Influence of graphite nodularity on microstructure and processing window of 1.5% Ni–0.3% Mo austempered cast iron

Abstract The effect of graphite nodularity on microstructure and processing window of austempered cast iron with four-different levels of graphite nodularity and a composition of 3.6% C, 2.2% Si, 0.2% Mn, 1.5% Ni, 0.3% Mo was studied. Microstructural observations and measurements of the retained austenite content and hardness, are reported as functions of austempering time at 440 and 300 °C after austenitising at 900 °C. The results of the experimental investigation indicate that graphite nodularity has considerable effect on the microstructure. After 8 min austempering time at 440 °C austempering temperature and 20 min austempering time at 300 °C austempering temperature, the amount of retained austenite (Xγ) increases with the graphite nodularity. The microstructural investigation and hardness test investigation indicate that the graphite nodularity has a significant effect on the time t 1 corresponding to the end of stage I (process window), whereas decreasing the graphite nodularity delays the end of stage I reaction. The starting time of reaction II, t 2 is determined from the variation of Xγ with austempering time, it is indicated that lower graphite nodularity accelerates the start of stage II reaction.