This thesis studies in-situ the phase transformations during heat treatment of two advanced steels: a supermartensitic stainless steels (SMSS), on which the main focus of this work is, and Fe-C-Mn-Si steels. A magnetic technique, based on the analysis of saturation magnetization, is utilized as the primary analysing tool. The scientific aim of this work is twofold: (i) to study the microstructural evolution involved in thermal processing of advanced steels based on optimising retained austenite and (ii) to optimise and extend the application of magnetic methods for these steels. For SMSS, the retained austenite fraction plays an essential role in controlling mechanical properties that often have a narrow tolerance window. Magnetic techniques are of increasing interest in the steel industry for monitoring the development of austenite on the basis of different magnetic properties of the phases in the steel. A new approach is proposed for determining the austenite fraction from in-situ thermo-magnetic measurements in SMSS. The formation of austenite during heat treatment of a SMSS is investigated and the kinetics of the martensite to austenite transformation, as well as the stability of austenite was established. The analysis of the Fe-C-Mn-Si steels provides a basis to further develop the analysis of austenite formation of multi-phase steels using thermo-magnetic techniques.@en

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