Visualization of austenite formation during heating in alloy steels

Experiments utilizing a hot-stage confocal scanning laser microscope (CSLM) have been carried out to observe phase transformations in two steels, 4118 and 4320. Austenite formation during continuous heating and isothermal conditions were investigated on the surface of samples that were etched to reveal the ferrite and eutectoid regions. It was found that the austenite precipitated at eutectoid/ferrite boundaries. Austenite would then proceed to cover the eutectoid regions first and thereafter a distinguishable γ/α front would proceed advance through the ferrite. Anywhere from 2 to 5 fronts could be observed in each ferrite grain, but did not originate preferentially based on surrounding microstructure. In most cases, only 1 austenite front was observed to advance through a eutectoid colony. The rates of front advancement occurred at higher temperatures and were slightly faster in 4118 than 4320 when subjected to non-isothermal growth conditions. Non-isothermal growth rates were generally non-linear. Front advancement rates under isothermal conditions were generally linear, were significantly faster in 4320 than 4118, and displayed a first order relationship to superheat (T - Tα→γ).