The interaction between proeutectoid ferrite and austenite during the isothermal transformation of two low-carbon steels — a new model for the decomposition of austenite

Abstract

Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been employed to examine the austenite to proeutectoid ferrite and ferrite/carbide reactions in two low-carbon (0.04 wt%) steels. It is demonstrated that proeutectoid ferrite (both polygonal and Widmanstätten) can “partition” the prior austenite grains into several smaller units or pools. It is also shown that prior to the initiation of the pearlite reaction, ferrite grain growth can occur. The pools of austenite exert a Zener-like drag force on the migrating ferrite grain boundaries. However, the ferrite boundaries can eventually break away and small pools of austenite become completely embedded in single proeutectoid ferrite grains. Subsequently, these small pools of austenite transform to discrete regions of cementite, together with epitaxial ferrite. Conversely, certain small pools remain in contact with the ferrite grain boundaries and it is considered that transformation of these latter pools will eventually lead to the formation of massive films of cementite at the ferrite grain boundaries. Larger pools of austenite prevent ferrite boundary breakaway, and these latter, austenitic regions eventually transform to pearlite.