Phase Equilibria and Phase Transformations in Stainless Steels Containing "Colossal" Concentrations of Interstitial Carbon

含有“大量”间隙碳的不锈钢中的相平衡和相变

• 批准号: 0706163
• 负责人: Arthur Heuer
• 金额: $ 56万
• 依托单位: Case Western Reserve University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-06-15 至 2011-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0706163&HistoricalAwards=false
• 关键词: Phase; Equilibria; Transformations; Stainless; Steels

TECHNICAL: Phase equilibria and phase transformations will be studied in stainless steels carburized at low temperatures (725-750K) under paraequilibrium conditions. Prior experience with austenitic stainless steels has shown that "colossal" concentrations of interstitial carbon can be achieved using paraequilibrium carburization and lead to very much improved properties, including "superhardness" and enhanced wear, fatigue, and corrosion resistance. Under a NSF SGER program, PI have demonstrated that low temperature paraequilibrium carburization is also possible for at least some ferritic and martensitic stainless steels and that a carburization-induced ferrite-to-austenite phase transformation can occur in some alloys. PIs have put forth a combined computational/experimental program to study low temperature paraequilibrium carburization of this class of stainless steels. The computational work will employ the CALPHAD formalism and explore the composition space where ferritic alloys should develop a colossal interstitial carbon supersaturation and remain ferritic through the entire process. A second group of ferritic alloys will transform to austenite during carburization, and the composition space where this is possible will likewise be explored computationally. A suite of ferritic alloys predicted to display both types of behavior will then be melted, rolled into sheet form, carburized, and characterized. A further important aspect of the prior work on austenitic steels is the strong compositional dependence of carbon diffusivity at the low temperatures and large carbon concentrations inherent in paraequilibrium carburization. This will be explored in detail. PI would be able to explore the concentration dependence of carbon diffusivity in ferritic alloys, again at the low temperatures and large carbon concentrations afforded by paraequilibrium carburization. Such studies of carbon diffusion would not be possible absent the greatly enhanced solubility of carbon in ferritic alloys that can be achieved by low temperature carburization. Stainless steels carburized under paraequilibrium conditions have the promise to greatly extend the application arena where these materials can be used. As mentioned already, the process is inexpensive, and uses conventional carburization gases at 1 atm pressure in a low temperature furnace. It is fully conformal and entails no dimensional changes or loss of component ductility. Hence, finished components can be carburized as a final step in their manufacture. NON-TECHNICAL: This transformative research will provide the scientific underpinning for an exciting disruptive technology. The use of carburized stainless steels with their improved properties will allow substitution of these materials for more expensive materials in many structural applications. The work on ferritic and martensitic stainless steels is quite novel, and reveals that an understanding of a materials system, particularly under conditions far from true thermodynamic equilibrium, can lead to exciting breakthroughs even for so-called "mature" materials and industries. The research suggested here will also have very positive impact on the graduate materials science and engineering program at Case Western Reserve University, particularly for the two graduate students who will be supported. The students' research will entail their acquiring skills in a host of mainstream materials science techniques, including modeling and microstructural characterization, and thermodynamic and kinetic analyses.

技术：将在准平衡条件下研究低温(725-750K)渗碳不锈钢的相平衡和相变。以前使用奥氏体不锈钢的经验表明，使用准平衡渗碳可以获得“大量”的间隙碳，从而极大地改善性能，包括“超硬”以及增强的耐磨性、疲劳性和耐腐蚀性。在NSF SGER计划下，PI已经证明，至少对一些铁素体和马氏体不锈钢来说，低温准平衡渗碳也是可能的，并且在一些合金中可以发生渗碳诱导的铁素体到奥氏体的相变。PIS提出了一个计算/实验相结合的程序来研究这类不锈钢的低温准平衡渗碳。计算工作将采用CALPHAD公式，并探索铁素体合金在整个过程中应发展成巨大的间隙碳过饱和度并保持铁素体的成分空间。第二组铁素体合金将在渗碳过程中转变为奥氏体，并同样将通过计算探索可能实现这一点的成分空间。一套预计将表现出这两种行为的铁素体合金随后将被熔化、轧制成片状、渗碳和表征。奥氏体钢之前工作的另一个重要方面是，在准平衡渗碳所固有的低温和高碳浓度下，碳扩散系数与成分有很强的相关性。我们将详细探讨这一点。PI将能够探索铁素体合金中碳扩散系数的浓度依赖关系，同样是在准平衡渗碳提供的低温和高碳浓度下。如果没有低温渗碳可以大大提高碳在铁素体合金中的溶解度，这样的碳扩散研究是不可能的。在准平衡条件下渗碳的不锈钢有望极大地扩展这些材料的应用领域。如前所述，该工艺成本低廉，在低温炉中使用压力为1大气压的传统渗碳气体。它是完全保形的，没有尺寸变化或部件延展性损失。因此，成品部件可以作为其制造的最后一步进行渗碳。非技术：这项变革性研究将为一项令人兴奋的颠覆性技术提供科学基础。使用性能得到改善的渗碳不锈钢，可以在许多结构应用中用这些材料替代更昂贵的材料。对铁素体和马氏体型不锈钢的研究是相当新颖的，它揭示了对材料体系的理解，特别是在远离真正热力学平衡的条件下，即使对于所谓的“成熟”材料和行业，也可以带来令人兴奋的突破。这里建议的研究也将对凯斯西储大学的材料科学和工程研究生项目产生非常积极的影响，特别是对将得到支持的两名研究生。学生的研究将需要他们获得一系列主流材料科学技术的技能，包括建模和微观结构表征，以及热力学和动力学分析。