Underlying interstitial carbon concentration dependence of transgranular fatigue crack resistance in Fe-C ferritic steels: The kinetic effect viewpoint

Tension tests at different strain rates for five water-quenched Fe-C fully ferritic steels with different interstitial carbon contents were conducted at room temperature (RT), which verified occurrence of dynamic strain aging (DSA). The threshold stress intensity factor ranges (ΔKth) of small crack for these materials were determined by using rotating bending fatigue tests on specimens micro-notched by a focused ion beam technique. The experimentally determined ΔKth increased, first steeply and then moderately, with increasing carbon concentration. The experimental ΔKth of a Fe-0.017C (wt.%) steel with maximum carbon content was approximately 40.5% higher than conventionally predicted results by Murakami's equation. The outstanding resistance to fatigue crack propagation probably stems from enhanced DSA, cohesive strength of grain boundary (GB) and material strength in the vicinity of GB with the growth of carbon content.