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 Select Effect of Nitrogen Content on Precipitation Behavior and Mechanical Properties of Mn18Cr18NAustenitic Stainless Steel Fengming QIN, Yajie LI, Xiaodong ZHAO, Wenwu HE, Huiqin CHEN Acta Metall Sin    2018, 54 (1): 55-64.   doi:10.11900/0412.1961.2017.00291 Accepted: 31 October 2017 Abstract （108）   HTML （0）    PDF （1226KB）（607）       Mn18Cr18N austenitic stainless steel with excellent mechanical properties and corrosion resistance is widely used in nuclear industries, power plants and medicine field. However, precipitation of the second phases during hot deformation deteriorates the mechanical properties and hot formability. In order to clarify the precipitation behavior of this steel, the precipitation behavior and its influence on mechanical properties of Mn18Cr18N austenitic stainless steel with different nitrogen contents were investigated by JmatPro software, OM, SEM and TEM analytical methods. The results indicate that precipitation phases consist of Cr2N and a few M23C6, in which Cr2N preferentially precipitates along grain bound aries and then grows up to the interior of austenite grain by discontinuous cellular. With increasing of ageing temperature, the precipitation of Cr2N became more sensitive. When the nitrogen content increases to 0.7%, the most sensitive precipitation temperature of Cr2N is 750 ℃ with an incubation period of 10 min. However, M23C6 mainly precipitates by granular at austenitic grain boundaries and maintains cube-on-cube orientation relationship with adjacent austenite grain. The results of mechanical property test indicate that the precipitation of Cr2N has a negligible effect on strength and obvious deterioration on plasticity of Mn18Cr18N austenitic stainless steel. The precipitation of Cr2N after ageing treatment leads to remarkable decrease in elongation and reduction of area, and the elongation reduced from 52.9% to 27.7%. Meanwhile, fracture mode also transformed from ductile fracture to intergranular fracture and transgranular fracture with the increasing of Cr2N. TEM analysis shows that solution treatment sample reveals good plastic deformation ability and coordinates deformation by slip and twinning, simultaneously. Nevertheless, dislocations slipped, propagated and eventually piled up between lamellas of Cr2N and around granular M23C6 after ageing treatment, which induce the degeneration of the plastic deformation capacity of Mn18Cr18N austenitic stainless steel.
 Select Research on Hot Working Behavior of Low-NickelDuplex Stainless Steel 2101 Yusen SU, Yinhui YANG, Jianchun CAO, Yuliang BAI Acta Metall Sin    2018, 54 (4): 485-493.   doi:10.11900/0412.1961.2017.00151 Accepted: 31 August 2017 Abstract （88）   HTML （2）    PDF （7433KB）（565）       The thermal deformation difference of two phases for duplex stainless steel (DSS) makes hot working difficult, 2101 DSS substitute Mn, N for Ni to stabilize austenite phase, which will significantly affect hot deformation behavior. Hot compression tests in the temperature ranging from 1123 to 1423 K and strain rate ranging from 0.001 to 10 s-1 were carried out on a Gleeble-3800 thermal simulator for 2101 DSS. At the same strain rate, the flow curve characteristics of 2101 DSS changed from dynamic recrystallization (DRX) to dynamic recovery with increasing deformation temperature. Increasing deformation stain rate from 0.001 s-1 to 0.01 and 0.1 s-1 increased DRX temperature range, but higher strain rate of 1 and 10 s-1 is not beneficial to DRX occurrence. In the deformation temperature region of 1253~1323 K and low strain rate of 0.001~0.1 s-1, the smaller strain value corresponding to the peak stress, the austenite DRX is more likely to occur, which is beneficial to the equiaxed recrystallized grains formation. At low strain rate, the recrystallization grain grows up with the increase of deformation temperature, the worse effect of austenite DRX is related to weakened austenite stabilized ability of Mn substitution for Ni at high Zener-Hollomon parameter values. Based on the thermal deformation equation, the apparent activation energy Q was calculated as 464.49 kJ/mol, which is slightly higher than that of 2205 DSS, and the constitutive equation of the peak flow stress was established. By combining with flow curve and microstructure analysis, the processing map exhibits the optimum processing conditions are in deformation temperature ranging from 1220 to 1350 K and strain rate ranging from 0.001 to 0.1 s-1 with high power dissipation coefficient of 0.40~0.47, under which the austenite DRX obviously occurred.
