ABSTRACT

Environmentally assisted cracking of high strength nickel based alloys under was investigated in 3.5wt% NaCI under cathodic polarization. Rising displacement tests were performed on C22HS and MP98T were performed at a slow K-rate of 0.005Nmm-3/2/s. The results suggest that there is no significant environmental effect for MP98T, however C22HS exhibited a decrease in the initiation toughness. Tests performed under K control under a range of conditions on 718 and K-500. In general, the crack growth rate (CGR) measured under constant K were lower than those obtained under rising displacement tests under similar conditions. The CGR in both 718 decreased with decreasing potential. The CGR was also observed to depend strongly on the loading mode. There was a strong effect of increasing K, with increasing K tests resulting in substantially higher CGR’s than at constant under similar conditions. The above results suggest that the rate limiting step for crack propagation may be the generation of hydrogen at the crack tip. A crack tip strain rate based model was applied to rationalize the data though more work is needed to establish the various parameters associated with the model.

INTRODUCTION

High strength nickel based alloys like Monel K-500, Inconel 718, Inconel 625+/725 have been known to be susceptible to hydrogen embrittlement/1-3/. Slow strain rate tests revealed susceptibility to intergranular cracking in K-500 in seawater over a range of applied cathodic potentials from -850mV to - 1000mV SCE/4/. Field failures of K-500 were attributed to hydrogen embrittlement in seawater under cathodic charging/5/. It was suggested that in these cases that high hardness associated with cold work from rolling threads on fasteners may have been the primary cause of enhanced susceptibility to hydrogen embrittlement/5/. The presence of carbon films along the grain boundaries was also attributed to increased susceptibility, though the exact mechanism that is operative was not clear/6/.

Constant load and double-cantilever-beam (DCB) testing performed on UNS N07718 and UNS N07725 demonstrated that under cathodic polarization of -1 V to -1.1 V SCE (in synthetic seawater) these materials are resistant to hydrogen embrittlement/3/. However, the observation of failures in the field suggest that there may be other factors responsible for cracking which were not simulated in the published work. It is possible that in constant load tests the absence of a pre-crack coupled with dynamic straining may be a reason for not observing cracking.

Slow rising displacement tests (K-rates of ~0.3 to 0.5MPavm/h) have been employed recently to evaluate susceptibility of nickel based alloys to hydrogen embrittlement in seawater under cathodic polarization/711/. The results indicate that materials such as K-500, and Inconel 718 exhibit susceptibility under these conditions, with Kth values significantly lower than measured in dead loaded/rising step loaded tests/7,9/. The slow rising displacement tests have been valuable in trying to discern susceptibility as measured via Kth. However, it is interesting to note that in a number of systems evaluated using the slow rising displacement method/7-12/, the measured steady plateau CGR even at low K-rates is on the order of 10” 6 to 10-4mm/s/8-12/. More recent work that has demonstrated that the CGR’s of Inconel 718 are sensitive to K-rates/7/ and when measurements were performed at K-rates significantly lower than 0.5MPaVm/6/. Similar data has been observed for K-500 when comparing the CGR data under rising displacement versus constant load conditions/7/. In this context, it is important to note the measured CGR’s at K-rates of 0.5MPaVm/h may not be meaningful for developing damage tolerant designs (CGR of 10-6mm/s would result in a crack extension of ~1inch/year). Under these conditions it is clear that hydrogen embrittlement is present, it is important to note that any predictive model needs to address the observed sensitivity to K-rate. Recent efforts to model the measured CGR have essentially focused on the relating the measured CGR to the effective diffusivity of H, and the concentration of hydrogen, without accounting for the observed strain rate dependencies in any manner/10-12/.

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