Anisotropy in Hydrogen Embrittlement Resistance of Drawn Pearlitic Steel Wire Evaluated by <i>in-situ</i> Miniature Test and Scanning Electron Microscopy during Plasma Hydrogen Charging

In drawn pearlitic steel wire, hydrogen embrittlement cracks are preferentially formed along cementite/ferrite interfaces perpendicular to the tensile stress, predicting an anisotropy in the hydrogen embrittlement resistance of the wire. To validate this prediction, notched miniature tensile test specimens were extracted from the wire with their orientation changed, and in-situ miniature tensile tests were conducted during plasma hydrogen charging in a scanning electron microscope. The fracture process was also investigated from secondary electron images simultaneously obtained during these in-situ miniature tensile tests. As a result, it was revealed that, in accordance with the prediction, the fracture stress is decreased by the plasma hydrogen charging for the specimens with the tensile direction perpendicular to the drawing direction of the wire whereas it is hardly changed for the specimen with the tensile direction parallel to the drawing direction. In the secondary electron images, a stationary subcrack was observed before the fracture only for the specimen with the tensile direction parallel to the drawing direction. Based on finite element method analysis results, the subcrack was found to be formed by the triaxial stress due to the notch. The difference in the presence and absence of the subcrack depending on the specimen orientation was also well explained from the preferential hydrogen embrittlement crack formation along the cementite/ferrite interfaces perpendicular to the tensile stress.