AUSTENITE TRANSFORMING IN CONTINUOUS COOLING PROCESS UNDER DIFFUSION CONTROL MODEL

Abstract

Austenite-ferrite transformation in low carbon steels has a fundamental role in phase transforma- tion and is industrial importance. The kinetics of austenite transformation can be described by the kinetics of aus- tenite-ferrite interface migration. Two classical models, the diffusion-controlled growth model and the interface- controlled model, can be used to describe the growth of proeutectoid ferrite during g→a isothermal transformation. The austenite transformation in continuous cooling process is more common in production. In continuous cooling process, the equilibrium carbon concentrations in austenite and ferrite change with temperature and the kinetics of austenite transformation is different from that in isothermal process. Based on the models for g→a isothermal trans- formation, a diffusion control model is established for the growth of proeutectoid ferrite during the decomposition of supersaturated austenite in continuous cooling process. The interface position of proeutectoid ferrite varying with temperature is described with the model. The soft impingement effect at the later stage of transformation is considered. The carbon concentration at the austenite side of interface is difficult to reach the equilibrium carbon concentration when the cooling rate is high. A parameter as the function of cooling rate is proposed to modify the carbon concentration at the austenite side of interface. The polynomial diffusion field approximation is assumed in front of the interface. Simulation is done by utilizing the model to analyze the growth of proeutectoid ferrite in con- tinuous cooling process with different bulk concentrations, austenite grain sizes and cooling rates. The interface po- sition of proeutectoid ferrite as a function of temperature or time is obtained under different cooling conditions. Al- so, carbon diffusion length at the austenite side of interface as a function of time and carbon profile as a function of