Brittle Fracture of Precipitation Hardening Stainless Steel

Fracture of a hook of a launching strop used for lifting a launch on a patrol vessel is investigated in detail. The hook was made of a precipitation hardening stainless steel (JIS SUS630) . No apparent deformation was observed in the hook and the fracture surface exhibited features of 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 from 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 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.