Characterization of the austemperability of partially austenitized ductile iron

Abstract

This study focuses on the characterization of the hardenability and the austemperability of partially austenitized ductile iron melts alloyed with small amounts of Cu, Ni and Mn. The hardenability was studied by using Jominy tests. The results show that the hardenability of partially austenitized ductile iron samples is significantly lower than that found on the fully austenitized samples. The partially austenitized samples do not show an initial plateau of hardness. This behavior can be caused by the presence of ferrite in the matrix before cooling. This ferrite facilitates the epitaxial growth of new ferrite during cooling, which may account for the drop in hardness and the poor hardenability. The austemperability was measured on round bars after austempering in a molten salt bath. The metallographic studies show that full dual phase microstructure could be obtained only at the smaller diameter rods. As the rod diameter becomes larger, pearlite precipitates. The cooling rate for the advent of pearlite precipitation identified in the Jominy hardenability tests was compared with that observed on the austempering treatments, observing a good correlation. It is proposed that the behavior of ductile iron austempered from partial austenitizing temperatures can be predicted with good accuracy by measuring the cooling rate for the onset of pearlite precipitation on Jominy tests.