Advanced Process Influenced by Holding Time in the (α + γ) Temperature Range, on the Mechanical Characteristics of Permanent Molded Toughened ADI

Austempered Ductile Iron (ADI) castings are produced for various engineering applications especially, gears, crank shafts, axles etc. The ADI castings made in permanent molds, show excellent properties in respect of strength, elongation, wear resistance, toughness and dimensional stability. While the heat treatment processes have been optimized for the ductile irons, the special thermal treatment in the (α + γ) temperature range with varied holding times, have not been widely reported and discussed. Keeping the above points in view, this study is mainly focused on enhancing the mechanical properties of permanent mold toughened austempered ductile iron (PMTADI), by following a special heat treatment cycle called as the “QB¹ process”. It consists of austenitizing the samples at 900 °C and subsequently oil quenched to room temperature. Following this, they are reaustenitized to (alpha + gamma) temperature range (740 °C) with the holding time varied at 60, 90, 120 and 150 min. The samples are then austempered at 350 °C in a salt bath for a constant duration of 90 min. Based on the experimental results, it is observed that PMTADI sample reaustenitized to 740 °C and having a holding time of 120 min, has revealed higher UTS and % elongation values compared to the samples with other holding times. The microstructures of the samples have been assessed using light microscopy for examining the phases present in the samples. The PMTADI sample with 120 min holding time has revealed fine spheroidal graphite in a bainite matrix with highest nodule count, lowest size and Retained Austenite of about 34%. Further, the tensile and impact damaged samples have been assessed using SEM to get good correlation between the features and the data.