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

The present study focuses on the effect of heating rate on ferrite to austenite transformation and carbide dissolution in a chromium-molybdenum low alloyed steel. Scope is to shed light in the mechanisms that lead to mixed microstructures with the coexistence of martensite, lower bainite and small amounts of retained austenite. For this purpose a conventional heating rate (10oC/s) was examined for comparison purpose to high heating rates (100 – 300oC/s). For both treatments, performed via dilatometry, the reheating was followed by a peak stay at the austenization temperature, ranging between 900 – 1080oC. The soaking time at the selected temperature varies from 2 – 300s. Using modelling tools, the kinetics of the phase transformations occurring during heating and short austenitization were thoroughly studied. Additionally, an initial simulation of the subsequent quenching step and the displacive martensitic transformation has been conducted according to a phase field modelling approach. The results revealed that the heating rate increase retards the carbide dissolution and the austenite formation since the required time for carbon diffusion is greater compared to the available time during heating with rates >100oC/s. In addition, chromium decreases the interfacial movement velocity of cementite/austenite due to «solute–drag» effect. The increase of chromium addition, as observed in M7C3 carbides, retards carbide dissolution and austenite formation. The study on coarse carbide dissolution proves that their dissolution and the austenite formation are both impeded as the required distance for carbon diffusion increases and a greater duration is required. The undissolved carbides impede austenite grain growth and induce chemical heterogeneity in austenite, which may lead to a small fraction of retained austenite after quenching also estimated using the Koistinen – Marburger equation. Simulation results agree with experimental examination of the obtained microstructures in optical (OM) and scanning electron microscopy (SEM, EBSD, TEM – EDS).