Effect of hydrogen on fracture behavior of a quenched and tempered medium-carbon steel
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recent years, a systematic understanding of the role of microstructure in hydrogen embrittlement has begun to emerge.~-3 This topic is of particular interest in the case of steels, 2-4 although in m a n y cases the evidence is incomplete and tends to be somewhat indirect. The most recent work has made progress by emphasizing relatively simple microstructures, such as spheroidized plain-carbon steels. 5-8 It is clearly of interest, however, to address the more complex microstructures such as pearlite, bainite, and tempered martensite which are important in practice. The present study was intended to make a beginning on this work while retaining compositional simplicity through use of a plain-carbon steel of martensitic structure. Fracture p h e n o m e n a in a relatively brittle material such as this are most conveniently studied by techniques which permit initiation and propagation of fracture events to occur in a well-defined way. Rice suggested several years ago 9 that an appropriate specimen design would include a notch which, f r o m the viewpoint of fracture mechanics, was fairly blunt. The specimen selected for the present work is the four-point bend bar with Charpy-type notch which has been characterized in some detail by Griffiths and Owen; l~ more bluntly-notched specimens have recently been used by Hirth and co-workers. 6,11,12 The fact that the peak stress in the present specimen occurs at a distance below the notch root l~ which is large compared to microstructural dimensions is expected 9 to facilitate identification of crack initiation site(s) and fracture paths. Moreover, since the peak strains occur
JAMES E. COSTA, formerly graduate student, Carnegie-Mellon University, is now with TIMET, Henderson, NV 89015. ANTHONY W. THOMPSON is Professor, Department of Metallurgy and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. Manuscript submitted August 8, 1980. METALLURGICAL TRANSACTIONS A
at the notch root itself, it should be possible to separate stress-controlled f r o m strain-controlled fractures through identification of the initiation locale. 6,~1,13 These observations can then be compared for specimens with and without hydrogen. Understanding of the fracture initiation process in the presence of hydrogen is of value beyond the initial m o m e n t of fracture, for there is suggestive evidence 14,15 that sustained-load cracking proceeds by a sequence of repeated subsurface reinitiation events. It was hoped that the present work would provide inf o r m a t i o n with a bearing on this process.
EXPERIMENTAL PROCEDURE The material used was SAE 1045 steel, the composition of which is shown in Table I. Material was received as 12.7 m m (0.5 in.) hot-rolled plate. The asreceived microstructure has been presented elsewhere.16 It comprised fine and coarse pearlite in a network of proeutectoid ferrite; no preferred orientation of prior austenite grain shape was exhibited. Sulfide inclusion stringers were elongated parallel to the rolling direction but were typically less than 5 ~m
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