 Select Morphological Characteristics and Size Distributions of Three-Dimensional Grains and Grain Boundaries in 316L Stainless Steel Tingguang LIU, Shuang XIA, Qin BAI, Bangxin ZHOU Acta Metall Sin    2018, 54 (6): 868-876.   doi:10.11900/0412.1961.2017.00318 Accepted: 23 January 2018 Abstract （123）   HTML （10）    PDF （2969KB）（808）       Three-dimensional characterization of grains and grain boundaries is significant to study the microstructure of polycrystalline materials, and is the key to advance the subject of three-dimensional materials science (3DMS). In this work, the technique of serial sectioning by mechanical polishing coupled with 3D electron backscatter diffraction (3D-EBSD) mapping was used to measure the microstructure of a 316L stainless steel in 3D. Volume of the collected 3D-EBSD microstructure is 600 μm×600 μm×257.5 μm, which is quite large to study the 3D microstructure of structural materials with conventional grain size (20~60 μm). Dream3D and in-house developed Matlab programs were used to process the 3D-EBSD data, and subsequently ParaView was used to visualize the grains and grain boundaries in 3D. Combined usage of these tools and in-house programs make the possibility that not only 3D grains but also 3D grain boundaries can be studied in both morphology and quantification. In total, 1840 grains and 9177 grain boundaries are included in the measured 3D-EBSD microstructure. The 3D morphological characteristics and size distributions of grains and grain boundaries in the 316L stainless steel were investigated, including 3D grain size, grain surface area, boundary quantity per grain, grain boundary size and the average boundary size per grain, as well as relationships between these morphological parameters were discussed. Results showed that distributions of all of these morphological parameters of 3D grains and grain boundaries in the polycrystalline 316L steel can be well represented by log-normal distribution, and all relationships of these parameters versus grain size can be well represented by power function. Additionally, the 3D morphologies of most grains in the 316L stainless steel deviate from the ideal equiaxed grains, having complex shapes due to existing of twins, such as semi-sphere shaped, plate shaped and some very complex grains. In many ways, the larger grains have more complex morphology with greater number of faces, larger surface area and larger deviation from equiaxed grains.
 Select Direct Synthesis of NiCo2O4 Nanoneedles and MoS2 Nanoflakes Grown on 316L Stainless Steel Meshes by Two Step Hydrothermal Method for HER Dan LI, Yang LI, Rongsheng CHEN, Hongwei NI Acta Metall Sin    2018, 54 (8): 1179-1186.   doi:10.11900/0412.1961.2018.00001 Accepted: 09 May 2018 Abstract （110）   HTML （3）    PDF （5085KB）（547）       The synthesis of nanostructures catalytic electrode for hydrogen evolution reaction (HER) plays an important role in national economy such as chlor-alkali industry, chemical power supply and fuel cell. Electro-splitting of water powered by electric energy has attracted extensive attention because this process can convert electric energy into chemical energy for easier storage and delivery. In this work, a facile and direct synthesis of NiCo2O4 nanoneedles and MoS2 nanoflakes grown on 316L stainless steel meshes substrate by two step hydrothermal method was reported. Initially MoS2 nanoflakes grown on the stainless steel (SS) meshes, and then NiCo2O4 nanoneedles were grown on MoS2/SS meshes at optimum conditions using hydrothermal method. The prepared nanostructures were characterized by SEM, TEM and XRD. Then a three-electrode system was used to test the property of HER. The results show that the as-prepared electrode exhibits good catalytic behavior towards HER. The onset overpotential and Tafel slope are 65 mV and 108 mV/dec respectively. When the current density reaches 100 mA/cm2, the overpotential is 219.6 mV. Furthermore, the composite structure exhibits good cycle stability in the same experimental conditions.
