Cracking Behavior In Inconel Alloy 690: Role Of The Environment In High Temperature H2-Supersaturated Steam

Nickel base alloys 600 and 690 have found widespread applications in nuclear reactor steam generators. In particular, alloy 690 with a major element composition of typically Ni-30Cr-9Fe, has shown an apparent improvement in performance when compared with alloy 600 in both primary water and caustic environments. Nevertheless, alloy 690 can also be susceptible of undergoing hydrogen induced stress corrosion cracking (SCC). However, the role of the microstructural state is not well defined, such as the effect of the presence or absence of carbide precipitation along grain boundaries on the overall susceptibility. In this work, self-loaded linear elastic fracture mechanics M-WOL specimens made of alloy 690 were tested in both, the solution annealed condition (minimal grain boundary carbide precipitation), and after a grain boundary carbide precipitation treatment. Tests were carried out at 320-400 degrees C in an instrumented autoclave used to 'in-situ'monitor crack growth rates. The environment was simulated by the generation of high-pressure supersaturated hydrogen steam. The tested M-WOL specimens were loaded under various applied KI values (between 50-80 MPa m1/2). This work discusses the role of the artificially imposed environment, as well as the kinetic and microstructural factors, which lead to crack propagation in alloy 690