Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Low-temperature surface carburization has proven to be one of the most effective techniques for improving the mechanical properties of 316-type austenitic stainless steel (Fe-Cr-Ni alloy), including surface hardness, fatigue resistance and wear resistance. It is well known that carbon diffusion in austenitic stainless steel is a very complicated process and still not fully understood. So it is of great importance to figure out the carbon diffusion mechanism in steel and establish a model that can predict the carbon concentration along the depth direction in any given carburization conditions. Studies in recent years reveal that trapping effect should be considered in carbon diffusion in austenitic steels at low temperature. In this work, low-temperature surface carburization treatment was carried out with 316L austenitic stainless steel, and the carbon concentration along the depth direction was measured. A kinetic model based on the "trapping-detrapping" mass transport mechanism for simulating the carbon fraction- depth profile was developed. This model considered that the diffusion of carbon under the influence of trap sites formed by local chromium atoms. Then the calculated carbon concentration was compared to the experimental results in order to check the validity of the model. The results show as follow: (1) in low-temperature-carburized 316L austenitic stainless steel, the carbon fraction-depth profile exhibits plateau-type shape which is not consistent with the standard analytic solution of the diffusion equation (Fick's law of diffusion); (2) carbon fractiondepth profile based on "trapping-detrapping" model is in good agreement with experimental carbon fraction-depth profile, which indicates the trapping effect plays an important role in carbon diffusion; (3) carbon diffusivity decreases by the trapping effect of Cr atoms, and the detrapping energy of carbon deduced from fitting experimental data is 165 kJ/mol; (4) the proposed model can only be used to describe the carbon diffusion in austenitic stainless steel during low-temperature surface carburization without chromium carbide precipitation. In addition, the influence of stresses induced by incorporating the carbon into austenite lattice on the carbon transport mechanism is not included in the trapping-detrapping model.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Based on the consideration of economy and being easy to construct on site, Cr5Mo/A302 dissimilar welded joints are widely used in elevated temperature applications such as coal- fired power station, nuclear plant and petrochemical industry. Because of the difference in carbide forming elements on both sides of welded joint fusion line, carbon diffusion will happen in service progress and induce premature invalidation. The calculation methods of carbon diffusion and its harm to creep life of dissimilar welded joints have been investigated in the past decades. Theoretically, at a certain temperature, creep damage mechanism changes according to stress levels. However, the previous works paid little attention on the effect of carbon diffusion to creep life at different stress levels and few studies have been done focusing on relationships among carbon diffusion degree, stress level and creep life. In this work, carbon diffusion behavior of Cr5Mo/A302 dissimilar welded joint was first studied by employing aging treatment test, micro-hardness measurement and Fick's second law. Subsequently, creep tests were performed to investigate the effect of 200 mm wide decarburized zone on the joint creep life at different stress levels. The results showed that carbon diffusion in Cr5Mo/A302 dissimilar welded joints became serious with the increase of aging treatment time. The widths of carbon rich zone and decarburized zone were both consistent with the parabola distribution law and could be simulated by Fick's second law. The decarburised zone, which formed in the process of aging, reduced the creep life of the joint greatly at high stress levels. However, with the decline of testing load, its effect became much smaller. When the stress level dropped to about 36% of the yield stress, its effect was negligible. Meanwhile, the relationship of stress and carbon diffusion impact factor S was established to determine the critical value of the stress below which the decarburised zone would not affect the high-temperature creep life of the dissimilar joint.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Because of excellent corrosion resistance, good toughness and machinability, austenitic stainless steels are widely used in many industries. In order to improve the corrosion resistance, the carbon content of austenitic stainless steel is ultra-low, resulting in low surface hardness, poor wear and fatigue resistance properties which limit its application. Low temperature colossal carburization (LTCC) is a kind of novel surface strengthening technology for significantly increasing the surface hardness of austenitic stainless steels, while keeping their original excellent corrosion resistance because of no formation of carbides. The wear, fatigue and corrosion resistance of austenitic stainless steel of low temperature carburized layer have been investigated in recent years. However, the researches on key technical parameters, especially the carburizing atmosphere and the alloying element, have been rarely reported due to intellectual property protection limits. In this work, OM, EPMA, XRD and IXRD are used to investigate the effects of CO concentration on the microstructure, carbon concentration distribution, phase constitution and residual stress of the carburized layer on 316L austenitic stainless steel surface. Based on thermodynamic theory, the model of carbon transfer and diffusion was also built by software DICTRA to calculate the distribution of carbon concentration and activity of low temperature carburized layer. The results reveal that S phase is detected on 316L austenitic stainless steel surface treated by LTCC, and the compressive residual stress is formed at the same time. The increment of CO concentration can significantly increase the carbon concentration of carburized layer, which improve the hardness and compressive residual stress. The simulated carbon concentration and activity distributions are in accordance with the experimental results when the carbon concentration is lower, but when the carbon concentration is higher, the simulated carbon concentration is lower than experimental results due to the decrease of trapping sites and high compressive residual stress.
