Method of Evaluating Hydrogen Embrittlement Susceptibility of Tempered Martensitic Steel Showing Intergranular Fracture
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TRODUCTION
QUASI-CLEAVAGE (QC) and intergranular (IG) fracture are the typical fracture modes involved in hydrogen embrittlement of high-strength steels. One important question in hydrogen embrittlement is the difference in the fracture mechanism between the two modes. QC fracture is a generic term that includes relatively flat and ragged fracture surfaces except for cleavage fracture. It has been shown that QC fracture propagates along {110} planes,[1–3] which are martensite lath boundaries or slip planes of bcc crystals. While some models[4–12] of the fracture mechanism have been proposed so far, it has been commonly accepted that plastic deformation, i.e., slip motion of dislocations dragging hydrogen, is related to QC fracture. IG fracture, on the other hand, has been considered as a brittle fracture caused by grain boundary decohesions such as temper embrittlement by phosphorus or
YU MATSUMOTO is with the Graduate School of Science and Technology, Sophia University, Tokyo 102-8554, Japan and also with the Nippon Steel & Sumitomo Metal Corporation, Chiba 293-8511, Japan. KENICHI TAKAI is with the Department of Engineering and Applied Sciences, Faculty of Science and Technology, Sophia University, Tokyo, 102-8554, Japan. Contact e-mail: [email protected] Manuscript submitted September 26, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
sulfur.[13–19] Lowering of surface energy,[14] decrease of binding energy, i.e., lattice decohesion,[15,16] and lowering of cohesive energy of grain boundaries[17–19] have been proposed as brittle models of IG fracture. IG fracture initiates at the inside of a notch tip,[20] where local stress is maximum, while QC fracture initiates at the notch tip,[21] where local strain is maximum. This indicates that hydrogen-related IG fracture is controlled by stress rather than by strain and has a more brittle nature than QC fracture. The authors have investigated the difference in fracture strength between tensile tests and constant load tests (CLTs) using tempered martensitic steel that showed QC fracture due to hydrogen and had tensile strength of 1450 MPa.[22] If the fracture strength in tensile tests did not depend on either the crosshead speed or the order of hydrogen charging and stress application, hydrogen embrittlement would be caused by decohesion, not by plastic strain. The fracture strength of both notched bar specimens and smooth ones did, in fact, depend on the crosshead speed. Moreover, fracture strength depended on whether hydrogen charging was conducted before or soon after stress application. If dislocations that were trapping hydrogen moved, hydrogen embrittlement was enhanced. If hydrogen, on the other hand, was trapped by dislocations that had moved and stabilized, hydrogen hardly affected fracture strength. Accordingly, QC fracture was shown to be related to plastic strain, and the difference in fracture
strengths among the testing conditions was investigated from the viewpoint of hydrogen–dislocation interactions. Since IG fracture is possibly caused by a differen
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