Heat Treatment of Metals

Formation mechanism of κ-carbide in FeMnAlC low density steel

Cao Chenxing, Wang Cunyu, Cao Wenquan

2021, 46(12): 1-6. doi:10.13251/j.issn.0254-6051.2021.12.001

Abstract ( 65 )

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FeMnAlC steel is a new type of high strength and low density steel. The high content of alloying elements makes the microstructure and properties of the FeMnAlC steel have a large regulatory space. κ-carbide is a unique precipitated phase in the steel, and its precipitation position and size have a great influence on the properties of the FeMnAlC steel. Reasonable regulation of κ-carbide precipitation can effectively improve the strength of the steel, and with high plasticity and toughness. The formation mechanism of κ-carbide in the FeMnAlC steel was summarized, and the morphology characteristics and control methods of κ-carbide were explored by combining different cooling rates and short-term aging tests.The results show that the κ-carbide is generated by spinodal decomposition, and the structure is coherent with the matrix, which will coarsen with the extension of aging time and the increase of temperature. Si and Ni elements will promote the precipitation of κ-carbide, and Mo, Cr and other elements will inhibit the precipitation of κ-carbide.

Effect of heat treatment process on precipitates and shape memory properties of Fe-15Mn-4.5Si-10Cr-5Ni shape memory alloy

Zhao Kai, Yang Zhongmin, Li Jiarui, Chen Ying, Cao Yanguang, Li Zhaodong

2021, 46(12): 7-12. doi:10.13251/j.issn.0254-6051.2021.12.002

Abstract ( 59 )

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Effects of direct aging, solution and aging heat treatment on the microstructure, precipitation of the second phase and shape memory effect of the tested alloy were studied by means of OM,TEM and physicochemical phase analysis in order to further improve the shape memory effect of Fe-15Mn-4.5Si-10Cr-5Ni shape memory alloy. The results show that,the shape memory performance of the specimen decreases obviously because the second phase particles redissolve greatly after solution heat treatment; At the same time, the direct aging treatment significantly increases the amount of Cr carbide second phase precipitation in the specimen compared with the solution and aging treatment, which greatly improves the shape memory performance of the specimen. The best shape recovery ultimately can be obtained by direct aging at 800 ℃.

Effect of aging treatment on microstructure and mechanical properties of high quality GH738 alloy

Wu Yubo, Yang Chengbin, Hou Weixue, Rong Yi, Li Chun, Lu Yunjie, Li Lingli

2021, 46(12): 13-18. doi:10.13251/j.issn.0254-6051.2021.12.003

Abstract ( 66 )

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Effect of aging temperature and time on microstructure and properties of high quality GH738 alloy was studied by means of mechanical testing and microstructure observation. The results show that the aging temperature and time both affect the fraction and size of γ′ phase. When the aging temperature is between 720 ℃ and 800 ℃, with the increase of aging temperature, the strength of the alloy decreases, the primary and secondary γ′ phases grow up by 30 nm and 8 nm, respectively. The volume fraction of the primary γ′ phase increases, and that of the secondary γ′ phase decreases. When the aging temperature is 800 ℃, the volume fraction of the primary γ′ phase reaches the peak value as about 8%. When the aging time is between 0 and 48 h, with the increase of aging time, the alloy strength first increases and then decreases. The two types of γ′ phase grow up by 20 nm and 6 nm, respectively. The volume fraction of the primary γ′ phase increases first and then decreases, and that of the secondary γ′ phase just changes as the opposite. When the aging time is 8 h, the volume fractions of two types of γ′ phase reach their peak/valley values respectively, as about 8% and 12%. The changes in the size and volume fraction of the γ′ phase, especially volume fraction, lead to different effects of two strengthening mechanisms of dislocations, resulting in change in strength.

Effect of cold deformation on microstructure and mechanical properties of 028 alloy containing nitrogen

Gong Fangyuan, Feng Hanqiu, Lang Yuping, Chen Haitao, Wang Baoshun, Zhong Qiang, Leng Chongyan

2021, 46(12): 19-23. doi:10.13251/j.issn.0254-6051.2021.12.004

Abstract ( 50 )

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Effect of cold rolling deformation on microstructure and mechanical properties of 028 alloy containing nitrogen was studied by means of optical microscope (OM), transmission electron microscope (TEM) and tensile test. The results show that when the amount of cold deformation is the same, the yield strength increases obviously with the increase of nitrogen content. When the cold deformation is increased from 0% to 70%, the tensile strength of the 0.25%N tested alloy is increased from 764 MPa to 1405 MPa, and the yield strength is increased from 390 MP to 1249 MPa. At low deformation, there are a large number of dislocations and plane slip structures in the alloy. With the increase of cold deformation, the number of deformation twins in the alloy increases gradually, and the dislocations multiply continuously. When the cold deformation continues to increase, the deformation twins are split and broken.

Effect of Nb on microstructure and impact properties of H13 steel

Ouyang Weihao, Zhou Jian, Gao Peng, Lin Peng, Chi Hongxiao, Wang Wenjun

2021, 46(12): 24-30. doi:10.13251/j.issn.0254-6051.2021.12.005

Abstract ( 69 )

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Microstructure and mechanical properties of the H13 tested steel with Nb content of 0, 0.067% and 0.270% (mass fraction) after quenching and tempering were comparatively studied by means of OM, SEM, EDS, phase analysis, hardness test and impact properties test. The results show that quenching hardness of the tested steel decreases after adding Nb. As the quenching temperature increases, the grain size of the Nb-containing tested steels is smaller than that of the 0Nb tested steel, but there are some undissolved carbides in the Nb-containing tested steels. The secondary hardening peak of the three tested steels after tempering all appear at 510 ℃. After quenching at 1050 ℃ and tempering at different temperatures, the impact absorbed energy of the 0.067Nb tested steel is higher than that of the 0Nb tested steel. Due to the influence of large-size carbides, when the quenching temperature is below 1080 ℃, the impact absorbed energy of the 0.27Nb tested steel is not as good as that of the other two tested steels.

Microstructure and mechanical properties of low density austenitic matrix hot rolling steel with different Ni contents

Wang Fengquan, Sun Ting, Wang Maoqiu, Yu Wenchao, Zhou Xiaolong

2021, 46(12): 31-39. doi:10.13251/j.issn.0254-6051.2021.12.006

Abstract ( 60 )

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The microstructure and mechanical properties of three kinds of Fe-18Mn-10Al-1C-(0, 3, 5)Ni-0.08V-0.03Nb (wt%) austenitic matrix low density dual phase steels after hot rolling were studied. The results show that the microstructure of the steel after hot rolling is composed of elongated austenite, band B2 and massive B2 particles along the grain boundaries of recrystallized austenite grains. In addition, nano-sized κ-carbide and DO₃ phase are formed in austenite grains and B2 grains, respectively. The yield strength of the tested 5Ni steel reaches 1352 MPa, which is mainly due to the precipitation strengthening effect of a large number of nano-sized B2 particles and VC phase in the austenite grain boundary. With the increase of Ni content, the strength and hardness of the steel increase, and the yield strength of 5Ni steel is 116 MPa higher than that of 0Ni steel, which is attributed to more B2 phase content (16.9%) in the 5Ni steel. However, while the strength of the Ni-containing steel increases, the plasticity of the steel is greatly lost, resulting in a very low elongation. The reason is that the banded B2 phase is mainly distributed at the austenite grain boundary, and the crack is easier to break along the grain boundary in the process of deformation. In the meantime, delamination can be observed on the fracture surface.

