The potential of mechanical twinning in ultrafine retained austenite to enhance high cycle fatigue property of advanced bainitic steels

We have studied here the high cycle fatigue properties and sub-surface fatigue crack initiation mechanism in advanced bainitic steel subjected to conventional bainitic austempering (BAT) and novel disturbed bainitic austempering (DBAT) processes. The retained austenite in DBAT steel mainly exhibited ultrafine film-like morphology, whereas a considerable amount of coarse blocky retained austenite was present in the BAT steel. The study indicated that the DBAT steel had higher fatigue strength and endurance, compared to the BAT steel. The enhanced fatigue properties are attributed to mechanical twinning that occurred in film-like retained austenite at the sub-surface fatigue crack initiation area.