The nature of intergranular stress corrosioncracking (SCC) of alloy X-750 was characterized in low-andhigh-temperature water by testing as-notched and precrackedfracture mechanics specimens. Materials given the AH, BH, andHTH heat treatments were studied. While all heat treatmentswere susceptible to rapid low-temperature crack propagation(LTCP) below 150 0C, conditions AH and BH were particularlysusceptible. Low-temperature tests under various loadingconditions (e.g., constant displacement, constant load, andincreasing load) revealed that the maximum stress intensityfactors (Kpmax) from conventional rising load tests provideconservative estimates of the critical loading conditions inhighly susceptible heats, regardless of the load path history. Forresistant heats, Kpmax provides a reasonable, but notnecessarily conservative, estimate of the critical stress intensityfactor for LTCP. Testing of as-notched specimens showed thatLTCP will not initiate at a smooth surface or notch, but willreadily occur if a cracklike defect is present. Comparison of thecracking response in water with that for hydrogen-prechargedspecimens tested in air demonstrated that LTCP is associatedwith hydrogen embrittlement of grain boundaries. Equivalentactivation energies for stage II LTCP rates (11.3 kcal/mol) andhydrogen diffusion (11.5 kcal/mol) indicate that hydrogendiffusion to the peak stress region ahead of a crack is the rate-controlling process. Auger analysis showed that variability inLTCP resistance is associated with phosphorus and sulfursegregation to grain boundaries. Above 150 0C, an increase infracture resistance and decrease in the degree of hydrogenenrichment precludes rapid intergranular cracking. The stresscorrosion crack initiation and growth does occur in high-temperature water (>250 0C), but crack growth rates are ordersof magnitude lower than LTCP rates. The SCC resistance ofHTH heats is far superior to that of AH heats as crack initiationtimes are two to three orders of magnitude greater and growthrates are one to two orders of magnitude lower.
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