Effect of specimen size on hydrogen embrittlement cracking of 2.25Cr-1Mo heavy section steel for pressure vessels

By means of charging large thick specimens with hydrogen, we investigated the effects of specimen size on hydrogen-embrittlement cracking. Crack extension in a hydrogen-charged 3.5T-CT specimen extended over a longer duration than was the case for a 1.0T-CT specimen. However, the values of the lower-bound threshold stress intensity factor (K _IH) for 1.0T-CT and 3.5TCT specimens were similar to one another when these were determined with a short-term rising load (dK/dt= 0.005 MPa·m ^1/2;· s ^-1). We conducted numerical analysis on the hydrogen diffusion and accumulation around a crack tip, taking into consideration the hydrogen distribution in the specimen. This analysis demonstrated that the maximum hydrogen concentration for cracking can be reached under the conditions present during a short-term rising load test (dK/dt= 0.005 MPa· m ^1/2/s). Thus, the results of the numerical analysis confirm that a minimum value of K _IH equivalent to that of a heavy section steel can be obtained with a small fracture mechanics specimen. We also attempted to explain long-term crack extension characteristics, taking into consideration hydrogen dissipation from a specimen. The analysis predicts that when the mean hydrogen concentration falls below a certain level (e.g., about 1.6 ppm under certain assumptions), the value of K _IH increases significantly. This increase in K _IH occurs because when the necessary stress intensity factor for cracking increases as a result of a decrease in the mean hydrogen concentration, the gradient of the maximum hydrostatic stress distribution becomes moderate, especially when the applied stress intensity factor is more than about 48 MPa·m _1/2. Finally, we propose a method for the prediction of the long-term crack extension behavior of a large thick specimen; the method takes into consideration the hydrogen dissipation curve and the effect on K _IH of a decrease in the mean hydrogen concentration.