Effect of heat treatment on microstructure and properties of 30Cr16Mo1VN steel

Cao Xin, Li Quan, Yang Yinhui

2021, 46(12): 40-45. doi:10.13251/j.issn.0254-6051.2021.12.007

Abstract ( 73 )

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In order to explore optimum heat treatment process of 30Cr16Mo1VN steel, the effect of quenching and tempering temperatures on the microstructure and mechanical properties was studied by impact and tensile tests, Rockwell hardness test, OM, SEM, XRD and TEM. The results show that the optimum austenitizing temperature for quenching of the steel is 1050 ℃, at which there are a small amount of M₂₃C₆ carbide and M₂N nitride to hinder the grain boundary migration in quenching, and the average grain size of prior austenite is 14.1 μm. However, most of the carbide and nitride are dissolved into the matrix, resulting in lowering of the Ms point and increasing the content of retained austenite up to 59.2%. After cryogenic treatment at -73 ℃, a large amount of retained austenite is quenched into martensite, and the hardness of the steel is increased to 57 HRC. After tempering at 300 ℃, the steel has a good match of strength and toughness, with the tensile strength and the area reduction up to 2030 MPa and 10.0% respectively. The tempering leads the matrix to recover and the dislocation density to decrease, while as the tempering temperature rises, the dispersed fine spherical carbide precipitates in the matrix to hinder the movement of dislocations and produce secondary hardening, and the hardness peak appears when tempering at 450 ℃. When the tempering temperature is lower than 500 ℃, the hardness is always greater than 55 HRC, the steel has good tempering resistance.

Thermal deformation behavior of as-cast and as-forged GH4738 alloy

Su Xing, Lü Xudong

2021, 46(12): 46-52. doi:10.13251/j.issn.0254-6051.2021.12.008

Abstract ( 51 )

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By using MTS thermal simulator, compression tests were carried out on the as-cast and as-forged GH4738 alloy under the conditions of deformation temperature of 1000-1150 ℃ and strain rate of 0.01-1 s^-1, in which the reduction is 10%, 30% and 50%, respectively. The results show that the stress-strain curves of the alloy in two states both have typical characteristics of dynamic recrystallization, and there are three stages: work hardening, flow softening and steady-state flow. According to the stress-strain curves, the thermal deformation activation energies of as-cast and as-forged GH4738 alloy are Q=575.89 kJ/mol and Q=588.04 kJ/mol, respectively. The EBSD analysis shows that the dynamic recrystallization of as-forged GH4738 alloy occurs earlier and more significantly than that of as-cast alloy under the same thermal deformation parameters.

Effect of tempering temperature on precipitation and mechanical properties of Ti-V-Mo microalloyed martensitic steel

Han Rong, Liu Hongxi, Yu Wenchao, Wang Maoqiu, Shi Jie

2021, 46(12): 53-60. doi:10.13251/j.issn.0254-6051.2021.12.009

Abstract ( 60 )

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Microstructure and precipitates of Ti-V-Mo microalloyed martensitic steel after quenching and tempering at different temperatures (550,600,650 ℃) for 1 h were characterized by microanalysis and physicochemical phase analysis, and the strengthening components were calculated. The results show that the steel has the best comprehensive mechanical properties when tempered at 600 ℃: the tensile strength is 1298 MPa, the yield strength is 1286 MPa and the elongation is 14%. The calculation results of strengthening component show that precipitation strengthening and fine grain strengthening are the main strengthening mechanism, accounting for about 40% and 30% of the total strength. And precipitation strengthening component σ_p is 517 MPa, provided by (Ti, V, Mo) C particles (mass fraction 22%) below 5 nm. When the tempering temperature increase from 550 ℃ to 600 ℃, the tensile strength and yield strength increase, but the elongation change little. The main reason is that as σ_P contributes to the yield strength increasingly, the plasticity is improved with the strength increasing.

Hot deformation behavior of A286 superalloy with two different smelting processes

Deng Shanshan, Sun Yongqing, Jiang Yehua, Liu Zhenbao, Liang Jianxiong, Wang Changjun

2021, 46(12): 61-71. doi:10.13251/j.issn.0254-6051.2021.12.010

Abstract ( 70 )

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In order to investigate the hot deformation behavior of A286 superalloy with two smelting processes of VIM+VAR (vacuum induction melting+vacuum arc remelting) and EAF+LF+VAR (electric arc furnace+ladle furnace+vacuum arc remelting), hot compression tests were carried out on a Gleeble-3800 hot simulation test machine at temperatures ranging from 950 to 1150 ℃ and strain rates ranging from 0.01 to 10 s^-1. The Arrhenius constitutive equations of the A286 alloy are established based on the true stress-true strain curves and work hardening rate curves modified by friction and adiabatic heating. The hot deformation activation energies of the VIM+VAR alloy and the EAF+LF+VAR alloy are determined to be 358.15 and 372.54 kJ·mol^-1, respectively. A new dynamic recrystallization model is established by introducing the dynamic recrystallization velocity parameter k_v with critical strain and 50% dynamic recrystallization volume fraction strain. Prasad criterion is used to draw the hot processing maps at 0.2, 0.5 and 0.9 strain of the A286 alloy prepared by the two smelting processes. Combined with the microstructure analysis, the optimal hot working conditions of the VIM+VAR alloy is 1050-1100 ℃, 0.01-1 s^-1 and 1100-1150 ℃, 0.1-10 s^-1, while that of the EAF+LF+VAR alloy is 1050-1100 ℃, 0.01-1 s^-1 and 1100-1150 ℃, 0.1-3 s^-1. It is concluded that the VIM+VAR alloy has a wider hot-working range and its hot-working performance is better than that of the EAF+LF+VAR alloy.

Effect of carbon content on wear resistance of metal clothing made of Nb microalloyed high carbon steel

Gu Weihua, Zhuo Chengzhi, Lu Zhong, Gao Li, Zhang Yongqing, Yong Qilong

2021, 46(12): 72-75. doi:10.13251/j.issn.0254-6051.2021.12.011

Abstract ( 48 )

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Based on the principle of microalloying, three types of high carbon steels containing 0.8%-1.0% C and 0.03%-0.04%Nb were designed. The results show that the increase of carbon content can completely eliminate the network cementite and make more carbon dissolve in austenite during heat treatment, so as to obtain higher quenching hardness. The comparison of quenching microstructure of steels with different carbon content shows that when the carbon content is 0.90%, the martensitic matrix can obtain the highest hardness while keep the extreme fine grain size. The card clothing made of this type of steel can reach an average hardness of 841 HV0.2 with grain size grade of 13.5. The wear resistance increases by about 30% compared with standard high-end steel 80WV applied to card clothing through quick wear tests and online tests in real carding machines.

Effects of nitrogen content and solution treatment temperature on microstructure and hardness of 21-6-9 austenitic stainless steel

Yan Zhikun, Chen Haitao, Lang Yuping, Qu Huapeng, Feng Hanqiu, Liu Rongpei

2021, 46(12): 76-80. doi:10.13251/j.issn.0254-6051.2021.12.012

Abstract ( 48 )

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In order to investigate the effect of nitrogen content and solution treatment temperature on microstructure and hardness of 21-6-9 stainless steel, three kinds of hot rolled 21-6-9 stainless steel with different nitrogen contents were solution treated at 950, 1000, 1050, and 1100 ℃ for 1 h, respectively, of which the microstructure was observed by optical microscope. Combined with Thermo-Calc thermodynamic calculation, the microstructure of the tested steel was analyzed and the hardness was tested. The results show that ferrite precipitates in the 21-6-9 stainless steel with 0.20%-0.28%N along the rolling direction after hot rolling, and the ferrite in the steel with 0.20%-0.28%N can be eliminated by solution treatment at 950-1000 ℃. When the N content reaches 0.34%, the ferrite no longer appears in the tested steel. With the increase of solution treatment temperature, grain growth occurs in the 21-6-9 stainless steel and the hardness decreases. With the increase of N content, the hardness of the 21-6-9 stainless steel in solution treated state can be significantly increased, while the increasing degree decreases with the increase of solution treatment temperature.

