Dualism of precipitation morphology in high strength low alloy steel

Transmission electron microscopy (TEM) images showed that interface precipitation and random precipitation can occur within the same ferrite grain. The reason is that the austenite decomposition rate varies with transformation temperature and time. An excessively fast austenite/ferrite interface movement velocity, which usually happens in small ferrite grains, would cause these ferrite grains with microalloying elements to exceed their solubility. Furthermore, these microalloying elements will be precipitated randomly after isothermal holding at longer times. Consequently, a large microhardness distribution can usually be detected in specimens with tiny ferrites because some ferrite grains are in a fresh state, without carbides, due to high austenite/ferrite interface movement velocities. Furthermore, one important technological limit that should be kept in mind is the difficulty of developing only one type of precipitation morphology (i.e., interface precipitation or random precipitation) within every ferrite grain.