EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Tempering of martensitic steels is commonly employed to improve their ductility and increase their strength by secondary precipitation. The different phenomena which occur during tempering were intensively studied and are well established in such steels. Nano-bainitic steels represent a new class of alloys, whose microstructure consists of nano-structured bainite formed at low temperature with a high amount of austenite. This imparts simultaneously high ductility and high tensile strength to the steel. Recently it has been shown that adding secondary carbide forming elements such as V, Mo increases the resistance to softening and hence the properties at higher temperature. Investigating the secondary carbide precipitation inside a nano-bainitic structure is thus necessary to optimize the microstructure and the thermal treatments for this promising new class of steels.
The study aims to combine the mechanical properties resulting from the nano-bainite microstructure with secondary hardening precipitation as observed in martensite steels. One steel grade was chosen whose composition is based on previously investigated nano-bainitic steel (0.67C-1.5Si-1.7Cr-1.3Mn wt.%). Mo and V elements are added to further secondary carbide precipitation. In addition to the nano-bainite microstructure, two martensitic microstructures are also investigated.
The tempering of both martensites and nano-bainite microstructures are investigated in situ during the tempering by dilatometry, synchrotron XRD and ex situ by high resolution TEM. The global kinetics, the sequence of precipitation are established quantitatively, as well as the evolution of ferrite/martensite and austenite phase fractions and cell parameters. A nucleation and growth model with physical basis was used to predict the kinetics of precipitation as well as the particle densities, radii and matrix and carbides composition evolutions considering their interactions during the tempering treatments and give satisfactory results. The model allowed to interpret further the microstructure evolutions.