Intergranular Fatigue Crack Initiation and Its Associated Small Fatigue Crack Propagation in Water-quenched Fe–C Fully Ferritic Steel

Intrinsic Factors that Trigger the Coaxing Effect in Binary Fe–C Ferritic Alloys with a Focus on Strain Aging

“…For instance, when the stress amplitude was increased by 5 MPa every 10 7 cycles, from 210 MPa, in a Fe-C ferritic alloy, the specimen did not show failure until 250 MPa. 7,8) In fact, the fatigue life without the coaxing effect was shorter than 2 × 10 6 cycles even at 240 MPa. 7,8) Utilization of the coaxing effect is expected to improve the robustness of the fatigue limit, particularly when the fatigue limit is controlled by the threshold stress for crack propagation.…”

“…The fatigue limit of the 17CWQ steel was controlled by two factors: 1) the transgranular fatigue crack non-propagation phenomenon and 2) fatigue crack initiation along grain boundaries and subsequent coalescence with the main crack. 7,8) Accordingly, the coaxing effect in the 17CWQ steel enhanced its resistance to transgranular fatigue crack growth and to intergranular fatigue crack initiation. In contrast, distinct fatigue crack non-propagation was not observed during the coaxing effect test in the 06CWQ steel, probably because of the frequent intergranular fatigue crack initiation and subsequent coalescence.…”

“…[4][5][6] In addition, suppression of fatigue crack initiation is crucial even for the crack propagation phenomenon, since fatigue cracks can propagate through the coalescence of cracks when subcracks form in the vicinity of the main crack. 7,8) Once the fatigue crack ceases to propagate without new crack initiation, the specimen does not show failure in an ideal constant loading condition. Interestingly, a specimen fatigued at the fatigue limit does not show failure even after a slight increase in the stress amplitude.…”

Fatigue Behavior in an Fe–N Binary Ferritic Steel: Similarity and Difference between Carbon and Nitrogen

“…Commonly, the term "small crack" in fatigue phenomena is understood from different viewpoints, such as microstructurally small cracks, 5) mechanically small cracks, 6) physically small cracks, 7) and chemically small cracks. 8) According to the previous studies, 2,3) the kinetic effect of solute atoms effectively contributes to the regimes of fatigue crack initiation and mechanically or microstructurally small crack growth. In this study, we therefore focused on the fatigue damage evolution from crack initiation to mechanically or microstructurally small crack growth that covers 70% of fatigue life in steels.…”

“…1) In contrast, interstitial carbon enhanced transgranular fatigue crack resistance and improved the fatigue limit of steels. 2,3) The dramatic effects of interstitial atoms in steels are believed to be a key for enabling safe mechanical design and the development of next-generation fatigue-resistant materials.…”