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Effect of microstructure on hydrogen diffusion behavior of GCr15 bearing steel

Fu Siyu, Du Yu, Huang Zhiyuan, Lu Jianing, Du Linxiu

Heat Treatment of Metals 2022, 47 (12): 175-180. doi:10.13251/j.issn.0254-6051.2022.12.030

Abstract （37）

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Electrochemical hydrogen permeation test of spheroidizing annealed, spheroidizing annealed+quenched and tempered (Q&T) GCr15 bearing steel was carried out to describe hydrogen diffusion behavior. The effects of microstructure and heat treatment status on hydrogen diffusion behavior were analyzed. The results show that the distribution of carbides in the spheroidizing annealed specimens has a greater effect on hydrogen diffusion behavior. Hydrogen diffuses fastest in banded carbides, while the network carbides along grain boundaries acts as hydrogen traps, trapping more hydrogen atoms. In the Q&T specimens, hydrogen diffuses fastest in banded undissolved carbides. The effective hydrogen diffusion coefficient decreases with the increase of the volume fraction of retained austenite. The hydrogen diffusion coefficient of the spheroidizing annealed tested steel is much higher than that of the Q&T steel. The number of hydrogen traps in Q&T specimens increases due to the presence of retained austenite, making hydrogen permeation more difficult.

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Microstructure and wear resistance of GCr15 bearing steel with low-temperature plasma sulfurizing coatings

Li De, Shan Qiongfei, Wang Xin, Liu Yong, Zhou Meng, Zhang Yi

Heat Treatment of Metals 2022, 47 (12): 181-187. doi:10.13251/j.issn.0254-6051.2022.12.031

Abstract （37）

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Microstructure and wear resistance of GCr15 bearing steel after different surface treatments were studied by means of OM, XRD, SEM and UMT friction and wear tester. The results show that compared with that carbonitrided, a FeS-based sulfurized layer with uniform and fine microstructure is formed on the surface of the GCr15 bearing steel after carbonitriding and low-temperature plasma sulfurizing, though the hardness decreases slightly, the friction coefficient and volume wear rate are significantly reduced, so that the ability of the bearing steel surface to resist scratching and to anti-biting is significantly improved, the service life of the bearing is also prolonged. During the wear process, the FeS layer falls off and produces a smearing effect, which significantly reduces the degree of abrasive wear. The analysis of the wear morphology confirms that the wear mechanisms are abrasive wear and adhesive wear.

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Characteristics of direct current field enhanced pack boron-aluminizing of 45 steel

Ren Zhonghua, Xie Fei, Pan Jianwei

Heat Treatment of Metals 2022, 47 (12): 188-195. doi:10.13251/j.issn.0254-6051.2022.12.032

Abstract （34）

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Direct current field enhanced pack boron-aluminizing (DCFEPBA) was carried out on 45 steel at 750 ℃ by using pack agent with content of aluminum powder varied from 0 to 1.5%, and compared with the corresponding conventional diffusion and direct current field enhanced boronizing. The phase structure, microstructure and hardness distribution of the specimen case at different positions in the direct current field were observed and analyzed by means of X-ray diffraction, optical microscopy and microhardness test. The results show that the direct current field has a significant enhancing effect on both pack boriding and pack boron-aluminizing. The formation characteristics of the direct current electric field enhanced case are not only related to the position of the specimen and the orientation of the diffusion surface, but also to the content of aluminum powder in the penetrant. When the content of aluminum powder in the penetrant increases from 0 to 1.5%, the thickness of the case of the cathode specimen facing the anode, and that of the intermediate specimen facing the anode increase first and then decrease; While the thickness of the case of the intermediate specimen facing the cathode increases slowly first and then increases significantly, and that of the anode specimen facing the cathode increases first, then decreases, and then increases. When the aluminum content is 0, the case of the anode specimen facing the cathode side and that of the intermediate specimen facing the cathode side are both serrated single-phase Fe₂B, and that of the cathode specimen facing the anode side and that of the intermediate specimen facing the anode side are composed of “serrated” FeB+Fe₂B. When the aluminum content is 0.3%, each case is composed of two-phase “serrated” (Fe, Al)B and (Fe, Al)₂B; and when the case is serrated, the peak hardness is 1400-1900 HV0.025. When the aluminum powder content is ≥ 0.5%, the “serrated” feature of the case gradually disappears from the cathode to the anode, the KAlF₄, AlF₃, and Fe₃Si phases gradually appear on the surface of the case of each specimen, and the hardness of the case decreases significantly, the peak hardness is 450-1400 HV0.025.

