抄録
The variation of threshold stress to SCC with Ni content of 25 Cr-0-9 Ni steels has been studied by constant load SCC test in 45% MgCl2 solution boiling at 427K. The lowest value is obtained at a content of about 2% Ni and the highest one at 6-8% Ni where the α-phase in the steel attains 40-50% in volume ratio. To consider the reason of high resistance of duplex stainless steels, WOL test, constant load test and electrochemical tests have been conducted using 25 Cr-6 Ni (α+γ) steel as well as 28 Cr-4 Ni (α) and 21 Cr-9 Ni (γ) steel which are equivalent on composition respectively to α- and γ-phases of the duplex steel determined by EPMA analysis. The duplex steel has higher value of KISCC and lower rate of crack propagation than those of any steel consisted of single phase. The cracks of duplex steels have been observed to propagate across α-phase and to be retarded by γ-phase. Electrochemical coupling of the γ-steel with the γ-steel while holding at each proof load makes no significant effects on their times to failure. The high SCC threshold stress of duplex stainless steels could be attributed to a keying effect of the γ-phase which is caused by mechanical effects based on mixed (α+γ) duplex structure rather than an electrochemical one.
