Abstract
The purposes of the present study are to identify the transition of fracture morphology and the mechanism of microstructurally sensitive cracking in 18%Ni maraging steel by systematically changing microstructure size and cyclic plastic zone size ω′. It has been widely known that the steel definitely consists of lath martensite, block, packet and prior austenite grain boundaries. Displacement controlled stress intensity factor range ΔK decrease tests were carried out with two kinds of stress ratios, 0.1 and 0.9, between ΔK=20 MPa·m1⁄2 and the threshold stress intensity factor range ΔKth. The lath martensite boundary cracking, the dominant fracture in the vicinity of the ΔKth value, is related to the ratio of the ω′ value to the lath martensite boundary size. Hence the three-dimensional morphology and the size of the lath martensite were identified by the three procedures: The morphology of the lath martensite boundary cracking was clarified. The lath martensite boundary lines were analyzed on cleavage cracks nearly parallel to the main crack. The distribution of the lath martensite size observed with a transmission electron microscope was evaluated. Consequently, the lath martensite boundary cracking was found to occur most frequently when the ω′ value was nearly equal to the lath martensite size.
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