Property Development in As-Cast Heavy Section SG Iron Castings by Alloying with Nickel and Copper

Abstract

Ever since its discovery in 1948, the application of ductile cast iron [also known as spheroidal graphite iron (SGI)] has been increasing day by day. This is due to the fact that the material exhibits an excellent combination of tensile strength, ductility, toughness in addition to castability, machinability and damping characteristics. It has also got considerable amount of resistance to corrosion. For these reason extensive works are being carried out now-a-days to produce ductile irons with enhanced mechanical and physical properties. An investigation was performed to improve the mechanical property of thick-walled/heavy section ductile iron castings in as-cast condition alloying with nickel and copper. The property enhancement can be done by heat treatment procedures or by the addition of suitable alloying elements. In the present work, effect of two important alloying elements Copper (Cu) and Nickel (Ni) has been studied in thick section (≥20mm) ductile iron castings. For this purpose fifteen different melts of SG iron were produced in a coreless medium frequency induction furnace having a capacity of 1000 kg. Five of them were almost without nickel and copper (they were present in a very little amount), five were with significant amount of Ni and another five were with significant amount of Cu. Different tests were performed to determine the different important mechanical properties like strength, ductility, toughness and hardness of the matrix structures of each of the melts in order to study the effect of the two alloying elements on those properties. The microstructures of all the different alloys were studied and a phase analysis was performed to get an idea about the structure - property relationship. The results showed that the small addition of Ni and Cu changed the as-cast mechanical properties owing to the different as-cast matrix micro-structures. The study of the tensile properties for the melts without Ni & Cu showed the low strength and hardness as well as high ductility and impact energy with feritic matrix. Some fractions of pearlitic structures were developed near the austenitic cell boundaries due to the presence of Ni in the melts with nickel content. The amount of pearlite increased with increase in Ni content but ferritic matrix rapidly changed to pearlite (for the melts with Cu content) due to the addition of Cu in the melt. The tensile properties for the melts C1 to C5 showed that 0.2% YS, tensile strength were increased with increasing the pearlite level in the matrix structure. The Brinell hardness value for all melts was found increasing from 126 to 252 due to increase in the pearlite content in the matrix as investigated. This change in hardness value reflects the change in the mechanical properties of as-cast heavy section SG iron castings.