20 April 2017, Volume 41 Issue 4
    

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  • Phase Structure and Electric Property of BiFeO3 Doped 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 Piezoelectric Ceramics
    LI Bo, WANG Yi-ping, YANG Ying, CHEN Jing
    Materials For Mechanical Engineering. 2017, 41(4): 1-5. https://doi.org/10.11973/jxgccl201704001
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    (1-x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xBiFeO3 (BNBT6-xBF, 0.02≤x≤0.10, molar fraction) lead-free piezoelectric ceramics were prepared by the conventional solid-state reaction method, and the effects of BiFeO3 doping content (namely x) on the crystal structure, the dielectric, ferroelectric and electric field induced strain response properties of the ceramics were investigated. The results show that the obtained BNBT6-xBF ceramics all had a pure perovskite structure. The crystal structure of the ceramics was still near the morphotropic phase boundary with x less than 0.08 while changed to pseudo cubic structure with x equal to 0.09. The doping of BiFeO3 was favorable for the increase of ceramic dielectric constant. With the increase of BiFeO3 doping content, a phase transition from morphotropic phase to pseudocubic phase was induced in the ceramics and the ferroelectric property decreased. The electric field induced strain first increased then decreased and reached the maximum value of 0.28% with x equal to 0.09, meanwhile the normalized piezoelectric constant was 440 pm ·V-1.
  • Microstructure Evolution of Delta-Processed GH4169 Alloy during Hot Deformation
    SI Jia-yong, CHEN Long, LIAO Xiao-hang, LIU Na, ZHONG Li-ping
    Materials For Mechanical Engineering. 2017, 41(4): 6-10. https://doi.org/10.11973/jxgccl201704002
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    The hot compressive deformation experiments were conducted on the delta-processed GH4169 alloy at the deformation temperatures of 900-1 060 ℃ and strain rates of 0.001-0.500 s-1. The effects of deformation temperature, strain rate and strain on the microstructure and the morphology and evolution of δ phase were studied and the evolution mechanism of microstructure was analyzed. The results show that a large number of plate-like or long-needle-like δ phase distributed in the microstructure of the delta-processed alloy. During the hot compressive deformation, with the increase of strain, the bending, kinking and spheroidization of the δ phase occured due to the combined effects of deformation and dissolution breakages. With the deformation temperature increasing or the strain rate decreasing, the content of δ phase decreased, while the grain size and the volume fraction of dynamic recrystallization increased.
  • Application of Combined Weighted Method Based on Weight Confidence on Selection of Engineering Materials
    CHEN Kui, ZHANG Tian-yun, ZHONG Yue-fu, HU Xue-wen
    Materials For Mechanical Engineering. 2017, 41(4): 11-14. https://doi.org/10.11973/jxgccl201704003
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    Referring to the question that it is hard to determine the index weight of comprehensive evaluation for the selection of engineering materials, combined weighted method based on weight confidence is put forward on the base of analyzing the theory and shortcoming of existing combined weighted method. Taking the choice of storage pot material under low temperature working condition as an example, subjective, objective and intelligent weights of eight kinds of evaluation indices, obtained by analytic hierarchy process, improved entropy method and back propagation network respectively, were combined with combined weighted method proposed above. Ten kinds of candidate materials were evaluated according to improved technique for order preference by similarity to ideal solution (ITOPSIS) with the combined weights obtained. The results show that the 301 stainless steel is the best storage pot material under low temperature working condition, this accords with industry practice. All above show that the weights of evaluation indices for engineering materials determined by the combined weighted method are reasonable and feasible.
  • Surface Strengthening Effect of Nickel-Aluminum Bronze Treated by Shot Peening at Different Intensities
    XIONG Di, WANG Li-qiang, XU Xiao-yan, LÜ Yu-ting, LÜ Wei-jie
    Materials For Mechanical Engineering. 2017, 41(4): 15-19. https://doi.org/10.11973/jxgccl201704004
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    The cast nickel-aluminum bronze was shot peened at shot peening intensities of 0.15, 0.20, 0.25 mm. The surface residual stress, micro-hardness, micromorphology and surface roughness of the nickel-aluminum bronze before and after shot peening were analyzed and compared, and the domain size and micro-strain in the surface layer were calculated. The results show that with the increase of shot peening intensity,the compressive residual stress and the level of work-hardening of the nickel-aluminum bronze both increased grandually. The surface roughness increased obviously, accompanied by the increase of micro-cracks when the shot peening intensity was too large, thus leading to the surface stress relaxation and resulting in the decrease of strengthening effect.
