Abstract
Ferallium 255 duplex stainless steel was cathodically precharged with hydrogen at 265 °C in a molten salt electrolyte. Sustained load tests were carried out in air at 0 °C, 25 °C and 50 °C with average hydrogen contents from 3 to 15 wt ppm. The DC potential drop method was calibrated with optical measurements to continuously monitor the crack position and allow calculation of crack velocity and stress intensity. The crack velocityvs stress intensity (K) curves generally rose gradually over a large range inK and had definite thresholds for subcritical crack growth. Second and third stages were not always clearly delineated. Threshold stress intensities decreased as hydrogen content increased. An identifiable stage II occurred most often for alloys containing about 10 wt ppm dissolved hydrogen. The crack growth velocities generally increased with increasing temperature or hydrogen content. As the dissolved hydrogen increased, the fracture mode changed from microvoid coalescence (MVC) to microcrack coalescence (MCC) with some tearing ridges. At high hydrogen content, both ferrite and austenite phases showed brittle morphology, which was identical to the fracture surface of the uncharged specimens tested in hydrogen gas at 108 kPa pressure. Comparing the embrittling effect of internal hydrogen with that of external hydrogen it is found that the threshold stress intensity in hydrogen gas at 1 atm is lower than that at the highest internal hydrogen concentration (15 wt ppm). In the case of external hydrogen, the hydrogen source is at the crack tip, whereas for the internal hydrogen case, outgassing reduces the hydrogen content in this region, even when the bulk hydrogen content is fairly high.
Published Version
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