Effect of Hydrogen Environment during Loading on Delayed Fracture of Ultrahigh Strength Maraging Steel

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The delayed fracture of an ultrahigh strength maraging steel in hydrogen gas was studied with notch tensile specimens at room temperature. It was shown that the delayed fracture behaviors in hydrogen gas were affected significantly by the environmental conditions during loading. When the specimens were loaded in hydrogen gas, the delayed fracture strength was extremely low. When they were loaded under vacuum and then exposed to hydrogen gas environment, the delayed fracture strength was considerably higher than that of the specimens loaded in hydrogen gas. This difference in the degree of embrittlement between them was able to be explained by the strong dependence of the hydrogen absorption at the fresh surface, that was produced by plastic deformation, on the environmental conditions during loading, that is, the delayed fracture was probably controlled by the hydrogen absorption. The delayed fracture strength evaluated by loading in hydrogen gas was approximately equal to the notch tensile strength obtained by the slow strain rate technique in hydrogen gas at pressures of both 6.67 kPa and 66.7 kPa, and both strength values in 6.67 kPa hydrogen gas were also equal to those in 66.7 kPa hydrogen gas. These strength values were estimated to be the lower critical stress below which the fracture of the notch tensile specimens did not occur in the hydrogen gas environment used in this study. This lower critical stress was discussed in view of the start of the plastic deformation at the notch root of the specimens.

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