A model for ferrite/pearlite band formation and prevention in steels
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20/1/04
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A Model for Ferrite/Pearlite Band Formation and Prevention in Steels P.E.J. RIVERA-DÍAZ-DEL-CASTILLO, J. SIETSMA, and S. VAN DER ZWAAG A model for predicting the conditions under which ferrite/pearlite band formation occurs, and therefore the conditions in which it can be avoided in steels, has been developed. The model requires as input the alloy composition and microchemical segregation wavelength, and provides in turn the homogenization temperature and time in which the alloy should be held in the austenite region for band elimination. The model was applied to three alloys and predicted with accuracy the conditions under which bands were observed to disappear in different investigations from literature. The conditions under which the model can be applied to any alloy are explored.
I. INTRODUCTION
FERRITE/PEARLITE and ferrite/martensite banding in steels is a well-known phenomenon that has been recognized for several decades.[1] In the quest for alloys with optimum mechanical properties, the factors influencing the formation of ferrite/pearlite bands have been the object of a great deal of attention.[2] Most of the work has relied on experimental verification of the conditions for band formation as a consequence of isothermal treatment and alloying.[3] A major problem on developing models for predicting band formation has been the lack of quantitative data for the evolution of the austenite-ferrite transformation leading to band formation, and a criterion for the degree of banding. By measuring the degree of anisotropy resulting from an unequal distribution of ferrite and cementite regions, Kop et al.[4] have provided a method to quantify the degree of banding due to the unequal dilatation behavior in directions normal and parallel to the bands. The evolution of the austenite : ferrite/pearlite transformation leading to banding has recently been studied by Offerman et al.[5] In an elegant method, they have measured the ferrite fraction formed as a function of time with neutron depolarization (ND) experiments during isothermal annealing, and combined this with electron probe microanalysis (EPMA) measurements for determining the influence of the solute local compositions on band formation. With the aid of a thermochemical database (MTData[6]), they provided solid evidence for linking band formation with the difference in the nucleation rates of ferrite resulting from the presence of microchemical bands. In their method, however, the driving force for ferrite nucleation was calculated by taking into account the influences of only a limited number of components (Fe, C, and Mn). In the present work, a more general thermodynamic/ thermokinetic model is developed for quantifying the formation and prevention of bands as a function of austenitization and transformation temperature and time, and alloy P.E.J. RIVERA-DíAZ-DEL-CASTILLO, Research Fellow, and S. VAN DER ZWAAG, Professor, are with the Faculty of Aerospace Engineering, Delft University of Technology, 2629 HS Delft, The Netherlands. C
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