ROLE OF CARBIDES IN STRESS-CORROSION CRACKING RESISTANCE OF ALLOY-600AND CONTROLLED-PURITY NI-16-PERCENT CR-9-PERCENT FE IN PRIMARY WATER AT 360-DEGREES-C

Intergranular stress corrosion cracking (IGSCC) of two commercial allo y 600 (UNS N06600) conditions (heat-treated at low temperature [600LT] and at high temperature [600HT]) and two controlled-purity Ni-16% Cr- 9% Fe alloys (carbon-doped mill-annealed [CDMA] and carbon-doped therm ally treated [CDTT]) were investigated using constant extension rate t ensile (CERT) tests in primary water (0.001 M lithium hydroxide [LiOH] + 0.01 M boric acid [H3BO3]) with 1 bar (100 kPa) hydrogen overpressu re at 360 degrees C and 320 degrees C. Heat treatments produced two ty pes of microstructures in the commercial and controlled-purity alloys: one dominated by grain-boundary carbides (600HT and CDTT) and one dom inated by intragranular carbides (600LT and CDMA). CERT tests were con ducted over a range of strain rates and at two temperatures with inter ruptions at specific strains to determine the crack depth distribution s. Results showed IGSCC was the dominant failure mode in all samples. For the commercial alloy and controlled-purity alloys, the microstruct ure with grain-boundary carbides showed delayed crack initiation and s hallower crack depths than did the intragranular carbide microstructur e under all experimental conditions. Data indicated a grain-boundary c arbide microstructure is more resistant to IGSCC than an intragranular carbide microstructure. Observations supported the film rupture/slip dissolution mechanism and enhanced localized plasticity. The advantage of these results over previous studies was that the different carbide distributions were obtained in the same commercial alloy using differ ent heat treatments and, in the other case, in nearly identical contro lled-purity alloys. Observations of the effects of carbide distributio n on IGSCC could be attributed more confidently to the carbide distrib ution alone rather than other potentially significant differences in m icrostructure or composition. Crack growth rates (CGR) increased with increasing strain rate according to a power law relation with a strain rate exponent between 0.4 and 0.64. However, CGR measured in m/unit s train decreased with increasing strain rate, indicating an effect of e nvironment or creep. Temperature dependence of CGR was consistent with the literature.