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

技术：将研究在准平衡条件下低温（725-750K）渗碳不锈钢的相平衡和相变。先前对奥氏体不锈钢的经验表明，使用准平衡渗碳可以获得“巨大”的间隙碳浓度，并导致性能的极大改善，包括“超硬度”和增强的磨损、疲劳和耐腐蚀性。在NSF SGER项目下，PI已经证明，至少对某些铁素体和马氏体不锈钢来说，低温过平衡渗碳也是可能的，并且渗碳诱导的铁素体到奥氏体相变可能发生在某些合金中。研究人员提出了一种计算与实验相结合的方案来研究这类不锈钢的低温准平衡渗碳。计算工作将采用CALPHAD形式，并探索铁素体合金在整个过程中应该形成巨大的间隙碳过饱和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，并且可能的成分空间也将通过计算来探索。一套铁素体合金预计将显示这两种类型的行为，然后将被熔化，轧制成片状，渗碳，并表征。奥氏体钢先前工作的另一个重要方面是，在低温和高碳浓度的准平衡渗碳过程中，碳扩散率对成分的依赖性很强。我们将对此进行详细探讨。PI将能够探索铁素体合金中碳扩散率的浓度依赖性，再次在低温和高碳浓度的准平衡渗碳。如果没有碳在铁素体合金中的溶解度可以通过低温渗碳得到极大的提高，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大大扩展这些材料的应用领域。如前所述，该工艺价格低廉，并在低温炉中使用1atm压力的传统渗碳气体。它是完全保形的，不需要尺寸变化或部件延展性的损失。因此，成品部件可以渗碳作为其制造的最后一步。非技术：这种变革性研究将为令人兴奋的颠覆性技术提供科学基础。在许多结构应用中，使用具有改进性能的渗碳不锈钢将允许用这些材料代替更昂贵的材料。铁素体和马氏体不锈钢的工作是相当新颖的，并揭示了对材料系统的理解，特别是在远离真正热力学平衡的条件下，甚至可以导致所谓的“成熟”材料和工业的令人兴奋的突破。这里建议的研究也将对凯斯西储大学的研究生材料科学与工程项目产生非常积极的影响，特别是对两位将获得资助的研究生。学生的研究将需要他们掌握大量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。

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.