Structure–property correlation in austempered alloyed hypereutectic gray cast irons

The austempering behavior of a series of hypereutectic alloyed gray iron compositions with carbon equivalent from 4.37 to 5.14 was studied to understand the influence of microstructure on its mechanical and wear properties. The alloying elements in the alloys included Ni, Mo, Cr and inoculation by micro-constitution of Ti, Nb and Ce. The alloys were austempered at 360 °C and upper bainitic type feathery ferrite was observed in the matrix. While the graphite content determined by optical metallography varied between 16 and 24 vol%. The volume of austenite determined by XRD analysis showed values between 20 and 26%. The ferrite lath size was determined using XRD peak broadening. The tensile property varying between 188 and 270 MPa, showed no significant variation with volume percentage of carbon or austenite in the ausferrite. However the wear rate varying between 0.5 and 2.6 × 10−7 g/Nm, showed a decreasing trend with graphite content attributed to the higher lubricating effect of released carbon during sliding wear. The specific wear rate of hypereutectic alloys, increased with increasing ferrite lath size due to enhanced softer ferrite phase on the sliding surface. The wear rate was found to increase with volume of austenite, austenite carbon content and austenite lattice parameter, which is attributed to increased stability of austenite against strain induced martensite formation and the increased formation of bainitic carbides in the second stage tempering. The various technical aspects in correlating the microstructure with the mechanical and wear properties of hypereutectic austempered gray iron are described.