Matrix of Austempered Ductile Cast Iron

Metallographic study on the matrix of austempered ductile irons with 3.43 to 3.76% C, 2.10 to 2.18% Si, 0.15 to 0.50%Mn, 0 to 0.05%Cr, 0 to 1.81%Ni, 0 to 0.2%Mo, 0 to 0.51 Cu and 0 to 0.005 % Bi was carried out. These irons were heated at 875℃ for 2 h and austempered at 235℃ and 375℃ for 1 to 3 h. The morphology and distribution of bainitic ferrite and retained austenite were examined and mean bainitic ferrite spacing (BS) was measured by use of scanning electron microscope and optical microscope. The retained austenite determination of irons using X-ray diffractometer was conducted. Γ-pool distributed in region of boundary of eutectic cell and much retained anstenite adjacent to lathlike bainitic ferrite was in irons. Bainitic ferrite coarsened as the austempering temperature rose from 235 to 375℃. The amount of retained austenite ranged in irons from about 14 to 31 vol. % and it decreased as the austempering time prolonged. The hardness (Hv) of irons was within 560 to 625 at 235℃ for 2 h while it was within 350 to 400 at 375℃ for 2 h.