Dynamic recrystallization behavior and processing map of 17Cr2Ni2MoVNb and 20Cr2Ni4A gear steels

Dong Mingzhen, Ouyang Xuemei, Liu Yangming, Gou Lei, Yuan Wufeng, Yan Yongming

2021, 46(12): 81-86. doi:10.13251/j.issn.0254-6051.2021.12.013

Abstract ( 54 )

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The flow stress curves of 17Cr2Ni2MoVNb and 20Cr2Ni4A gear steels at 1000-1150 ℃ and 0.01-10 s^-1 were obtained by means of thermal simulation Gleeble-3800, and the dynamic recrystallization Avrami dynamics model and the processing map of the two steels were constructed. The results show that both steels are susceptible to dynamic recrystallization at high deformation temperatures and low strain rates. The higher Nb and Mo content in 17Cr2Ni2MoVNb steel has a greater inhibitory effect on dynamic recrystallisation than the high Ni content in 20Cr2Ni4A steel, which results in a smaller dynamic recrystallisation volume fraction in 17Cr2Ni2MoVNb steel than in 20Cr2Ni4A steel under the same heat deformation conditions. The optimal hot processing parameters of 17Cr2Ni2MoVNb steel are in the processing temperature range of 1050-1150 ℃ and at the strain rate of 0.1-0.6 s^-1, while those of 20Cr2Ni4A steel are in the processing temperature range of 1100-1150 ℃ and at the strain rate of 3.3-5.5 s^-1.

Effect of heat treatment on mechanical and magnetic properties of 0Cr13 steel

Ma Yuanyuan, Bao Hansheng, Gong Zhihua, Zhao Jiqing, Yang Gang

2021, 46(12): 87-93. doi:10.13251/j.issn.0254-6051.2021.12.014

Abstract ( 93 )

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Effect of different recrystallization annealing process and quenching and tempering process on mechanical and magnetic properties of the 0Cr13 stainless steel at room temperature was studied. The results show that after quenching at 980 ℃ for 1 h, water-cooling, tempering at 725 ℃ for 2 h, water-cooling and tempering at 400 ℃ for 2 h, furnace-cooling, the 0Cr13 steel can obtain ferrite and martensite duplex microstructure, and the mechanical properties and magnetic properties are better matched. After quenching at 980 ℃ for 1 h, water-cooling, tempering at 725 ℃ for 2 h, water-cooling and tempering at 870 ℃ for 2 h, furnace-cooling, the 0Cr13 steel has excellent magnetic properties and smaller coercive force, but the strength decreases significantly. After recrystallization annealing at 820 ℃ for 5 h and furnace-cooling, the 0Cr13 steel obtains organic and uniform equiaxed ferrite microstructure, strength of which is lower than that after quenching and tempering, but has good soft magnetic properties. After quenching and tempering at 870 ℃, the coercive force H_c and residual magnetic induction B_r of the material can be significantly reduced, and the magnetic field can be rapidly demagnetized. In the 0Cr13 steel, the more the amount of ferrite is, the better the weak magnetic properties; and the higher the mass of martensite is, the higher the strength. There is a certain contradictory relationship between magnetic properties and strength, and better magnetic properties will lose a certain strength and vice versa.

Preparation and high-temperature tribological properties of WS₂ reinforced CoCrTi composites

Qian Yu, Cui Gongjun, Bian Canxing, Kou Ziming

2021, 46(12): 94-99. doi:10.13251/j.issn.0254-6051.2021.12.015

Abstract ( 43 )

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CoCrTi-(2.5, 4.0, 6.0) WS₂ composites were prepared by hot pressing sintering technology, and the content of WS₂ was optimized. The tribological properties of the composites from room temperature to 1000 ℃ were studied by ball-disk high temperature friction tester. The microstructure and phase composition of the composites were analyzed by X-ray diffractometer and scanning electron microscope. The results show that the suitable addition of WS₂ significantly improves the hardness and tribological properties of the composites. The friction coefficient and wear rate of the three composites have roughly the same trend as follows. The friction coefficient decreases with the increase of temperature from room temperature to 400 ℃, while the wear rate changes in the opposite trend. The friction coefficient increases slightly with the increase of temperature from 400 ℃ to 1000 ℃, while the wear rate first decreases, then increases, and finally decreases with the increase of temperature, and the wear rate reaches the maximum value at 800 ℃. Under the given test conditions, the composites with WS₂ content of 4.0wt% has the best high-temperature tribological properties. The specimens show different degree of abrasive wear at low temperatures, while the wear mechanism at high temperatures is oxidative wear.

Effect of Mg content on microstructure and properties of Al-Zn-Mg aluminum alloy

Huang Yuanchun, Li Xiangyue, Tan Weiyang, Hu Jiawei, Huang Juan, Zou Ti

2021, 46(12): 100-106. doi:10.13251/j.issn.0254-6051.2021.12.016

Abstract ( 56 )

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By changing the magnesium content, four compositions of Al-6.0Zn-xMg alloys were designed. The effect of Mg content on microstructure and properties of the as-cast and homogenized Al-Zn-Mg alloy and T6 state properties was studies by means of optical microscope (OM), scanning electron microscope (SEM), differential scanning calorimetry (DSC), hardness, electrical conductivity and room temperature tensile tests. The results show that there are a large number of eutectic network structures and chain-like second phases near the grain boundaries of the four as-cast alloys, mainly α (Al)+ternary T (AlZnMg) phases, and a small number of Al₃ (Zr, Ti) phases and iron-rich phases, and increasing the Mg content will increase the non-equilibrium eutectic phase in the alloy structure. After homogenization treatment and followed air-cooling, a large number of fine and dispersed needle-like η(MgZn₂) phases are precipitated in the alloy matrix, and with the increase of Mg content, the number of precipitation of this needle-like η(MgZn₂) phase also gradually increases. As the Mg content increases, the hardness gradually increases, while the electrical conductivity gradually decreases, and the hardness and electrical conductivity of the homogenized alloy are higher than that of the as-cast alloy. Among the four alloys at T6 state, the Al-6.0Zn-2Mg alloy has the best comprehensive mechanical properties.

Effect of N content on corrosion resistance of 13Cr super-martensitic stainless steel

Li Miaomiao, Zou Dening, Tong Libo, Pang Yang

2021, 46(12): 107-112. doi:10.13251/j.issn.0254-6051.2021.12.017

Abstract ( 50 )

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Nitrogen alloying of 13Cr super-martensitic stainless steel was carried out, quenching and partitioning process was adopted to study the influence of different N content on microstructure and electrochemical corrosion behavior, the results show that with the increase of N content, the lath martensite shows an obvious fining behavior. The austenite content is increased through the formation of VN, which prevents the combination of N and Cr to form Cr₂N, and the Cr forms a corrosion product film dominated by Cr₂O₃ on the surface of the steel, thus improving the corrosion resistance of the steel. The surface of the tested steel is dominated by local corrosion, and pitting corrosion occurs in the specimens. With the increase of N content, the porosity of the corrosion product film decreases, and the surface passivation film is of an electric double layer structure, which increases the stability of the passivation film, and the number of pitting pits is significantly reduced and the pits are smaller. The increase of N content results in the improvement of pitting resistance of the specimens, the corrosion products on surface of the tested steel containing 0.35%N are firmly attached, smooth and compact, and the grain size is uniform, playing a good protective role.

Effect of Ni-CNTs on interface behavior of Sn-3.0Ag-0.5Cu lead-free solder joints

Chen Jin, Qu Min, Cui Yan

2021, 46(12): 113-118. doi:10.13251/j.issn.0254-6051.2021.12.018

Abstract ( 41 )

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Lead-free solder Sn-3.0Ag-0.5Cu with different contents(0,0.05,0.1,0.2,0.5wt%)of nickel-plated multi-walled carbon nanotubes (Ni-CNTs) was fabricated by mechanical mixing method, and reflow soldered with F4N furnace. The solder joints were aged in a vacuum drying oven at 170 ℃ for 0 and 48 h. The effect of Ni-CNTs content on wettability, melting point of Sn-3.0Ag-0.5Cu solder and the intermetallic compound (IMC) layer at the solder joints was studied by means of DTA, SEM, EDS and other analytical methods. The results show that the Ni-CNTs can significantly improve the wettability of the composite solder and reduce the melting point, also inhibit the growth of the interface IMC layer and change its composition. The optimal addition amount of Ni-CNTs is 0.05%. Compared with Sn-3.0Ag-0.5Cu solder, the wetting angle of Sn-3.0Ag-0.5Cu-0.05(Ni-CNTs) is reduced by 49.76%, the melting point is reduced by 0.331 ℃. The thickness of the interface IMC layer after aging is 4.292 μm (t=0 h) and 5.238 μm (t=48 h), which is reduced by 34.26%(t=0 h) and 36.55% (t=48 h), respectively, compared with Sn-3.0Ag-0.5Cu solder joints interface IMC thickness of 6.529 μm (t=0 h) and 8.255 μm (t=48 h), and the phase composition of the interface IMC layer changes to (Cu_1-xNi_x)₆Sn₅ and (Cu_1-xNi_x)₃Sn.

