The microstructure and mechanical properties of austempered ductile iron

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Department of Materials Science and Engineering


This paper represents a summary of experimental results dealing with the time dependence of microstructure and mechanical properties during austempering, and with the austempering temperature dependence of microstructure and mechanical properties. Alloys with a nominal compositions of 3.7 C, 2.5 Si and various controlled amounts of manganese, molybdenum, and nickel were prepared in the MTU foundry. Austenitization at 927° C (1700°F) and 871°C (1600°F) was followed by austempering at temperatures between 230° C (446° F) and 420° C (788° F) at 10°C (18°F) intervals for one hour, and at 316°C (601°F) and 371°C (700° F) for various times from 2 min to 1440 min. Optical and electron microscopy as well as x-ray metallography were used to determine the kinetics and details of the transformations during stages I and II. It is shown that the carbon gradient within the austenite during the transformation controls the rate of Stage I and alloy content controls the rate of Stage II. Interdendritic segregation of alloying elements leads to the presence of significant quantities of untransformed austenite, especially at early austempering times. It is shown that these volumes constitute convenient crack paths, thereby reducing ductility. Minimizing the continuity of those volumes increases ductility, a job aided by a lower austenitizing temperature and a minimum alloy (especially manganese) content. A processing window concept used to optimize ductility at austempering temperatures in excess of 350° C (662° F) is defined by the times needed to avoid excessive untransformed austenite volume (UAV) (the minimum time) and to avoid excessive decomposition of austenite (the maximum time). Tensile strength and ductility are shown to be a function of austenite volume fraction, scale of the microstructure, alloy content, the presence of carbide formed during the austenite transformation, and the presence of intrinsic defects such as eutectic alloy carbides.

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Journal of Heat Treating