A study of fatigue crack propagation in prior hydrogen attacked pressure vessel steels

A study has been made of the effects of prior hydrogen attack damage on fatigue crack propagation behavior in commercial pressure vessel steels. Quenched and tempered Mn-Mo-Ni steel (ASTM A533B Class 2) and normalized and tempered 2.25Cr-1Mo steel (ASTM A387 Class 2 Grade 22) were exposed to gaseous hydrogen atmospheres for up to 1480 hours at hydrogen pressures of 12.4 to 17.2 MPa and temperatures of 550° to 600°C and tested in fatigue. Mild degrees of hydrogen damage, characterized by limited methane bubble formation with no appreciable decarburization, were found to increase growth rates slightly at near-threshold stress intensities. Severe degrees of hydrogen damage, characterized by extensive intergranular bubble formation and decarburization with associated large reductions in strength and toughness, were found to have no further influence on near-threshold growth rates. The minor influence of prior hydrogen damage on fatigue crack extension, even for cases of severe attack, is attributed to result from two mutually competitive mechanisms, namely, the creation of methane-filled voids on prior austenite grain boundaries, which increases growth rates, and the enhancement in crack closure from decarburization-induced softening and rough cavitated intergranular fracture surfaces, which decreases growth rates.