NDE Observations of Hydrogen Effects in 4340 Steel

Abstract

AbstractCathodic charging of hydrogen into 4340 steel produces drastic deterioration of mechanical properties such as the notch tensile strength, depending on current density, charging time, and the poison used as an inhibitor of hydrogen recombination and bubble formation at the specimen surface. Fusion of charged samples permitted hydrogen content to be measured and revealed a linear increase in the latter with charging current density. The change in the Doppler spectrum sharpness parameter Δ(P/W) deviates negatively with cathodic charging time but inversely with the charging current density (fugacity). Thus a low-fugacity hydrogen charging appears merely to screen existing positron trap sites, whereas a high-fugacity charge also creates new defect traps. This would result in a lesser net screeninig effect which would be seen as a smaller negative deviation in Δ(P/W) as observed. For cumulative charging, however, one finds oscillatory behavior in (P/W) as in earlier cumulative charging experiments with Ni. This behavior indicates that hydrogen screening of positron traps was followed by new positron trap site creation and that this sequence of events occurred repeatedly and in that order.