H<SUB>2</SUB>SO<SUB>4</SUB>-NaCl溶液中における18-8鋼の応力腐食割れにおよぼす加工組織の影響*

Abstract

The stress corrosion cracking (S.C.C.) behavior of 18-8 stainless steel in a 5 N H2SO4-0.4 M NaCl solution at −325 mV (vs S.C.E.) was investigated to examine whether the strain-induced martensite would may give rise to S.C.C.As prestraining, the elongation of 27% in tension was made at various temperatures between 20 and 160°C. X-ray diffraction and transmission electron microscopic observations showed that the specimen prestrained at 40°C contained α′ and ε-martensitic phases in the austenitic matrix. The S.C.C. susceptibility of the specimens varied with the annealing temperature ranging from 20°C to 1100°C for 120 min. S.C.C. was observed for specimens treated at temperatures below 600°C where α′-martensite persisted. The number of cracks on the specimen after the S.C.C. test decreased with increasing temperature of prestraining and S.C.C. was not observed at temperatures above 140°C. α′ and ε-martensitic phases were not detected by electron diffraction analysis in the specimens prestrained above this temperature.The corrosion striation on the specimens surface, which was prestrained at a temperature below 120°C, developed during immersion in a 5 N H2SO4-0.4 M NaCl solution at −325 mV for 120 min. The same striation was found near the crack tips, when the cross section of the specimen after the S.C.C. test was polished and immersed in the solution.It is concluded that the occurrence of martensite structure in 18-8 stainless steel may enhance the S.C.C. susceptibility in H2SO4-NaCl solutions.