３０４ステンレス鋼の応力腐食割れ破断寿命の確率分布に及ぼすＣａＣｌ２濃度の影響

Abstract

In order to compare the failure time distribution of a laboratory accelerated test with the field failure time distribution of stress corrosion cracking of Type 304 stainless steel, the effect of CaCl2 concentration on the probability distribution of the failure time Tf, crack initiation time Ti and crack propagation time Tp for smooth plate specimens with or without crevice under a constant condition of 373K and 200MPa has been analyzed. Two parameter Weibull distribution fitted approximately to all the distributions obtained. The apparent shape parameter mi' for Ti of both type of the specimens with or without crevice was found unity, indicating that a chance failure mode was operating. On the other hand, mf' and mp' for Tf and Tp show a maximum value of 4 or 6 at an intermediate concentration range of 30 or 40mass% CaCl2, corresponding to a wearout failure mode, and decrease to unity with a decrease of CaCl2 concentration. This fact means that SCC fracture and propagation at a low concentration range take place by a change failure mode. Since the same chance failure mode had been reported for the field failure time distributions of Type 304 and 316 stainless steel heat exchanger tubes, an approximate acceleration factor of the laboratory test conducted under the condition of 25 mass% CaCl2, 373K, 200MPa with crevice was estimated to be ca. 104 by comparing the mean failure time of both distributions of the field and the accelerated test.