  • Effect of Resistance Spot Welding Process on Property of GH163 Nickel-based Superalloy Joint
    ZHOU Qing-quan, SHUAI Ge-wang, LIU Ze-min
    Materials For Mechanical Engineering. 2017, 41(4): 20-23. https://doi.org/10.11973/jxgccl201704005
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    The resistance spot welding was conducted on 1.5 mm thick GH163 nickel-based superalloy, and the effects of welding current, electrode pressure and welding time on the microstructure, microhardness and tensile shear of the joint were studied. The relatively good welding parameters were obtained. The results show that the fine dendritic columnar crystals was uniformly distributed in the nugget zone of the joint, with the growth direction perpendicular to the nugget arc line, which was the same direction as the heat radiating of the nugget. Meanwhile with the current increase, the grain size decreased. The microhardness of the joint increased with the shortening distance to the nugget center while showed an increasing trend with the increasing welding current. The tensile shear of the joint first increased then decreased, and then increased again with the welding current increase, and gradually increased with the prolongation of welding time while decreased with the increase of electrode pressure. When the electrode pressure was 11.0 kN, welding current was 7.1 kA and welding time was 0.50 s, the comprehensive properties of the joint were the best, with the nugget diameter of 7.14 mm, the room and high temperature tensile shear force of 15.34 kN and 13.13 kN, and the tensile fracture mode of base metal fracture.
  • Static Softening Behavior of a High-Strength and High-Toughness Low-Alloy Martensite Steel
    ZHAO Yan-jun, MENG Qing-xue, MA Ben-li, ZENG Jian-min, JIANG Chang-biao
    Materials For Mechanical Engineering. 2017, 41(4): 24-28. https://doi.org/10.11973/jxgccl201704006
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    With the new developed high-strength and high-toughness 20SiMn3NiA low-alloy martensite steel as a research object, the double-pass compression tests were conducted on the steel by a thermal simulator at 900-1 000 ℃ and strain rate of 1.0 s-1 for pass intervals between 1 s and 100 s. And the static softening behavior of the steel was studied. The results show that at the deformation temperature of 900 ℃, with the pass interval prolonging, the true stress-true strain curves during second pass deformation of the tested steel exhibited a change from dynamic recrystallization character (namely greater trend of softening than hardening) to static recrystallization character (namely greater trend of hardening than softening). The static recrystallization fraction increased from 6.48% with pass interval of 1 s to a nearly stable value of 85%. At the deformation temperature of 1 000 ℃, the true stress-true strain curves during second pass deformation had a static recrystallization character. The static recrystallization fraction increased from 84.48% with pass interval of 1 s to 96% with pass interval of 100 s. The static active energy of the tested steel was 448 kJ ·mol-1.
  • Hot-Compression Deformation Constitutive Equation of 60Si2CrVAT High-Strength Spring Steel
    ZHOU Qiu-yue, WU Xiao-dong, LIANG Yu, XIE Jian-feng, WU Shun, ZOU Lei
    Materials For Mechanical Engineering. 2017, 41(4): 29-32,57. https://doi.org/10.11973/jxgccl201704007
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    The hot compression deformation of 60Si2CrVAT high-strength spring steel was carried out on thermo-simulation machine at different temperatures (900,950,1 050,1 150℃) and strain rates of 0.1-10 s-1, the influences of deformation temperature and strain rate on hot deformation behavior of the steel were investigated. The hot-compression deformation constitutive equation of 60Si2CrVAT spring steel was established based on hyperbolic sine equations of Arrhenius type. The results show that the flow stress of the spring steel increased with the increase of deformation rate and decreased with the increase of deformation temperature, it was found that dynamic recrystallization was more likely to occur at high deformation temperature and low strain rate. And the activation energy was calculated to be 372 kJ ·mol-1 when true strain was 0.2, the calculated flow stress well agreed with the experimental results and the average relative error between them was 4.89%.
