In-situ high-temperature EBSD study of austenite reversion from martensite, bainite and pearlite in a high-strength steel

The austenite (gamma) reversely transformed from lath martensite (LM), lath bainite (LB), granular bainite (GB) and pearlite + ferrite (P + F) in a high-strength steel was studied at high temperatures using in-situ electron backscatter diffraction (EBSD). The memory effect of initial gamma significantly affects the nucleation of the reverted gamma in LM and GB structures, while a weak influence on that of LB and P + F structures. This results in a significant difference in gamma grain size after complete austenitization, with the first two obtaining larger gamma grains while the latter two are relatively small. Crystallographic analysis revealed that the reverted gamma with acicular morphology (gamma A), most of which maintained the same orientation with the prior gamma , dominated the reaustenitization behavior of LM and GB structures through preferential nucleation within gamma grains and coalesced growth modes. Although globular reverted gamma (gamma G) with random orientation or large deviation from the prior gamma can nucleate at the grain boundaries or within the grains, it is difficult for it to grow and play a role in segmenting and refining the prior gamma due to the inhibition of gamma A coalescing. For LB and P + F structures, the nucleation rate of intragranular gamma G increases with increasing temperature, and always shows a random orientation. These gamma G grains can coarsen simultaneously with the intergranular gamma G , ultimately playing a role in jointly dividing and refining the final gamma grains. Research also found that the differences in the effects of four different microstructures on reverted gamma nucleation are closely related to the variant selection of the matrix structure, as well as the content and size of cementite (theta). High density of block boundaries induced by weakening of variant selection and many fine theta formed in the lath are the key to promoting LB structure to obtain more intragranular gamma G formation, as well as the important role of the large-sized theta in P + F structure. (c) 2024 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.