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Discussions of physical stability and increasing routes of sintered rare-earth NdFeB magnets

Wu Yucheng, Liu Jiaqin

Heat Treatment of Metals 2023, 48 (10): 8-14. doi:10.13251/j.issn.0254-6051.2023.10.001

Abstract （65）

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Sintered rare-earth NdFeB is an important functional material, which is broadly applied in automobile, medical equipment and aerospace fields etc. The preparation, application and control of physical stability were reviewed through involving in optimizing the composition and tailoring grain boundary structure, exploring the effects on micro mechanism of the stability through powder metallurgy route. The influence of chemical composition and microstructure on the stability of the magnetic fields and temperature was concluded by analysing the stability of sintered Nd-Fe-B magnets under different conditions.

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Heat treatment technologies for toughening of cold working die steel DC53

Yuan Zhizhong, Chen Lu, Zhang Bocheng, Wang Mengfei, Liu Haiming, Niu Zongran, Wang Zhiyuan, Cheng Xiaonong

Heat Treatment of Metals 2023, 48 (10): 15-22. doi:10.13251/j.issn.0254-6051.2023.10.002

Abstract （79）

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Duplex phase heat treatments of austempering-quenching-austempering (A-Q-A) and austempering-quenching-tempering (A-Q-T) were processed to improve the toughness of cold working die steel DC53. The influence of heat treatments on microstructure and mechanical properties of the DC53 steel was analyzed and compared with conventional heat treatment of quenching-high temperature tempering (Q-T). The results show that lower bainite/martensite (LB/M) multiphase structures with retained austenite (A_R) content of 21.7% and 16.5% is resulted from the A-Q-A and the A-Q-T specimens,respectively. The hardness of the A-Q-A and the A-Q-T specimens is 59.6 HRC and 59.9 HRC, respectively, slightly lower than that of Q-T, i.e. 62.3 HRC. However, the impact absorbed energy of these two processes are 84.9 J and 87.3 J, respectively, much higher than that of Q-T, i.e. 35.6 J. It is due to the fine grain strengthening of the duplex phase microstructure,deformation strengthening and the content increase in A_R. Therefore, excellent coordination of strength and toughness can be achieved by LB/M duplex phase heat treatment, which enable the DC53 steel to be used in service environments with impact.

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Effect of different heat treatment processes on microstructure and properties of large-thickness bridge steels

Yang Ying, Xu Hongliang, Wang Yubo, Wu Huibin, Mao Xinping

Heat Treatment of Metals 2023, 48 (10): 23-28. doi:10.13251/j.issn.0254-6051.2023.10.003

Abstract （54）

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Effect of different heat treatment processes on microstructural characteristics of large-thickness weathering bridge steels with a C-Mn-Nb-V composition system were investigated. The microstructure evolution of the steel under three different processes (TMCP, TMCP+tempering, and TMCP+quenching and tempering) was investigated by means of optical microscope, scanning electron microscope (SEM), and electron backscattered diffraction(EBSD). The results show that the microstructure is composed of granular bainite, polygonal ferrite, and M/A islands after the TMCP process. After the TMCP+tempering process, the degree of recovery and recrystallization is high, M/A islands disappear, and the microstructure is fine and uniform, which is granular bainite and polygonal ferrite. After the TMCP+quenching and tempering process, a complex phase of granular bainite, polygonal ferrite, and a small amount of M/A islands is obtained, and the grains grow slightly after recrystallization. Therefore, the microstructure and comprehensive properties of the steel plate is the best after TMCP+tempering process.

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Microstructure and properties of cold-sprayed metal coating optimized by heat-treatment: A review

Li Bo, Wang Hao, Liu Bo, Wu Lijuan, Zhang Qunli, Yao Jianhua

Heat Treatment of Metals 2023, 48 (10): 29-36. doi:10.13251/j.issn.0254-6051.2023.10.004

Abstract （36）

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Cold gas dynamic spray, also known as cold spray (CS), is a solid-state deposition technology based on spraying material plastic deformation, which can be used for the preparation of functional coatings on the surface of metal components and metal additive manufacturing/remanufacturing. However, there are still problems including high porosity, weak interface bonding and poor plasticity in cold spraying technology. Subsequent heat treatment is often used as an effective means to regulate the microstructure and performance of cold spraying deposition layers. Therefore, the current research status of using subsequent heat treatment to control the microstructure and properties of cold sprayed metal deposition coating was reviewed. Through heat treatment, the microstructure, phase and stress state of the cold sprayed deposition coating can be optimized, thereby improving the microhardness, wear/corrosion resistance, thermal/electrical conductivity, strength and plasticity of the deposition coating.

