Austempered Compacted Graphite Iron — Influence of austempering temperature and time on microstructural and mechanical properties

This study investigates the effect of austempering temperature and time on the microstructural and mechanical properties of unalloyed Compacted Graphite Iron (CGI) with an initially ferritic matrix structure. The as-cast CGI samples were first austenitised at 900 °C for 60 min in a furnace, then austempered in a closed salt bath at three austempering temperatures – 275, 325, and 375 °C – for different times; 30, 60, 90, and 120 min. Tensile properties, Brinell, Vickers and Rockwell C hardness values were evaluated for the as-cast and austempered CGI ones. LOM and SEM, EBSD analysis techniques were used for microstructure and phase analysis. A mixture of acicular ferrite and retained austenite was achieved in the austempered CGI samples. In general, a decrease in austempering temperature resulted in a decrease in retained austenite content, corresponding improvements in hardness and tensile strength, and a decrease in elongation values. This research was supported by the Stiftelsen för kunskaps- och kompetensutveckling (KK-Foundation), Sweden under the ProSpekt scheme under grant number [GNR. 20170021]. Federal-Mogul AB, Bodycote, and Dollet Shifo Rapheal are greatly acknowledged for their material support and helping with the heat treatments, respectively. The authors thank Ingvar L. Svensson for providing the Matlab Script for analysing the tensile curves.