Effect of Ni alloying on thermal stability and soft magnetic properties of Finemet alloy

Zhang Chaohan, Tao Pingjun, Zhu Kunsen, Huang Wenhao, Yang Yuanzheng

2021, 46(12): 119-123. doi:10.13251/j.issn.0254-6051.2021.12.019

Abstract ( 46 )

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Fe_73.5-xSi_13.5B₉Cu₁Nb₃Ni_x (x=0, 1, 2, 3) alloy strips were prepared by copper roller melt-spinning method and then treated by annealing. The phase structure, thermal stability and soft magnetic properties of the strips were tested by XRD, DSC, VSM and soft magnetic DC tester. The results show that the alloy strips are completely amorphous in the quenched state, and the two-phase coexistence structure of amorphous and α-Fe nanocrystalline are formed in the alloy after annealing at 560 ℃ for 60 min. With the increase of Ni content, the first initial crystallization temperature (T_s1) and the second initial crystallization temperature (T_s2) of the alloy strips decrease first and then increase on the whole. The difference value between the two initial crystallization temperatures (ΔT_s) shows a downward trend as a whole, from 166.0 ℃ to 132.8 ℃, and the thermal stability decreases. In the quenched state, the addition of Ni makes the soft magnetic properties of the amorphous strips deteriorate. After annealing, the soft magnetic properties of the strips are obviously improved. When the Ni content is x=1, the strip has better soft magnetic properties: saturation magnetization of 157.7 emu/g and the coercive force of 6.8 Oe.

Effect of Nb on microstructure and properties of medium-low carbon super-bainitic steel

Gao Shi, Pan Xuefu

2021, 46(12): 124-129. doi:10.13251/j.issn.0254-6051.2021.12.020

Abstract ( 54 )

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Based on non-Nb and 0.05wt%Nb steels, effect of Nb on microstructure and properties of 0.25wt%C super-bainitic steel was studied. The test results show that bainite structure can be obtained for the both steels by rolling, rapid cooling and then slow cooling isothermal process. When isothermal treated at 300 ℃ for 8 h, bainite content in Nb containing steel is more than 80%, yield strength is increased by 12% (109 MPa), impact absorbed energy reaches 52 J. The result of calculation by Thermal-Calc software and observation by TEM indicates that tiny carbides formed by Nb-Mo and other elements are precipitated, that pin lath boundary, inhibit lath merging and coarsening, and improve the stability of the lath. Size of bainitic ferrite lath in Nb containing steel is in the range of 150-200 nm by holding for 8 h. By using BSE and EBSD analysis, because of Nb element promoting the precipitation of carbides, reducing the stability of undercooled austenite, promoting bainite transformation, inhibiting martensite transformation, refining residual austenite, the uniformity and stability of microstructure is improved, that is the main mechanism to improve properties.

Effect of La-Ce mischmetal on microstructure and properties of AZ91D alloy

Chen Lichao, Su Zhicheng, Ren Jingtao, Fang Liqiang, Wang Zichao, Wang Yuyu, Zhao Liping

2021, 46(12): 130-134. doi:10.13251/j.issn.0254-6051.2021.12.021

Abstract ( 53 )

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Taking AZ91D magnesium alloy as the research object, the effect of La-Ce mischmetal content on microstructure and properties was investigated. The results show that with the adding of La-Ce mischmetal, the as-cast microstructure is network β-Mg₁₇Al₁₂ phase and a few rod-shaped rare earth compounds. With the increase of rare earth content, the β-Mg₁₇Al₁₂ and rod-shaped rare earth compounds are refined. After solid solution and aging, with the increase of rare earth content, the elongation of the tested alloy increases, the hardness and tensile strength first increase and then decrease. When the La-Ce mischmetal content is 2.38%, the mechanical properties of the alloy are the best.

Phase transformation and corrosion resistance of Q450 acid-resistant steel

Wang Zhen, Hu Deyong, Sun Chao, Gao Xiuhua, Wu Hongyan, Du Linxiu

2021, 46(12): 135-141. doi:10.13251/j.issn.0254-6051.2021.12.022

Abstract ( 49 )

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A new type of acid-resistant steel containing 0.09%Bi for railway open car was developed. The dynamic CCT curve was drawn through thermal simulation test, and the sulfuric acid corrosion resistance of the steel was studied by means of full immersion test and electrochemical test. The results show that with the increase of cooling rate, the microstructure changes from pearlite+ferrite to bainite and is refined. After hot rolling in the laboratory, water cooling to 622 ℃, and then asbestos coil cooling to room temperature, the microstructure obtained is ferrite and a small amount of bainite. After full immersion corrosion test, the corrosion products on the surface of the steel is smooth and the corrosion is light, and the mechanical properties and sulfuric acid corrosion resistance of the steel meet the requirements of Q/ASB 174—2016 standard. The electrochemical corrosion test show that the self-corrosion potential, self-corrosion current and corrosion rate of the steel are better than that of the Q345B steel, and the oxide film with dense corrosion products rich in Cu₂S on the surface significantly improves the corrosion resistance.

Micromorphology and wear resistance of selective electrochemically deposited nickel coating

Bai Xuechen, Liang Guoxing, Lü Ming

2021, 46(12): 142-148. doi:10.13251/j.issn.0254-6051.2021.12.023

Abstract ( 55 )

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Nickel coating was deposited on the 45 steel substrate by the selective electrochemical deposition technique, the growth law of the electrochemical deposited nickel coating in the selected area was explored by the morphology observation and energy spectrum analysis, and the wear resistance of the coating and the substrate was compared. The results show that the growth process selective electrochemical deposition is gradual by stacking of small particles into a dense coating; the growth of the grains is spirally stacked, with microcracks inside and on the surface of the spherical grains. Oxygen is present in the gaps between the grains, and oxidation reaction inhibits the mutual fusion of small crystal grains; the structure of electrochemically deposited nickel coatings in selected areas is denser, and there is a clear boundary between the coating structure and the matrix structure, and the coating structure has better wear resistance than the matrix material.

Law of continuous cooling transformation and isothermal transformation of titanium-free hot stamping steel

Geng Wubin, Cui Qingling, Liang Jiawei, Zhu Zongwei, Zhang Longchao

2021, 46(12): 149-155. doi:10.13251/j.issn.0254-6051.2021.12.024

Abstract ( 46 )

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Using Thermo-Calc thermodynamics software (TCFE 9 database), DIL805A/D deformation thermal expansion phase changer and field emission scanning electron microscope (FE-SEM), the law of microstructure evolution of a titanium-free hot stamping steel during continuous cooling transformation and isothermal transformation was studied. The results show that the Ac₁ and Ac₃ of the steel is 749 ℃ and 863 ℃, respectively, and the corresponding CCT curve and TTT curve are drawn. The martensitic transformation start temperature of the titanium-free hot stamping steel is 385 ℃, and the martensitic transformation finish temperature is 130 ℃. During the cooling process of supercooled austenite, the critical cooling rate for martensite transformation is 5 ℃/s; when the isothermal temperature is higher than 750 ℃, the full martensite structure can be obtained after hot stamping.

