The effect of microstructural banding on failure initiation of HY-100 steel
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MICROSTRUCTURAL banding of hot-rolled, lowalloy steels is a common occurrence and is associated with the chemical banding of substitutional alloying elements.[1–5] The phenomenon can be understood in terms of the decomposition modes of the chemically inhomogeneous austenite under certain combinations of grain size and cooling rate.[4,5] For example, when these steels are slowly cooled from the austenite region, proeutectoid ferrite first forms within those regions, where the content of austenite-stabilizing elements is relatively small. The subsequent growth of proeutectoid ferrite creates essentially carbide-free regions within the microstructure. Finally, pearlite forms in the region of high solute (austenite stabilizer) content, thus creating a banded microstructure consisting of pearlite and ferrite bands. The HY-100 steel may be characterized as a hardenable steel which depends on Ni, Cr, Mn, and Mo contents for hardenability. Considering that the chemical banding gives rise to the microstructural banding, and given the relatively high content of the substitutional alloying elements, HY-100 is expected to show a high susceptibility to microstructural banding. Microstructural banding has not been previously studied in HY-100 steel. Furthermore, while the phase transformations associated with microstructural banding have been studied in some detail in low-alloy steels, the few reports of the mechanical properties of microstructurally banded steels[6,7] are limited to the effects on tensile ductility and impact resistance.[6] None of the previous research has addressed the issue of the effect of microstructural banding on the micromechanism of failure at a high-stress triaxiality. The tensile-failure behavior of a quenched and tempered HY-100 steel plate has recently been examined as a function of the stress state.[8,9,10] The results of these studies show D. CHAE and A.L. WILSON, Graduate Research Assistants, and D.A. KOSS and P.R. HOWELL, Professors, are with the Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. Manuscript submitted August 4, 1999.
METALLURGICAL AND MATERIALS TRANSACTIONS A
that, for loading parallel to the long transverse direction of the plate, failure at high-stress triaxialities occurs as a result of localized shear deformation, as voids link due to a voidsheeting mechanism.[8,9,10] Such failure has been shown to depend on the presence of elongated MnS inclusions, which are oriented normal to the tensile axis and which nucleate large, elongated, cylindrically shaped voids. Material separation subsequently occurs as shear localization develops between the elongated, primary voids, resulting in the formation of carbide-nucleated secondary voids. In such a failure process, the carbides serve as secondary void sites but are active in initiating voids only after large local strains are induced, due to deformation localization in the ligament between the primary voids. Therefore, void-sheet failure may be delayed, but not avo
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