  • Microstructure and Property of New Ultra-high Strength Steel Joint Welded by MAG Welding using Different Welding Wires
    JIANG Zhuo-jun, LI Jing-yong, QI Yan-chang, CUI Bing, MA Cheng-yong
    Materials For Mechanical Engineering. 2017, 41(4): 33-38,43. https://doi.org/10.11973/jxgccl201704008
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    A new ultral-high strength steel was welded by metal active gas (MAG) welding with Ni-Cr-Mo and Ni-Mo welding wires, respectively, under the same heat input. The microstructures and mechanical properties of the two welded joints were compared and studied. The anti-crack sensitivity of two joints welded with the two welding wires was assessed by the Y-groove cracking test. The results show that weld microstructure of the welded joint with Ni-Cr-Mo welding wire mainly consisted of coarse lath martensite and small amount of bainite, while that with Ni-Mo welding wire as composed of fine acicular ferrite and small amount of block proeutectoid ferrite. The high strength joint of the steel can been obtain by the MAG welding with the two welding wires. But the toughness of the welded joint with Ni-Mo welding wire was better than that with Ni-Cr-Mo welding wire and the softening phenomenon in tempering zone was observed in both welded joints. The crack resistance of the welded joint with Ni-Mo welding wire was better than that with Ni-Cr-Mo welding wire, and the cold crack fracture patterns of Y-groove welded samples with the two welding wires were mainly intergranular fracture and little transgranular fracture.
  • Effect of Pre-oxidation Temperature on High-Temperature Cyclic Oxidation Resistance of Ni-20Cr-5Al-0.2Y2O3 Alloy
    SUN Duan-jun, LIANG Chun-yuan, SHANG Jin-long, SONG Ya-ru, YIN Ji-hui, ZHANG Xiu-hai
    Materials For Mechanical Engineering. 2017, 41(4): 39-43. https://doi.org/10.11973/jxgccl201704009
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    Ni-20Cr-5Al and Ni-20Cr-5Al-0.2Y2O3 (mass fraction/%) alloys were prepared by powder metallurgic technology. The Ni-20Cr-5Al-0.2Y2O3 alloy was first pre-oxidized at different temperatures from 500 ℃ to 1 100 ℃, and then the un-oxidized and pre-oxidized alloys were cyclic oxidized at 1 150 ℃ for 15 h. The effects of Y2O3 and pre-oxidation temperature on the high-temperature cyclic oxidation resistance of the alloys were studied. The results show that the high-temperature cyclic oxidation resistance of Ni-20Cr-5Al alloy was improved with the addition of 0.2wt% Y2O3. The pre-oxidation can improve the high-temperature oxidation resistance of Ni-20Cr-5Al-0.2Y2O3 alloy effectively. Furthermore, when the pre-oxidation temperature was 700 ℃, the alloy achieved the best high-temperature cyclic oxidation resistance. The reason is that a dense protective oxide scale was formed on the surface of the alloy after pre-oxidation, which prevented the alloy from the further oxidation.
  • Electrochemical Anti-scale Effect of Special Brass Alloy Used for N80 Steel
    FAN Zhou, FU Jin-wen, LIU Jian-yi, HU Xiao-gang, LI Jia-lian
    Materials For Mechanical Engineering. 2017, 41(4): 44-48. https://doi.org/10.11973/jxgccl201704010
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    A special brass alloy contained rare earth cerium was prepared by high temperature melting method, and the brass alloy and C7701 alloy were immersed in simulated oil gas field production water, and then Ca2+ concentration in the production water was measured by EDTA complex formation titration method and the weight loss method. N80 steel sheet was immersed into simulated oil gas field production water with the two alloys treatment for 72 h to calculate the scaling rate on the surface of N80 steel sheet. The microstructure of the scale was observed, and the phase of the scale was analyzed. Anti-scale effect of special brass alloy was studied by combining with the effect of metal cations and galvanic cell action on the scale. The results show that, in the simulated oil and gas field production water, a relatively large amount of Zn2+ was produced by the special brass alloy, which hindered the nucleation of CaCO3 crystals; after immersing in simulated oil gas field production water with special brass alloys for 72h, the scaling rate on N80 steel sheet surface decreased by 20%. More calcite calcium carbonate can transform into aragonite calcium carbonate with the special brass alloy's action, so the special brass alloy has a better anti-scale effect than C7701 alloy.
