The effect of heat treatment, mechanical deformation, and alloying element additions on the rate of bainite formation in austempered ductile irons

Abstract

Several Alloys of Ductile Cast Iron containing various amounts of manganese, molybdenum, and nickel were austempered in the temperature range 316° to 427 °C. The rate and morphology of ferrite platelet formation (bainite reaction) were studied by optical metallography, x-ray diffraction, and hardness measurements. Austenitizing temperature, austempering temperature, and deformation by rolling were used as variables to control the kinetics of ferrite formation, stage I of the austempering reaction.

Vickers hardness, austenite carbon content, and volume fraction all change rapidly during the formation of the ferrite platelets, reaching a plateau when stage I is completed. The rate of this reaction is increased dramatically by deformation of the austenite prior to the reaction, is retarded somewhat by alloy additions, and increases with decreasing austenitization temperature. In addition, as the rate of the bainite reaction increases, with its higher ferrite plate density, a more uniform microstructure results, thus minimizing the incidence of martensite in the higher-alloy-content interdendritic volumes.