The effect of cooling rate on the microstructures formed during solidification of ferritic steel
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I. INTRODUCTION
THE commercial corrosion resistant Fe-Cr-Ni alloys (.11.5 pct Cr) containing very little Ni (,2.5 pct) are generally termed martensitic or ferritic stainless steels. The composition of the alloys is one of the major factors that determines the amount of martensite/d-ferrite formed; e.g., austenite forming elements (e.g., C, N, and Ni) promote the formation of austenite phase, while ferrite-forming elements (e.g., Cr, Mo, and V) discourage austenite formation.[1–4] In the ferritic stainless steel, the austenite has often been observed to precipitate at the d-ferrite grain boundaries and transform into martensite on cooling to room temperature. The morphologies of the grain boundary precipitates have been earlier examined and classified for isothermally transformed alloy steel by Dube´ et al.[5] for ferrite morphology and later modified by Aaronson[6] for ferrite and cementite morphologies. Based on these investigations, it has been suggested that the morphologies of the grain boundary precipitates consist of[6] (1) allotriomorph, (2) primary and secondary sideplates (sideneedles), (3) primary and secondary sideteeth, (4) idiomorphs, (5) intergranular Widmansta¨tten plates, and (6) massive structure. Recently, three-dimensional analyses were conducted to determine more precisely the classification of the precipitates.[7] The results of these investigations indicated that only dendritic grain boundary precipitates and Widmansta¨tten precipitate morphologies exist and that the intergranular precipitates were not observed. The morphology can strongly affect the mechanical properties of the alloy. Knowledge of the morphology formation can therefore provide a way of controlling and optimizing the mechanical properties of the alloy.[8] N.H. PRYDS and X. HUANG, Senior Scientists, are with the Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark. Manuscript submitted February 18, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
During the last 3 decades, a considerable effort has been devoted to study the effect of rapid solidification on Fe-base alloys mainly due to the inherent technical importance of this class of materials.[9–12] It is known that rapid solidification processing can result in microstructure refinement, formation of metastable phases and morphological changes. Although rapid solidification enables production of more uniform and refined microstructure in comparison with conventional solidification, the understanding of the local inhomogeneity of the products during rapid solidification and their influence on the subsequent precipitation processes is also of technical importance. In recent studies, the microstructural development has been characterized in 12 pct Cr stainless steels produced by different rapid solidification processes, namely, gas atomizing, melt spinning, and laser surface remelting, showing that the solidification conditions have a strong effect on the structures that are formed.[13,14] The present study was undertaken to obtain a detailed und
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