Conventional high-strength nodular graphite iron as a substitute for austempered ductile iron (ADI)

Abstract

Cast iron materials are becoming more widely used for large and complex components in mechanical engineering and drive technology. As a consequence of steadily increasing requirements, high-strength nodular graphite irons are in great demand. In order to fully exploit the potential of these materials, an in-depth characterization of their mechanical properties is necessary. This allows for reduced safety margins. In this study, fatigue tests and accompanying microstructural characterization were performed comparatively on conventional nodular graphite iron EN-GJS-700-2. The material was characterized by very fine nodular graphite. The material has been cast into patterns of differently sized planetary carriers. Fatigue specimens were prepared from the cast components and tested in order to determine the fatigue limit. Microstructural characterization and fractography including optical and scanning electron microscopy were carried out. As a key result, the correlation between mechanical and microstructural properties reveals the importance of fine and nodular graphite accompanied by the absence of microporosities or other casting defects in order to achieve high fatigue strengths. Moreover, a minor influence of the (pearlitic or ausferritic) matrix structure, in which the graphite is embedded, was determined. Consequently, the fatigue strength of conventional high-strength nodular cast iron may be rendered adequate by improving the graphite quality and eliminating casting defects. By understanding the correlations between mechanical properties and its microstructure, a precise load-carrying capacity can be obtained. Thus, the accurate design process of a cast component may avoid additional alloying elements and the expansive heat treatment in a salt bath that is necessary for ADI. The implementation of these findings can enhance the efficiency of cast components while reducing the costs.