Spheroidization of binary Fe-C alloys over a range of temperatures

Abstract

The spheroidization of cementite in binary Fe-C alloys, 0.24, 0.42, and 0.79 wt pct C, was investigated over a range of temperatures, 594°, 649°, and 704°C, for times up to about 10⁶ sec. Quantitative metallography techniques were used to obtain the following microstructural data on the cementite particles: shape, size distribution, mean size, number of particles per unit volume, and growth (and shrinkage) rates of various sizes in the size distribution. The variations of these microstructural parameters were analyzed in terms of existing models for the spheroidization process. The Lifshitz-Wagner analysis is shown to have limited applicability to the spheroidization of cementite in binary steels, since the required steady-state size distribution is not attained in times less than about 10⁶ sec. An analysis similar to that of Lifshitz and Wagner, but requiring no specification of the shape of the size distribution, is shown to apply and indicates that the observed spheroidization was diffusion-controlled. The effective diffusion coefficient was between the values for the diffusion of carbon and iron in ferrite and approximated the coupled diffusion coefficients developed by Oriani and Li, Blakely, and Feingold.