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 (θ) and retained austenite (RA) on the kinetics and microstructure evolution in austenite (γ) 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 θ precipitation, enhanced the formation of globular γ 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 γ at θ / ferrite interface results in the formation of globular γ. Theoretical analysis reveals that coarser θ particle has a higher nucleation potency for globular γ, and thus coarsening of θ particles by pre-tempering or austempering promotes its formation. On the other hand, the precipitation of θ particles before γ reversion is suppressed when a large amount of RA is present, thus globular γ 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 γ because of enhanced Mn enrichment into θ and coarsened θ particle.