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Formation mechanism of dispersed carbide in carburized case of 20MnCr5 gear steel

Li Yang, Hou Shengwen, He Erkang, Zhang Xiaotian

Heat Treatment of Metals 2022, 47 (12): 196-200. doi:10.13251/j.issn.0254-6051.2022.12.033

Abstract （31）

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20MnCr5 gear steel was carburized and quenched by the newly developed diffusion carburizing process, and the microstructure of precipitates was studied by means of metallographic microscope. The results show that within a certain depth range of the carburized case, two kinds of carbide with different dispersion distribution are precipitated, which is mainly large particles with a size of 1-5 μm at the grain boundary, while at the intragranular the carbide is mainly small in size, which is less than 1 μm in size and accounts for more than 80%. The precipitation mechanism of two different types of carbides is also revealed.

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Evolution of microstructure and properties of nickel-based N06022 alloy pipe during solution treatment

Qin Xingwen, Ding Wenyan, Tu Zhengping, Shen Chengfei

Heat Treatment of Metals 2022, 47 (12): 201-209. doi:10.13251/j.issn.0254-6051.2022.12.034

Abstract （33）

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Influence of solution treatment on microstructure evolution and properties of nickel-based N06022 alloy pipe was investigated by metallographic microscopy, scanning electron microscopy, energy spectrum analyzer, mechanical and corrosion performance test. The results show that the cold-rolled N06022 alloy pipe contains a large amount of precipitated phases, which have a pinning effect on grain boundaries, and the average grain size of the steel is hardly affected by the holding time, when the solution temperature is below 1100 ℃. When solution temperature increases to 1150 ℃, the grain size and growth rate increase sharply, which is mainly related to the dissolution of the precipitated phase. In the solution temperature range of 1000-1200 ℃, the activation energy of grain growth for 15 min and 30 min is Q_B=646.56 kJ/mol, Q_C=566.45 kJ/mol, respectively, and the relationship between the properties of the alloy and the grain size satisfies the Hall-Petch relationship. With the increase of solution temperature, the intergranular corrosion rate of the alloy decreases first and then stabilizes, the strength and hardness show slow-fast downward trend, and the elongation shows slow-fast upward trend. When the holding time is 15 min and the solution temperature is about 1150 ℃, the intersection of hardness curve and elongation curve appears, and the corrosion rate is stable, which is the optimal solution treatment process.

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Microstructure and tribological properties of amorphous coatings by supersonic atmospheric plasma spraying

He Tao, Wang Xuepeng, Liu Haibo, Liu Qi, Jia Hua, Liu Na, Ding Fei

Heat Treatment of Metals 2022, 47 (12): 210-215. doi:10.13251/j.issn.0254-6051.2022.12.035

Abstract （31）

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Fe-based and Mo-based amorphous coatings are fabricated by the new generation of supersonic atmospheric plasma spraying (SAPS), and microstructure and tribological properties of the coatings were comparatively analyzed. The results show that a small number of pores in the SAPS sprayed Fe-based and Mo-based amorphous coatings lead to high structural density. Compared with Fe-based coating, the Mo-based coating with lower porosity has a lower wear rate (1.1×10^-4μm·N^-1·s^-1) during friction, resulting in a better wear resistance. The wear mechanism of the coatings is mainly abrasive wear and fatigue wear, accompanied by delamination and debris oxidation.

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Microstructure and mechanical properties of MgAlSnZnCu lightweight high entropy alloys

Yang Tuoyu, Wang Kehong, Zhang Deku, Chen Feng, Guo Chun, Zhang Yajing

Heat Treatment of Metals 2022, 47 (12): 216-221. doi:10.13251/j.issn.0254-6051.2022.12.036

Abstract （33）

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In order to improve the stability of the barrier layer, MgAlSnZnCu series of lightweight high entropy alloys were prepared by electromagnetic induction heating method under the protection of a covering agent and argon gas. The as-cast microstructure, annealed microstructure and phases of the alloys were characterized and analyzed by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy spectrum analysis (EDS), and microhardness test was used to study its hardness. The results show that the microstructure of as-cast MgAlSnZnCu alloy is composed of (AlMgZnCu)-FCC phase and Mg₂Sn phase. The morphology of Mg₂Sn phase is 2-6 μm granular and irregular block. When the content of Mg decreases from 22% to 12%, the hardness of the alloy increases from 277 HV0.2 to 326 HV0.2, and the compressive strength increases from 223 MPa to 237 MPa. Needle-like and flake-like impurity phases appear in the microstructure after the decrease of Sn and Zn contents, and the dispersion of hardness values is large, after annealing at 450 ℃ for 6 h, the impurity phases in the microstructure tend to accumulate and grow obviously. The mechanical test results show that the annealing have little effect on compressive strength and hardness. The MgAlSnZnCu series alloys can maintain good thermal stability at 450 ℃.