Subjects: Biology >> Botany >> Applied botany submitted time 2022-07-05 Cooperative journals: 《广西植物》
Abstract： The spatial distribution pattern of population is an important means to describe population characteristics, interspecific interactions, and its relationship between population and environment. Cyclobalanopsis glauca is a constructive population of climax community in karst ecosystems, in order to further understand the spatial distribution pattern of its populations. According to the sample plot investigation specification, a sample plot of 60 m × 60 m was set up, and the woody plants in the plot were spatially located and investigated for each tree. The C. glauca population was divided into three different diameter classes in the study, belonging to saplings (DBH<2 cm), small trees (2 cm≤DBH<10 cm), adult trees (10 cm≤DBH). Using the point pattern analysis method, g (r) and L12 (r) functions based on Ripley’sK function was used for the analysis of the spatial distribution pattern and correlation analysis of C. glauca population in different diameter classes. The results were as follows: (1)The diameter class structure of C. glauca population is in an inverted “J” distribution. (2) On the small scale, the stage of saplings, small trees and adult trees, the spatial distribution pattern tends to cluster distribution，and the small trees, large tree stage tends to random distribution on the large scale. (3) The performance of spatial correlation between different diameter classees is different, the seedling and sapling are positively related on the scale of 0-15 m; sapling and adult trees reflected in the scale of 0-10 m is no correlation, it showed a positive correlation on the scale of 10-15 m; Seedling and adult trees performance on the scale of 0-2 m is no correlation, it showed positive correlation on the scale of 2-15 m. The spatial distribution pattern and correlation of C. glauca population in the karst area of Guilin change with the spatial scale and development stage, which was the result of the combined effects of the population’s bioecological characteristics, intraspecific and interspecific relationships, and environmental conditions. The results of the study can provide basic information for the formation and maintenance mechanism of plant community and resource utilization research of karst hills.
Subjects: Biology >> Botany >> Applied botany submitted time 2021-04-02 Cooperative journals: 《广西植物》
Abstract：落叶阔叶林是亚热带岩溶石山典型的森林植被类型之一，为了了解此类森林植被的群落类型和组成结构，本研究在桂林阳朔县和灵川县岩溶生境典型的落叶阔叶林内设置了 49 个 20 m× 20 m 的随机样方，在野外调查的基础上进行群落数量分类和排序，对其群落类型进行划分。在此基础上对不同类型的群落种类组成、结构及其分布与环境因子的耦合关系进行深入分析。结果表明：（1）桂林岩溶石山落叶阔叶林可划分 3 个群丛组类型，即群丛组 A：南酸枣-喜树+巴豆群丛组(ASS. Choerospondias axillaris-Camptotheca acuminata+Croton tiglium)，群丛组 B：光皮梾木-喜树群丛组(ASS. Swida wilsoniana-Camptotheca acuminata)，群丛组 C：麻栎-檵木群丛组(ASS. Quercus acutissima-Loropetalum chinense)。（2）不同群落类型中常绿和落叶物种的丰富度、多度、胸高断面积及重要值的变化：在乔木层中，3 个群丛组均为落叶物种占优势；在灌木层中，群丛组 A、B 以落叶物种占优势，群丛组 C 以常绿物种占优势。（3）在 1~5 cm 和5~10 cm 径级内，群丛组 A 和 B 的落叶物种在 4 个度量指标上均大于常绿物种，群丛组 C 除了物种丰富度以外，均为常绿物种大于落叶物种；在 ≥10 cm 径级中，除了群丛组 B 的物种丰富度外，3 个群丛组中的各项指标均为落叶物种占主导地位。（4）岩石裸露率、海拔、坡向、土壤含水量、土壤 pH、土壤有机质、全氮、速效氮、速效钾、林冠开阔度和人为干扰是影响研究区不同类型群落分布的主要环境因子。
Subjects: Biology >> Ecology submitted time 2018-05-21 Cooperative journals: 《广西植物》
Abstract： 研究群落水平上的植物功能性状特征随坡向梯度的变化规律，对认识不同坡向上的植物群落形成及其应对环境变化的机制具有重要意义。本研究以桂林岩溶石山不同坡向的植物群落为研究对象，探讨了植物功能性状(比叶面积(specific leaf area，SLA)、叶绿素含量(leaf total chlorophyll content，CC)、木材密度(wood density，WD)和环境因子随坡向的变化规律。结果表明：比叶面积和叶绿素含量为阴坡˃半阴坡˃阳坡，且阴坡和半阴坡分别与阳坡差异性显著；木材密度为阴坡
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：针对316L奥氏体不锈钢表面强化，进行了低温表面渗碳实验并测量了渗后试样渗碳层内C浓度沿深度方向的分布。建立了以“陷阱-去陷阱”质量传递机制为基础的动力学模型，该模型认为Cr原子在C的扩散过程中对C原子具有陷阱作用，利用该模型计算出渗后试样中C浓度沿渗层深度方向的分布，并与实验结果进行比较。结果表明，实验测得的C浓度沿渗层深度方向的分布形状呈现凸状，与简单Fick定律得出来的结果不同，而基于“陷阱-去陷阱”模型得到的计算结果与实验结果符合较好，表明陷阱作用在C扩散过程中扮演着重要的角色。Cr原子通过对C原子陷阱作用，会降低C的扩散系数，且对实验数据拟合得到C的去陷阱摩尔激活能为1.65 kJ/mol。所提出模型仅适用于未发生碳化物析出的低温渗碳，且并未考虑扩散应力的影响。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-05 Cooperative journals: 《金属学报》
Abstract：基于时效处理实验、微观硬度测量和Fick第二定律，对Cr5Mo/A302异质焊接接头的C扩散行为进行研究，随后对已存在200 μm脱碳层的异质焊接接头和没有脱碳层的焊态接头进行持久实验，研究了脱碳层在不同应力水平下对接头寿命的影响。结果表明，随时效时间的延长，增、脱碳现象加剧，增、脱碳层的宽度变化均遵循抛物线型规律，实验测量结果与Fick第二定律拟合结果基本一致。预先形成的脱碳层在高应力条件下，可以成倍降低接头持久高温寿命，导致接头发生提前断裂，但是随着应力水平不断降低，断裂时间不断提高，预先形成的C扩散对接头持久寿命的影响程度不断下降，当应力降低到接头抗拉强度的36%以下后，其影响可以忽略。建立了C扩散影响因子S与应力的关系，并确定了已存在脱碳层对高温寿命不产生影响的临界应力值。
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