Influence of slip ratio on the phase stability of austenitic steel AISI 304 during rolling contact wear

This study uses rollers with curved profiles to explore the phase stability of austenitic matrix AISI 304 exposed to the rolling contact wear under the variable slip ratios in the range from −0.23% up to 8.81%. The wear track is formed under the cyclic surface delamination and the asymmetric extrusion of matrix towards the track side. The size of wear track and the fraction of extruded materials grow along with the slip ratio. Severe plastic deformation makes the parental matrix susceptible to phase transition when the ferromagnetic martensite replaces the paramagnetic austenite. The extent of this transition and the preferential alignment of neighbouring phases were found to be more complex under the higher slip ratios. The matrix is preferentially strained along the wear track width when the slip ratio is close to zero, whereas the presence of friction components at the higher slip ratios preferentially strains the matrix closer to the rolling direction (RD). The sandwich-like microstructure is formed when the extremely refined layer containing a mixture of strain-induced martensite (SIM) and austenite on the free surface is replaced by the thermally softened one due to friction heating, containing mostly strain-induced martensite, followed by the plastically strained region with prevailing austenite fraction. The heat release due to plastic deformation of austenite, phase transformation, and friction contributes to elevated temperatures, especially at the higher slip ratios. The enhanced thermal softening at the higher slip ratios reduces the compressive residual stresses, but the dislocation density in austenite as well as martensite is increasing. The study also demonstrates that this phase transformation can be reliably monitored by magnetic Barkhausen noise (MBN), which increases with the slip ratio due to the growing preferential straining of both phases into the direction of rolling. The high sensitivity of this non-destructive magnetic method is based on the medium-increasing fraction of the strain-induced martensite and its distinct preferential straining along the rolling direction at higher strain rates. The influence of residual stress state on magnetic emission is only minor.