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Microstructure and mechanical properties of high strength low alloy thick steel plates for offshore engineering

Cui Shugang, Shi Changxin, Gu Guochao, Xu Wenhua, Lü Yupeng

Heat Treatment of Metals 2023, 48 (10): 37-44. doi:10.13251/j.issn.0254-6051.2023.10.005

Abstract （36）

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A 420 MPa grade high strength low alloy heavy plates with a thickness of 80 mm were produced by the process of TMCP plus tempering at 600 ℃. The microstructure and mechanical properties along the thickness direction were analyzed by using optical microscopy, scanning electron microscopy, transmission electron microscopy, electron back-scattered diffraction and mechanical tests. The results show that a fine-grained structure with an average grain size of 6-10 μm can be obtained by TMCP. After tempering, the microstructure near the surface of the plate is dominated by bainite, while the core is mainly characterized by aggregated carbides and ferrite. After tempering at 600 ℃, cementite precipitates on the ferrite grain boundary, and compound precipitation of carbides of Nb and Ti elements is found. Tensile tests show a decrease in yield strengths from 542 MPa on the surface to 384 MPa at the core. The fracture model of the plate is dominated by micro-hole coalescence fracture. Micro-holes are mostly formed between ferritic laths and near inclusions. The grain refinement strengthening and precipitation strengthening dominate the contribution of each strengthening mechanism to the yield strength.

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Comparison of austenite grain growth behavior of 20CrMnTi steel and 20 steel

Xue Li, Zhang Liwen, Ding Haochen, Zhang Chi, Song Kangjie

Heat Treatment of Metals 2023, 48 (10): 45-49. doi:10.13251/j.issn.0254-6051.2023.10.006

Abstract （38）

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Austenite grain growth behavior of 20CrMnTi steel and 20 steel under different holding temperatures (1000-1200 ℃) and different holding time (0-300 s) was studied. Based on the experimental results, a Sellars model describing austenite grain growth behavior was established. By comparing the average relative error (AARE) and correlation coefficient (R) between the calculated values of austenite grain growth model and the experimental values, the reliability of the model was verified. The experiment and simulation results show that the grain size of the steel increases obviously with the increase of heating temperature, and increases rapidly in the first 60 s with the increase of holding time, and then slows down. However, under the same test conditions, the austenite grain size of the 20CrMnTi steel is obviously smaller than that of the 20 steel and no abnormal grain growth occurs,indicating that the element addition of Ti has obvious effect on refining the microstructure.

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Microchannel surface terminal and heat transfer performance of manifold all-diamond

Feng Xurui, Wei Xinyi, Zhang Jianjun, Zheng Yuting, Chen Liangxian, Liu Jinlong, Li Chengming, Wei Junjun

Heat Treatment of Metals 2023, 48 (10): 50-58. doi:10.13251/j.issn.0254-6051.2023.10.007