Microstructure and properties of nanocrystalline Cu-Nb thin films prepared by unbalanced magnetron sputtering ion plating

Wang Yupeng, Wu Hongjun, Bao Mingdong, Tian Wu, Zhou Bing

2021, 46(12): 156-161. doi:10.13251/j.issn.0254-6051.2021.12.025

Abstract ( 59 )

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Influence of Nb content on microstructure and properties of the nanocrystalline Cu-Nb film was studied. The Cu-Nb nanocrystalline films with different Nb content were prepared on single crystal Si substrate with (100) crystal plane and glass substrate by unbalanced magnetron sputtering ion plating technology, and the effect of Nb content on their microstructure and properties was studied. The specimens were annealed at 400 ℃ in a horizontal vacuum annealing furnace. The microstructure, mechanical properties and electrical properties of the films before and after annealing were analyzed by using a field emission scanning electron microscope equipped with energy dispersive X-ray spectrometer, atomic force microscope, X-ray diffraction and nanoindenter and four-probe resistivity tester. The results show that the as-deposited Cu-Nb film surface is composed of dense nanocrystallines without obvious defects such as holes and cracks, and with the highest surface roughness of only 8.54 nm. With the increase of Nb content, the average grain size of the film is decreased by 5 nm. The hardness of the film is also increased due to fine-grain strengthening, which showing a maximum value of 4.9 GPa with a Nb target current of 1.3 A. Compared with the as-deposited specimen, the as-annealed specimen has a little changes in hardness, elastic modulus, average grain size and surface roughness, which indicates that the as-annealed Cu-Nb film exhibits excellent thermal stability. The addition of Nb can effectively refine the grains and increase the hardness of the film. At the same time, the immiscibility of Cu-Nb allows the nanocrystalline film to maintain better thermal stability at high temperatures. When the sputtering current of Nb target is 0.5 A, the performance of the Cu-Nb film is the best: for the as-deposited Cu-Nb the resistivity is the lowest of 3.798×10^-7 Ω/m, the film thickness is 1050 nm, the roughness is 4.70 nm, and the hardness and elastic modulus are as high as 4.6 GPa and 139.5 GPa, respectively.

Deformation microstructure of biomedical 316L austenitic stainless steel

Li Pengtao, Liu Jinwang, Luo Xian, Chen Jianxin

2021, 46(12): 162-167. doi:10.13251/j.issn.0254-6051.2021.12.026

Abstract ( 54 )

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Deformation behaviour in engineering strain range of 2%-40% of the 316L biomedical austenitic stainless steel was investigated in depth at multiple scales by means of EBSD, SEM and TEM. The results indicate that when the strain is more than 20%, the <001> and <111> orientations in 316L austenitic stainless steel are gradually parallel to the tensile direction, and a large number of deformation twins and martensite appear during the deformation process. From the micro-scale and nano-scale, based on the detailed analysis of the twins and martensitic transition, it is found that the deformation firstly induces the formation of deformotiou twins. The twin boundary reduces the mean free path of dislocations and causes dislocation pile-up packing, which further induces the formation of deformation twins. With the increase of deformation, more twins and α-martensite appear. The process of martensitic transition is only γ→α, which α-martensite is mainly distributed near the twin boundaries, especially at the twin intersection. The orientation relationships between the austenitic matrix and the α-martensite are [011]_γ//[011]_α and (420)_γ//(123)_α.

Microstructure and properties of laser clad WC/TiC/Co coating on YG8 cemented carbide surface

Liang Weiyin, Liang Guoxing, Dong Lijun, Liu Donggang, Wang Shiying

2021, 46(12): 168-174. doi:10.13251/j.issn.0254-6051.2021.12.027

Abstract ( 73 )

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WC/TiC/Co coating on YG8 cemented carbide surface was prepared by laser cladding, and the microstructure and the phase composition were analyzed by means of scanning electron microscope(SEM), energy disperse spectroscopy(EDS) and X-ray diffraction(XRD), respectively. In addition, microhardness, hardness distribution and wear resistance were measured or tested. The results show that the coating surface is flat and bonded to the matrix tightly, the cross section morphology is fine without obvious defects. WC particles can be find in surface layer and edges while disappear in clad layer, and new homogeneous microstructure is observed in clad layer. In heat affected zone, WC grains take place recrystallization and gather because of heat effect of laser. The phases in clad layer are mainly composed of WC, W₂C, (Ti,W)C_1-x, M₆C(Co₄W₂C, Co₃W₃C), etc, which result in dispersion strengthening, greatly enhance the performance of the coating. After detection, the microhardness of the clad layer distributes in the range of 1700-1800 HV0.5, and the maximum reaches 1783 HV0.5, which is obviously higher than the common YG8 cemented carbide, while the microhardness of heat affected zone and matrix is slightly decreased. Wear resistance of the clad layer is heightened effectively, compared with the YG8 cemented carbide, wear volume is reduced by 90.67%, average friction coeffiaiont is 0.293, and the abrasive wear is major wear mechanism.

Cellular automaton simulation of microstructure evolution of Hastelloy C276 superalloy during hot processing

Jin Chen, Du Yanxin, Zhang Chi, Zhang Liwen

2021, 46(12): 175-179. doi:10.13251/j.issn.0254-6051.2021.12.028

Abstract ( 53 )

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Single-pass and double-pass hot compression tests for Ni-Cr-Mo-based superalloy Hastelloy C276 were conducted by Gleeble thermal-mechanical simulator, and the flow stress curves and microstructure under different hot deformation conditions were obtained. On this basis, the physical metallurgy models as well as the parameters were determined, and then the cellular automaton model was established for topological evolution of microstructure. The results show that the dynamic recrystallization is prone to occur of the Hastelloy C276 superalloy during high temperature compression. When the dynamic recrystallization is not complete, the recrystallized grains will further grow rapidly during the heat preservation or the hot compression pass intermittence, so that metadynamic recrystallization will occur. The recrystallization behavior of the Hastelloy C276 superalloy is sensitive to the hot processing parameters, including the deformation temperature, strain rate, strain and so on. The dislocation density evolution, recrystallization nucleation, and migration of grain boundary, etc during the heat preservation and the hot compression pass intermittence can be calculated simultaneously by the constructed cellular automaton model, which can be used to predict both the microstructure topological structure evolution and the stress-strain response of the Hastelloy C276 superalloy during high temperature compression effectively.

Microstructure and properties of annealed silver-copper plate for silver-copper foil rolling

Sun Yumei, Wang Xiaowen, Gong Benkui, Feng Rui, Fei Xiangyu, Zhao Weichao

2021, 46(12): 180-186. doi:10.13251/j.issn.0254-6051.2021.12.029

Abstract ( 48 )

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Evolution of microstructure and microtexture, and their effects on Vickers hardness and corrosion resistance of rolled silver-copper plate during the annealing process were studied by means of optical microscope, electron backscattering diffraction technique, Vickers hardness tester and electrochemical workstation. The results show that during the annealing process, the average grain size increases. Moreover, the grain morphology evolves from rolled strip grains to equiaxed grains, at the same time, the texture type develops from deformed texture (copper texture, brass texture, S texture and R texture) to annealed texture (cube texture and {025}<001> texture). With the increase of annealing temperature, the content of small-angle grain boundary and the Vickers hardness of the silver-copper plate decrease, simultaneously, the content of ∑3 grain boundary increases, and the corrosion resistance first weakens and then increases. The content of cube texture is capable of reaching the highest when the silver-copper plate is heated at 300 ℃ for 10 min. Under this condition, the silver-copper plate exhibits excellent corrosion resistance, and the Vickers hardness reaches 60 HV0.3.

Research progress on damping and mechanical properties of iron-based damping alloy

Wang Shuliang, Chen Luyu, Jiang Xiaosong

2021, 46(12): 187-192. doi:10.13251/j.issn.0254-6051.2021.12.030

Abstract ( 41 )

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Iron-based damping alloy, which converts part of vibration energy into heat energy by its own mechanism, can weaken the impact of vibration and noise on equipment and workers effectively, and it has characteristics of superb processability, excellent corrosion resistance, wide operating temperature range and low price. However, the rational matching of damping and mechanical properties of iron-based damping alloy is still a difficult problem that has not been effectively solved and cannot meet the practical engineering application. Heat treatment, element doping and other methods were carried out to optimize the damping performance or mechanical properties of iron-based damping alloys, but almost all of them deteriorated the damping performance when improving the mechanical properties. In order to provide a suggestion for exploring effective process methods to control damping and mechanical properties, the damping mechanism, influencing factors of damping performance and mechanical properties of iron-based damping alloys are analyzed. Finally, the research directions of iron-based damping alloys are prospected.

