Microstructural Evolution in High-Carbon Quench and Partitioning Steels: The Role of Chemical Composition

Abstract

This study investigates the microstructural evolution of high-carbon quench and partitioning (Q&P) steels and the effect of the chemical composition using techniques such as dilatometry, optical microscopy (OM), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). The applied Q&P treatments, starting from full austenitization, were applied to the developed steel, leading to microstructures containing volume fractions of retained austenite between up 0.15-0.30. During the partitioning step, a large fraction of the austenite was sufficiently carbon enriched to be retained at room temperature. In some alloys, depending on chemical composition and fraction of austenite at quench, bainite formation occurred during isothermal holding. Notably, no fresh martensite was formed. The microstructure showed pronounced microstructural banding due to the segregation of alloying elements such as Mn and Si. Solute-rich bands show a coarser microstructure of tempered martensite (TM), relatively large austenite islands and bainite. While solute-lean regions show a finer microstructure of TM and fine film-like retained austenite. Increasing C content resulted in a decrease of the martensite start temperature (𝑀𝑠 ) leading to a higher fraction of untransformed austenite at similar quenching temperatures as well as tetragonal martensite. At higher fractions of untransformed austenite the microstructural difference between solute-rich and solute-lean bands is more pronounced. Alloying elements, such as Mn and Ni, play a critical role in refining the microstructure and stabilising austenite.