THE INTERACTION BETWEEN PROEUTECTOID FERRITE AND AUSTENITE DURING THEISOTHERMAL TRANSFORMATION OF 2 LOW-CARBON STEELS - A NEW MODEL FOR THE DECOMPOSITION OF AUSTENITE

Both scanning electron microscopy (SEM) and transmission electron micr oscopy (TEM) have been employed to examine the austenite to proeutecto id ferrite and ferrite/carbide reactions in two low-carbon (0.04wt%) s teels. It is demonstrated that proeutectoid ferrite (both polygonal an d Widmanstattan) can ''partition'' the prior austenite grains into sev eral smaller units or pools. It is also shown that prior to the initia tion of the pearlite reaction, ferrite grain growth can occur. The poo ls of austenite exert a Zener-like drag force on the migrating ferrite grain boundaries. However, the ferrite boundaries can eventually brea k away and small pools of austenite become completely embedded in sing le proeutectoid ferrite grains. Subsequently, these small pools of aus tenite transform to discrete regions of cementite, together with epita xial ferrite. Conversely, certain small pools remain in contact with t he ferrite grain boundaries and it is considered that transformation o f these latter pools will eventually lead to the formation of massive films of cementite at the ferrite grain boundaries. Larger pools of au stenite prevent ferrite boundary breakaway, and these latter, austenit ic regions eventually transform to pearlite.