In Situ SEM/EBSD Observation of the Reverse Transformation Behavior of Austenite From Lath Martensite in C–Ni Steels

The formation mechanism of austenite ( γ ) reversely transformed from lath martensite in 0.003C–9Ni and 0.03C–9Ni steels is studied using an in situ scanning electron microscopy/electron back scattering diffraction technique to reveal the mechanisms for controlling the fine γ microstructure. The formation of granular γ at the martensite packets and prior γ grain boundaries by a diffusionless shear mechanism in the 0.003C–9Ni steel affords γ grains with various crystallographic orientations. However, the reversely transformed γ phase in the 0.03C–9Ni steel has the same grain structure and crystallographic orientation as those of the prior γ phase before the first martensitic transformation, due to the formation of granular and acicular γ phases identified as the “ γ memory effect.” This study reveals that the γ memory effect in the 0.03C–9Ni steel is attributed to cementite; the crystallographic orientation relationships between cementite, tempered lath martensite, and acicular γ phases restrict the crystallographic orientation of acicular γ grains to match that of the prior γ grains. Therefore, the orientation restriction mechanism is the most valid mechanism for the formation of acicular γ grains with the same crystallographic orientation as the prior γ grains. This study will help the further development of functional steels.