Mechanical response and constitutive modeling of heat treated 22MnB5 steel

Hu Zehao, Ma Honghao, Liu Jun, Cheng Liu, Zhang Liqiang, Guo Pengcheng

2021, 46(12): 193-198. doi:10.13251/j.issn.0254-6051.2021.12.031

Abstract ( 69 )

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The cold-rolled 22MnB5 steel sheet was water quenched by heating at 750 ℃, 850 ℃ and 950 ℃ for 30 min, respectively, and then stretched at room temperature with strain rates of 0.0005, 0.001, 0.01 and 0.1 s^-1, respectively. The results show that as the heating temperature increasing, the yield strength and tensile strength also increase significantly due to the increase of lath martensite. In addition, as the increase of strain rate, the yield strength and tensile strength of the steel first increase and then decrease. Finally, based on the Voce constitutive model, the constitutive equation which can describe the mechanical behavior of 22MnB5 steel under different heat treatment processes and strain rates is constructed by introducing the strain rate term of Johnson-Cook constitutive model. The correlation coefficient (R) and average absolute relative error (AARE) are 0.993 and 3.15%, respectively.

Microstructure and tribological properties of laser clad Co-Ti₃SiC₂ composite coating on Ti6Al4V alloy

Meng Xiangjun, Shen Ying, Zhang Shaoyu, Xu Hongliang, Wang Yang, Liu Xiubo

2021, 46(12): 199-203. doi:10.13251/j.issn.0254-6051.2021.12.032

Abstract ( 39 )

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To improve the tribological properties of Ti6Al4V alloy, the laser clad composite coatings were prepared on the surface of Ti6Al4V alloy using pure Co powder, Co-2%Ti₃SiC₂, Co-5%Ti₃SiC₂, and Co-8%Ti₃SiC₂ mixed powder as raw materials. The phase composition, microstructure and tribological properties of the coatings at room temperature were analyzed by means of X-ray diffraction (XRD), scanning electron microscope (SEM) and friction wear testing machine. The results show that all the composite coatings are bonded well with the substrate, with some porosities observed, and the main phases of the pure Co coating consist of γ-Co, CoTi, and CoTi₂, while the phases of the Co-Ti₃SiC₂ coatings consist of γ-Co, CoTi, CoTi₂, TiC, Ti₅Si₃ and residual Ti₃SiC₂. The hardness of the coatings is increased by 1.90-2.15 times compared with the matrix, and the wear resistance is correspondingly improved by 3.02 to 5.44 times.

Grain boundary and texture evolution of a high magnetic grain-oriented silicon steel during rolling and heat treatment

Yu Chunming, Zhang Jiming, Dang Bo, Zhang Yi

2021, 46(12): 204-208. doi:10.13251/j.issn.0254-6051.2021.12.033

Abstract ( 69 )

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Coincident site lattice (CSL) boundary and texture of a high magnetic grain-oriented silicon steel in different rolled and heat treatment states were studied by means of electron backscatter diffraction (EBSD). The results show that the cross-section texture of hot-rolled oriented silicon steel is distributed in layers, the surface layer of which is mainly {110}<001> Goss texture, and the quarter of the thickness is mainly {001}<110> cubic texture, {112}<111> copper type texture and {110}<001> Goss texture, while the core forms strong {112}<111> copper type texture, {111}<110> deformation texture and {110}<001> Goss texture. After normalizing, the cross-sectional texture gradient is not obviously changed, but the {112}<111> texture at the center position changes to {110}<001> Goss texture. The texture components are {110}<001> Goss texture, {112}<111> texture and {111}<110> deformation texture in the cold rolled and annealed state steel. After secondary recrystallization, a strong {110}<001> Goss texture is produced. With the change of texture, the CSL grain boundary undergoes a significant transformation. ∑3-∑29 appear in the CSL boundaries in the hot-rolled state, with low proportions. The proportion of CSL grain boundary increases after normalizing. The proportion of CSL boundary increases greatly in the cold rolled and annealed state, especially ∑3, ∑7, ∑9 and ∑15 boundaries. Due to the transformation of CSL boundaries, the type of CSL boundary reduces during second recrystallization, while the proportions of ∑3, ∑13 and other grain boundaries increase, but the ∑9 grain boundary disappears.

Microstructure, properties and safety evaluation of low hardness P91 steel tube after long service

Deng Hui

2021, 46(12): 209-213. doi:10.13251/j.issn.0254-6051.2021.12.034

Abstract ( 43 )

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Microstructure, properties and safety evaluation of low hardness P91 steel after 10⁵ h service were studied by means of microstructure observation, energy dispersive spectrum (EDS) analysis, room temperature and high temperature mechanical property test, room temperature impact test and high temperature stress rupture test. The results show that the microstructure of low hardness P91 steel tube (157 HBW) after long service is composed of massive ferrite and large size M₂₃C₆ phase. Compared with the normal hardness tube, its mechanical properties at room temperature and short time mechanical properties at high temperature decrease greatly, and its extrapolation value of 10⁵ h endurance strength is 36% lower than that of the standard recommended value. The residual life of low hardness straight tube section of high-pressure steam pipe is 54 075 h, which means the operation of the power plant unit has great security risks.

Ductile-brittle transition temperature of a grain-oriented silicon steel and its influencing factors

Li Li, Jia Zhiwei, Zhang Yifan, Luo Li, Jiang Qiwu

2021, 46(12): 214-217. doi:10.13251/j.issn.0254-6051.2021.12.035

Abstract ( 76 )

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Ductile-brittle transition temperature and its influencing factors of 0.27 mm high magnetic grain-oriented silicon steel plate of Ansteel were determined by measuring the mechanical properties of specimen at different normalizing temperatures and observing the microstructure and fracture morphology. The results show that the critical breaking strength of the tested steel is about 590 MPa, and the ductile-brittle transition temperature range is 60-80 ℃. With the increase of test temperature in the range of 0-100 ℃, the phosphorus concentration decreases from about 90% to about 65%, and the corresponding equilibrium time is short, which is beneficial to improve the grain boundary bonding strength and to weaken the brittle fracture tendency. In addition, the coarsening of normalized grains and the microcracks caused by uneven deformation of ferrite and pearlite are the main structural factors of embrittlement, which are not conducive to cold rolling of grain-oriented silicon steel.

Effect of hot rolling on microstructure and properties of 65Mn steel

Yang Yingying, Wu Hongyan, Qi Min, Du Linxiu

2021, 46(12): 218-222. doi:10.13251/j.issn.0254-6051.2021.12.036

Abstract ( 52 )

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Effect of hot rolling process on microstructure and mechanical properties of 65Mn steel was experimentally studied. Electron probe microanalyzer (EPMA) and transmission electron microscope (TEM) were used to characterize the hot-rolled microstructure. The results show that two initial structures of pearlite and bainite can be obtained by controlling the hot rolling process. With the decrease of final cooling temperature and final rolling temperature, the proeutectoid ferrite content and pearlite interlamellar spacing gradually decrease, and the strength and hardness gradually increase. Compared with pearlite, hot-rolled bainite has higher strength, with tensile and yield strengths of 943 MPa and 648 MPa, respectively.

