The influence of the austenite dispersion on phase transformation during the austempering of ductile cast iron having a dual matrix structure

Abstract

Abstract In the present work, an unalloyed ductile cast iron containing 3.50 C wt.%, 2.63 Si wt.%, 0.318 Mn wt.% and 0.047 Mg wt.% was austempered for various times at both the conventional austenitizing temperature (single phase austenitic region) and the different intercritical austenitizing (partially austenitizing) temperature ranges to investigate the effect of parent austenite dispersion at the austenitizing temperature on the nature of austenite decomposition during austempering. A microstructure map was created to illustrate the transformation of products quantitatively as a function of austempering time for a particular austenitizing temperature and austempering time. It was concluded that in addition to proeutectoid and bainitic ferrite, the new ferrite (also called epitaxial ferrite) introduced into the partially austenitized structure during austempering and new ferrite formation and its content was dependent on the scale of the parent austenite dispersion at the austenitizing temperature, variation in austenitizing temperature and austempering time. The new ferrite formed only in the structure with coarse parent austenite dispersion.