Abstract
Environmentally assisted cracking of high strength PH nickel based alloys 725, 945X, and 955 were evaluated under subsea HPHT environments. The two primary environments of interest in this effort were in 3.5wt% NaCl, pH = 8.2 under cathodic polarization at low temperature 40°F and in sour service environments at elevated temperature and pressure (350°F/400°F and 125psia CO2/0.08psia H2S). Fatigue crack growth rate and static crack growth rate measurements were performed in both the environments. In sour environments, the alloys exhibited excellent resistant to environmentally assisted fatigue and stress corrosion cracking. This was evidenced by little or no frequency dependence of the fatigue crack growth rate over a range of ΔK values. The alloys did not exhibit any evidence of static crack growth rate even when tested to high values of stress intensity factor (90ksi√in).
However, tests performed under cathodic polarization revealed that the fatigue crack growth rate exhibits a strong dependence on frequency for various ΔK values. In all cases, stable static crack growth rate (CGR) was observed for all the alloys tested. A comparison of the fatigue crack growth rate behavior suggests that the 725 exhibited the highest susceptibility, while 955 and 945X appeared to exhibit similar fatigue crack growth rate behavior. A static crack growth rate of 10-6mm/s (under constant K conditions) was obtained at 50ksi√in (55MPa√m) on 725, while it required K values of about 80ksi√in (88MPa√m) for 945X and 955. The effect of applied potential on the static crack growth rate behavior of 945X was also explored.
