Thermally activated growth of lath martensite in Fe- Cr-Ni-Al precipitation hardenable stainless steel

The austenite-to-martensite transformation in a partially hardened stainless steel containing 17wt %Cr, 7wt %Ni and 1wt % Al was investigated with Vibrating Sample Magnetometry and Electron Back-Scatter Diffraction. Magnetometry demonstrated that measurable martensite formation can be suppressed on fast cooling to 77K as well as on subsequent fast heating to 373 K. Surprisingly, martensite formation was observed during moderate heating from 77 K, instead. Electron Back-Scatter Diffraction demonstrated that the morphology of martensite is lath type. The kinetics of the transformation is interpreted in terms of athermal nucleation of lath martensite followed by thermally activated growth. It is anticipated that substantial autocatalytic martensite formation occurs during thermally activated growth. The observation of several retardations and accelerations of the transformation rate during slow isochronal cooling is interpreted in terms of the contribution of self-induced mechanical stabilization of austenite during martensite formation.