Paraequilibrium Carbides in Stainless Steels

技术概述：铁素体、马氏体和双相不锈钢，在低于500°C的温度下进行最佳渗碳，具有硬化的壳体，其中包含一类不降解的显著碳化物相，并且在某些情况下实际上提高了这些重要钢的耐腐蚀性。对这些碳化物的分析已经确定，它们与形成它们的大块不锈钢具有相同的金属成分，因此被称为准平衡碳化物。由于这些不平衡碳化物的形成不会耗尽周围的铬基体，因此不会发生“敏化”和耐腐蚀性的丧失。提出的研究将全面表征这些副平衡碳化物的结构、微观结构和化学计量学，以及它们对间隙硬化钢表面的电化学、摩擦学和机械性能的影响。一系列的加工参数，例如，时间/温度和工艺气体气氛中的碳活性，将应用于这些不锈钢，以产生广泛的准平衡碳化物相。热力学和动力学建模将与加工和表征活动并行进行，以提供对这一激动人心的技术的深入了解。非技术概述：奥氏体（高镍含量）不锈钢的低温渗碳现在已在工业规模上应用于许多重要的应用。类似的技术目前还不能用于铁素体、马氏体和双相不锈钢。这类重要的磁性不锈钢的成功低温渗碳——由于其镍含量大大降低，成本更低——将产生广泛的影响。适当硬化的渗碳表面将使这些不锈钢建立新的应用和新的市场，基于改进的耐磨性和耐腐蚀性，以及其他有用的性能。最后，学生研究人员将在约束优化项目的背景下，进行关于该主题的论文研究，并体验涉及处理，表征和建模的丰富活动。

TECHNICAL SUMMARY: Ferritic, martensitic, and duplex stainless steels, optimally carburized at temperatures below 500 °C, possess a hardened case containing a remarkable class of carbide phases that do not degrade, and in certain cases actually improve the corrosion resistance of these important steels. Analyses of these carbides have determined that they have the same metal compositions as those of the bulk stainless steels in which they formed, and thus are termed paraequilibrium carbides. Inasmuch as the formation of these paraequilibrium carbides do not deplete the surrounding matrix of Cr, "sensitization" and loss of corrosion resistance does not occur. The proposed research will fully characterize the structure, microstructure, and stoichiometry of these paraequilibrium carbides, and their effect on electro-chemical, tribological, and mechanical properties of the interstitially hardened steel surfaces. A range of processing parameters, e.g., time/temperature and carbon activity in the process gas atmosphere, will be applied to these stainless steels to generate a broad spectrum of paraequilibrium carbide phases. Thermodynamic and kinetic modeling will be performed in parallel with the processing and characterization activities to provide in-depth understanding of this exciting technology.NON-TECHNICAL SUMMARY: Low-temperature carburization of austenitic (high nickel content) stainless steels is now employed on an industrial scale in a number of important applications. Similar technology is not now generally available for ferritic, martensitic, and duplex stainless steels. Successful low-temperature carburization of this important group of magnetic stainless steels---which furthermore are less costly due to their greatly reduced nickel content---will have broad impact. A properly hardened carburized surface will enable these stainless steels to establish new applications and new markets, based upon improved wear and corrosion resistance, in addition to their other useful properties. Finally, the student researchers doing their thesis research on this topic will experience a rich array of activities involving processing, characterization and modeling, all against the backdrop of a constrained optimization project.