Ultrafine Ductile and Austempered Ductile Irons by Solidification in Ultrasonic Field

Abstract

In this research, ultrasonic melt treatment (UST) was used to produce a new ultrafine grade of spheroidal graphite cast iron (SG iron) and austempered ductile iron (ADI) alloys. Ultrasonic treatment was numerically simulated and evaluated based on acoustic wave streaming. The simulation results revealed that the streaming of the acoustic waves propagated as a stream jet in the molten SG iron along the centerline of the ultrasonic source (sonotrode) with a maximum speed of 0.7 m/s and gradually decreased to zero at the bottom of the mold. The metallographic analysis of the newly developed SG iron alloy showed an extremely ultrafine graphite structure. The graphite nodules’ diameter ranging between 6 and 9 µm with total nodule count ranging between 900 to more than 2000 nodules per mm2, this nodule count has never been mentioned in the literature for castings of the same diameter, i.e., 40 mm. In addition, fully ferritic matrix was observed in all UST SG irons. Further austempering heat treatments were performed to produce different austempered ductile iron (ADI) grades with different ausferrite morphologies. The dilatometry studies for the developed ADI alloys showed that the time required for the completion of the ausferrite formation in UST alloys was four times shorter than that required for statically solidified SG irons. SEM micrographs for the ADI alloys showed an extremely fine and short ausferrite structure together with small austenite blocks in the matrix. A dual-phase intercritically austempered ductile iron (IADI) alloy was also produced by applying partial austenitization heat treatment in the intercritical temperature range, where austenite + ferrite + graphite phases coexist. In dual-phase IADI alloy, it was established that introducing free ferrite in the matrix would provide additional refinement for the ausferrite.