Role of cementite and retained austenite on austenite reversion from martensite and bainite in Fe-2Mn-1.5Si-0.3C alloy

The effects of cementite (theta) and retained austenite (RA) on the kinetics and microstructure evolution in austenite (gamma) reversion of an Fe-2Mn-1.5Si-0.3C (mass%) alloy have been studied by investigating the reversion from as-quenched and tempered martensite, and bainite. Pre-tempering of martensite or prolonged austempering, both resulting in theta precipitation, enhanced the formation of globular gamma at high reversion temperature, while the presence of RA in carbide-free bainite structure obtained by short time austempering suppressed its formation. High-temperature pre-tempering or prolonged austempering retarded the reversion kinetics, while low-temperature pre-tempering or the presence of RA have negligible influences on the kinetics. Nucleation of gamma at theta / ferrite interface results in the formation of globular gamma. Theoretical analysis reveals that coarser theta particle has a higher nucleation potency for globular gamma, and thus coarsening of theta particles by pre-tempering or austempering promotes its formation. On the other hand, the precipitation of theta particles before gamma reversion is suppressed when a large amount of RA is present, thus globular gamma formation is suppressed. It was concluded that the retardation on reversion kinetics by high-temperature pre-tempering or austempering is mainly owing to the suppression on the growth of reverted gamma because of enhanced Mn enrichment into theta and coarsened theta particle. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

The effects of cementite (theta) and retained austenite (RA) on the kinetics and microstructure evolution in austenite reversion have been studied in an Fe-2Mn-1.5Si-0.3C alloy. The results indicate that pre-tempering or prolonged austempering enhance the formation of globular gamma, while the presence of RA in short time austempered bainite structure suppresses it. High-temperature pre-tempering or prolonged austempering delays reversion kinetics, while low-temperature pre-tempering or the presence of RA have minimal effects on kinetics.