Effect of quenching temperature on microstructure and properties of novel NM450 corrosion-wear resistant steel

Peng Siyuan, Deng Xiangtao, Liang Liang, Fu Tianliang, Liu Ning, Wang Zhaodong

2021, 46(12): 223-228. doi:10.13251/j.issn.0254-6051.2021.12.037

Abstract ( 45 )

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Effect of quenching temperature on microstructure and mechanical properties of NM450 corrosion-wear resistant steel was studied by means of scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and mechanical property testing. The results show that the tempered lath martensite microstructure is obtained by quenching at 840-960 ℃ and low temperature tempering. When the quenching temperature is at or lower than 870 ℃, the dispersed second phase appears in the microstructure, and the Cr content is obviously higher than that of the matrix. When the quenching temperature is higher than 900 ℃, the second phase disappears and the austenite grains begin to grow obviously. With the increase of quenching temperature, the strength and hardness of the tested steel tend to decrease, and the impact absorbed energy reaches the highest at 900 ℃. According to the orientation distribution and grain boundary distribution, it can be found that when quenched at 960 ℃, the effective grain size is the largest, the proportion of large angle grain boundary is the lowest and the impact property is the worst. When quenched at 900 ℃, the effective grain size is close to that at 840 ℃, but the microstructure is more uniform, and the proportion of large angle grain boundary increases, which is the main reason for the high impact property when quenched at 900 ℃.

Evolution of microstructure and properties of 1RK91 stainless steel during cold rolling and aging

Tian Shuoshi, Ma Fengcang, Liu Ping, Liu Xinkuan, Li Wei, Zhang Ke

2021, 46(12): 229-235. doi:10.13251/j.issn.0254-6051.2021.12.038

Abstract ( 40 )

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Effects of strain-induced martensitic transformation during cold rolling and aging process on mechanical properties of the 1RK91 stainless steel were studied by means of optical microscope (OM), X-ray diffraction (XRD), field emission scanning electron microscope (SEM), Vickers hardness tester and universal material testing machine. The results show that the strain-induced martensite transformation occurs during cold rolling of the 1RK91 stainless steel. The nucleation of strain-induced martensite depends on the proliferation and movement of dislocations during cold rolling and is affected by the deformation temperature. The tensile strength and dislocation density of the material increase with the increase of cold rolling deformation, and the tensile strength after solid solution and 75% cold rolling are 903 MPa and 1195 MPa, respectively, the dislocation densities are 5.75×10¹⁰ cm^-2 and 8.84×10¹⁰ cm^-2, respectively, and the calculated cold-working strengthening coefficient of the steel is 3.89 MPa/%. Under conventional aging, the peak hardness of solid solution treated and 75% deformed steel are 499 HV0.2 and 610 HV0.2, respectively, and after graded aging, the peak hardness are 513 HV0.2 and 639 HV0.2, respectively. With the increase of deformation, the peak hardness of the conventional aged material increases, and the temperature to reach the peak hardness decreases. After graded aging, the material has higher peak hardness and better anti-over-aging ability on the whole.

Influence of heat treatment on microstructure and properties of spray formed 7A50 aluminum alloy

Yang Chunmiao, Wang Shuhui, Liu Wenwen, Li Yanzhen

2021, 46(12): 236-240. doi:10.13251/j.issn.0254-6051.2021.12.039

Abstract ( 53 )

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Influence of heat treatment on microstructure and mechanical properties of spray formed 7A50 aluminum alloy was investigated. The microhardness and mechanical properties were tested. The OM, SEM(EDS) and TEM(SAED) were used to characterize the microstructure. The results indicate that the dendrite grains in the spray formed 7A50 aluminum alloy are eliminated, and the grain size reaches 60-120 μm. Pre-aging (PA) treatment enhances the initial hardness of the alloy and decreases the hardening rate during the natural aging (NA) process. Compared to the solid solution treated (SS)+NA state, the final hardness decreases by 19 HV0.5 after 12 days NA process. Under the SS+PA+NA+bake hardened (BH) conditions, the yield strength and tension strength of the alloy reach their peak values 475 MPa and 525 MPa respectively, the BH value of yield strength is 165 MPa, while the elongation decreases by 10%. Under different heat treatment conditions, the fracture morphologies all show ductile characteristics and have a large number of dimples. After SS+PA+NA treatment, the secondary phase in the matrix is dispersed and fine, with the size ranging from 5 nm to 15 nm.

Influence of heat treatment process on microstructure and properties of Ti-3Al-6Mo-2Fe-2Zr alloy

He Chaowei, Zhang Kezhao, Yang Dan, Niu Hongzhi, Yan Chunyan, Bao Yefeng

2021, 46(12): 241-246. doi:10.13251/j.issn.0254-6051.2021.12.040

Abstract ( 46 )

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Influences of solution and aging treatment on microstructure, mechanical properties and corrosion resistance of near-β titanium alloy (Ti-3Al-6Mo-2Fe-Zr) were studied by means of optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD). The results show that with the increase of solution temperature, the content of primary α phase gradually decreases. After solid solution treatment at 930 ℃, the alloy phase composition becomes a single β phase. When the solution temperature is below 830 ℃, the primary α phase gradually transforms into β phase with the increase of solution temperature. The strengthening effect of the second phase is weakened, resulting in a decrease in strength and an increase in plasticity. When the solution temperature is above 830 ℃, the β-phase grains gradually coarsen with the increase of solution temperature, leading to a decrease in strength and plasticity. The fracture mode also changes from micropore coalescence fracture to cleavage fracture. The corrosion resistance is improved as a result of the decrease of content of α phase and β/α phase boundary, as the solution temperature increases from 780 ℃ to 930 ℃. After the solution treatment at 780 ℃ for 1 h followed by water quenching and aging treatment at 500 ℃ for 6 h followed by furnace cooling, the fine needle-like secondary α phase precipitates within the metastable β phase, resulting in an increase in strength and a decrease in plasticity.

Fracture resistance mechanism of high strength structural steel Q690GJ treated by intercritical quenching process

Deng Wei, Qin Xiaomei

2021, 46(12): 247-251. doi:10.13251/j.issn.0254-6051.2021.12.041

Abstract ( 67 )

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Effects of QT (quenching+tempering) and QLT (quenching+intercritical quenching+tempering) process on microstructure and low temperature toughness of high strength structural steel Q690GJ were investigated. The microstructure of the low temperature impact specimen and the NDT (nil-ductility transition) specimen was analyzed by means of optical microscopy and scanning electronic microscopy. The results show that the low temperature toughness of the steel treated by QLT process is obviously better than that treated by QT process. The microstructure obtained by QLT process is lath martensite+retained austenite. More stable retained austenite that responsible for the enhanced plastic formability before crack initiation is transformed through intercritical quenching. While, chaotic martensite lath bundles formed by QLT process effectively prevent crack propagation. Then the low temperature toughness and NDT temperature of the steel is improved and reduced, respectively by QLT process.

Process of ion nitro-carbon titanizing for industrial pure iron

Liu Yan, Zhang Chengxing, Li Shuangxi

2021, 46(12): 252-255. doi:10.13251/j.issn.0254-6051.2021.12.042

Abstract ( 50 )

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Aiming at the problem of low hardness and poor wear resistance of nitrided layer of the electromagnetic pure iron treated by the binary ion nitrocarburizing, the surface modification treatment of ion nitro-carbon titanizing was carried out. The results show that under the same nitriding process, the surface hardness of nitrided layer is increased effectively due to the addition of titanium sponge. After nitriding at 520 ℃ for 8 h, the measured hardness of the nitrided layer is up to 513 HV0.05. But ternary ion nitro-carbon titanizing decreases the magnetic properties of pure iron, especially low magnetic properties, which is lower than the requirements of GB/T 3656—2008, and the medium and high magnetic properties still meet the requirements of the standard.

Effect of tempering temperature on mechanical properties of 18Cr2Ni4WA steel after carburizing and quenching

Li Feng, Li Shuangxi, Li Jiqiang, Wang Hongwei, Liu Qiaoqiao, Wang Dongzhu, Miao Weiwei

2021, 46(12): 256-261. doi:10.13251/j.issn.0254-6051.2021.12.043

Abstract ( 75 )

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In order to study the effect of different tempering temperatures on the mechanical properties of carburized surface and non carburized part (core) of 18Cr2Ni4WA steel after carburizing and quenching, the mechanical properties of specimen at different tempering temperatures after carburizing and quenching were studied by means of microhardness tester, tensile testing machine and impact testing machine. The properties of gear specimen at different tempering temperatures were simulated and analyzed by ABAQUS simulation software. The results show that with the increase of tempering temperature, the hardness and strength of the specimen decrease, and the impact absorbed energy of the core also decreases significantly. At 300 ℃, its impact absorbed energy value is close to the minimum value specified in the technical requirements. Considering the furnace temperature and the non-uniformity of material composition, the tempering temperature of this kind of steel after carburizing and quenching should not be higher than 260 ℃ under certain conditions.

