of reheating quenching temperature (880-980 ℃) and tempering temperature (200-650 ℃) on microstructure and mechanical properties of a high strength construction machinery steel with yield strength of 1100 MPa were studied. The results show that with the increase of reheating quenching temperature from 880 ℃ to 980 ℃, the average austenite grain size of the tested steel increases from 8 μm to 24 μm, the yield strength and tensile strength first increase then decrease and with that at 920 ℃ being the maximum, while the impact absorbed energy at -40 ℃ continues to decrease. When quenched at 920 ℃ and then tempered at 200-650 ℃, with the increase of tempering temperature, the martensite laths of the tested steel are merged, the lath morphology is gradually blurred, and the number and morphology of carbides also change accordingly; and the strengths of the tested steel decrease greatly, while the plasticity and toughness decrease first and then increase. The optimal heat treatment process of the tested steel is: quenching at 920 ℃ and tempering at 200-250 ℃.

Effect of hot rolling + quenching and partitioning on microstructure and properties of Q235 steel

Meng Xiaoyue, Lin Wanming, Ning Angang, Ma Rongze, Zhang Zhiwei

2021, 46(11): 71-77. doi:10.13251/j.issn.0254-6051.2021.11.011

Abstract ( 141 )

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Effect of partitioning temperature during one-step quenching and partitioning (Q&P) process and one-step hot rolling + quenching and partitioning (HR-Q&P) process on microstructure and mechanical properties of the Q235 steel was studied by means of optical microscope (OM), scanning electron microscope (SEM), electronic universal testing machine and digital microhardness tester. The results show that the grains of the tested steel are obviously refined by HR-Q&P process, and the microstructure is composed of martensite, ferrite and bainite. When partitioned at 350 ℃ by HR-Q&P process, the yield strength and tensile strength of the steel reach the maximum, the values of which are 449 MPa and 560 MPa, respectively, while the elongation decreases by 8% compared with the as-received one, but still exceeds 30%. The hard phases of martensite and bainite appear at the same time during HR-Q&P process, resulting in secondary cracking. Under the one-step Q&P process, compared with the as-received tested steel, the tensile strength increases by about 32%, the yield strength increases by nearly one times, and the elongation remains above 26%.

Effects of accumulative roll bonding temperature on microstructure and mechanical properties of AZ31 magnesium alloy

Guan Di, Qu Meijing, Hua Fuan

2021, 46(11): 78-83. doi:10.13251/j.issn.0254-6051.2021.11.012

Abstract ( 63 )

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Effects of accumulative roll bonding temperature on grain size, basal texture, interfacial bonding and mechanical properties of the AZ31 magnesium alloy were investigated by using optical microscopy, tensile test at room temperature, microhardness tester, X-ray diffractometer and scanning electron microscope. The results show that the grain refining effect of AZ31 magnesium alloy after accumulative roll bonding three-pass is obvious, and the hardness increases. As the accumulative roll bonding temperature increases, the grain refining effect and the increase trend of hardness weakens. The increase of accumulative roll bonding temperature can weaken the basal texture. The comprehensive mechanical properties are capable of reaching the best when the AZ31 magnesium alloy sheets accumulative roll bonded three-pass at 450 ℃, i.e. the microhardness of 70.64 HV0.05, tensile strength of 288.64 MPa, yield strength of 203.76 MPa, elongation of 16.96% and interfacial bonding strength of 21.53 MPa.

Effect of annealing temperature on mechanical properties of ECAP high-nitrogen austenitic stainless steel

Hou Junfeng, Xu Wang, Zhang Mingxu, Li Yongze, Dong Fuyuan

2021, 46(11): 84-89. doi:10.13251/j.issn.0254-6051.2021.11.013

Abstract ( 67 )

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In order to study the effect of equal channel angular pressing (ECAP)+ annealing process on mechanical behavior of the high-nitrogen steel, single pass ECAP deformation was performed on high-nitrogen steel at room temperature, and then the steel with different N contents was annealed at 700, 800, 850, 900 ℃, respectively, to analyze the change rules of mechanical properties. The results show that the yield strength and tensile strength of the high-nitrogen steel decrease on the whole with the increase of annealing temperature, while the plasticity increases. The tensile strength and uniform elongation of the high-nitrogen steel at different annealing temperatures show the traditional inverse relationship, and the strength and uniform elongation increase synchronously with the increase of N content. The tensile fracture morphology of high nitrogen steel ECAP + annealed at low temperatures is intergranular, and intergranular fracture tendency is more obvious with the increase of N content.

Influence of process parameter on laser quenching effect of 42CrMo steel

Jiao Yongxiang, Deng Dewei, Sun Qi, Li Tianshu, Yang Shuhua, Zhang Yong, Ma Yushan

2021, 46(11): 90-96. doi:10.13251/j.issn.0254-6051.2021.11.014

Abstract ( 81 )

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Influence of laser quenching power and scanning speed on surface hardness and hardened depth of the 42CrMo steel was studied by applying different process parameters, and the variation trends of hardening performance under constant laser energy density and different laser quenching parameters were analyzed. The results show that the hardened depth and hardness of the hardened layers can be improved by increasing laser power and decreasing scanning speed. There is a stronge correlation between the energy density parameters calculated by combining the laser power and scanning speed and the quenching effect. Under the same energy density, the high power and high speed parameters can increase the hardness, hardened depth and processing efficiency.

Effect of dew point on surface oxidation behavior of DP590 dual phase steel

Yang Peiyuan, Huo Qinghui, Chen Zhuo, Zhou Wei, Wang Hongbin

2021, 46(11): 97-102. doi:10.13251/j.issn.0254-6051.2021.11.015

Abstract ( 67 )

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Oxidation of alloy elements is inevitable in continuous annealing process of the DP590 duplex steel. The DP590 steel after annealing at different dew points was characterized and analyzed by means of scanning electron microscope (SEM), X-ray energy spectrometer (EDS), glow discharge spectrometer (GDS), and X-ray photoelectron spectrometer (XPS). The results show that during annealing process, as the dew point temperature increases, the DP590 steel changes from external oxidation to internal oxidation, the oxidation degree of the steel plate surface shows a trend of first increasing and then decreasing. The numerical simulation results indicate that the critical transition dew point of internal and external oxidation of the DP590 steel is -8.26 ℃, which is consistent with the test results. The surface oxide of the DP590 steel is mainly manganese oxide. With the increase of dew point temperature, the enrichment peaks of Mn and O elements on the surface of DP590 steel increase first and then decrease, while that of Si and Al elements show a gradually decreasing trend. The XPS data show that the oxides formed on the surface of DP590 steel mainly exist in the form of MnO, Mn-Si-O oxide and Si-O oxide.

