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

Heat treatments containing interrupted quenching and bainitizing below the primal Ms temperature were investigated for EN X38CrMoV5-1 steel. Via complex treatment triple-phase microstructures including martensite, bainitic ferrite and austenite were obtained. Prior martensite provided additional surfaces for bainite heterogenous nucleation. In this way, bainitic transformation was supported by anterior segment. Low temperature bainitization, below the primal Ms temperature, allowed to decrease bainitic sheaves size and to refine retained austenite. On the other hand, the presence of martensite limited the volume of austenite from which bainite could be formed. Moreover, martensitic ferrite appeared as a possible source of carbon atoms partitioning to austenite during isothermal hold. Thus presence of martensite promoted the austenite stabilization and extinction of the bainitic transformation. Designing a complex heat treatment therefore required taking into consideration opposing, mutually compensating phenomena and finding a compromise between them. The combination of dilatometric study with magnetic tests allowed to investigate the kinetics of the phase transformations and to estimate evolution of the phase composition during individual treatment segments. Then appropriate process temperature-time conditions were selected to achieve intended amounts of martensite, bainitic ferrite and retained austenite. The microstructures obtained via designed heat treatments were investigated using SEM and TEM. Also tensile and impact toughness tests as well as hardness measurements were carried out to examine mechanical properties of obtained materials. The achieved results were compared with the properties of the steel after conventional quenching and tempering treatment. As it was noticed, the designed complex treatments allowed to obtain higher impact strength and elongation at comparable hardness and tensile strength, however leading to the decrease in yield strength.