100kg/mm²級ステンレス鋳鋼の疲労強度

抄録

The influences of cast skin, inner defects such as shrinkage, blowholes and mechanical notches on zero-tension fatigue strength of a 100kg/mm² 13 Cr stainless cast steel were investigated.
Three grades of cast steels, A, B and C, were produced by three kinds of casting procedures, varying the size and distribution of inner defects. The highest grade in the cast steels is C, followed by B and A.
Five types of specimens employed were unnotched specimens with or without cast skin (machined and no machined), circular notched specimens with or without cast skin, and Weibull notched specimen without cast skin. All five types of specimens made of A, three types of specimens without cast skin made of B, and an unnotched specimen without cast skin made of C were tested.
The results are summarized as follows.
(1) The fatigue limits of unnotched specimens were 21, 31 and 39kg/mm² for A, B and C, respectively. These values corresponded well to the respective elongation values. The ratio of the fatigue limit of C to a 13 Cr stainless steel, HUS 80, was 0.72.
(2) It was shown that the effects of cast skin, inner defects and mechanical notches on the fatigue strength can be illustrated by a schematic diagram in which the fatigue strength reduction factor is considered to be closely related to the highly stressed volume or suface area in a specimen.
(3) It was indicated that fatigue cracks started at inner defects in the case of unnotched specimens made of A and B. Thus, the estimation of their fatigue life was made using fracture mechanics, in which it was assumed that (a) the initial crack size equals the maximum inner defect size on the fracture surface, (b) the fatigue life is determined only from the crack propagation life. It was found that fracture mechanics is a useful tool to predict the fatigue life in such a cast steel.