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Effect of matrix microstructure on mechanism of strength and ductility for 800 MPa grade dual phase steel

Hou Xiaoying, Wang Jun, Ding Mingkai, Liu Wanchun, Sun Weihua, Kang Huawei

Heat Treatment of Metals 2022, 47 (12): 222-227. doi:10.13251/j.issn.0254-6051.2022.12.037

Abstract （25）

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Two heat treatment processes were adopted to obtain 800 MPa grade dual phase steels with different matrix microstructure, and the microstructure morphology characteristics of the matrix and its effect on the mechanism of strength and ductility were studied systematically. The results show that the matrix microstructure has a significant indigenous effect on the plastic deformation mechanism of 800 MPa grade dual phase steels, resulting in differences in the properties. The (F+M) dual phase steel is composed of polygonal ferrite and about 28% second-phase martensite, with yield ratio of 0.540 and elongation of 23.3%. The (BF+γ) dual phase steel is composed of bainite ferrite matrix structure and about 24% second-phase retained austenite, with yield ratio of 0.702 and hole expansion ratio of 56%. During the plastic deformation process of (BF+γ) dual phase steel, the stress concentration at the crack tip can be effectively decomposed by the γ phase with a thickness of about 60-150 nm, and the crack propagation energy is also consumed. At the same time, the cracks generated by microcracks can be bridged by the volume expansion caused by the induced martensite transformation of residual austenite. Under the coordinated deformation mechanism of α phase BF and residual austenite phase, it is beneficial to improve the strength, plasticity and stretch-flange property. In addition, the proportion of large angle grain boundaries in (BF+γ) dual phase steel increases to 63.1%, and higher dislocation density exists in the matrix, the driving energy required for micro-crack propagation is weakened effectively, and the energy required for further propagation is also increased, the stress concentration generated during deformation or hole expansion is alleviated.

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Phase transformation behavior of non-quenched and tempered NM400 multiphase wear-resistant steel

Li Guannan, Lei Minggang, Chen Haotian, Lü Dewen

Heat Treatment of Metals 2022, 47 (12): 228-233. doi:10.13251/j.issn.0254-6051.2022.12.038

Abstract （33）

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In order to further optimize the different microstructure ratios of non-quenched and tempered NM400 multiphase wear-resistant steel, the Gleeble-3800 thermal simulation testing machine was used to explore the microstructure transformation law of the tested steel under continuous cooling conditions, and the metallographic method and the hardness method were combined to draw the dynamic continuous cooling transformation (CCT) curves. The results show that when the cooling rate is lower than 1 ℃/s, the microstructure of the steel is composed of ferrite+granular bainite+pearlite, there are some coarse austenite grains transform into granular bainite and pearlite either. When the cooling rate is 5-40 ℃/s, the pearlite transformation no longer occurs, and the microstructure is composed of ferrite+bainite+martensite. With the increase of cooling rate, the martensite content increases and the hardness increases. In addition, under different step cooling schemes, the lower middle cooling temperature and longer air cooling time are beneficial to the transformation of ferrite and bainite, and the retained austenite content increases with the increase of ferrite content. Due to higher Ms point, the martensite lath of the steel is wide, and self-tempering phenomenon occurs in the steel.

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Tempering brittleness of 2Cr13 stainless steel

Zhu Hongwei, Wang Ping, Hu Wanting, Chen Xin, Huang Yanjun, Yu Ruixing, Zhuang Yuanhong, Chen Yongshuan

Heat Treatment of Metals 2022, 47 (12): 234-236. doi:10.13251/j.issn.0254-6051.2022.12.039

Abstract （215）

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Changes of hardness, impact property and fracture morphology evolution of the 2Cr13 stainless steel after tempering near the tempering brittle temperature interval were studied. The results show that hardness of the 2Cr13 stainless steel after tempering at 490-550 ℃ is 40-50 HRC, the corresponding impact property shows a downward trend; After tempering at 570-620 ℃, the hardness of the 2Cr13 stainless steel is 30-38 HRC, the impact property gradually increases. Through the observation of the impact fracture morphology, after tempering at 490-560 ℃, the fracture mode first changes from cleavage fracture to intergranular fracture. With the increase of the tempering temperature, it eventually changes to a fracture mode along the grain+cleavage. Microscopically, the size of the fracture surface along the grain gradually decreases, and the number of cleavage gradually increases, which is manifested by the improvement of the impact performance on the macroscopic level.

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