Fine granular area linked to very high cycle fatigue in martensitic and bainitic steels: Characterization by means of EBSD-dictionary indexing

Abstract In martensitic/bainitic bearing steels, non-observable critical cracks can initiate at inner inclusions well below the endurance limit (at 107 cycles), leading to very high cycle fatigue failure. The stability of these cracks is not ensured even if they have a stress intensity factor lower than the threshold for long crack propagation, strongly compromising the safety of the mechanical component. Propagation of such cracks is reported to be preceded by the formation of a fine granular area (FGA) structure. Due to the high demanding characterization and testing experimental circumstances, the crystallography of FGA is far from understood. Recent advances have come hand by hand from an experimental procedure strategy which triggers the FGA formation at predicted crack initiation sites. The novel EBSD-dictionary indexing has proven to be a powerful tool from which FGA formation acquires new value and opens the door for a systematic microstructural characterization at different crack length values.