The fatigue crack growth rate was studied for high strength steel of different prior austenite grain sizes, from 34.2 to 238μm, produced by changing austenitizing temperature. The effect of tempering temperature and stress ratio on the growth rate was also examined.
It was found that the variation in prior austenite grain size and the changes of tempering temperature had little effect on the fatigue crack growth rate at the mid-range of ΔK. At low (ΔK<30kgmm-3/2) and high (ΔK>80kgmm-3/2) ΔK regions of the materials tempered at 200°C, however, the crack growth rate was increased as the prior austenite grain size became fine. The threshold stress intensity factor, ΔKth, decreased with reducing grain size, and with increasing yield strength and stress ratio.
Intergranular facets were formed in the both low and high ΔK regions regardless of tempering temperature. In this case, the area percentage of intergranular facets produced at the low ΔK region was correlated with ΔK, whereas the percentage at the high ΔK region was dependent on Kmax Morphology of intergranular facets seemed to be divided into four types which correspond to ΔK level. At the low ΔK region, a trace of twin deformation or smooth feature was found on the facets, while dimples and tongues were formed at the high ΔK region.