11 / 2025-02-21 17:23:31

Thermally activated martensite formation: an exception or the rule?

martensite; cryogenic treatment

Generally, the formation of martensite in steel is represented by horizontal lines in the CCT and TTT diagrams implying that martensite formation is athermal, i.e., independent of time. Nevertheless, hardening of precipitation hardening stainless steels that contain a significant fraction of austenite after cooling to room temperature is achieved by isothermal treatment in the sub-zero Celsius temperature regime. In the last 15 years, we investigated the kinetics of martensite formation with in-situ magnetometry, complemented by electron microscopy and X-ray diffraction studies. Our investigations revealed that thermally activated formation of martensite in a wide variety of steels and model alloys is the rule rather than the exception. For all the investigated steels and model alloys it was observed that martensite can form during cooling, isothermally as well as during heating. The kinetics of martensite formation depends on the steel chemistry and relates intimately to martensite’s internal structure. In this overview of our work, the data is put into perspective and it is demonstrated that Zhao & Notis’ presentation of lath and plate martensite as individual C-shaped curves can be extrapolated to the sub-zero Celsius regime to provide a comprehensive understanding. This paves the way for development of new steel compositions.