Thermally activated martensite formation: a comprehensive revision of the physical metallurgy of martensitic transformations in steels

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, precipitation hardening stainless steel that contains a significant fraction of austenite after cooling shows extensive isothermal transformation of austenite into martensite at sub-zero Celsius temperature. Isothermal martensite formation is inconsistent with the representation in CCT and TTT diagrams. In the last 10+ years, our team has investigated the kinetics of martensite formation with in-situ magnetometry, complemented by advanced microscopy and X-ray diffraction studies. Our investigations demonstrate that thermally activated martensite formation is the rule rather than the exception. For all the investigated steels and model alloys martensite can form during cooling, isothermally as well as during heating. The activation energy for martensite formation depends on the steel’s chemistry, particularly the content of interstitials, and relates to martensite’s internal structure. In this overview, the data is put into perspective and framed around a comprehensive revision of the physical metallurgy of martensitic transformations in steels.

Keywords:

Martensitic transformations, Steel, Heat-treatment, Materials characterization

Topic:

Physical metallurgy in heat treatment and surface engineering; cryogenic