ＳＵＳ３１６（ＬＣ）鋼のＳＣＣ感受性に及ぼす金属組織の影響

Abstract

Susceptibility to stress-corrosion cracking (SCC) of type 316 (LC) stainless steel was investigated and discussed in this study in terms of microstructure. Five kinds of material with different microstructure such as as-cold-rolled, recovered, with grain size differing from 15 to 150 μm, were prepared by the combination of cold rolling and heat treatment. Seven specimens were examined for each material. The specimen was a flat plate with dimensions of 50 mm long, 10 mm wide and 2 mm thick. The creviced bent beam (CBB) test was performed to evaluate the susceptibility to SCC. The CBB test is a bend test with an artificial crevice under steady strain of approximately 1%, and one of the accelerated tests to evaluate the SCC susceptibility of a material. The specimens were fixed on CBB fixtures and set in an autoclave. The test was then performed for 3.6 Ms at 561 K in high-purity water containing dissolved oxygen of approximately 8 ppm under 8 MPa pressure. The specimens after the test were sectioned on the centerline. They were mounted, grinded and then polished. The polished cross-section was examined by an optical microscope. Crack depth was measured and the data were classified into a frequency distribution histogram with 50 μm width. The recovered specimens that were cold rolled and then heat-treated at 673 K for 3.6 ks, showed apparently high susceptibility to SCC. On the other hand, though the macro structure of as-cold-rolled specimens and the recovered specimens were very similar, a significant difference was observed in the susceptibility to SCC. Observed cracks initiated from the specimen surfaces, transverse in grains, and then propagated along grain boundaries. No obvious relationship between the susceptibility and the grain sizes was observed.