Abstract
The intention of the paper was to improve the mechanical properties of ductile cast iron by a duplex matrix which is used in steels.
Ferritic ductile cast iron was heat-treated to produce the following duplex matrices of various proportions; ferrite-bainite, ferrite pearlite and ferrite-tempered troostite.
The tensile and impact tests were performed on the irons with a duplex matrix. The 0.2% proof stress, tensile strength and hardness increase with increasing volume fraction of the second phase, but there is no linear relationship known as the law of mixture. The harder the second phase is, the higher the strength becomes. In the elongation and impact energy of the alloy with higher silicon content, two peaks appear at volume fractions of upper bainite of about 50 and 95%, and the transition temperatures drop to minimums; the elongation values are 18 and 12%, the absorbed and upper shelf energies are 14.5 to 15kg•m/cm2 and the transition temperatures are -45° to-47°C in the un-notched specimen.
Thus, the strength and toughness of ductile iron can be improved by the proper second phase of a proper volume fraction in ferritic structure. The improvement comes from the fine duplex matrix structure and low carbon content of the second phase, which is a characteristic in the austenitizing of ferritic ductile cast iron, and it also comes from the second phase with high strength and high ductility, such as upper bainite, formed mainly around the graphite nodules, probably because of preventing a crack initiation at the graphite-second phase interface.