Interfacial reaction of Sn-Ag-Cu solder with magnetron sputtered Cu-Cr alloy film during aging treatment

Wang Yupeng, Wu Hongjun, Bao Mingdong, Yang Wenhao, Zhao Guohua, Zhou Bing

2021, 46(11): 103-109. doi:10.13251/j.issn.0254-6051.2021.11.016

Abstract ( 58 )

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Cu-Cr alloy films with different content of Cr was deposited by magnetron sputtering. The solder joint structure was formed by reflow soldering with Sn-Ag-Cu (SAC) solder at 240 ℃, and then the specimens were vacuum aged at 180 ℃. The soldering reliability of the Cu-Cr alloy as under bump metallization layer (UBM layer) forming solder joint with SAC was studied. The morphology of interfacial intermetallic compound (IMC) and the shear strength of solder joint were analyzed by field emission scanning electron microscope equipped with energy dispersive X-ray spectrometer and multifunctional thrust tester. The results show that needle-like IMC, which is different from the traditional SAC/Cu interface scallop-like IMC, is formed in the SAC/Cu-Cr solder joint after reflow soldering. After aging, the segregation of Cr at the grain boundary forms a chromium rich layer, which acts as a diffusion barrier to prevent the diffusion of Cu into IMC, so that the growth of Cu₃Sn and Kirkendal void is inhibited. The shear strength test results show that the SAC/Cu-Cr specimen after reflow soldering has higher shear strength than that of the SAC/Cu specimen. The solder joint structure formed by the Cu-Cr alloy UBM layer with a Cr target current of 1.5 A has smaller IMC thickness and the highest solder joint shear strength. It is confirmed that the Cu-Cr alloy UBM layer is beneficial to improve soldering reliability.

Microstructure, hardness and growth kinetics of 65Mn steel borided layer

Wei Xiang, Jiang Yanqing, Yu Lingjie, Peng Guangwei, Yu Hongbin, Hao Penglei

2021, 46(11): 110-119. doi:10.13251/j.issn.0254-6051.2021.11.017

Abstract ( 45 )

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Effects of boriding temperature (800-1000 ℃) and boriding time (2-8 h) on the thickness, microstructure and hardness of the borided layers on 65Mn steel by pack boriding and its growth kinetics were systematically investigated by optical microscope, XRD, EPMA and Vickers hardness tester, etc. The results show that with the increase of boriding temperature or time, the thickness of the borided layers increases continuously, but the amount, size and depth away from the surface of borided layer of black holes also increase gradually when the boriding temperature exceeds 900 ℃. The layers are composed of Fe₂B columnar crystals, Fe₃(B,C) phase, Fe-Si binary compounds and Fe-C-Si ternary compounds, and the later three are formed between and in the front of Fe₂B columnar crystals. The hardness of the layers (800-1590 HV0.05) is greatly higher than that of the 65Mn steel (238 HV0.05). However, it is found that the hardness values of the borided layers are not monotonically decreasing when the distance from the surface of the layers is increasing, which should be attributed to the Fe₃(B,C) and Si-rich phases with lower hardness distributed between the Fe₂B columnar crystals with higher hardness. There is a liner relationship between square of the thickness of borided layer and boriding time, and the calculated diffusion activation energy of boron atoms in borided layer on the 65Mn steel is 220.96 kJ/mol.

Characterization of plasma nitrided layer on 3Cr13 steel in different atmospheres at 450 ℃

Li Rui, You Yuan, Yan Mufu, Yan Jihong, Chen Hongtao, Wang Chaohui, Liu Dongjing, Hong Lin

2021, 46(11): 120-125. doi:10.13251/j.issn.0254-6051.2021.11.018

Abstract ( 43 )

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Nitrided layers on the 3Cr13 steel after nitriding for 4, 8 and 12 h in ammonia and in (ammonia+nitrogen) mixed atmosphere respectively at 450 ℃ were compared. The cross section microstructure, microhardness, phase composition and corrosion resistance of the nitrided layers were characterized by metallographical microscope, microhardness tester, X-ray diffractometer and electrochemical workstation. The nitrided layer obtained in ammonia is composed of a compound layer and a bright white layer. However, there is no bright white layer in the nitrided layer obtained in ammonia and nitrogen mixed atmosphere. The surface hardness of the nitrided layer (12 h) in ammonia is 1050.0 HV0.05. After ammonia nitriding, the compound layer's main phase is ε-Fe_2-3N, the secondary phase is γ′-Fe₄N, a small amount of CrN appears, and the phase composition of the bright white layer is γ′-Fe₄N. After nitriding in ammonia, the passivation zone of the polarization curve widens, the self-corrosion current density decreases and the corrosion resistance increases. The surface hardness of the nitrided layer (12 h) in ammonia and nitrogen mixed atmosphere is 998.0 HV0.05. Because of the increase of nitrogen concentration in the mixed atmosphere, the CrN content increases in the nitride layer after nitriding for 8 h, and the secondary phase transforms from γ′-Fe₄N, which is secondary phase after nitriding in ammonia for 8 h, to CrN. The self-corrosion current density of the nitrided layer decreases when the nitriding time prolongs to 12 h in ammonia and nitrogen mixed atmosphere, the width of the passivation zone widens, and the corrosion resistance slightly improves.

Effect of MAO voltage on friction and wear properties of TC4 alloy oxide film

Yu Guoqing, Jing Ran, Wang Guodi, Zhang Manxue, Feng Tian, Xie Niansuo

2021, 46(11): 126-130. doi:10.13251/j.issn.0254-6051.2021.11.019

Abstract ( 48 )

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Effect of micro-arc oxidation(MAO) voltage on friction and wear properties of the TC4 alloy oxide film prepared via MAO technology was studied. The morphology, phase composition, hardness and friction and wear properties of the oxide film were tested by means of laser confocal microscope, scanning electron microscope, X-ray diffractometer, micro-hardness tester and high temperature vacuum friction and wear tester. The results show that with the MAO voltage increasing, the content of Anatase-TiO₂ and Rutile-TiO₂ in the MAO-TC4 alloy surface oxide film increases, and the surface roughness, microhardness and average friction coefficient of the oxide film also increase, while the wear rate of which first decreases and then increases. When the MAO voltage is 280 V, the optimum friction and wear property of the MAO-TC4 alloy oxide film is obtained, with the minimum wear rate of 2.8 mg/cm².

