Microstructure-based kinetic model of austenitization of steels

Austenitization is a critical stage in thermal processing of steels. Recently, rapid thermal processing techniques such as electromagnetic induction and laser processing impose a great need for more reliable and accurate characterization of the austentizing phenomena. Besides, a better understanding of the phenomenon and an accurate modeling of the austenitization process is likely to result in higher quality products and lower cost processes in many commercial operations such as steel making and heat treatment.; The objective of the present study is to establish a quantitative relationship between the microstructurel features and the kinetics of austenitization. The austenitization kinetics from initial pearlite microstructures in eutectoid steels has been investigated. Six different initial pearlite microstructures are obtained in carefully controlled isothermal processes carried on the Gleeble 3500 Then-no-Mechanical Simulator. Isothermal austenitization experiments are also carried out on the Gleeble 3500 with a high-speed dilatometer. The kinetic data of austenitization from the six initial pearlite microstructures fit very well with the Avrami equation. The average error is 4.2%. Based on the fitted results, isothermal transformation diagrams and the progress of austenitization during continuous heating at constant heating rates have been constructed for six different initial pearlite microstructures. A microstructure-based kinetic model has also been established in the present study.; A FEM-based model has been developed to address both cementite dissolution and carbon homogenization during isothermal austenitization of a lamellar pearlite microstructure at temperatures higher than 913°C. The dependence of the diffusion coefficient of carbon in austenite on the temperature and carbon concentration has been taken into account. The relations of time, temperature and transformation as well as the redistribution of carbon concentration are obtained in the form of TTT diagram in the temperature range between 913°C and 1148°C.