Effect of specimen preparation on contact fatigue wear resistance of austempered ductile cast iron

Austempered ductile iron (ADI) has been tested as a wear resistance material for applications where the applied loads are very high and cyclic, such as gears. This paper analyzed the endurance lifetime of this material in wear testing equipment that applies contact fatigue stresses in a lubricated ball-on-flat system. The tests were performed at 3.0 or 3.7 GPa of maximum Hertz pressure, using ISO 46 lubricant at 85 °C, until spalling occurrence. The effect of specimen preparation was studied for ground or polished specimens. The worn surfaces were characterized by means of an optical and electron microscope and by the difference between unworn and worn surface profiles. It was found that the graphite nodules were exposed at the surface in two ways: cracked and partially exposed. These morphologies were not found in polished specimens, but the wear process in all test conditions produces them. The low endurance lifetime observed in ground specimen was explained based on these defects, which were present before the tests. The slope of Weibull curves was related to the width of the worn track. For polished specimens the slope was smaller than that observed for ground conditions. Thus, any manual process for preparation can be considered as forbidden in order to obtain better results.