Effect of process parameters on interfacial microstructure of WC particle reinforced surface composite and steel substrate

Zhang Zhanzhan, Ning Jiaqing, Zuo Lingli, Liao Haiyang

2021, 46(11): 131-136. doi:10.13251/j.issn.0254-6051.2021.11.020

Abstract ( 65 )

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WC particle reinforced surface composite was prepared on 45 steel as substrate material by spark plasma sintering and then casting process. The effects of various casting process parameters on the interface bonding and microstructure between the WC/Fe composite and the steel substrate were studied. The results show that when the casting amount increases, the numerical simulation ingot mold temperature may reach 1493 ℃, and the high temperature residence duration is about 734 s, providing favorable conditions for realizing the metallurgical bonding of molten steel and WC/Fe composite material. However, due to the large casting quantity, the microstructure of the WC/Fe composite material changes noticeably, and the WC strengthened phase is hardly observed, while a lot of fishbone carbide Fe₃W₃C appears in the microstructure. When the casting quantity is set to 2/5 of the mold volume, a good macroscopic interface between the WC/Fe composite and the steel substrate is formed, and the interface reaction product Fe₃W₃C increases, but the original morphology of the reinforced particles is still preserved.

Maximum exemption thickness of post weld heat treatment for pressure vessels

Cui Qingfeng, Fang Wunong

2021, 46(11): 137-142. doi:10.13251/j.issn.0254-6051.2021.11.021

Abstract ( 72 )

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Engineering practice shows that modern steel quality and welding technology are sufficient to ensure that the structural steel is exempt from post weld heat treatment at larger thicknesses. Relevant thickness is much larger than the provision of maximum exemption thickness of PWHT in the specifications, which originates from engineering experience. In the light of the potential brittle fracture failure of pressure vessel, the method for determining maximum exemption thickness of PWHT was suggested according to the principle of fracture mechanics. Combined with performance data of the Q345R steel, the engineering applicability of the specific thickness values was demonstrated. The results show that the conservative brittle fracture prevention requirements results in the maximum exemption thickness of PWHT in Chinese pressure vessel code being less than that of European and American codes.

Novel Q&P process, microstructure and mechanical properties of a 980 MPa grade hot-dip galvanized steel

Wang Kaitai, Xu Yunbo, Xu Bing, Gu Xingli, Wang Yuan, Li Jiayu, Ni Shenya, Gong Dianyao

2021, 46(11): 143-146. doi:10.13251/j.issn.0254-6051.2021.11.022

Abstract ( 64 )

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A novel high-quenching-temperature quenching and partitioning (HQ&P) process was proposed to investigate the microstructure and mechanical properties of a 980 MPa grade C-Mn-Si-Al hot-dip galvanized steel by comparison with the conventional quenching and austempering (QAT) process. The results show that the desired duplex microstructure consisting of blocky ferrite, bainite, and some M/A islands is achieved in the tested steel. With the increase of annealing temperature during QAT process, the content of ferrite gradually decreases while the content of bainite increases. During the HQ&P process, the bainitic transformation occurs twice, which results in a more uniform microstructure with more bainite and a small amount of retained austenite. The best mechanical properties of the tested steel treated by HQ&P process are obtained with the tensile strength of 1005 MPa and the elongation of 26.1%.

Effect of reversed austenite on low temperature impact fracture process of medium Mn steel

Li Jing, Yang Yuehui, Zhang Xiaojuan, Liang Guoli, Yuan Shaoqiang

2021, 46(11): 147-151. doi:10.13251/j.issn.0254-6051.2021.11.023

Abstract ( 51 )

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Ultra low carbon 7%Mn steel was tempered at different temperatures and the content of reversed austenite in the steel and the impact absorbed energy at -60 ℃ and -100 ℃ were measured, and the microstructure near impact fracture was observed, then the effects of content and stability of reversed austenite on low temperature impact fracture process of the tested steel were discussed. The results show that the effect of reversed austenite on low temperature toughness of the tested steel has pros and cons. It can alleviate the stress concentration at the crack tip and improve the low temperature toughness of the steel through phase transformation. On the other hand, when the stability of reversed austenite is lower, a large amount of reversed austenite can transform to martensite easily under stress, resulting in the reduction of low temperature toughness. The plastic deformation area near the impact fracture is small, which indicates that it is difficult to achieve toughening by a wide range TRIP effect during impact fracture.

Microstructure and properties of pure copper wire after high temperature long time annealing

Xiao Qiulei, Mao Xiqin, Ou Meigui, Liang Yu, Chen Desong

2021, 46(11): 152-156. doi:10.13251/j.issn.0254-6051.2021.11.024

Abstract ( 89 )

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Multi-pass cold drawing deformation for ø8 mm annealed T2 pure copper bar was carried out by the industrial wire drawing machine, and the drawn specimen of ø3.5 mm was finally obtained. The annealing treatments at 600 ℃ for different time were carried out, and the relationship between the microstructure and properties was analyzed by observing the morphologies and testing the mechanical and electrical properties. The results show that the new recrystallization grains are formed and accompanied by increase of annealing twin ratio after annealing. As the annealing time increase, the recrystallized grains grow continuously, the tensile strength and elongation after breaking fluctuates slightly. The average tensile strength and the elongation of annealed wires are about 67.3% and 8 times of that of as-drawn wires, respectively, and the average conductivity of annealed wires is about 3.3% higher than that of as-drawn wires, and the conductivity is gradually improved with the annealing time increase.

