Comparative Assessment of High‐Cycle Fatigue and Failure Mechanisms in 9Cr‐1Mo Steel due to Hydrogen at Ambient and Elevated Temperature

ABSTRACT This study examines the high‐cycle fatigue behavior of 9Cr‐1Mo steel in as‐received (AR) and hydrogen pre‐charged (HC) conditions at 25°C and 530°C under a stress ratio of 0. HC specimens exhibited accelerated fatigue degradation, as indicated by a lower fatigue‐strength coefficient and reduced microhardness at both temperatures. SEM analysis of AR specimens tested at 25°C showed typical fatigue features, while HC specimens exhibited multiple hydrogen‐induced crack initiation and failure assisted by the co‐occurrence of hydrogen‐enhanced localized plasticity and hydrogen‐enhanced decohesion mechanisms. At 530°C, the surface crack initiation failure mode transitioned to necking‐induced failure in AR specimens, and the inclusion promoted a subsurface secondary crack‐initiated fatigue source, resulting in inferior fatigue performance. However, HC specimens exhibited surface and subsurface crack initiation, with fissures driven by the high‐temperature hydrogen attack. EDS results indicate that inclusions and metal‐carbonitrides serve as stress concentration sources in the presence of hydrogen, thereby deteriorating fatigue performance.