Microstructural effects on the cleavage fracture stress of fully pearlitic eutectoid steel
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I.
INTRODUCTION
THEREhas
been extensive research on the deformation and fracture behavior of pearlitic 1080 (eutectoid) steel. Although general agreement exists as to the microstructural parameters controlling the yield strength and Charpy impact toughness, research continues on identifying the microstructural features controlling cleavage fracture. Previous work showed that the pearlite interlamellar spacing controlled the yield strength, ~-~5 although variations in prior austenite grain size may have an indirect influence. 5 The interlamellar spacing, in turn, is controlled by the (isothermal) transformation temperature, while the austenitization temperature controls the prior austenite grain size. This allows somewhat independent variation of these two microstructural parameters and the mechanical properties that correlate with them. Yield stress varies approximately inversely with interlamellar spacing in 1080 steel, although the exact mathematical form describing this relationship is under discussion. 3'4'6'7'12'13 In addition, the deformation and work hardening behavior of the pearlitic structure also appears to vary with strength level (i.e., interlamellar spacing). 16 Figure 1 summarizes a proposed dislocation model for ductile fracture in pearlitic microstructures.17 Coarse pearlitic structures possess low stength and initially deform by dislocation generation at ferrite/cementite (a/Fe3C) 18A9interfaces. As deformation proceeds, slip localizes into slip bands which create offsets in the cementite. 16.20These offsets act as local stress concentrators, thereby increasing the fiber-loading stresses in the lamellae. Fracture of the cementite ensues, creating an easy path for further deformation. Intense shear in the adjoining ferrite along the slip band follows, fracturing adjacent cementite lamellae. 16 J.J. LEWANDOWSKI, formerly Graduate Student, Camegie Mellon University, is Assistant Professor, Department of Metallurgy and Materials Science, Case Western Reserve University, Cleveland, OH 44106. A.W. THOMPSON is Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted June 17, 1985. METALLURGICALTRANSACTIONSA
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Fig. 1--Proposed dislocation mechanism for shear cracking in pearlitic microstructures. 17
Eventually, voids form in the ferrite at the fractured ends of the cementite lamellae. The local stresses are concurrently elevated by work hardening, even during void growth, eventually producing rapid brittle fracture in a tensile specimen when the local critical stress for cleavage is reached. As such, the void growth process is interrupted by another fracture mode (i.e., cleavage). Fine pearlitic structures, on the other hand, possess higher strength and deform in a more homogeneous manner. 16'iv Slip is more homogeneously distributed in the form of less intense, but more closely spaced, slip bands. The finer cementite also behaves in a more ductile manner, as it may rupture by necking rather
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