Influência do número de nódulos de grafita nas propriedades mecânicas do ferro fundido nodular austemperado

Abstract

In recente years, austempered ductile cast irons (ADI) has emerged as a major engineering material because of its excellent mechanical properties. These properties, as result of a microestructure containing spheroidal graphite and ausferrite, include high strength, ductility, toughness, good wear resistance and machinability. Through the years, the study of influence of process variation on these properties has had special attencion. However, during solidification, a high graphite nodule count exerts an important role in microestructure formation because it decreases the segregation regions contributing to obtain a homogeneous microestructures. The present work describes the influence of graphite nodule count on the ultimate tensile strenght, yield strenght, elongation and Brinell hardeness of an austempered ductile iron. It was obtained 43 samples with between 87 and 784 graphite nodule/mm2 using different cooling rate, inoculation rate and nodularization process variations. All the samples, having the same chemical composition, were austenitized at 885oC for 2 hours, then quenched and kepted in a salt bath of 360oC for 1,5 hours. Metallography, scanning electron microscopy (SEM) and X-ray diffraction analysis were performed to correlate the mechanical properties attained with the microestructural observation. The results of this insvestigation indicated a tendency of the ultimate tensile strength to increase with the nodule count while the yield strength didn´t change. It was observedthat the elongation increased significantly and the Brinell hardness decreased as nodule count was increased.