Influence of intermediate annealing on Roping pattern, microstructure and properties of cold rolled 6016 aluminum alloy plate

Li Dalin, Zhu Pengcheng, Wang Ping, Lü Zhengfeng, Zhang Shanshun

2021, 46(11): 157-160. doi:10.13251/j.issn.0254-6051.2021.11.025

Abstract ( 61 )

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Influence of intermediate annealing on Roping pattern, microstructure and properties of the 6016 aluminum alloy was investigated by using optical microscope and TEM. The hemming property of the plate was tested, and its microstructure was characterized. The results indicate that after intermediate annealing at 450 ℃ for 1 h, the banded distribution of grains with different orientations formed by rolling deformation in the matrix structure is weakened, and the uneven yield extension of the matrix structure is weakened. The intermediate annealing treatment at 450 ℃ makes the Roping pattern on the surface of 6016 aluminum alloy plate disappear completely, the hemming value is only 0.4, and the hemming property is the best. After intermediate annealing at 350 ℃ and 400 ℃, the surface coarse grain is obvious, and the grain size reaches 400-550 μm. The size and quantity of dispersed phase in the matrix are large, and the size reaches 50-100 nm. Coarse surface grains and a large number of precipitates at grain boundaries are the main reasons for the deterioration of hemming property.

Effect of annealing temperature on microstructure and properties of large-diameter TA31 titanium alloy seamless tube

Deng Yajie, Huang Haiguang, Zhang Haoze, Shi Yaming, Zhang Yuqin, Jiang Yehua

2021, 46(11): 161-165. doi:10.13251/j.issn.0254-6051.2021.11.026

Abstract ( 72 )

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Ti-6Al-3Nb-2Zr-1Mo(TA31) titanium alloy round ingots were cast by using vacuum consumable electric arc furnace and electron beam cooling bed furnace, and a ø178 mm×12 mm large-diameter seamless tube was obtained by directly cross piercing, then the effect of annealing temperature (800, 850, 900, 950 ℃) on microstructure and properties of the seamless tubes was studied. The results show that the rolled seamless tube in deformed Widmanstätten structure is mainly composed of lamellar α-phase colonies and original β-phase grain boundaries. After annealing, the lamellar primary α-phase reduces and the original β-phase grain boundaries disappear, with the microstructure being gradually homogenized. But when the annealing temperature exceeds 900 ℃, the α-phase colonies are coarsened and transform into a basketweave structure. With the increase of annealing temperature, the tensile strength and yield strength of the tube first decrease slightly and then slowly increase, while the elongation first increases and then decreases, and the morphology of fracture changes from ductile + quasi-cleavage mixed fracture to ductile fracture and then to ductile + quasi-cleavage mixed fracture. The comprehensive analysis shows that the suitable annealing temperature for the TA31 titanium alloy large-diameter seamless tube prepared in a short process is about 900 ℃, at which the average values of tensile strength, yield strength and elongation are 873 MPa, 785 MPa and 12.8%, respectively.

Microstructure and properties of TC4 titanium alloy bar for aerospace fasteners after solid solution treatment and aging

Wu Chen, Ma Baofei, Xiao Songtao, Li Minna

2021, 46(11): 166-169. doi:10.13251/j.issn.0254-6051.2021.11.027

Abstract ( 79 )

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TC4 titanium alloy bars for aerospace fasteners were treated by solid solution treatment and aging, the miorostructure, hardness and room temperature tensile properties at different positions of the alloy bar were observed and examined. The results show that the microstructure and properties of the alloy bar after solid solution and aging treatments are significantly different from surface to core because of the cooling rate difference. After solid solution treatment and aging, the microstructure consists of stable equiaxed α, dispersed α′ martensite and substable β phases. There is no significant difference in morphology and content of secondary α phase in the end section of the specimen because of the similar cooling rate. From the upper part of the middle section to the core, the secondary α phase increases in content gradually, and the secondary α phase layer increases in thickness step by step and tends to be equiaxed. There is no obvious difference in microhardness at different positions of the end section, but the microhardness value from edge to core on the middle section shows a general decreasing trend, and the microhardness at edge on the middle section is close to that of the end section. Because of the slow cooling of the specimen core during solution treatment, the tensile properties of the whole specimen at room temperature are obviously poorer than those of the specimen with its core part removed.

Effect of pre-treatment temperature on strength of Cr-Ni-Co-Mo maraging stainless steel

Ji Yurui, Yang Zhuoyue, Tan Honglin, Ding Yali

2021, 46(11): 170-173. doi:10.13251/j.issn.0254-6051.2021.11.028

Abstract ( 60 )

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Effect of pre-treatment temperature on strength of Cr-Ni-Co-Mo maraging stainless steel was investigated, including the effect on heritability of the original forged coarse grains and spontaneous recrystallization, as well as the effect of pre-treatment temperature on the amount of retained and reversed austenite and the mechanical properties after the final heat treatments. The results show that, the pre-treatment below 900 ℃ inherits the forging coarse grains, that is, the austenite with high defect density is formed by the α′→γ shear mechanism. The pre-treatment temperature rises, the amount of retained austenite after solid solution at 750 ℃, cryogenic treatment at －73 ℃, and the amount of retained and reversed austenite after aging at 500 ℃ decrease, so the final tensile and yield strength increase. The pre-treatment above 800 ℃ does not influence much on retained and reversed austenite amount after final heat treatment, and the tensile and yield strengths tend to stabilize. The spontaneous recrystallization of the inherited forged coarse grains via pre-treatment above 900 ℃ refines the grains and reduces the density of defects in the formed austenite, and finally reduces the density of defects accumulated in the austenite after solution treatment at 750 ℃, so the final aging strengthening effect is reduced, resulting in that the grain refinement is not enough to improve the final strength.

Measurement of complete dissolution temperature of γ′ phase in nickel-based P/M superalloy FGH4096

Li Chang, Chen Leilei, Qu Zonghong, Lai Yunjin, Liang Shujin

2021, 46(11): 174-177. doi:10.13251/j.issn.0254-6051.2021.11.029

Abstract ( 63 )

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Equilibrium phase diagram of the nickel-based P/M superalloy FGH4096 was tested by JmatPro software, and the γ′ phase dissolution results at different solid solution temperatures were studied by using OM and SEM methods according to the γ′ phase equilibrium dissolution temperature. The results show that the γ′ phase in the FGH4096 alloy calculated by the thermodynamic software JmatPro is dissolved in a large amount between 930-1100 ℃, and the complete dissolution temperature is above 1100 ℃. From 1110 ℃ to 1130 ℃, the grain size increases from 16.1 μm to 18.2 μm, and the change is not obvious, but from 1130 ℃ to 1160 ℃, the grain size quickly increases from 18.2 μm to 28.6 μm. While the γ′ phase in the nickel-based P/M superalloy FGH4096 plays a strengthening role, it also hinders the growth of grains. As the solid solution temperature increases from 1110 ℃ to 1130 ℃, the content of the γ′ phase decreases. The pinning effect is reduced, resulting in grain growth. Above 1130 ℃, the γ′ phase is completely dissolved, the pinning effect of the γ′ phase disappears, and the grains grow rapidly. Finally, it is determined that the actual complete dissolution temperature of γ′ phase in the FGH4096 alloy is between 1130-1140 ℃.

