SGER: A Novel Thermomagnetic Austempering Process

这项由材料研究部授予韦恩州立大学的探索性研究(SGER)小额赠款是为了探索一种名为“热磁等温淬火”的新过程。假设在磁场中采用两步等温处理将得到具有高屈服强度、疲劳强度、断裂韧性和冲击强度的铁-碳合金，这些性能在结构合金中通常是不能获得的。有了这个奖项，普塔通达教授将把这一创新工艺应用于等温淬火球墨铸铁(ADI)，这种材料之所以被选中，是因为它在交通、国防和制造业中的大量应用。作为该工艺发展的一个组成部分，发展了奥氏体相变到铁素体和高碳奥氏体相变自由能变化的热力学分析。分析表明，在外加磁场的作用下，铁素体形核的驱动力会增大，从而导致材料的组织非常细小。将审查这一分析模型的适用性以及在推导该模型时所作假设的有效性。这项研究将确定ADI的热磁等温淬火是否会在奥氏体相中产生含碳量非常高的超细晶组织。确定了碳在ADI组织发展中的作用，以及细化铁素体和奥氏体对ADI断裂韧性和高周疲劳强度的影响。这项研究将有助于提高黑色金属物理冶金的知识现状。该项目将确定磁场中两步等温工艺的可行性，并确定合适的工艺参数组合，以创建由(A)非常大体积分数的细小铁素体和奥氏体组成的独特显微组织，(B)奥氏体中非常高的碳含量，以及(C)稳定的奥氏体。这种独特的显微组织将为ADI合金提供高屈服强度、疲劳强度、冲击强度和断裂韧性。热磁等温淬火工艺可提供高可靠性、低成本、高耐久性的工程合金，从而对材料加工行业产生重大影响。此外，有可能将这种新方法应用于其他金属合金，如碳钢、铁镍合金和镍基高温合金，以生产具有独特机械性能组合的合金。学生培训和教学将是这一进程发展不可或缺的一部分。

This small grant for exploratory research (SGER) award by the Division of Materials Research to Wayne State University is to explore a new process termed "thermomagnetic austempering." It is hypothesized that applying a two-step austempering treatment within a magnetic field will result in iron-carbon alloys with high yield strength, fatigue strength, fracture toughness and impact strength, a combination of properties not normally obtained in structural alloys. With this award, Professor Putatunda will apply this innovative process to austempered ductile cast iron (ADI), a material chosen because of its numerous applications in the transportation, defense and manufacturing industries. As an integral part of the development of this process, a thermodynamic analysis of free energy change for the phase transformation of austenite to ferrite and high carbon austenite has been developed. This analysis indicates that the driving force for ferrite nucleation should increase under an externally applied magnetic field, resulting in a very fine-grained microstructure in the material. The applicability of this analytical model and the validity of the assumptions made in deriving it will be examined. The research will establish if thermomagnetic austempering of ADI creates an ultra-fine grained microstructure with very high carbon content in the austenite phase. The role of carbon in the development of the microstructures of ADI and the influence of refined ferrite and austenite on the fracture toughness and high cycle fatigue strength of ADI will be established. The research will contribute to advancing the present state of knowledge in ferrous physical metallurgy.The project will establish the feasibility of the two-step austempering process in a magnetic field and the right combination of processing parameters that creates a unique microstructure consisting of (a) a very large volume fraction of fine ferrite and austenite, (b) a very high carbon content in austenite, and (c) a stable austenite. This unique microstructure will provide high yield strength, fatigue strength, impact strength and fracture toughness in ADI alloys. The thermomagnetic austempering process could have a significant impact in materials processing industries by providing high-reliability, low-cost engineering alloys with great durability. Furthermore, it may be possible to apply this new approach to other metal alloys such as carbon steels, iron-nickel alloys, and nickel-based superalloys to produce alloys with unique combinations of mechanical properties. Student training and teaching will be integral part of this process development.