ISSN 0412-1961
CN 21-1139/TG
Started in 1956

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 , Volume 56 Issue 5 Previous Issue    Next Issue
 Select Effect of Cold Deformation and Solid Solution Temperature on σ-phase Precipitation Behavior in HR3C Heat Resistant Steel CAO Tieshan, ZHAO Jinyi, CHENG Congqian, MENG Xianming, ZHAO Jie Acta Metall Sin, 2020, 56 (5): 673-682.  DOI: 10.11900/0412.1961.2019.00267 Abstract   HTML   PDF (3791KB) ( 314 ) HR3C steel, widely applied in ultra-supercritical power plant, suffers an intergranular embrittlement problem during long-term high-temperature ageing or service, which will be enhanced by the precipitation of σ phase. Research has showed that the precipitation behaviors of σ phase are different significantly as the difference of manufacturers, which relates to the preparation process of cold-deformation & solid-solution treatment. In this work, the effects of cold deformation and solution treatment on the precipitation kinetics of σ phase and related mechanical properties for HR3C steel during the ageing process were studied. The results show that cold-deformation and solid solution temperature both have a significant influence on the precipitation of σ phase in the steel. The increase of cold-deformation will promote the precipitation of σ phase, and rising solution temperature helps to inhibit the growth of σ phase but increase the grain size. The precipitation kinetics study of σ phase in HR3C steel with different pre-treatment shows that σ phase growths slowly at first, and then gets into a rapid precipitation period, and finally reaches a steady-state with a value of about 5.7% (volume fraction). The impact toughness analysis shows that the increase of cold-deformation would lower down the impact toughness of HR3C steel during the ageing procedure, while the rise of the solid-solution temperature increases the impact toughness before ageing and reduces it during ageing.
 Select Tensile Properties of Selective Laser Melted 316L Stainless Steel YU Chenfan, ZHAO Congcong, ZHANG Zhefeng, LIU Wei Acta Metall Sin, 2020, 56 (5): 683-692.  DOI: 10.11900/0412.1961.2019.00278 Abstract   HTML   PDF (3487KB) ( 493 ) Selective laser melting (SLM), as the most common additive manufacturing (AM) method, is capable of manufacturing metallic components with complex shape layer by layer. Compared with conventional manufacturing technologies such as casting or forging, the SLM technology has the advantages of high degree accuracy, high material utilization rate and environmentally friendly, and has attracted great attention in the fields of aerospace, nuclear power and medicine. The 316L austenitic stainless steel is widely used in the industrial field because of the excellent corrosion resistance and plasticity. It is also one of the commonly used material systems for SLM. In this work, the tensile properties and fracture mechanism of 316L stainless steel fabricated via SLM technology were investigated. The microstructure of the SLMed 316L specimens after tensile fracture was characterized and analyzed. The results show that the SLMed 316L stainless steel has a relatively desirable combination of strength and ductility, and its tensile performance is obviously better than that of 316L stainless steel prepared by traditional methods. The nanometer-scale cell structure inside the grain contributes to the improvement of strength. Deformation twins were observed in the SLMed 316L stainless steel after tensile test. The appearance of twins is oriented-dependent, and it is easy to occur in the grain with the direction near <110>-<111>.