Effect of quenching temperature on microstructure of low alloyed corrosion resistant 27CrMo48VNb steel OCTG

Gu Shunjie, Lü Chuantao, Xiao Xu, Ding Lei, Li Yang, Chen Pengfei

2021, 46(11): 178-181. doi:10.13251/j.issn.0254-6051.2021.11.030

Abstract ( 44 )

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A series of quenching treatment at different temperatures were performed on low alloyed corrosion resistant 27CrMo48VNb steel OCTG. The microstructure, prior austenite grain and precipitates of the tested steel quenched at different temperatures were characterized by optical microscope and transmission electron microscope. The effect of quenching temperature on microstructure, grain size and precipitate was analyzed. The results show that the martensite can be observed in the tested steel after quenching. The size of microstructure and prior austenite grain gradually increases with the increase of quenching temperature. When the quenching temperature is 890-1000 ℃, the grain size increases slightly with the increase of quenching temperature; when the quenching temperature exceeds 1000 ℃, as the quenching temperature increases, the prior austenite grains are significantly coarsened. The change trend of microstructure and prior austenite grain size versus quenching temperature is related to the dissolution and precipitation behavior of precipitates at high temperature. Therefore, the quenching temperature from 890 ℃ to 1000 ℃ of the tested steel is recommended.

Effect of tempering temperature on induction quenched and tempered microstructure and mechanical properties of 42CrMo steel bar

Zhang Qing, Cao Pei

2021, 46(11): 182-185. doi:10.13251/j.issn.0254-6051.2021.11.031

Abstract ( 75 )

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Effect of tempering temperature (500, 550, 600, 650, 700 ℃) on microstructure and mechanical properties of the 42CrMo steel bar was studied by medium frequency induction quenching and tempering. The results show that as the tempering temperature increases, the microstructure of the 42CrMo steel keeps as tempered sorbite. The carbides gradually change from unevenly distributed fine needle shape to short rod shape, and the aspect ratio decreases, then transform into a dispersed granular shape at 600 ℃, and segregate at 650 ℃, coarsen rapidly at 700 ℃. With the increase of induction tempering temperature, the hardness, tensile strength and yield strength show a continuous downward trend, while the elongation after fracture and the percentage reduction of area show a continuous small upward trend.

Effect of heat treatment process on mechanical properties and corrosion resistance of 5Cr steel

Liu Yong, Zhang Zhihui, Gao Yan, Zhao Jinbin, Wu Huibin

2021, 46(11): 186-190. doi:10.13251/j.issn.0254-6051.2021.11.032

Abstract ( 78 )

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Effect of heat treatment process on microstructure and mechanical properties of the 5Cr steel was studied by using optical microscopes, tensile and impact testing machines and other instruments. The corrosion process of 5Cr steel was simulated and the resistance of hot rolled and quenched + tempered steels was compared and analyzed by using high temperature and high pressure reactors and combing with electrochemical tests. The results show that after quenching and tempering treatment, the 5Cr steel has carbides dispersed and precipitated in the structure and has high strength and good plasticity and toughness. The corrosion rate of hot rolled steel is 0.188 mm/a, and that of quenched + tempered steel is 0.158 mm/a. In the quenched + tempered steel, the carbide (Fe₃C) as the cathode is less and uniformly dispersed, the self-corrosion potential is higher and the corrosion current density is lower of the structure, the degree of galvanic corrosion is shallow, and the corrosion resistance is better.

Influence of retrogression treatment on microstructure and mechanical properties of 7A20 aluminum alloy during RRA treatment

Wang Shan, Yang Chunmiao, Zhang Riqiang, Wang Shuhui, Teng Dunbo, Xiang Qingyi

2021, 46(11): 191-194. doi:10.13251/j.issn.0254-6051.2021.11.033

Abstract ( 51 )

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Influence of retrogression treatment on microstructure and mechanical properties of the 7A20 aluminum alloy during RRA treatment was investigated. The mechanical properties and conductivity were tested. The microstructure at grain boundary and inner grain was characterized by using OM and TEM (HRTEM). The results indicat that the tensile strength decreases from 630 MPa to 525 MPa under 160 ℃ retrogression treatment with the increase of time. While tensile strength increases from 645 MPa to 710 MPa after reaging treatment at 120 ℃ for 12 h. The longer the retrogression time, the greater the increase of intensity caused by reaging, which reaches 185 MPa after 80 min. At a retrogression temperature of 200 ℃ and a retrogression time of 10 min, the peak strength is 715 MPa. As the retrogression temperature increases, the conductivity first increases and then decreases, which has a positive correlation with retrogression time. After retrogression at 160, 200 and 240 ℃ corresponding to the peak strength, the grain sizes of matrix are 55, 65 and 70 μm, respectively with a little difference. After retrogression treating at 200 ℃ for 10 min and reaging treatment, there is discontinuous MgZn₂ phase precipitated at grain boundaries with the size of 10-20 nm, which is incoherent with the matrix. There is dispersive nano-sized η′ strengthening precipitates inner the grains, which is coherent or semi-coherent with the matrix. There is a PFZ region with a width of 45-55 nm at the grain boundary.

As-cast microstructure and high temperature solution treatment process of 254SMO stainless steel

Gao Ang, Qian Zhangxin, Liu Dexue

2021, 46(11): 195-198. doi:10.13251/j.issn.0254-6051.2021.11.034

Abstract ( 59 )

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Effect of solution treatment at different temperatures and time on the as-cast microstructure of super austenitic stainless steel 254SMO was studied by microstructure observation and back scattering electron scanning, and the σ phase dissolution process in the steel was studied by electron microprobe. The results show that with the increase of solution treatment time and temperature, the as-cast dendrite structure and the segregation gradually disappear, and the σ phase gradually dissolves and transforms into δ phase (ferrite). The optimum solid solution treatment process is 1280 ℃×9 h.

