Microstructure, wear behavior and surface hardening of austempered ductile iron

Austempered ductile iron has been considered as a substitution to replace conventional steel castings, steel forgings and even aluminum in many applications. Its excellent mechanical properties are mainly attributed to its unique ausferritic structure containing acicular ferrite and carbon saturated austenite. Some ADI components such as camshaft and engine valve commonly experience surface contact and relative motion under external load and pressure. Outstanding wear resistance of ADI is required to ensure the satisfactory working performance and service life of these key components. In order to meet the advanced requirements of mechanical properties and wear behavior of ADI, a dual-step austempering process and additional surface hardening treatments are suggested to be utilized. In this review, conventional single-step austempering process and corresponding ausferritic structure are introduced. Second, the positive effects of a novel dual-step austempering process on microstructure and mechanical properties of ADI are provided. Next, wear behavior of single-step and dual-step ADIs and wear mechanisms are discussed. Then, studies regarding the applications of surface hardening treatments on wear resistance of ADI are reviewed. Finally, a summary and expectations are discussed. The information presented in this paper would be beneficial to future studies and applications for any grade of ADI.