Fatigue Cracking Evolution in H13 Cr‐Mo Steel Under UNSM Treatment: Comparative Analysis of Surface MSFCs and Internal Fisheye Cracks

ABSTRACT This study investigates the fatigue crack behavior of H13 Cr‐Mo steel under both untreated and ultrasonic nanocrystal surface modification. Three distinct crack propagation mechanisms were identified: (1) Surface Inclusions: When inclusions were present on the surface, fatigue cracks initiated and propagated as single dominant cracks, resembling those observed in artificial pit specimens. (2) Absence of Surface Inclusions: In the absence of inclusions, fatigue life was governed by the initiation, growth, and coalescence of multiple small fatigue cracks (MSFCs). These cracks exhibited depth‐wise propagation, influenced by the formation of large elliptical crack tips where stress concentration occurred, accelerating internal crack growth. (3) Fracture Characteristics: The final fracture behavior of both surface and internal cracks was found to be similar, characterized by rapid crack growth near the end of fatigue life. Notably, fisheye cracks accounted for approximately 93%–95% of the total fatigue life, a result consistent with previous findings and validated through comparative analysis.