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

The fatigue crack initiation and propagation behavior of a water-quenched binary Fe-C fully ferritic steel was investigated though rotating-bending fatigue testing. Intergranular and transgranular crack initiation and propagation were observed. The intergranular crack propagation did not stop, while the transgranular crack propagation was retarded by crack closure and strain aging. As a result, intergranular cracking was the dominant cause of fatigue damage in the steel. A considerable number of cracks were initiated; these propagated through coalescence, which occurred mainly at the grain boundaries. Dominancy of the intergranular fatigue crack propagation increased with increasing stress amplitude. In addition, the steel showed coaxing effect significantly. The coaxing effect suppresses crack initiation as well as crack propagation.