 Select Distribution Characteristics of Twin-Boundaries in Three-Dimensional Grain Boundary Network of 316L Stainless Steel Tingguang LIU, Shuang XIA, Qin BAI, Bangxin ZHOU, Yonghao LU Acta Metall Sin    2018, 54 (10): 1377-1386.   doi:10.11900/0412.1961.2018.00062 Accepted: 27 June 2018 Abstract （120）   HTML （6）    PDF （5151KB）（392）       Grain boundaries are sources of failure and weakness due to their relatively excess free volume compared to the lattice of polycrystalline materials exposed to aggressive environment. The control of grain boundary degradation has become one of the key issues of materials science and engineering. It has been found that the coincidence site lattice (CSL) boundaries, especially Σ3 (the twin boundaries), have stronger resistance to intergranular degradation than random boundaries. Materials with a high proportion of CSL boundaries that could disrupt the connectivity of random boundaries have better performance to resist intergranular failures. However, the distribution characteristics of twin boundaries in grain boundary network are still unclear. In this work, three-dimensional electron backscatter diffraction (3D-EBSD) was used to map the 3D grain boundary network of a 316L stainless steel. The topological characteristics of triple junction and quadruple junction in the presence of twin boundaries were investigated. The distribution of twin boundaries around grains and grain boundaries was analyzed. The results show that the twin boundary number fraction in the 3D grain boundary network is lower than the measured twin boundary area fraction, indicating that the average area per twin boundary is larger than random boundary. Most of triple junctions in the 316L stainless steel have one twin boundary. The proportion of triple junctions with two twin boundaries is about 9.4%. A quadruple junction has three twin boundaries at most. Most of quadruple junctions have one or two twin boundaries. About 7.9% of quadruple junctions have three twin boundaries. The 3D-EBSD data of 316L includes 1840 grains, 7353 random boundaries and 1824 twin boundaries. On average, a 3D grain in the 3D microstructure has 11 faces (39.85 neighboring faces that includes all boundaries of the grain and all boundaries that connected with the grain by lines or points), in which the number of twin boundaries is 2.03 (8.02) on average. A 3D grain boundary has 9.35 neighboring boundaries, in which the number of twin boundaries is 1.99 on average.
 Select Research and Development of Maraging Stainless Steel Used for New Generation Landing Gear Ke YANG, Mengchao U, Jialong AN, Wei NG Acta Metall Sin    2018, 54 (11): 1567-1585.   doi:10.11900/0412.1961.2018.00356 Accepted: 30 August 2018 Abstract （120）   HTML （22）    PDF （11433KB）（730）       Properties of landing gear are closely related to the service safety of aircraft. Thus, it is essential to improve the comprehensive properties of the material used for landing gear. This article briefly introduces the application status and existing problems of currently used landing gear materials, and then proposes future developing directions of landing gear materials. Finally, a new maraging stainless steel with high strength, high toughness and good corrosion resistance, which can be a promising steel for the new generation landing gear material, is introduced.
 Select Precipitate Evolution in a Modified 25Cr-20Ni Austenitic Heat Resistant Stainless Steel During CreepRupture Test at 750 ℃ Guodong HU, Pei WANG, Dianzhong LI, Yiyi LI Acta Metall Sin    2018, 54 (11): 1705-1714.   doi:10.11900/0412.1961.2018.00361 Accepted: 13 September 2018 Abstract （151）   HTML （2）    PDF （9763KB）（533）       25Cr-20Ni austenitic heat resistant stainless steels are widely used as structural materials in nuclear industries and power plants for their excellent corrosion resistance and creep properties at elevated temperature. It is generally accepted that the precipitation during creep is a key factor influencing the creep properties. However, the evolution of precipitates is complicated due to the interaction of the alloy elements. To investigate the precipitation behaviors, a modified 25Cr-20Ni austenitic heat resistant stainless steel has been crept at 750 ℃ under different stresses varying from 100 MPa to 180 MPa. The microstructure observation indicates that M23C6 and (Nb, V)(C, N) precipitates are formed during 32.6 h creeping deformation under 180 MPa. M23C6 precipitates are mainly generated at grain boundaries and (Nb, V)(C, N) particles are dispersively distributed in austenitic matrix. The grain boundary M23C6 carbides are significantly coarsened and Ostwald ripening process happens during 98.1 h creeping deformation under the stress of 150 MPa and 353.0 h creeping deformation under stress of 120 MPa, while (Nb, V)(C, N) carbonitrides show high dimensional stability. With the creep rupture time further prolonging to 353.0 h and 752.3 h under the creep stress of 120 and 100 MPa, respectively, σ-phases are generated first at grain boundaries and then at inner grains. Meanwhile, large amounts of σ-phases are formed around (Nb, V)(C, N) particles, indicating the σ-phase precipitation is accelerated by (Nb, V)(C, N) carbonitrides. Composition analysis and thermodynamic calculation are subsequently performed to elucidate the precipitation mechanism of σ-phase. Carbon and nitrogen depleted zone is detected at the interface between (Nb, V)(C, N) precipitates and austenitic matrix. A correlation between σ-phase and C/N contents has been calculated by Thermo-Calc, which shows that the mass fraction of σ-phase increases with the decreasing C/N contents. According to the thermodynamic calculations and experimental results, it is reasonably inferred that the formation of σ-phase is induced by the carbon and nitrogen depletion in austenitic matrix. Additionally, the fracture surfaces of creep specimens show intergranular fracture under all creep stresses. When the creep time is comparatively short, cracks are inclined to propagate along grain boundaries owing to the low cohesion between grain boundary M23C6 precipitates and austenitic matrix, resulting in intergranular creep fracture. With the precipitation of σ-phase at grain boundaries after long time creep, the cracks are primarily generated from σ-phase, further deteriorating the creep elongation.
