Effect of the Number of Nodular Graphite Particles per unit Area on the Distribution of Hard Phase in Ferritie-Martensite Dual-Phase Ductile iron

In this research, effect of the number of nodular graphite particles, as an important characteristic of ductile cast irons, on the distribution of martensite in dual-phase ductile iron has been studied. Ductile iron samples were produced by step casting and the samples were then annealed in order to achieve a fully ferritic structure. A special heat treatment was conducted to produce dual matrix structures (DMSs) in the ductile iron samples. This heat treatment consisted of preheating the samples at 600 C for 10 minutes, partial austenitizing at 900 C in molten aluminum bath for 10, 15, 20, and 25 seconds, and quenching in oil. The volume fractions of different phases were determined according to ASTM-E582 standard. The microstructures of the samples were examined using the optical microscope as well as the scanning electron microscope equipped with the energy dispersive spectrometer (EDS) to determine the composition of the regions around the graphite nodules and the intercellular areas. The results showed that for all the austenitizing times, the martensite volume fraction in the samples increases with an increase in the thickness of the steps. It was also shown that for the thick steps, the graphite nodules are surrounded by the martensite phase whereas for the case of thin steps, the martensite phase has been formed mostly at the intercellular regions.