Susceptibility of micro-alloyed steel to hydrogen-assisted cracking under short- and long-term gas pre-charging

Hydrogen-assisted cracking (HAC) of materials with elastic–plastic behavior and small-scale yielding at the crack tip can be evaluated in gaseous hydrogen using the J-integral method and compact tension (CT) specimens. However, the duration of pre-charging the CT specimens in hydrogen gas can affect the results. Therefore, this paper investigates the HAC susceptibility of P460QL micro-alloyed steel at room temperature in gaseous hydrogen at the pressure of 200 bar by considering two different conditions: (i) short-term pre-charging for 20 min and (ii) long-term pre-charging for 72 h. Moreover, the hydrogen content of flat specimens exposed to different charging durations was measured using thermal desorption analysis (TDA), showing that after charging for 72 h the steel was almost saturated with hydrogen. Because of plastic deformation (blunting) at the crack tip, the cracks did not grow in CT specimens tested in air even at high J-integral values of 700 kJ/m 2 . In contrast, CT specimens tested in hydrogen after short-term as well as long-term hydrogen pre-charging showed considerable HAC, as the average critical J-integral value was only about 115 kJ/m 2 . Hence, the investigated micro-alloyed steel was basically susceptible to HAC, although the equivalent stress-intensity factor in gaseous hydrogen was about 163 MPam 0.5 . Increasing the pre-charging duration from 20 min to 72 h influenced the stable crack extension, however, it did not have any significant effect on the measured J-integral values. The findings of this study demonstrate that long-term pre-charging in gaseous hydrogen prior to J-integral testing may not be necessary for such steels. Furthermore, measuring only the bulk hydrogen content does not provide a clear indication of the risk of HAC.