Fatigue Crack Propagation in Pearlitic Steel under Pressurized Gaseous Hydrogen: Influences of Microstructure Size and Strength Level

For the wall-thickness reduction of the components destined for pressurized gaseous hydrogen, widespread use of high-strength martensitic steels has long been desired. However, their strong susceptibility to hydrogen-assisted fatigue crack growth (HA-FCG) is still limiting their proactive applications. Here, we instead focused on pearlite as another potential reinforcing agent for the development of new hydrogencompatible steels with acceptable cost performance. Fatigue crack growth (FCG) behavior of three eutectoid steels with different microstructure sizes (i.e., ferrite/cementite interlamellar spacing, colony and block sizes) and strength levels was investigated in a 90 MPa hydrogen gas, an essential evaluation when attempting to perform a defect tolerant design of the components used for high-pressure gases. The pearlitic steels clearly exhibited the acceleration of their FCG rate in hydrogen gas up to a hundred times that in air, wherein its magnitude was greater in the material with finer microstructure and concomitant higher strength. The delamination of ferrite/cementite lamellae, which were inclined largely from the loading axis, was determined to be the primary cause of such HA-FCG in pearlitic steels. Nevertheless, the extent of FCG acceleration was minor with respect to martensitic steels. The fact was ascribed to the barrier role of the cementite platelets oriented nearly perpendicularly to the crack as well as to the geometrical retardation effects arising from the crack deflection and blanching. Ultimately, pearlite was superior to martensite from the perspective of HA-FCG resistance; besides, the superiority was more substantial as the loading rate became slower.