  • Preparation and Property of Graphene/Polylactic Acid Composite
    WU Hai-hua, WU Zhao, LÜ Jia-wei, WANG Dao, LIU Ning
    Materials For Mechanical Engineering. 2017, 41(4): 49-53. https://doi.org/10.11973/jxgccl201704011
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    Using polylactic acid with two particle size (20, 125 μm) as substrate material, respectively, and graphene as reinforcing material, the graphene/polylactic acid composite powders were prepared by ball milling, then pressed to two graphene/polylactic acid composites at 150 ℃. The bending performance and conductivity of the composites were studied. The results show that the graphene was coated on the surface of polylactic acid particles to form composite particles under the combining effects of mechanical interlocking and electrostatic adsorption. With the increase of graphene content, the bending strength of the two composites first increased then decreased, and the conductivity increased first rapidly then gently, and achieved relatively good values using polylactic acid with relatively small particle size. But too much graphene resulted in the defects on the surface of composites, such as cracks and broken. With the increase of molding pressure, the conductivity of the composite increased slightly, but the bending strength changed not obviously.
  • Preparation of Surface Modified SrTiO3 Nanopowder
    HAI Shen-wei, LI Wen-ge, YAO Wei-hua, ZHANG Ke-min
    Materials For Mechanical Engineering. 2017, 41(4): 54-57. https://doi.org/10.11973/jxgccl201704012
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    Using strontium nitrate, tetrabutyl titanate and potassium hydroxide as raw materials, SPAN80 surfactant modified SrTiO3 nanopowders were prepared by the hydrothermal method. The phase composition, morphology and particle size of the powder were analyzed by X-ray diffraction, transmission electron microscope, energy disperse spectroscope and laser particle size tester. The results show that after modification with 10vol% SPAN80 surfactant, SrTiO3 powder particles were spherical and the average particle size with a relatively narrow grain-size distribution was lower than that of unmodified powder. The surfactant had no effect on the phase composition of the powder. When the addition of SPAN80 surfactant was higher than 10vol%, the particle size of the SrTiO3 powder showed an increasing trend.
  • Hot Corrosion Behavior of 12Cr1MoV Steel in High Temperature Mixed Alkali Chloride Salt
    XIONG Wei-zhou, HE Jian-jun, ZHANG Wei, REN Yan-jie
    Materials For Mechanical Engineering. 2017, 41(4): 58-63. https://doi.org/10.11973/jxgccl201704013
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    Hot corrosion behavior of 12Cr1MoV low alloy steel in mixed salts of 70%NaCl-30%KCl, 50%NaCl-50%KCl, 30%NaCl-70%KCl (mass fraction) at 600 ℃ and 700 ℃ was studied by the corrosion kinetic method, respectively. The corrosion morphology at the surface and cross section was observed, the composition of the corrosion products was analyzed and the corrosion mechanism was investigated. The results show that the 12Cr1MoV steel was corroded seriously at the surface and in the substrate in the high temperature mixed salts. The produced Cl2 inside the steel was the main reason for the initiation and propagation of defect structure such as cracks and pores. With the increase of KCl content in mixed salts and of the temperature, the corrosion rate of sample increased rapidly. The corrosion mode in the substrate changed from intergranular corrosion to integral corrosion due to the relatively high content of KCl.
