Effect of hardness differential on metal-to-metal fretting damage

During the development of a standardized laboratory fretting test it was observed that the fretting damage to austenitic stainless steel samples taken from wrought bar stock was significantly different from damage to the same alloy in cold-finished strip form. These unexpected results suggested that hardness differences between the mating members might play a significant role in how a couple damages under fretting motion. Is a hardness differential between contacting members better from the damage standpoint than mating members at the same hardness?The newly developed ASTM G 204 standard fretting test was used to address this question. This test uses a reciprocating ball on a flat with amplitude of 50 μm, normal force of 10 N, and duration of one million rubbing cycles at 13 Hz. A hardened steel ball rider (52100 steel at 670 HV) was mated with type O1 tool steel at different hardnesses (690 HV to 280 HV) and the relative fretting damage was assessed. Tests were conducted in air and in light mineral oil.The oil significantly reduced system damage at all hardness differentials. In addition, the damage went from fretting corrosion to fretting wear. In both air and oil a hard/soft couple produced a significant increase in system damage compared to a hard/hard couple. Both air and oil tests suggested that a slightly reduced hardness (10%) of one member may reduce damage by about 10%, but the benefit was marginal. The test conducted in mineral oil demonstrated that adhesive wear is the prevailing mechanism for damage to both members. Adhesive wear appeared to be the origin of the pitting that is often typical in fretting contacts.