Tribological behaviour of shot peened Cu-Ni austempered ductile iron

Wear and fatigue properties of power transmission components are usually improved by various surface engineering processes. One surface modification process is shot peening which is generally carried out to improve bending fatigue. However there are contrasting studies meant to investigating whether shot peening actually increases the sliding wear resistance of austempered ductile iron (ADI). Unlubricated wear tests were conducted on ground ADI and shot peened ADI pins. Hardness measurements of the worn ADI surfaces showed a 19% increase in hardness after testing at low loads, possibly due to strain hardening and frictional heating. Metallography of the worn surfaces showed a distorted microstructure at the surface, indicative of surface flow. On the other hand, samples tested at high loads showed a 73% increase in hardness. A white non-etchable layer which was identified as untempered martensite formed upon cooling of wear test samples. Calculation of the wear factors and friction coefficients showed that shot peening does not improve the wear resistance. This has been attributed to the fact that the potential advantages resulting from the higher hardness at the surface, stress-induced austenite to martensite transformation and the residual compressive stresses of the shot peened specimens are counteracted by the induced surface roughness.