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 with respect to the lamellar interface 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 monotonically 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 load reduction with respect to the displacement 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.

Preferable Resistance against Hydrogen Embrittlement of Pearlitic Steel Deformed by Caliber Rolling

“…10(b). This seems to be consistent with the finding by Tomatsu et al 22) with their microbending test to clarify the isotropic property of the directional alignment of elongated pearlitic steel. These microstructural features prevent the cracking path from going along the radius direction so that the rolled pearlitic steel shows the preferential HE susceptibility.…”

“…Lately, the study with the small-scale test in the electron microscope clarified that the HE resistance of the cold-drawn pearlite shows anisotropic behavior of fracture, supporting the importance of lamellar structure. 22) Other paper discussed the characteristic lattice defects is the dominant factor for the HE resistance of the cold-drawn pearlite. 23) However, still the detailed mechanism for the superior HE resistance has not been clarified adequately.…”

Fatigue Crack Propagation in Pearlitic Steel under Pressurized Gaseous Hydrogen: Influences of Microstructure Size and Strength Level

“…The highly fracture-resistant nature of axially aligned pearlite with H-absorption was also reported recently by Ueji et al, even in the macroscale specimen of a caliber rolled C-Si-Mn-Cr steel 55) . Even though our two-dimensional observation was not capable of assessing the fracture behavior of the other case where the lamellar is stacked parallel to the CT specimen thickness, the previous study notably clarified a comparable fracture resistance of such parts to LT 33) because the tearing of cementite into its longitudinal axis is similarly required for the crack to propagate forward.…”

“…In order to figure out the impact of microstructural anisotropy of pearlite on the resistance to Hassisted crack propagation, Tomatsu et al carried out micro-scale bending tests of a cathodically-charged notched cantilever under various combinations of crack growth direction and lamellar orientations 33) . They identified superior fracture resistance when the stacking direction of the lamellar and the crack growth direction was mutually parallel (i.e., LT in the present expression), besides the resistance became the lowest accompanying ferrite/cementite interface failure when the crack plane laid parallel to the lamellar plane (i.e., LD).…”

“…Notwithstanding, despite its appreciable strength level equivalent to martensite, pearlite exerts a much superior resistance to hydrogen-assisted fracture under static tensile and constant stress loading conditions 31,32) . Such a lower susceptibility to hydrogen has been attributed to the absence of prior austenite grain boundaries (PAGBs), which act as a preferential propagation pathway of hydrogen-induced cracks in martensite, as well as to the barrier effect of hard cementite (Fe 3 C) lamellae oriented perpendicularly to the crack propagation direction 31,33) . Also, in the FCG tests performed under a 90 MPa hydrogen gas environment, the authors recently reported that a lower FCG acceleration rate could be obtained in pearlitic steel with 1080 MPa tensile strength compared with martensite having an identical strength level and even with ferrite 19) .…”