 Select Effect of Hot Band Annealing Processes on Texture and Formability of 19Cr2Mo1W Ferritic Stainless Steel Houlong LIU,Mingyu MA,Lingling LIU,Liangliang WEI,Liqing CHEN Acta Metall Sin    2019, 55 (5): 566-574.   doi:10.11900/0412.1961.2018.00540 Accepted: 04 March 2019 Abstract （71）   HTML （3）    PDF （13885KB）（385）       Low-cost ferritic stainless steels with excellent oxidation resistance and anti-corrosion ability are widely used in the fields of household appliances, hardware decoration, architectural structures, fuel cells and automobile exhaust systems. In order to achieve good formability of the ferritic stainless steel, the annealing process of hot-rolled sheet is crucial. As a newly developed 444-type heat-resistant ferritic stainless steel containing W and Ce, however, the influence of hot band annealing process of 19Cr2Mo1W ferritic stainless steel on its formability is not clear and need to have a deep understanding. In this work, the effect of annealing temperature of hot band on the microstructure, texture and formability of this steel was studied by means of XRD, EBSD, roughness measurement and formability test. The results indicated that although annealing processes were carried out at different temperatures after hot rolling, the characteristic of texture in the hot-rolled and annealed sheet was inherited to the cold-rolled sheet to some extent. The increased intensities of {223}<1$1ˉ$0> and {111}<0$1ˉ$1> texture components in the hot-rolled and annealed sheet were beneficial to improvement of the γ-fiber texture in the cold-rolled and annealed sheet. The extent of deviation from γ-fiber texture in the cold-rolled and annealed sheet was increased with increasing the intensities of {001}<1$1ˉ$0>~{115}<1$1ˉ$0> texture components in the cold-rolled sheet. An increased annealing temperature of the hot-rolled sheet could effectively weaken the intensities of {001}<1$1ˉ$0>~{115}<1$1ˉ$0> texture components in the cold-rolled sheet. In addition, the banded microstructures in the hot-rolled and annealed sheet were significantly reduced by increasing annealing temperature of the hot-rolled sheet, which improved the microstructure uniformity and formability of the cold-rolled and annealed sheet.
 Select Intrinsic Increment of Plasticity Induced by TRIP and Its Dependence on the Annealing Temperature in a Lean Duplex Stainless Steel CHEN Lei , HAO Shuo , MEI Ruixue , JIA Wei , LI Wenquan , GUO Baofeng Acta Metall Sin    2019, 55 (11): 1359-1366.   doi:10.11900/0412.1961.2019.00108 Accepted: 02 September 2019 Abstract （58）   HTML （0）    PDF （6212KB）（290）       Recently, advanced lean duplex stainless steels (LDXs) with exceptionally good tensile properties by transformation-induced plasticity (TRIP) have been developed to respond to the skyrocketing raw material cost. In these new alloys, TRIP in the metastable austenite phase is expected to dominate overall deformation of the steels. Solution annealing, as a critical step of production processing, affects the austenite characteristics in LDXs, such as volume fraction and mechanical stability of austenite, which in turn influences its TRIP behavior. In order to further develop advanced LDXs, an assessment in the plastic increment of TRIP and its dependence on solution treatment are necessary. In this work, the tensile deformation test of a LDX which was annealed in the range of 1000~1200 ℃ was carried out on a Gleeble-3800 machine. The microstructural mechanism of work hardening characteristics was characterized by TEM, and the saturation of strain-induced martensite (SIM) under different conditions was calculated by XRD. Some quantitative indicators which can characterize the plastic increment of TRIP were proposed, including apparent plastic increment ($Δe$), average plastic increment ($Δeˉ$) induced by unit volume SIM and intrinsic plastic increment ($Δe*$) related only to mechanical stability of austenite. Meanwhile, their dependences on annealing temperature were discussed. The results show that SIM can develop in two ways of γ→ε→α′ and γ→α′ whereby the work hardening of the LDX exhibit a "three-stage" characteristic. There is a critical deformation temperature (Md) where the TRIP is absent at every annealing temperatures. The higher the annealing temperature is, the smaller the Md and the $Δe$are. As annealing temperature increases, $Δeˉ$ increases, while $Δe*$ decreases, indicating a fact that the more stable the austenite is, the smaller the intrinsic plastic increment of TRIP is. In addition, both $Δeˉ$ and $Δe*$ show a linear relationship with the austenite stability coefficient (k).