Effects of prior austenite grain size on reversion kinetics of different crystallographic austenite in a low carbon steel

The effects of prior austenite grain (PAG) size on reversion kinetics of different crystallographic austenite (gamma) with acicular and globular morphologies in a low carbon steel with the structure of pre-tempered martensite have been studied. It is unexpected that the coarse PAG (C-PAG) shows a strong inhibition on the reversion kinetics of globular gamma, while a weak influence on that of acicular gamma. Intragranular globular gamma is hardly to be formed in the low carbon steel, and the grain boundary (GB) ones are dominantly formed. C-PAG provides low-density PAGBs for GB globular gamma formation, leading to slower reversion kinetics. Theoretical analyses revealed that the low-density and fine cementite particles result in a low nucleation potency for globular gamma thus less intragranular ones were formed. On the other hand, C-PAG leads to coarse martensite structure, which was expected to have a strong retardation on the reversion kinetics of acicular gamma. However, acicular gamma is mainly nucleated at block boundaries for the cases of (Fine) F-PAG, and it is almost equally nucleated at block and sub-block boundaries for C-PAG. This decreased the differences in nucleation density and growth kinetics of the acicular gamma between the C-PAG and F-PAG, and the influences of PAG size on the reversion kinetics is weakened.