Thermally activated growth of lath martensite in Fe–Cr–Ni–Al stainless steel

Abstract

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.

Anomalous kinetics of lath martensite formation in stainless steel

“…In this alloy, the martensitic transformation proceeds from face centred cubic c to body centred tetragonal a´, and martensite has lath morphology. 18,37 Martensite formation occurs isothermally at temperatures lower than ~310 K, 18 and isothermal martensite formation is industrially exploited at subzero Celsius temperature. 38,39 Austenite is metastable at room temperature and transforms into a9 martensite upon mechanical deformation, 38,39 ion implantation 40,41 and electropolishing.…”

“…After austenitisation and room temperature storage, the samples are dual phase, composed of austenite and martensite. 18,38,39 Martensite formation was investigated by vibrating sample magnetometry (VSM) reflected light microscopy (RLM) and scanning electron microscopy (SEM). Microscopy was performed on the surface of the material as prepared before austenitisation.…”

“…These values, which progressively increase with the degree of transformation, are consistent with literature data for isothermal martensite formation in Fe-Ni-Cr alloys and suggest that the movement of dislocations in austenite could be the rate limiting step for thermally activated martensite formation. 18,50,51 In particular, an increase in the activation energy for martensite formation during the transformation was reported in Ref. 50 and interpreted in terms of progressive hardening of the austenite phase as caused by martensite formation.…”

“…10, the units of martensite are not distributed over the majority of (prior) austenite grains but are grouped in clusters, which nucleate independently, while their growth is thermally activated and extends over several (prior) austenite grains. This interpretation is based on previous work on the formation of lath martensite in a Fe-Cr-Ni-Al stainless steel, 18 where clusters of martensite units were observed at the surface of the material together with numerous unaffected austenite grains. Moreover, two transformation rate maxima were revealed by magnetometry upon isochronal cooling of the material at 0.1K min -1 .…”

“…Moreover, two transformation rate maxima were revealed by magnetometry upon isochronal cooling of the material at 0.1K min -1 . 18 The interpretation of the transformation mechanism reported in Ref. 10 takes into account the effects of the transformation induced elastic (i.e.…”