EFFECT OF MICROSTRUCTURE ON THE CHARACTERISTICS OF SMALL FATIGUE CRACK-GROWTH IN DUAL-PHASE STEELS

The influences of martensite morphology on the characteristics of small fatigue crack growth were investigated under rotating bending using two kinds of dual-phase steels: Steel A, which has a microstructure containing isolated martensite phases in a ferrite phase, and Steel B, which has a continuous martensite phase filled with ferrite phases. Most of the fatigue life in plain specimens is occupied by the growth of small cracks less, than 1 mm in the two steels, and in the region corresponding to about 50% of the total life, small-crack growth is affected strongly by microstructures such as the ferrite grain boundary and martensite phase. Even in the region where the growth behaviors are affected by microstructures and the fluctuation of growth rate is marked, the average crack-growth rate can be considered to be proportional to the crack length. That is, the resistance to the crack growth is approximately evaluated through the static strength. On the other hand, the fatigue limits in the steels are determined by each limit for crack growth which is affected by the microstructure. Consequently, there is a large difference between fatigue ratios (sigma(W)/sigma(B)) of the two steels (0.39 in Steel A, 0.49 in Steel B).