Effect of prior-austenite grain size and transformation temperature on nodule size of microalloyed hypereutectoid steels
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INTRODUCTION
THERE has been a significant interest in the addition of microalloying elements, particularly vanadium, to medium and high carbon steels with predominantly pearlitic microstructures,[1,2,3] since it has been recognized that the formation of a fine dispersion of vanadium carbide precipitates within the pearlitic ferrite lamellae increases strengthening.[1,4,5] Similarly, Tarui et al.[6] have shown that increasing the silicon content from 0.2 to 1.5 pct in an eutectoid carbon steel improved both the tensile yield and ultimate strengths, primarily due to solid solution strengthening of the pearlitic ferrite by silicon addition. In these cases, the increased strengthening in microalloyed steels has been related to the main microstructural constituent, pearlite, which consists of two phases, ferrite and cementite, as alternating lamellae. Generally, pearlite is considered to be a transformation product of the decomposition of austenite by a diffusional process involving nucleation followed by growth.[7–11] The initiation of a pearlitic microstructure from austenite entails the formation of neighboring nuclei of ferrite and cementite on an austenite grain boundary. Supposing that initially only one nucleus of cementite forms on this boundary, this region will be locally rich in carbon, which has obtained from the immediate surroundings. This then reduces the carbon content on either side of the cementite nucleus and encourages the adjacent nucleation of ferrite, as shown in Figure 1. If this occurs and the cooperative process continues, adjacent nuclei of alternating ferrite and cementite are formed. These can then grow by a relatively short-range diffusion process in which carbon diffuses parallel to the reaction front, from in front of the growing ferrite to the growing cementite. More importantly, the diffusion distance does not increase with time, as would be the case for a continuous precipitation A.M. ELWAZRI, Research Associate, and S. YUE, Professor, are with the Department of Metals and Materials Engineering, McGill University, Montreal, PQ, Canada H3A 2B2. Contact e-mail: [email protected] P. WANJARA, Research Officer, is with the Aerospace Manufacturing Technology Centre, Institute for Aerospace Research, National Research Council of Canada, Montreal, PQ, Canada H3T 2B2. Manuscript submitted May 14, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
process. This plausible progression leads to the transformation of austenite to a lamellar product that has a specific spatial orientation in which the ferrite and cementite phases are parallel to each other within a colony region. In reality, for each colony of pearlite, the lamellae are mostly parallel and are frequently curved. However, different pearlite colonies have different lamellae orientations, and, as the transformation progresses, neighboring colonies of lamellae join together and continue to advance into the austenite, such that when the transformation occurs at low degrees of undercooling the colony groups advance by a boundary
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