The nature of quasicleavage fracture in tempered 5.5Ni steel after hydrogen charging
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I. INTRODUCTION W H E N high strength martensitic steels are embrittled by hydrogen they may fail either by intergranular separation along the prior austenite grain boundaries or by transgranular fracture along planes which traverse the prior austenite grains. The transgranular failure mode superficially resembles the transgranular cleavage fracture found in similar steels when they are broken below their ductile-to-brittle transition temperatures. Recent research has shown, however, that there are significant differences. The most striking of these is the fracture plane itself, which is predominantly {100} in low-temperature fracture, but is often found to be {110} or {112} after hydrogen embrittlement. ~-4 While the {100} plane is the natural cleavage plane in bcc iron, the {110} and {112} are the dominant slip planes. The coincidence between the fracture plane and the dominant slip plane in steels that have been embrittled by hydrogen has led several authors to suggest that embrittlement occurs through a "glide plane decohesion" resembling that occasionally found in the basal plane fracture of hexagonal close-packed metals. 1'2'5'6 The evidence supporting this hypothesis is indirect, however, and there is an' alternative explanation for the crystallography of transgranular fracture which is based on the prevalence of {110} and {112} planes in the boundaries of martensite plates and subgrains in high strength steel. The {110} plane is the usual low angle boundary plane separating laths in lath martensitic steels; the {112} plane is the boundary plane when adjacent laths are twinrelated. 7'8 Both the {11(3} and {112} planes are common boundary planes for the subgrains formed in iron after severe deformation or tempering. 9 It is therefore possible that transgranular fracture in embrittled high strength steels follows lath or subgrain boundaries, and hence resembles intergranular fracture more than either transgranular cleavage or glide plane decohesion. To clarify this issue we carried out high resolution studies of the fracture path in a 5.5Ni steel which had been emY. H. KIM, Graduate Research Assistant, Lawrence Berkeley Laboratory, and J.W. MORRIS, Jr., Professor, are both with the University of California, Berkeley, CA 94720. Manuscript submitted November 16, 1982. METALLURGICALTRANSACTIONSA
brittled by hydrogen charging. 5.5Ni is a high alloy steel that is sold commercially for structural use at cryogenic temperatures. Its microstructure is well known, and typically contains well-defined laths of dislocated martensite that are organized into packets (Figure 1). The laths within a packet are separated by highly dislocated, low angle boundaries that tend to follow {110} planes (Figure 2). 7 Below its ductile-to-brittle transition temperature 5.5Ni steel fractures in a brittle transgranular mode along {100} cleavage planes. 7'8 Since the microstructure and mechanical properties of 5.5Ni steel are well known and techniques have been developed for high resolution studies of its transgranular fracture features, t
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