Abstract （36）

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Diamond thick film was prepared by DC arc plasma spraying, and Z-type manifold-type all-diamond microchannel was formed by high-energy laser. The diamond microchannel was treated with hydrogen plasma, acid and fluorine plasma to obtain hydrogen-terminal, oxygen-terminal and fluorine-terminal. Through the two-phase heat transfer experiment, the change mechanism of the surface modification technology in the sustained stability of the diamond microchannel radiator was analyzed. The researches find that in the process of constant heat flow treatment, with the increase of operating time, the hydrophobic properties of the hydrogen-terminated diamond microchannels decrease rapidly and are not suitable for heat transfer research. The hydrophilic properties of the oxygen terminal decrease significantly and then tend to be stable. The fluorine-terminated surface is the most stable, and its hydrophobicity remains constant after a slight decrease in the initial stage. At the same time, it is found that the heat transfer of the oxygen terminal and fluorine terminal manifold diamond microchannels is dominated by nuclear boiling, including bubble flow and elastic flow. After further heating, a large number of bubbles converge to form annular flow, and the evaporation of thin film is the dominant mechanism. The hydrophobic fluorine terminal microchannel will accelerate the nucleation of bubbles, and the heat transfer performance is better than that of fluorine terminal in flow boiling state. The excessive bubble formation causes the occurrence of reverse flow, and the oxygen terminal microchannel can delay the occurrence of reverse flow, showing a higher boiling starting point and critical heat flux. The heat transfer coefficient of the fluorine terminal microchannel is higher than that of the oxygen terminal in boiling state, so the fluorine terminal can be more suitable for the surface modification technology of preparing hydrophobic diamond heat sink.

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Heat treatment and microstructure-property regulation of dual phase steels with identical composition

Zhang Yu, Wang Lingyu, Yang Kai, Lu Qi, Xu Wei

Heat Treatment of Metals 2023, 48 (10): 59-65. doi:10.13251/j.issn.0254-6051.2023.10.008

Abstract （41）

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Dual phase steels with three strength levels of 590, 780 and 980 MPa were achieved simultaneously by regulating the isothermal temperature of a low-carbon Si-Mn-Cr steel under different heat treatment processes. The results show that the martensite content in the final microstructure is increased with the increase of isothermal temperature, and with a consequent increase in the tensile and yield strengths and a decrease in the total elongation of the tested steel. Compared with the intercritical isothermal process, the inclusion of the overaging step requires a higher isothermal temperature and a higher fraction of martensite in the microstructure to achieve the same strength. As the overaging temperature increases, the isothermal temperature required to reach the same strength level further increases and the martensite fraction further increases. Dual phase steels with three different strength levels of 590, 780 and 980 MPa are prepared, respectively, by using the identical composition with both heat treatment processes. However, the 980 MPa steel after the intercritical isothermal-overaging process has nearly full martensite microstructure, which does not meet the microstructure characteristics of the actual dual phase steel.

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Effect of aging process on microstructure and properties of 2060 Al-Li alloy formed at cryogenic temperature

Jia Yanzhen, Yi Youping, Huang Shiquan, Dong Fei, Huang Ke

Heat Treatment of Metals 2023, 48 (10): 66-71. doi:10.13251/j.issn.0254-6051.2023.10.009

Abstract （26）

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Effects of aging temperature and aging time on the mechanical properties and microstructure evolution of cryogenic-formed 2060 Al-Li alloy were investigated by means of transmission electron microscope (TEM) and scanning electron microscope (SEM). The optimum aging process parameters for cryogenic temperature forming were obtained. The results show that at the initial stage of aging, the precipitated phase of the alloy is mainly spherical δ′ phase and acicular T₁ phase. With the extension of aging time, T₁ phase continues to nucleate and grow, while the number of δ′ phase decreases, the hindrance of slender T₁ relative to dislocation movement increases, and the strength of the alloy increases and reaches the peak value. When the aging time is further prolonged, the number of T₁phase does not change, but the morphology becomes thicker, and the strength of the alloy decreases. When aged at 165 ℃, 175 ℃ and 185 ℃, respectively, the time for the alloy to reach the peak aging becomes shorter with the increase of aging temperature, but compared with the higher temperature aging, the morphology of the precipitated phase after lower temperature aging is longer, the distribution is more dispersed, and the peak strength of the alloy is higher. After cryogenic temperature forming, the best comprehensive mechanical properties of 2060 Al-Li alloy can be obtained by aging at 165 ℃ for 35 h. The yield strength, tensile strength and elongation are 494 MPa, 538 MPa and 6%, respectively.

