Abstract
In the previous paper, the authors reported about the peculiar phenomena with regard to an 18 Ni maraging steel with low temperature aged structure, that KIC was strongly dependent upon the prior γ grain size and the delayed failure crack was observed in front of the fatigue precrack. This study has been made in order to investigate these phenomena in more details and to discuss the mechanism by which KIC is increased.
It is made clear that an increment in KIC is completely associated with slow crack growth of delayed failure which precedes the unstable fracture. However, its increment is not an increment in true KIC as a material constant but an increment in apparent KIC.
This may be due to the blunting at crack front caused by the microbranching of the delayed failure crack, the degree of which also depends upon the prior γ grain size.
Tensile ductility also markedly decreases under the same condition in which an apparent KIC increases. This is attributed to the occurence of the delayed failure crack at the specimen surface adjacent to or in the necking region. Thus, the 18Ni margaing steel with low temperature aged structure shows the complicated behavior on toughness and ductility because of the high sensitivity to delayed failure.
