SGER: A Novel Thermomagnetic Austempering Process

Wayne State University, Detroit MI

Investigators

Abstract

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.