A large 3.5 inch thick compact tension specimen (CT) was hydrogen pre-charged in an extra high capacity autoclave in order to introduce hydrogen into steel, then hydrogen cracking tests were conducted for an extended period of time in ambient air. The anticipated out gassing from the specimen was significantly less in the 3.5T-CT specimen than in the conventional 1.0T-CT specimen. The residual hydrogen after 2 weeks of exposure to ambient air was as much as 80% (≥2 ppm) of the original hydrogen introduced. The threshold stress intensity factor for the onset of cracking (= KIH) for the high toughness, recently manufactured 2.25Cr-1Mo steel was severely degraded to KIH = 42 MPa√m (46 ksi√in) under ultra slow strain rate (dK/dt = 0.005 MPa√m/sec. (0.006 ksi√in/sec)) and subcritical cracking continued over 100 hours. The crack growth rate was kept almost constant regardless of slow change of increase or decrease in K. On the other hand, the temper embrittled 60’s 2.25Cr-1Mo steel showed brittle, unstable fracture at very low stress intensity factor (KIC-H = 33 MPa√m(36 ksi√in)) with no subcritical crack occurring before the fracture event. The fracture point KIC-H turned out to be as low as 10% of the fracture toughness KIC. Finally, comparisons were made between 2.25Cr-1Mo and 2.25Cr-1Mo-1/4V steels by tests of small specimens, since the 2.25Cr-1Mo-1/4V steel substantially retains hydrogen for its lowest diffnsion coefficient. The degree of hydrogen embrittlement is higher at room temperature for both steels.

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