  • Effect of Composite Powder Pretreatment on Service Life of WC-10Co Ultrafine Cemented Carbide Cutting Tool
    CHENG Jian-bing, PANG Si-qin, WANG Xi-bin, YU Qi-xun, WEI Chong-bin
    Materials For Mechanical Engineering. 2017, 41(4): 64-68. https://doi.org/10.11973/jxgccl201704014
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    With un-pretreated and pretreated (heating at 1300℃ for 1h) WC-10Co powder, WC-10Co ultrafine cemented carbides were prepared by means of direct low pressure sintering and then were manufactured into cutting tools. Then the continuous cutting experiments were conducted on GH2132 superalloy by the cutting tools. The effects of the pretreatment on the microstructure and service life of the cutting tool were studied and the wear mechanism was analyzed. The results show that the average grain size of the cutting tool manufactured with pretreated composite powder was slightly larger while the service life was longer than that with un-pretreated powder. With the increase of cutting speed, the service life of the two cutting tools both decreased, but the life decreasing range of the cutting tool made from pretreated composite powder was relatively small. The adhesive wear and spalling wear were two main wear mechanisms for the two cutting tools. The distributions of WC grain size and of the adhesive phase Co in the cutting tool made from pretreated powder was uniform, resulting in the higher wear resistance than that with un-pretreated powder.
  • Corrosion Resistance and Damping Characteristic of Metal Rubbers Made of Different Materials in Neutral Salt Solution
    WANG Peng, LI Jing-yuan, LU Cheng-zhuang, HE Rong-hui, LI Yi
    Materials For Mechanical Engineering. 2017, 41(4): 69-73,79. https://doi.org/10.11973/jxgccl201704015
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    Solid circular pie-like metal rubbers were made of three stainless steel wires, namely 0Cr18Ni9,10Cr12Mn16Ni and 10Cr21Mn16Ni, and then cycle soak corrosion tests were carried out on these metal rubbers in 3.5wt% NaCl solution for 336 h. The corrosion resistance of the metal rubbers and their static compression and damping characteristics before and after corrosion were studied. The results show that the salt corrosion resistance of the metal rubber made of 10Cr21Mn16Ni stainless steel wire was the strongest, followed by that of 0Cr18Ni9 stainless steel, and that of 10Cr12Mn16Ni stainless steel wire was the worst. Comparing with those before corrosion, the static compression stiffness of three metal rubbers after corrosion all increased while the loss factor kept unchanged or went down. The static compression stiffness of the metal rubber made of 10Cr21Mn16Ni stainless steel wire had the smallest increase range of 4.5% and the loss factor kept unchanged. Meanwhile the stiffness of the metal rubber made of 10Cr12Mn16Ni stainless steel wire had the largest increase range of 150.0% and the loss factor decreased by 31.0%.
  • Corrosion Behavior and Influencing Factors of Strain Clamp for High Voltage Electricity Transmission
    CHEN Guo-hong, JI Kun, YAN Bo, ZHAN Ma-ji, WANG Ruo-min, ZHANG Tao, DU Xiao-dong
    Materials For Mechanical Engineering. 2017, 41(4): 74-79. https://doi.org/10.11973/jxgccl201704016
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    JY-120/20 press-connect-tube and LGJ-120/20 steel-cored aluminum strand were pressed to strain clamps. The corrosion behavior of the strain clamp was investigated by the electrochemical measurement and dry-wet immersion test in a 5wt% NaCl solution with various pH values at different temperatures. The resistance was tested and the corrosion morphology was observed. The influence factors of corrosion were also analyzed. The results show that the resistance of strain clamp generally showed an increasing trend with the elongation of corrosion time. However, due to the formation and spalling of corrosion products during the corrosion process, the secondary fluctuations in the increasing trend were observed. The corrosion preferentially occurred at the inner surface defects of the press-connect-tube, and then extended to the non-defect area. At the spalling area, the corrosion developed into the inner of the tube. The corrosion rate of the strain clamp increased with the decrease of pH value and increase of temperature of the corrosion solution.