 Select Relationship of Inclusions and Rolling Contact Fatigue Life for Ultra-Clean Bearing Steel SUN Feilong, GENG Ke, YU Feng, LUO Haiwen Acta Metall Sin, 2020, 56 (5): 693-703.  DOI: 10.11900/0412.1961.2019.00337 Abstract   HTML   PDF (2358KB) ( 340 ) The cleanliness of bearing steels produced in China has been greatly improved due to the significant progress in the steelmaking technologies in the past decade, leading to their total oxygen (T.O.) contents lowered to no more than 6×10-6. Under such a high cleanliness, it is then expected that the influence of non-metallic inclusions on fatigue property should be different from the previous knowledge, because both the size and quantity of inclusions are reduced greatly. Therefore, both inclusions and fatigue properties for three ultra-clean GCr15 (100Cr6) bearing steels containing T.O. around 6×10-6, which were manufactured via different industrial production processes, were studied for this purpose. First, inclusions were characterized by ASPEX SEM and then statically analyzed by the statistics of extreme values (SEV) and the generalized Pareto distribution (GPD). Next, their rolling contact fatigue lives (RCF) L10 and L50 were measured by flat washer tests. Only the largest inclusion in each sample is required for predicting the characteristic sizes of maximum inclusion (CSMI) for the three steels using the SEV method. The calculated CSMIs, however, are not consistent with the variation of either L10 or L50, indicating they are not relevant. In contrast, the types of inclusions above threshold (u) size can be classified and their number density of inclusions quantified when the GPD method is employed. In particularly, the CSMIs of different types of inclusions can be determined. In this case, it has been found that the CSMI of TiN inclusion, which is the most dangerous for initiating cracking, is in a good agreement with the low probability rolling fatigue life (L10), suggesting that they are very correlated. This, however, cannot explain the variation of high-probability fatigue life (L50). Instead, the density of total inclusions also played an important role on the L50 of ultra-clean bearing steels in addition to the CSMI of TiN inclusions. This is reasonable because cracking shall be initiated at not only the most dangerous TiN inclusion during the early failure but also some other highly dense inclusions particularly during the late failure. Therefore, it is then concluded that the L10 is much more related to the CSMI of most dangerous TiN inclusion; whilst the L50 is strongly affected by the number density of total inclusions.
 Select Solidification Structure Refinement in TWIP Steel by Ce Inoculation LI Gen, LAN Peng, ZHANG Jiaquan Acta Metall Sin, 2020, 56 (5): 704-714.  DOI: 10.11900/0412.1961.2019.00288 Abstract   HTML   PDF (2643KB) ( 288 ) Twinning-induced plasticity (TWIP) steel represents a novel grade of advanced high strength and ductility with significant potential for automotive industry. However, high alloying in TWIP steel leads to the inhomogeneous solute distribution and anisotropic local deformation. It is well known that the refinement of solidification structure is an effective solution to the above defects. Much attention has been paid to heterogeneous nucleation by Ce particles, acting as nucleating sites in liquid steel. The present work focuses on how Ce content and casting parameters affect the refinement of solidification structure in Fe-22Mn-0.65C TWIP steel, aiming to provide an effective technology in high alloy steel production. The reaction products of Ce inoculation were predicted by thermodynamics software FactSage 7.0 and their effectiveness of heterogeneous nucleation was estimated by lattice misfit model. The solidification structure refinement by Ce inoculation under different conditions was experimentally studied by OM, SEM, EBSD and EPMA. The results show that, with increasing Ce content the reaction products transferred from Ce2O3 to Ce2O3+a small amount of Ce2O2S, and both kinds of particles can act as heterogeneous nucleation cores theoretically. For as-cast solidification structure, the ratio of equiaxed grain area increased from 25% to 72%, average equiaxed grain size decreased from 480 μm to 130 μm and the segregation ratio of Mn decreased from 1.61 to 1.41. Meanwhile, the tendency of particle agglomeration was weakened by lowering inoculation temperature, resulting in the improvement structure refinement. In this work, the recommended inoculation parameters are concluded as (0.02%~0.04%)Ce with superheat of 20 ℃.