Optimization of heat treatment process of 3379BA1 steam turbine blade steel

Lu Chunguang, Mei Guojun, Lin Shengxiu, Qian Cairang, Li Yang, Lu Xiaoxue

2021, 46(11): 199-201. doi:10.13251/j.issn.0254-6051.2021.11.035

Abstract ( 55 )

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Orthogonal test method was used to study the relationship between heat treatment process and mechanical properties of the 3379BA1 steam turbine blade steel. The results show that the order of factors affecting the mechanical properties of the tested steel is tempering temperature, quenching temperature, tempering time, quenching time, and the optimal heat treatment process parameters are obtained as follows: quenching at 1050 ℃(holding for 60 min, oil cooling) and then tempering at 700 ℃(holding for 120 min, air cooling). Through test verification, the 3379BA1 steel treated under the optimal heat treatment can meet the property requirements, the average value of impact absorbed energy is higher by about 10 J than before the process optimization, and the yield ratio reaches 87.3%.

Effect of laser shock peening on residual stress and fatigue property of 7050 aluminium alloy with hole

Han Peipei, Jiao Qingyang, Quan Chunyi, Zhao Dong, Sun Rujian, Che Zhigang

2021, 46(11): 202-206. doi:10.13251/j.issn.0254-6051.2021.11.036

Abstract ( 65 )

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Effect of laser shock peening on microhardness, residual stress and fatigue property of the 7050 T7451 aluminium alloy with hole was studied. The results show that when the laser energy is 30 J, circular spot diameter is 4 mm and the shock times are twice, the microhardness of the 7050 aluminium alloy increases obviously, the surface microhardness increases by 12% compared with the base material, with the hardened layer of 1 mm. The amplitude of residual compressive stress exceeds 300 MPa, and the influence depth of residual compressive stress can reach about 1 mm, which is significantly greater than the depth of influence layer strengthened by shot peening. The residual compressive stress can improve the initiation resistance of fatigue crack, and the deeper influence layer of residual compressive stress is beneficial to prolong the crack growth life. Laser shock peening can obviously improve the fatigue life of the aluminum alloy with hole, which is increased by 4.7 times to 17.6 times compared to that of the untreated specimen, and the fatigue life gain and stability are obviously better than those of shot peening.

Oxidation resistance of hot-dip aluminized coating on cast K438 superalloy

Deng Pengfei, Zuo Linchun, Chen Xingwei, Xiong Wei, Liu Yan, Liao Conglai, Li Zhi

2021, 46(11): 207-212. doi:10.13251/j.issn.0254-6051.2021.11.037

Abstract ( 58 )

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In view of the insufficient oxidation resistance of K438 nickel-based superalloy at high temperature, the aluminized coating was prepared on its surface by hot dip aluminizing technology. After vacuum diffusion annealing treatment at 1000 ℃ for 1 h, the high temperature cyclic oxidation tests at 1000 ℃ for 250 cycles were carried out on the superalloy with hot-dip aluminized coating. The cross-sectional microstructure of the coatings after high temperature oxidation was studied by means of scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD). The results show that the oxidation mass gain curve of the substrate alloy begins to decline after oxidation at 1000 ℃ for 40 cycles, and the oxidation resistance at high temperature is poor, and the oxidation products such as Al₂O₃, TiO₂, Cr₂O₃, NiO and spinel NiCr₂O₄, NiAl₂O₄ are formed after oxidation at 1000 ℃ for 250 cycles. However, the oxidation mass gain changes a little after oxidation at 1000 ℃ for 40 cycles, the Cr₂O₃ is not found in the oxidation products of hot-dip aluminized coatings after oxidation at 1000 ℃ for 250 cycles, and its high temperature oxidation resistance is significantly higher than that of the substrate alloy, among which the coating hot-dip aluminized for 90 s has better oxidation resistance at high temperature.

Effect of pulsed bias frequency on microstructure and properties of TiSiN film

Wei Yongqiang, Song Rufan, Jiang Zhiqiang

2021, 46(11): 213-219. doi:10.13251/j.issn.0254-6051.2021.11.038

Abstract ( 63 )

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Pulsed bias arc ion plating technology was used to deposit TiSiN film on M2 high-speed steel substrate by changing pulsed bias frequency. Effect of pulsed bias frequency on surface and cross-sectional morphologies, element composition and phase structure of the TiSiN films, and the nanohardness and elastic modulus of the TiSiN films were tested by means of nanoindenter, scanning electron microscope (SEM), energy spectrometer (EDS), X-ray diffractometer (XRD) and other instruments. Within the statistical field of view(9×10³ μm²), the macro-particle diameter on the surface of the TiSiN films varies between 0.30 and 7.26 μm. With the increase of pulsed bias frequency from 40 kHz to 60 kHz, the number of macro-particles on the surface of the TiSiN films reduces from 495 to 356. As the pulsed bias frequency increases to 80 kHz, the number of macro-particles increases to 657. With the pulsed bias frequency of 60 kHz, the numbers of macro-particles and micro-pit defects on the surface of the TiSiN films are the least and the Si atom content reaches the minimum value of 0.46%. When the pulsed bias frequency is 50 kHz, the TiSiN films grow in a non-columnar structure, and the thickness reaches the minimum value of 1.63 μm. When the pulsed bias frequency is 60 kHz, the columnar crystal structure is refined, and the density of the films increases. The TiSiN films has a preferred orientation at the (111) crystal plane under different pulsed bias frequencies, but no Si peak is detected. Amorphous Si₃N₄ exists in the TiSiN films, the TiN crystals and Si₃N₄ amorphous structure form the composite structure. At the pulsed bias frequency of 60 kHz, the mount of the macro-particles on the TiSiN films surface reaches the minimum value. The nanohardness of the TiSiN films at the pulsed bias frequency of 60 kHz reaches 34.56 GPa, which is about three times higher than that of the M2 high-speed steel substrate. When the pulsed bias frequency is 50 kHz, the corrosion potential of the TiSiN film reaches the maximum value －0.352 V(vs SCE), which is 723 mV higher than that of the substrate, and the corrosion current density reaches the value of 0.73 μA/cm². When the pulsed bias frequency is 70 kHz, the corrosion potential of the TiSiN film reaches the value －0.526 V(vs SCE), and the corrosion current density reaches the minimum value of 0.66 μA/cm².

