析出硬化型ステンレス鋼製フックの破断事故に関する調査

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Fracture of a hook of a launching strop during lifting a launch on a patrol vessel is investigated in detail. The hook was made of a precipitation hardening stainless steel (JIS SUS630) and therefore this accident may be one of the typical problems accompanying the use of high-performance materials. There was no apparent deformation in the hook and the fracture surface exhibited features commonly observed in brittle fracture, such as chevron patterns in macroscopic observations and quasi-cleavage fracture in scanning electron microscopy. Hardness measurement revealed that the material was harder than expected for the prescribed heat treatment conditions. Charpy impact energy was less than one-half of the value in the literature. Elemental distribution maps obtained by electron-probe microanalysis showed segregations of the major additives including chromium, nickel and copper. The distribution patterns suggest that the segregations were likely formed during the die forging. These segregations may have formed low-toughness phases in the matrix. It is concluded that non-uniform microstructure with substantial segregation is responsible for the brittleness, or low toughness, probably caused by inadequate forging conditions such as low temperature and high pressing speed. Finally, more careful temperature control during the manufacturing, particularly during the forging, is proposed for prevention of this type of fracture.
独立行政法人海上技術安全研究所の論文
1998-03-11

析出硬化型ステンレス鋼製フックの破断事故に関する調査

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