 Select Microstructure and Corrosion Behavior of Fe-15Mn-5Si-14Cr-0.2C Amorphous Steel ZHAO Yanchun, MAO Xuejing, LI Wensheng, SUN Hao, LI Chunling, ZHAO Pengbiao, KOU Shengzhong, Liaw Peter K. Acta Metall Sin, 2020, 56 (5): 715-722.  DOI: 10.11900/0412.1961.2019.00275 Abstract   HTML   PDF (1970KB) ( 250 ) Amorphous steels exhibit ultra-high strength but room-temperature brittleness and strain-softening behavior as loading, which restricted the application of amorphous steels as high-performance structural material. Developing in situ crystals is an effective way to toughen the amorphous alloys. However, the crystals may sacrifice the corrosion resistance of amorphous steels. In this work, austenite and ferrite duel phases were introduced to the amorphous phase, via transformation induced plasticity (TRIP) of the austenite as loading, to enhance the ductility and improve the work-hardening behavior; and via the synergy of ferrite and amorphous phase to ensure the corrosion resistance. A novel amorphous steel Fe-15Mn-5Si-14Cr-0.2C was fabricated by magnetic suspension melting in a water-cooled copper crucible, and negative pressure suction casting into a copper mold. The microstructure and mechanical properties of the amorphous steel were characterized by XRD, EBSD and the electronic universal testing machine. The corrosion behavior in artificial seawater was studied on an electrochemical work station with a three-electrode system, and the corrosion morphology and corrosion products were characterized by SEM with EDS analysis. The results showed that the as-cast amorphous steel consisted of the amorphous matrix, CFe15.1 super-cooled austenite and Fe-Cr ferrite phases. From surface to inner, amorphous phases mainly exist in the margin, while crystalline phases are abundantly distributed in the center. The amorphous steel exhibited excellent comprehensive mechanical properties at room temperature, and its yield strength, fracture strength and plastic strain were up to 978 MPa, 2645 MPa and 35.8%, respectively. In artificial seawater, compared with 304 stainless steel, the amorphous steel showed high self-corrosion potential, low self-corrosion current density and high polarization resistance, large resistance arc radius, only one high frequency resistance arc and low corrosion kinetic rate. Moreover, the stable and dense passivation film was observed on the corrosion surface. Their excellent corrosion resistance and mechanical properties endow the amorphous steel with the potential to become a novel corrosion-resistant structural material for marine engineering.
 Select Deformation Mechanism and Dynamic Recrystallization of Mg-5.6Gd-0.8Zn Alloy During Multi-Directional Forging ZHANG Yang, SHAO Jianbo, CHEN Tao, LIU Chuming, CHEN Zhiyong Acta Metall Sin, 2020, 56 (5): 723-735.  DOI: 10.11900/0412.1961.2019.00292 Abstract   HTML   PDF (5277KB) ( 340 ) Multi-directional forging (MDF) is an effective way to fabricate wrought magnesium alloy with ultrafine grains and random texture. Therefore, microstructure evolution and dynamic recrystallization (DRX) of magnesium alloys during MDF process have been widely investigated. Mg-Zn-RE alloys containing long-period stacking ordered (LPSO) phase have received considerable attention owing to their excellent mechanical properties. In addition, LPSO phase has great effects on the deformation mechanism and DRX behavior. Still, limited comprehensive studies can be found in the literature dealing with the microstructure evolution, deformation mechanism and DRX of magnesium alloys containing LPSO phase in MDF deformation. In this work, MDF was applied to a Mg-5.6Gd-0.8Zn (mass fraction, %) alloy containing LPSO phase. Microstructure characteristics, deformation mechanism and DRX behavior of the material in different passes were examined. Results show that there are several stages of the microstructure evolution. Twinning was activated only in a small part of grains in the early stage of deformation. As the forging direction changes, the number of twinned grains and the volume fraction of DRX grains increased. A mixed structure with coarse deformed grain and DRX grains was sustained till last forging pass, and the average size of DRX grains is about 4 μm with a random orientation. {$101ˉ2$} tensile twinning is the main deformation mechanism and the selection of twin variants was dominated by the Schmid law. Change in forging direction is beneficial to twinning stimulation in grains of different orientations. Kink and slipping deformation could effectively accommodate the plastic strain where the operation of twinning was hindered. Kink deformation resulted in lattice rotation predominately about the <$101ˉ0$> axis. DRX grains nucleated at different places during the forging process. Not only the grain boundaries and the twinned region, but also kink boundaries can induce the nucleation of DRX grains. Eventually, the twinned regions were transformed to a strip-like recrystallization structure. Under the combined influence of twinning and kinking, as well as DRX induced by twins, kink bands and grain boundaries, the initial coarse grains were significantly refined.