Microstructure evolution of Al-Si coating on hot stamping steel during austenitizing

Wang Junhui, Cui Qingling, Zeng Linlin, Gao Xinyu, Liang Jiawei

2021, 46(11): 220-225. doi:10.13251/j.issn.0254-6051.2021.11.039

Abstract ( 54 )

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Microstructure evolution of Al-Si coating during austenitization process on the 22MnB5 hot stamping steel was studied by means of differential scanning calorimeter, field emission electron probe and laser confocal microscope. The results show that during the heating process of the coated plate, the Al-Si coating melts at about 570 ℃. Due to the lower temperature, the diffusion of Al, Fe, and Si atoms is blocked by Fe₂SiAl₇. When the temperature rises to about 610 ℃, the diffusion of Al atoms into the coating increases, which makes Fe₂Al₅ further grow; Si atoms diffuse to the outer surface of the coating and the substrate. Because Fe₂Al₅ has a weak ability to dissolve Si atoms, a layer is formed at the Fe₂Al₅ grain boundary in which precipitate is FeSiAl₂, and the remaining Si atoms diffuse on the surface of the coating to form Fe₂SiAl₇. At 750 ℃, Al atoms diffuse into the matrix to form Fe₃Al; the increase of Fe atoms in the coating makes Fe₂Al₅ and FeAl₂ continue to grow; due to the lower solubility of Si atoms in the Fe₂Al₅ and FeAl₂ phases, Fe₃SiAl₅ precipitates will form at the grain boundaries. Compared with Fe₂Al₅, Fe₃SiAl₅, Fe₃Al, the growth rate of FeAl₂ phase is faster, and it occupies the largest volume of Al-Si coating. This is because of FeAl₂ orthorhombic crystals, the high vacancy rate (30%) along the c-axis in the grid leads to stronger growth kinetics of the FeAl₂ phase.

Microstructure and properties of laser clad Ni-Al composite coatings on Cr12MoV steel

Chen Zixin, Zhou Houming, Zhu Yuxu, Zhang Zhehao, He Fangjia, Xu Caixing, Liu Gang

2021, 46(11): 226-231. doi:10.13251/j.issn.0254-6051.2021.11.040

Abstract ( 57 )

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Ni-Al composite coatings on Cr12MoV steel were fabricated by laser cladding technology and the effect of Al content on structure and mechanical properties of the clad layer was studied. The results show that when the Al content is more than 14%, cracks begin to appear. There is no obvious inclusion in the clad layer, but there are hard phases Al₂O₃ and intermetallic compounds Ni₃Fe and AlNi₃in the clad layer. As the Al content increases, the boundary of molten pool gradually disappears, and the hardness and wear resistance decrease firstly and then increase. On the whole, the clad layer with 14% Al has the best performance.

Effect of real TGO interface topography on interface stress of thermal barrier coatings

Guo Huimin, Li Bo, Zhang Liqun, Tao Jinglei, Huang Wei, Pei Xiuyuan, Xiao Junfeng, Nan Qing

2021, 46(11): 232-235. doi:10.13251/j.issn.0254-6051.2021.11.041

Abstract ( 62 )

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Effect of real thermally growth oxide (TGO) interface topography on interface stress of thermal barrier coatings on the surface of first-stage blade of a heavy-duty gas turbine was studied by using finite element analysis software MSC.MARC. The results show that in the top coat (TC) layer at the TC/TGO interface, the tensile stress in normal stress σ₂₂ is located in the trough area, and the compressive stress is located in the peak area; while in the bond coat (BC) layer at the BC/TGO interface, the σ₂₂ stress distribution is opposite to that of the TC layer. The σ₁₂ stress distribution of the BC layer is the same as that of the TC layer, the distribution of shear stress σ₁₂ in the TC layer is that the stress direction on the left side of the trough is negative, and that on the right side is positive. The normal stress σ₂₂ at the peak and trough in the TGO increases with the thickness increase of the TGO; and when the thickness of TGO is constant, with the increase of the amplitude of the BC/TGO interface, the σ₂₂ stress in TGO and BC increases.

Influence of fabrication technology on surface morphology of giant magnetostrictive Fe-Ga alloy film

Shen Zhi, Yan Jianwu, Jin Kang, Zhou Yingli, Yin Jian

2021, 46(11): 236-240. doi:10.13251/j.issn.0254-6051.2021.11.042

Abstract ( 47 )

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Fe-Ga alloy films were prepared by JZCK-600F multifunctional coating equipment, and the effect of sputtering technology on deposition rate and surface morphology of the Fe-Ga alloy films was investigated. The surface morphology and composition of the films were studied by SEM and EDS. The results show that sputtering time and power are the main factors that affect the thickness and growth rate of Fe-Ga alloy film when other process parameters are not changed. The thickness and deposition rate of the films increase with the increase of sputtering time and power and showing a positive proportional relationship between the film thickness and sputtering time and power. However, when the film thickness is too large, the increased internal stress may make the film peel off. When the sputtering power is too high, the internal stress may also induce inner cracks in the thin film. The magnetic domain images of Fe-Ga alloy films show obvious contrast between light and dark. The shape of the domain is irregular and round, similar to the structure of coral. The Fe-Ga alloy films prepared by magnetron sputtering have good crystallization and growth, and the morphology of the films is uniform and compact granular structure. The optimized sputtering parameters are sputtering power of 80 W, sputtering pressure of 0.6 Pa, sputtering time of 60 min and Ar gas flow of 25 mL/min. By which, the deflection of magnetostrictive thin film cantilever fabricated is 69.048 μm, which can meet the requirements of the fabrication of micro-devices.