  • Effect of Calcium Carbonate Whisker Content on Properties of Rubber Based Friction Materials
    WEN Guo-fu, WANG Xiu-fei, YIN Cai-liu, LAN Qi, ZHU Ren-jun, MENG Jie-li, LI Guang-feng
    Materials For Mechanical Engineering. 2017, 41(4): 80-83,88. https://doi.org/10.11973/jxgccl201704017
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    Taking calcium carbonate whisker as reinforced material, rubber based friction materials are fabricated using semi-wet technology and hot pressing method. Effects of calcium carbonate whisker's added content (0-25wt%) on hardness, density, impact strength, tensile strength and friction and wear properties of the friction materials were studied. The results show that with the increase of calcium carbonate whisker added content, hardness, density, tensile strength, wear extent of friction materials appear downward trend. When the mass fraction of calcium carbonate whisker is 15wt%, the friction material has the best comprehensive performance.
  • Microstructure Characteristic of Adiabatic Shear Band in 55vol% TiB2 Particles Reinforced Aluminum Matrix Composite at High Strain Rates
    ZHENG Zhen-xing, ZHU De-zhi
    Materials For Mechanical Engineering. 2017, 41(4): 84-88. https://doi.org/10.11973/jxgccl201704018
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    In order to research the new-found phase transformation band with the main characteristic of melting in aluminum matrix composite, microstructure characteristics of adiabatic shear band in 55vol% TiB2 particles reiforced aluminum matrix composite at varied strain rates (7×10-4-2×103s-1) were researched by using split Hopkinson pressure bar (SHPB), optical microscope (OM) and scanning electron microscope (SEM). The results show that molten aluminum bands or accumulation area of molten aluminum cells at approximate 45° to the compression axis, which was called phase transformation band, were found on the shear plane at high strain rates of (1-2)×103s-1. The formation of phase transformation band was related with adiabatic temperature-rise, which caused the aluminum alloy matrix melting, after quick cooling, the phase transformation formed. Adiabatic shear failure for the experimental aluminum matrix composite was controlled by thermal softening effect.
  • Wear Resistance of 20CrMo Carburized Steel after Boriding and Sulfur Nitrogen Boriding Treatment
    AN Ya-jun, WU Yue, MA Zhuang, ZHANG Bo
    Materials For Mechanical Engineering. 2017, 41(4): 89-93. https://doi.org/10.11973/jxgccl201704019
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    Using ferroboron and B4C as a boron source respectively and adding moderate nitrogen source, sulphur source and fillers, the boriding layers and sulfur nitrogen boriding layers on the surface of 20CrMo carburized steel were prepared by the powder embedding method, and then the phase composition, hardness and wear resistance of the two layers were analyzed. The results show that the boriding layers were both composed of Fe2B, FeB and Fe3C phases while the sulfur nitrogen boriding layers were composed of Fe2B, Fe3N, FeS and Fe3C phases without the formation of brittle FeB phase. The surface hardness, abrasive and adhesive wear resistance of the boriding layers and the sulfur nitrogen boriding layers were all higher than those of the carburizing layer. Moreover, the surface hardness and abrasive wear resistance of the boriding layers were higher while the adhesive wear resistance was lower than those of the sulfur nitrogen boriding layers. The abrasive wear resistance of the boriding layers prepared with ferroboron as a boron source was higher while the adhesive wear resistance was lower than those with B4C as a boron source.
  • Effects of Welding Speed on Microstructure and Mechanical Properties of Laser Welded Joint of TRIP590 High Strength Steel
    YANG Zhi-hua, YANG Shang-lei, TUO Wen-hai, JIANG Yi-shuai, WANG Yan, ZHANG Dong-mei
    Materials For Mechanical Engineering. 2017, 41(4): 94-97,102. https://doi.org/10.11973/jxgccl201704020
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    1.5 mm thick TRIP590 steel plates were butt welded by fiber lasers. Then the macro-morphology and microstructure of the joint was observed, the hardness and tensile properties were tested, and the influences of welding speed (0.050, 0.067, 0.083 m ·s-1) on the weld seam formation and on the microstructure and mechanical properties of welded joint were analyzed. The results show that a relatively low and high welding speed was not good for the formation of weld seam and led to relatively large depressions on weld seam surface. The welding speed had little influence on the microstructure and properties. The microstructures of weld seam and heat affected zone near weld mainly consisted of martensite, resulting in relatively high hardness of the two zones, which was nearly twice as high as that of base metal. The microstructure of heat affected zone near base metal was mainly composed of martensite and ferrite, and the martensite amounts decreased gradually with the increase of distance to the weld center, leading to the sharp decrease of hardness. Both the yield strength and tensile strength of the joints were slightly higher while the plasticity was slightly lower than those of base metal. The joints fractured in base metal and showed a ductile fracture.
