Boro-Austempering Treatment of Ductile Cast Irons

Samples of ductile cast irons alloyed with Cu, Cu-Ni and Cu-Ni-Mo were austempered, bonded and boro-austempered and characterized for hardness and micro-adhesive wear behavior. The kinetics of layer formation was also studied. The bonding method used was molten borax bath, in periods of 2 and 4 hours at temperatures of 850, 900 and 950 degrees C. The direct austempering treatment was performed immediately following the bonding treatment (from 850, 900 and 950 degrees C) using molten salt baths at temperatures of 240, 300 and 360 degrees C for 4 hours (boro-austempering treatment). For comparative purposes, the conventional austempering treatment was also conducted. Optical microscopy, scanning electron microscopy, Brinell hardness measurements (on the substrate) and Vickers (on the layers) were performed, along with micro-adhesive wear tests. The bonding treatment resulted in the formation of high hardness layers, in the range of 1450 to 1750 HV, with high wear resistance. The wear resistance of bonded and boro-austempered samples were increased by 40 times when compared to as cast or austempered samples, indicating the high efficiency of this type of treatment in increasing the wear resistance of this material.