Properties of laser clad AlCoCrFeNiW_x high entropy alloy coatings on H13 steel

Wu Tao, Wu Mengting, Chen Yunxiang, Hua Qianfeng, Shi Shuqin, Li Jiheng, Mao Pengzhan, Chen Xiaoming

2021, 46(11): 241-244. doi:10.13251/j.issn.0254-6051.2021.11.043

Abstract ( 147 )

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Hot working die steel H13 was used as the matrix, and AlCoCrFeNiW_x (x=0, 0.5) high-entropy alloy coatings were prepared by laser cladding, and the influence of W element addition on its structure, thermal stability and wear resistance was researched. The phase structure of the laser clad AlCoCrFeNiW_x high-entropy alloy coatings changes with the addition of W element. The addition of W element will promote the formation of the BCC phase, and the second phase rich in W forms at the grain boundary. The growth direction forms elongated dendrites along the cooling direction, and these second phases play a role of wear-resistant and reinforcing the coatings. The tempering stability of the high-entropy alloy coatings was studied after long-term heat preservation at 800 ℃. The results show that the AlCoCrFeNiW_x high-entropy alloy coatings can maintain high hardness and wear resistance, and the hardness after 14 h heat preservation is still above 400 HV0.2, the coating added with W element has stronger resistance to tempering and softening, and the friction and wear resistance is more than twice that of the substrate after heat preservation at 800 ℃ for 14 h.

Crack cause analysis of 20Cr2Ni4A steel gimbal axis

Luo Zixiang, Liu Ke, Yang Bing, Deng Yaoyao, Yuan Wufeng, Xiang Zhendong

2021, 46(11): 245-249. doi:10.13251/j.issn.0254-6051.2021.11.044

Abstract ( 81 )

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During the assembly process of 20Cr2Ni4A steel gimbal shaft, it was found that cracks were located at the oil hole and pin hole on the outer side of the frame. The physical and chemical analysis results show that there are suspected carburized layers and high hardness phenomenon. The simulation analysis shows that the stress values of the oil through-hole and the pin hole in the carburized and non-carburized states are different, and that of the carburized state is significantly higher than that of the non-carburized state. By quenching test, it is found that when the temperature is kept at 800 ℃ for 60 min and a higher carbon potential is applied, the surface of the 20Cr2Ni4A steel have different degrees of carburization. Based on the above analysis and production verification, it can be concluded that the cause of crack formation is the existence of carburized layer on the wall of pin hole and oil hole, and the stress reaches the cracking critical state during quenching, which leads to cracks in part of the gimbal axis.

Fracture failure analysis of 2Cr13 steel bolt in a 600 MW subcritical unit

Li Mengyang

2021, 46(11): 250-253. doi:10.13251/j.issn.0254-6051.2021.11.045

Abstract ( 61 )

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Fracture causes of ground bolts made of 2Cr13 steel for circulating water pump, were studied by the systematic failure analyzing including chemical composition, mechanical properties, microstructure and so on. The results indicate that not only the C content of the batch bolts is lower, but also the tensile strength, yield strength, hardness and impact property at room temperature are lower than the standard requirements. The fracture analysis shows that the material is very brittle. At the same time, the microstructure analysis shows that the heat treatment process of the 2Cr13 heat-resistant steel is as-annealed, and the comprehensive properties of the as-annealed 2Cr13 heat-resistant steel are obviously lower than those of the quenched and tempered steel. Therefore, the main cause of the bolt fatigue fracture is that the microstructure and properties do not meet the relevant requirements due to the improper heat treatment.

Failure analysis of compressor impeller blade made of 17-4PH stainless steel

Kang Zhenwei, Zhou Genshu, Liu Lijun, Zhang Xiaoke, Wang Mengjie

2021, 46(11): 254-257. doi:10.13251/j.issn.0254-6051.2021.11.046

Abstract ( 63 )

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Blades on the impeller of some kind of nitrogen compressor failed in early stage of service life. The chemical composition, metallographic structure, mechanical properties, macroscopic and microscopic fracture morphologies of the blades were analyzed for the failure causes. The results show that the impeller blade material is 17-4PH stainless steel and there are obvious fatigue characteristics on the blade fracture, indicating that the fracture is a fatigue fracture. The chemical composition, microstructure and properties of the blade material meet the standard requirements. The main cause of fracture is vibration of the blades.

Metallographic examination of aluminum alloy piston of an internal combustion engine

Zhang Zhenwei, Zhu Yujin, Jiang Rui, Zhao Jie, Chen Yun, Li Yulei, Ding Boyuan

2021, 46(11): 258-261. doi:10.13251/j.issn.0254-6051.2021.11.047

Abstract ( 62 )

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Macrostructure and microstructure of the piston of an internal combustion engine were analyzed by means of metallographic examination. The results show that the dispersed porosity and the concentrated porosity in the piston matrix are level 1 and level 1, respectively. The dispersive hole and the concentrated hole in the composite position are grade 1 and grade 1, respectively. The α-solid solution in the piston specimen is large, eutectic silicon is in short strip shape, some are in small block shape, and primary silicon is in medium block shape, and slightly aggregated, so is rated as grade 3. The fishbone-like iron inclusion and acicular iron inclusion are not obvious, so the iron inclusions are rated as grade 1.

Design and fluid-thermal coupling of a strong quenching tank with double vortex flow field

Qu Zhe, Zhu Xiaoshuo, Xing Ruofei, Fu Yudong

2021, 46(11): 262-269. doi:10.13251/j.issn.0254-6051.2021.11.048

Abstract ( 52 )

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In view of the lack of strong quenching equipment with uniform, high-speed and stable flow field, a physical model of quenching tank of strong quenching equipment with double vortex flow field was designed. The fluid mechanics analysis was carried out by using Fluent software, and the internal flow field of quenching tank of strong quenching equipment was studied. Based on the three-dimensional Reynolds average conservation N-S equation and the standard k-ε turbulence model, the flow field generated in the quenching tank was numerically calculated with the water inlet velocity of 3.0, 3.5, 4.0 m/s. The temperature field for specimen of 0.45% carbon steel in the quenching tank was calculated by the fluid-thermal coupling analysis. The results indicate that the liquid phase medium flow forms a double vortex fluid field in the quenching tank and a circulation field around part forms with high speed and stable flow. For the optimal process, the speed of the circulation field is 2.0 m/s when the water inlet speed is 4.0 m/s. This speed makes the cooling rate of 650 ℃/s for 0.45% carbon steel specimen, which fully meets the requirement of strong quenching.

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