Anisotropy in Hydrogen Embrittlement Resistance of Drawn Pearlitic Steel Investigated by <i>in-situ</i> Microbending Test during Cathodic Hydrogen Charging

Abstract

To investigate causes of superior hydrogen embrittlement resistance of drawn pearlitic steel, notched microcantilevers with different notch orientations were fabricated by focused ion beam, and microbending tests were conducted in air and during cathodic hydrogen charging by electrochemical nanoindentation. In air, indentation load increased with increase in indentation displacement, and no crack appeared for any notch orientations. During hydrogen charging, indentation load declined, and a crack appeared. The degree in the load reduction was larger, and the crack was deeper for the notch parallel to the lamellar interface than that normal to the lamellar interface. Furthermore, stationary cracks in the microcantilevers were observed by scanning electron microscopy and scanning transmission electron microscopy. For the notch parallel to the lamellar interface, a sharp long crack was identified along the lamellar interface. The crack stopped at the position where the cementite lamellae are disconnected. In lattice images, cementite was identified in one side of the crack, and ferrite in another side of the same crack. On the other hand, for the notch normal to the lamellar interface, a blunt short crack was identified. Thus, it was concluded that the ferrite-cementite interface is a preferential crack path, and hydrogen embrittlement resistance in the direction parallel to the lamellar interface is superior to that normal to the lamellar interface. The present results also indicate that directional lamellar alignment of the drawn pearlitic steel suppresses crack propagation in the radial direction of the drawn wire, improving the hydrogen embrittlement resistance in the drawing direction.

Hydrogen Embrittlement Susceptibility of Linear Friction Welded Medium Carbon Steel Joints

“…Focusing on the subcrack tip, crack-tip blunting was observed perpendicular to the cementite lamella. Tomatsu et al reported that the ferrite/cementite interface was the preferential crack propagation path due to hydrogen embrittlement in drawn pearlitic steel (carbon content: 0.94 %C) 21) . Regarding the S45C pearlite in this study, the crackpropagation path was along the lamellar interface of the pearlite, and the cementite located in a direction different from the crack-propagation direction might have hindered crack propagation.…”