SGER: Development of Austempered Ductile Cast Iron with Simultaneous High Yield Strength, Fatigue Strength, and Fracture Toughness by a Two-Step Austempering Process

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The focus of the high risk, high potential research is to investigate a means of improving the mechanical characteristics of a relatively new, widely used material, ductile cast iron, through a new and novel heat-treating process. Higher yield strengths generally reduce the fracture toughness of materials. Austempered ductile cast iron (ADI) has emerged as a promising engineering material, however in manufacturing situations, simultaneous high yield strength, high fatigue strength and high fracture toughness have not been obtained in ADI. In this exploratory research, a novel and innovative concept of two-step austempering is proposed to solve this problem. The project will establish the feasibility of two-step austempering process and will also establish the right combination of processing parameters so that unique microstructure consisting of (a) very large volume fraction of fine ferrite (b) very fine austenite and (c) very high carbon content in austenite and (d) stable austenite can be produced. This unique microstructure should provide simultaneous high yield strength, fatigue strength and fracture toughness in ADI. This will lead to a high reliability, low cost engineering material with greater durability. Furthermore, it will generate a new class of structural material. Once the validity of this concept of two step austempering has been established, it should be possible to apply this approach in other materials such as low and medium carbon and alloy steels, to produce materials with simultaneous high yield strength, fatigue strength and fracture toughness.