Displacement boundary conditions in heat treatment simulation analysis of 17CrNiMo6 steel helical gear

Liang Ruijun, Wang Zhiqiang, Li Huawen

2021, 46(12): 262-267. doi:10.13251/j.issn.0254-6051.2021.12.044

Abstract ( 49 )

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Based on the mathematical models of heat transfer, structure field transformation and thermo-elastoplasticity in 17CrNiMo6 steel heat treatment process, a finite element model of the helical gear heat treatment was established. The heat treatment process of the 17CrNiMo6 steel helical gear was simulated, and the influence of the three displacement boundary conditions (fixed constraint, symmetric constraint and friction constraint) on the distortion of heat treatment simulation was analyzed. The results show that the surface on the 17CrNiMo6 steel helical gear is subjected to compressive stress after quenching, but the core is subjected to tensile stress, and the compressive stress at the root of the tooth is greater than that at the top of the tooth. Compared with the commonly used boundary conditions of fixed or symmetrical constraints, the simulation results of friction constraints are more in line with the actual results.

Simulated research on influence of carburized layer thickness on quenching distortion of 18Cr2Ni4WA steel gear

Kong Lingli, He Ruijun, Li Guifa

2021, 46(12): 268-275. doi:10.13251/j.issn.0254-6051.2021.12.045

Abstract ( 49 )

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Influence of carburized layer on the temperature field, stress field and strain field of the 18Cr2Ni4WA steel arc-gear during gas quenching process was analyzed by means of finite element simulation software, and the action mechanism was explored further in combination with the first-principles method. The results show that the influence of depth of carburized layer on the temperature field of 18Cr2Ni4WA arc gear is small. But the stress field of surface layer in gear fluctuates is obvious when the depth of carburized layer is smaller than 0.5 mm, which increases from 86.7 MPa at the thickness 0.1 mm of the carburized layer to 278.6 MPa at 2.0 mm. The results of the strain field show that when the thickness of the carburized layer is 2.0 mm, the initial value of the equivalent strain on the surface of tooth top reaches 2%. The first-principles calculation results show that with the increase of carbon concentration, the bond strength between Fe-Fe atoms weakens. However, the newly formed Fe-C bond and Cr-C bond are obviously strengthened, and the electron density between Fe and C atoms shows obvious directionality. Therefore, the weakness of the Fe-Fe bond causes expansion coefficient of the 18Cr2Ni4WA steel to increase with the addition of carbon concentration. Then it will cause the heating-up distortion of the gear be not capable of recovering during the gas quenching and cooling process for its excessively thick carburized layer. As a result, the fluctuation of gear geometric dimension will be aggravated obviously.

Effect of process parameters on interface heat transfer efficiency of aluminum alloy during forced air cooling

Xu Rong, Li Luoxing

2021, 46(12): 276-281. doi:10.13251/j.issn.0254-6051.2021.12.046

Abstract ( 47 )

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The heat transfer process of high-speed gas impact on the hot metal surface was studied through a series of air-cooled quenching experiments. The inverse heat transfer method was used to calculate the interfacial heat flux(q) and the interfacial heat transfer coefficient(h). The effects of initial quenching temperature of specimen, surface roughness and air flow density on specimen surface on heat transfer process were studied. The results show that the initial quenching temperature of specimen has a significant effect on the heat transfer at the air-cooled quenching interface. When it increases from 470 ℃ to 520 ℃, the maximum value of both q and h is increased by about 50%, and the average cooling rate of quenching surface temperature, when decreases to 200 ℃, is increased by about 43%. With the increase of the air flow density on specimen surface, the interfacial heat exchange shows a tendency of first increasing and then decreasing, that is, when the interfacial heat transfer efficiency is highest, there is a critical air flow density on specimen surface corresponding to it, and the closer the injection angle is to 90°, the smaller the critical value is. As the surface roughness of the specimen increases, the interfacial heat transfer decreases continuously, which may be attributed to the more obvious the rougher surface pinning effect on the fluid in the boundary layer, the more unfavorable it is to improve the interfacial heat transfer efficiency. Moreover, in the range between 250-380 ℃, there is generally a depressed region in the curve of h with surface temperature, which may be related to the precipitation of secondary phase during quenching and cooling of aluminum alloy.

Blanking failure analysis of 25CrMnB track steel

Tian Yu, Yu Hao

2021, 46(12): 282-288. doi:10.13251/j.issn.0254-6051.2021.12.047

Abstract ( 54 )

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Distribution law of microstructure and hardness of different parts in the 25CrMnB track steel was studied by means of metallographic and hardness tests, and the cause of blanking fracture of hot-rolled 25CrMnB track steel was analyzed. The results show that the hardness of the water-cooled specimen is higher than that of the air-cooled specimen, and the hardness of the inducing tooth parts of the track steel where the failure occurs firstly is more likely to be abnormally high. The reason of blanking fracture of the track steel is the appearance of mixed grain structure, reticular ferrite and granular bainite. High stress concentration could occur at the abnormal microstructure when the material is stressed, which leads to the initiation and propagation of microcracks and the blanking failure of track steel. The reason for the appearance of mixed grain structure and reticular ferrite is too high final rolling temperature. On the basis of EDS analysis, the main reason for the appearance of granular bainite is the segregation of microscopic elements of Mn and B.

Failure analysis of pitting on working surface of die-casting mold made of 8407 steel

Meng Xinpei, Jin Linkui, Lin Quan, Huang Yihao, Fang Manting, Li Xiaohui, He Ping

2021, 46(12): 289-293. doi:10.13251/j.issn.0254-6051.2021.12.048

Abstract ( 53 )

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The die-casting mold made of 8407 steel was re-polished to remove the surface pits appeared in service, however, the pitting appeared again on the working surface in re-using of the die. The chemical composition, surface defect features and microstructure of failed die-casting mold parts were analyzed by means of metallographic microscope, scanning electron microscope and others. The results show that there are different degrees of cracking around and at the bottom of the surface pits, and slope-like indentations are formed on the edges of the pits. No chemical reactions such as oxidation and corrosion are found in the pits on the working surface of the mold, which implies that the pits are caused by extrusion deformation. The reason for the pitting on the working surface of the die-casting mold is that the polishing process is too strong, so that bright quenching structure forms in the surface layer of the mold working surface caused by high temperature and rapid cooling, and the resulted stress causes transverse crack of working face and form local longitudinal cracks. Under the influence of the heat from surface polishing, the secondary surface layer of the working surface is softened by annealing, thus forming a deformed fibrous structure. In the using process of the die, the longitudinal cracks on the working surface are crushed to form pits, which causes the surface macro-pits on the working surface of the mold, leading to the raised appearance defects on the surface of die-casting products of zinc alloy.

Cracking failure analysis of 40Cr steel forgings for automobile front axle

Cheng Yuan, Hu Fangzhong, Yang Shaopeng, Wang Kaizhong, Jin Guozhong, Chen Shijie, Wang Lianglin

2021, 46(12): 294-297. doi:10.13251/j.issn.0254-6051.2021.12.049

Abstract ( 89 )

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Cracking reason of 40Cr steel forgings used for automobile front axle was analyzed by means of chemical composition, microstructure and hardness analysis. The results show that the crack is a quenching crack occurred at the R angle of the forgings. The elemental segregation at the R angle of the forgings results in significantly higher hardenability than that of other locations around. Surface defects are produced in the forging process, so that the volume stress generated by martensitic transformation during quenching exceeds the strength of the material, resulting in the generation of quenching cracks. By increasing the R angle radius, controlling the chemical segregation of raw materials and adjusting the forging process, the risk of quenching cracking of 40Cr steel forgings for automobile front axle after heat treatment can be reduced.

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