Effects of boron addition and austempering time on microstructure, hardness and tensile properties of ductile irons

Abstract

The present work analyzes the effect of boron addition to an Austempered Ductile Iron, in amounts from zero to 120ppm. It has been found that boron has a strong effect on the equivalent carbon content, resulting in an increase on the precipitated graphite volume and a decrease in the dissolved carbon content in the matrix. This in turn, increases the ferrite volume fraction in the as-cast conditions from 0.24 in the base alloy to 0.78 for the iron with 120ppm of boron. Furthermore, a decrease in the nodularity from 100% in the base alloy to 83% with 120ppm of boron has been observed. During austempering, the transformation to ausferrite was faster and lower volumes of martensite and unstable austenite were detected when boron increased; this promoted lower hardness values, 239HV for the base iron and 189HV for the 120ppm boron alloy. The increase in hardness and strength, typical for the start of bainite formation, were not observed in the boron added irons, but just in the base alloy. Under this basis, it is assumed that at least the addition of 60ppm of boron extended the optimal processing window. The higher values of strength and ductility were obtained for the alloy with 60ppm of boron; these results are discussed in terms of the graphitizing effect of boron in these irons and the reduced amount of carbon dissolved in austenite.