 Select Effects of Artificial Ageing on Mechanical Properties and Precipitation of 2A12 Al Sheet LIANG Mengchao, CHEN Liang, ZHAO Guoqun Acta Metall Sin, 2020, 56 (5): 736-744.  DOI: 10.11900/0412.1961.2019.00293 Abstract   HTML   PDF (2909KB) ( 305 ) 2A12 Al alloy has been widely applied in the fields of aviation, aerospace, and vehicles due to its light weight, high specific strength and good corrosion resistance. The solution and ageing treatments are usually performed after the processing on 2A12 Al alloy, and the ageing parameters greatly affect the final mechanical properties. In the present study, the artificial ageing was performed on the cold rolled 2A12 Al sheet under various holding temperatures and holding time. The mechanical properties were evaluated by micro-hardness and tensile tests. Moreover, the evolution of microstructure and precipitations during ageing with different holding time were characterized. The results showed that the 2A12 Al sheet had the sole peak ageing, and the higher the temperature, the shorter time was required for the peak ageing. Both the holding temperature and holding time significantly affected the mechanical properties. The optimal ageing parameters were determined as holding at 185 ℃ for 16 h, and the corresponding yield strength, ultimate tensile strength and elongation along rolling direction were 381 MPa, 476 MPa and 13.6%, respectively. S (Al2CuMg) phase gradually precipitated during ageing process, and the size and distribution of S phase greatly affected the fracture mechanism. At the initial stage of ageing, S phase precipitated near grain boundaries, and the ductility fracture could be observed. With the extension of holding time, the coarsening of S phase took place, and the fracture was gradually transformed to intergranular and transcrystalline modes. Cu-Mg cluster was the main strengthening mechanism at the initial stage of ageing. Both Cu-Mg cluster and GPB zone contributed to the strengthening under the peak ageing, and the precipitations were transformed to stable S phase under the over ageing. Considering the combined effects of homogeneous and inhomogeneous nucleation, the precipitation during ageing of cold rolled 2A12 Al sheet followed the sequence of supersaturated solid solution (SSS)→Cu-Mg cluster+Sinhomo→Cu-Mg cluster+GPB zone+Sinhomo→Cu-Mg cluster +GPB zone+ Shomo+Sinhomo→S.
 Select Dimensional Effect on Thermo-Mechanical Evolution of Laser Depositing Thin-Walled Structure WANG Xia, WANG Wei, YANG Guang, WANG Chao, REN Yuhang Acta Metall Sin, 2020, 56 (5): 745-752.  DOI: 10.11900/0412.1961.2019.00317 Abstract   HTML   PDF (2076KB) ( 175 ) To accurately predict and effectively control temperatures, stresses and distortions are key problems existing in laser deposition manufacturing technology. The mechanism of thermo-mechanical evolution during the metal depositing process is not yet clear. In order to study the dimensional effect on thermo-mechanical evolution when TC4 single-pass and thin-walled structures are manufactured by laser deposition, finite element simulations and experiments are combined to explore the influence of the structures' length on temperature, stress and distortion of the substrates. The model reliability is validated by thermocouple temperatures and the residual deformations of substrates. The results show that the temperatures of molten pools increase periodically according to the depositing layers. As soon as the laser is terminated, the maximum temperatures of builds decline at high speed, but the minimum temperatures continue to rise in the form of parabola. When the lengths of thin-walled structures increase, the thermal extremes of molten pools are not affected, but the curvatures of cooling curves diminish, the steady cooling rates accelerate, meanwhile the temperature gradients increase. The initial stresses when depositing the first layer are maximum during manufacturing, the stresses decline progressively with the increasing layer numbers, but recover during cooling. While the lengths expand, stresses of the first layer increase. At the same time both low stress regions during depositing and high stress areas during cooling are enlarged which are around the depositing structures, but the lengths of thin-walled structures appear to have a minimal impact on the stress magnitudes. During deposition, the out-of-plane distortions of the substrates oscillate up and down, after cooling the directions of deformations are fixed towards the light source. The out-of-plane distortions are more obvious as the lengths increase. During cooling the substrates' deformations reach equilibrium earlier than temperatures and stresses.