  • Effects of Two-Stage Solution Treatment on Microstructure and Mechanical Properties of 7A04 Aluminum Alloy
    CHEN Ling, HE Huan, FU Yue-chun, SHEN Xiao-ming, WU Zhong-yi, ZENG Jian-min
    Materials For Mechanical Engineering. 2017, 41(4): 98-102. https://doi.org/10.11973/jxgccl201704021
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    A two-stage solution and aging treatment was conducted on 7A04 aluminum alloy, and then the effects of solution temperature and time on the microstructure and mechanical properties of the alloy were investigated by the orthogonal experiment. The optimal two-stage solution process was obtained. The results show that the optimal two-stage solution process was determined as: 450 ℃×30 min+505 ℃×60 min. Under this optimal process and followed by aging at 120 ℃ for 24 h, the tensile strength, yield strength, elongation and impact energy of the aluminum alloy reached 658.43 MPa, 593.07 MPa, 10.25% and 9.681 J, respectively. The first stage solution is the main influence factor on the mechanical properties of 7A04 aluminum alloy. However, the second stage solution at a relatively high temperature can remarkably improve the dissolution of the second phase, eutectic inclusions and intermetallic compounds, which precipitate as strengthening phases during the following aging treatment, and then results in the improvement of comprehensive performance of the alloy.
  • Accelerated Aging Behavior of Nitrile-Butadiene Rubber in Hydraulic Oil
    YU Jin-tao, WANG Zhan-bin, FAN Jin-juan, YUAN Li-hua
    Materials For Mechanical Engineering. 2017, 41(4): 103-105,110. https://doi.org/10.11973/jxgccl201704022
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    The high-temperature accelerated aging tests in hydraulic oil with temperatures of 110 ℃, 130 ℃ and 150 ℃ were carried out on a nitrile-butadiene rubber. Then the effects of aging time on the tensile property and hardness of the rubber were studied and the aging behavior was analyzed. The service life of the rubber was also predicted by using Arrhenius equation. The results show that during the aging process, under the combined effects of oil swelling, crosslinking and chain broking, the tensile strength of the nitrile-butadiene rubber first kept unchanged then decreased, the hardness first decreased then increased and the maximum percentage elongation decreased exponentially. According to the maximum percentage elongation data, the service life predicted by Arrhenius equation of the nitrile-butadiene rubber in 25 ℃ hydraulic oil was 2 836 d, which was equivalent to the real life. The prediction result was relatively accurate.
  • Effect of Carbon Content on Microstructure and Properties of Cr25 High-Chromium Cast Steel
    FU Li-ming, CHEN Shan-ping, ZHANG Rui-na, AN Miao, SUO Zhong-yuan, SHAN Ai-dang
    Materials For Mechanical Engineering. 2017, 41(4): 106-110. https://doi.org/10.11973/jxgccl201704023
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    The effects of carbon content (0.13wt%, 0.25wt% and 0.32wt%) on the microstructure, high-temperature oxidation resistance and high-temperature wear resistance of Cr25 high-chromium cast steel were investigated. The results show that the microstructures of the cast steels with different carbon content are composed of ferrite matrix and (Cr ·Fe)23C6 carbide. The morphology of carbides changes from discontinuous rod-like to continuous net-like, and the precipitation way changes from intergranular to intergranular and intragranular precipitation. In the early stage of oxidation at 1 200 ℃, the tested steel is oxidizing rapidly. After oxidation for 40 h, the oxidation rate tends to remain stable and the tested steel is in a stable oxidation stage due to the chromium oxide protective layer formed on the surface. With the increase of carbon content, the high-temperature oxidation resistance of the tested steel decreases while the wear resistance increases, and the wear mechanism changes from adhesive wear to the combining of adhesive and abrasive wear.
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