Sliding wear behavior of austempered ductile iron microalloyed with boron

This work analyses the wear resistance under dry sliding conditions by the pin-on-ring method of an austempered ductile iron (ADI) micro-alloyed with different amounts of boron. The main observed effect of boron was to accelerate the transformation of austenite to ausferrite during the austempering heat treatment; this caused lower values of retained austenite in the irons with boron additions. Lower retained austenite amounts, in turn, caused higher wear rates since the amount of austenite able to transform to martensite by plastic deformation is lower. It was observed that wear loss is a function of the iron hardness. For short austempering times higher values of hardness were obtained which produced lower wear; and the main observed wear mechanism was oxidation. On the other hand, for longer austempering times hardness decreased but the ductility was improved (more ausferrite, less martensite). With the increase in load, a higher deformation under the worn surface was observed and wear was slightly increased. It was observed that boron additions increased the carbon diffusion and accelerated the ausferrite transformation promoting a more homogeneous microstructure. Due to this, a higher ductility but lower values of hardness were obtained, which changed the wear behavior. The higher hardness values of the base alloy are related to a low friction coefficient and lower wear. On the other hand, boron added irons were softer than the base alloy, but the higher volume of graphite acting as a lubricant and the finer matrix contributed to give wear resistance, giving a good balance of mechanical properties.