Image analysis of microscopic crack patterns applied to thermal fatigue heat-checking of high temperature tool steels

► Heat-checking of hot forming tool steels is investigated under thermal fatigue. ► The heat-checking formation and evolution is characterized by SEM image analysis. ► The crack density follows a sigmoidal evolution, achieving an asymptotic value. ► The saturated crack pattern is quasi-isotropic, with mostly trivalent junctions. ► Geometrical and topological parameters stabilize after a critical number of cycles.