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Effect of quenching temperature on microstructure and properties of steel treated by heterogeneous quenching and partitioning

Zhang Chao, Yu Hebao, Xiong Zhiping

Heat Treatment of Metals 2023, 48 (10): 72-77. doi:10.13251/j.issn.0254-6051.2023.10.010

Abstract （36）

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By using a fast quenching & partitioning process to treat a partitioned Mn pearlite steel to obtain Mn-heterogeneous austenite, the effect of quenching temperature on the microstructure evolution and mechanical properties of the steel was systematically investigated. The results show that when high temperature austenite inherits the Mn distribution in Mn-rich cementite and Mn-poor ferrite in pearlite, ghost pearlite which consists of alternative Mn-enriched film retained austenite(RA) and Mn-depleted lath martensite can be obtained after quenching. With the increase of quenching temperature, the driving force of austenite-to-martensite transformation decreases, which leads to the decrease of ghost pearlite fraction and the increase of fraction and size of blocky RA. Due to the decrease of ghost pearlite fraction, the average width of martensite lath increases, resulting in the decreased yield strength. Moreover, the increased fraction and size of blocky RA ensure an increased uniform elongation by transformation-induced plasticity effect, whereas the transformation product (i.e., fresh martensite) is detrimental to the post-uniform elongation. Therefore, tuning the quenching temperature provides an effective strategy to tailor yield strength and uniform elongation while maintaining large ultimate tensile strength (about 1600 MPa) and total elongation (about 20%).

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Crack initiation and propagation behavior in tensile deformation of high-strength titanium alloy with lamellar microstructure

Huang Xiaowen, Liu Leliang, Liu Jixiong, Wang Xiaoxiang, Sun Qiaoyan

Heat Treatment of Metals 2023, 48 (10): 78-86. doi:10.13251/j.issn.0254-6051.2023.10.011

Abstract （72）

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Crack initiation and propagation mechanism in tensile deformation of Ti1300 alloy with lamellar microstructure after solution and aging treatment was investigated in detail, and the influence of grain boundary with Widmanstätten α phase (GBWα) on crack initiation and propagation was revealed. The results show that, the tensile fracture exhibits a mixed morphology of transgranular dimples and intergranular fracture facets, while the statistical analysis indicates a reduction in the number of intergranular fracture facets is related to an increase in the elongation of the alloy. By counting the locations of secondary crack initiation near the fracture, it is found that about 77% of secondary cracks near the fracture surface initiate predominantly at β grain boundaries, while about 23% of secondary cracks are initiated within β grains. By proposing a crack initiation strain parameter to assess the corresponding strain magnitudes for crack initiation, it is found that the strain parameter for cracks initiated at grain boundaries is lower than that for cracks initiated within β grains, which indicating a lower strain requirement for crack initiation at grain boundaries. As for cracks initiated at grain boundaries, 88% of them initiate at the grain boundaries without GBWα, while only 12% at those with GBWα, which indicating that the GBWα can inhibit the crack initiation. The crack propagation path in the fracture process is a mixture of extending through the grains and along the grain boundaries, and the GBWα can deflect the crack path.

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An improved U-Net algorithm for extracting carbides from spray formed high speed steel

Chen Jiashu, Hou Guodong, Zhou Jikuan, Liu Tianqi, Deng Baichuan, Zhang Xianglin

Heat Treatment of Metals 2023, 48 (10): 87-93. doi:10.13251/j.issn.0254-6051.2023.10.012

Abstract （23）

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To address the issues of low accuracy, poor extraction effect, and large analysis errors caused by using digital image processing algorithms and deep learning models for quantitative analysis of carbides in steel, a modified jet forming high speed steel carbides segmentation algorithm based on U-Net (GSG-Unet) was proposed. The aim is to accurately and efficiently segment and extract different types of carbides in steel for automated quantitative analysis. The model is strengthened by adding ConvNext module and CBAM attention mechanism to enhance its feature extraction capability and ability to handle missed detections, resulting in significant improvement in segmentation performance. The results show that the improved model has an accuracy of 91.31%, recall rate of 87.52%, class-average intersection over union of 84.89%, and Dice of 83.16%, which are significantly higher than those of the original model. This improved model can accurately segment MC and M₆C carbides from martensite matrix, providing strong technical support for rapid and accurate automated quantitative analysis of carbides in high speed steel.

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