Effect of austempering on the microstructural evolution and tribological mechanisms of Si-modified PM M35 high-speed steel

This study examines the influence of the heat-treatment route on the microstructural evolution, mechanical response, and tribological behavior of Si-modified (+1.5 wt.%) powder-metallurgy (PM) M35 high-speed steel. Since the chemical composition was kept constant for all specimens, the observed differences are attributed solely to martempering (MT) and isothermal austempering (AT). While martempered M35 steel provides high hardness, its predominantly martensitic microstructure limits fracture resistance under severe tribological loading. Microstructural characterization using scanning electron microscopy (SEM) and X-ray diffraction (XRD) revealed that silicon effectively suppresses cementite precipitation during austempering, enabling controlled carbon partitioning and promoting the formation of a refined bainitic/martensitic (B/M) microstructure with an increased fraction of stabilized retained austenite. Compared with the martempered condition, the austempered specimens exhibited a reduced hardness but a significantly enhanced transverse rupture strength, indicating an improved balance between strength and toughness associated with transformation-induced plasticity (TRIP). Tribological tests conducted using a ball-on-disk configuration combined with three-dimensional surface profilometry demonstrated a lower coefficient of friction, reduced wear depth, and improved surface integrity for the austempered condition. Wear mechanism analysis revealed a transition from brittle fracture-dominated delamination in the martempered microstructure to plasticity-controlled ploughing in the austempered B/M structure. These findings highlight austempering as an effective pathway for enhancing wear resistance and damage tolerance in Si-modified PM M35 steel without altering the alloy chemistry.