EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The standard types of graphite particles in cast iron are classified as spheroidal, compacted and lamellar graphite particles. The compacted graphite is considered as an intermediate shape between spheroidal and lamellar graphite. In spite of the many years of research, little is known about the physical phenomena leading to the evolution of compacted graphite (CG) within liquid melt during solidification, in particular how net-like features evolve from microstructures filled with melt and austenite, and how local variations at micrometer length scale affect this growth process. We present the first time-resolved three-dimensional (3D) measurements that allow direct observation of the evolution of CG and relate this dynamic process to the local surrounding microstructures in the bulk of a ductile cast iron sample during repeated melting and solidification. In situ synchrotron tomography combined with a novel high temperature environment cell is used for the examination, allowing us to map a gauge volume in the bulk of the sample in situ during repeated melting and solidification. We found that more than 67% of the graphite particles nucleating in the initial stage of the solidification tend to form spheroidal graphite, while those forming later likely develop into compacted graphite. It is also observed that the evolution of compacted graphite involves the nucleation, growth, development of branches, and interconnection of graphite particles, resulting in widely spread network structures and low sphericities. The development of branches is considered to be induced by high carbon concentrations or thin melt channels. The direct visualization of the dynamic evolution of compacted graphite provides new insights into correlating factors such as local variations of chemical compositions, and validation of simulation on the solidification process.