The addition of Si to steels is a well stablished method to delay cementite precipitation, allowing for carbon partitioning from martensite to retained austenite during tempering. It has been argued that carbon enrichment and stabilization of austenite leads to increased ductility and toughness. This has been the main motivation for the development of novel heat treatments, such as quenching and partitioning. High carbon steels can also benefit from improved ductility provided by the presence of stabilized retained austenite. However, the process of carbon partitioning is less understood due to the increased tendency for competitive carbide formation with increasing carbon content. The present work investigates the austenite carbon partitioning and austenite decomposition phenomena in a modified 1.82 wt.% Si hypereutectoid bearing steel during tempering. Dilatometry, in-situ and ex-situ synchrotron X-ray diffraction, 3D atom probe tomography, scanning electron microscopy, and hardness measurements were used. The results are discussed based on different equilibrium states between α' and carbides. It was found that carbon partitioning towards retained austenite occurs for several minutes without significant phase decomposition at temperatures lower than 300 °C. A transition temperature between prevalent austenite carbon enrichment and austenite decomposition occurs at 350 °C. Secondary cementite precipitation inside martensite, and at the α'/γ interfaces, is observed during tempering at temperatures above 400 °C. Results from constrained carbon equilibrium modeling with carbide presence indicate that homogeneously dispersed spheroidized primary cementite has little influence in the carbon partitioning phenomenon.

A novel combined process of Cold Stamping (CS) and Hot Stamping (HS) with Quenching and Partitioning (Q&P) treatment applied to advanced TRIP-assisted steel has been conducted by thermomechanical simulation to evaluate the influence of CS or HS in the Q&P processing. With this purpose, Q&P, CSQ&P, and HSQ&P cycles were designed to obtain multiphase microstructures containing ferrite, martensite, bainitic-ferrite, and the maximum retained austenite (RA) fraction after the processes. The objective was to investigate the effects of the variables involving the heat treatments, such as the intercritical austenitization temperature, the isothermal and non-isothermal deformation, the amount of deformation, and the temperature and partitioning times, and to analyze their influence on the microstructural and mechanical responses. Time-resolved X-ray diffraction using synchrotron radiation was undertaken in a thermomechanical simulator coupled to the synchrotron light source to understand the influence of time, temperature, and strain on the level of carbon enrichment in austenite. In addition, the in situ austenite transformation kinetics and lattice parameter evolution were tracked, making it possible to optimize the RA fraction at room temperature after Q&P processing. The newly developed combined process is promising as the transformation-induced plasticity phenomenon during deformation can contribute to the formability and energy absorption. The results also indicate that the deformation of austenite promotes the ferrite transformation while suppressing the bainite transformation. It was possible to plot the results in an elongation-mechanical strength diagram, coupled to material property charts, also known as, ‘banana curve’, allowing us to identify and correlate the thermal or thermomechanical treatment conditions that led to an increase in ductility or strength according to the volume fractions of the resulting phases. Comparing the results for the HSQ&P treatments, it was observed that isothermal strains at higher temperatures (≥800 °C) are more advantageous to increase mechanical strength, while non-isothermal strains (starting at 750 °C) are suggested if the objective is the increase in ductility, with mechanical strength being slightly sacrificed.

The modification of the matrix of ductile cast irons by heat treatments has been of interest of researchers for many years. Among these treatments, in the last years the Quenching & Partitioning (Q&P) process has emerged as a viable way to produce microstructures containing controlled amounts of martensite and retained austenite, providing a good combination of strength and ductility. In this work, the different mechanisms of phase transformations occurring during the Q&P heat treatment applied to a ductile cast iron alloy is investigated. Microsegregation, inherent to cast irons, was analyzed by means of Electron Probe Microanalysis (EPMA). Microstructural characterization was performed with Scanning Electron Microscopy (SEM) and Electron Backscattered Diffraction (EBSD), while kinetics of carbon redistribution and competitive reactions were studied using dilatometry and in situ synchrotron X-ray diffraction. It was found that either transition carbides or cementite precipitate in martensite depending on the partitioning temperature. Despite of carbides precipitation, evidence of carbon partitioning from martensite to austenite was obtained. Formation of bainitic ferrite occurs during the partitioning step, further contributing to carbon enrichment of austenite. The experimental results are compared with a local field model that computes the local kinetics of carbon redistribution by simultaneously considering carbides precipitation and growth of bainitic ferrite. Results showed that kinetics of carbon partitioning from martensite to austenite depends on the carbides free energy. More stable carbides do not dissolve and prevent the escape of carbon from martensite. Fast carbon partitioning occurs by dissolution of less stable carbides, but it is slowed down as growth of bainitic ferrite proceeds. This result is explained by the overlapping of the diffusion fields (soft impingement) of the carbon partitioned from martensite and the carbon rejected from growth of bainitic ferrite.

In the present study the microstructural evolution and kinetics of carbon redistribution during the partitioning step of the Quenching and Partitioning process are investigated by means of a modeling approach that simultaneously considers the martensite-austenite carbon partitioning and the decomposition of austenite into bainitic ferrite. The development of the phase fractions, interface position, and carbon compositions are analyzed for two different binary Fe-C alloys with distinct initial carbon compositions and simulation geometries. The composition dependence of carbon diffusivity in austenite is taken into account for solving the diffusion field equations. Simulations indicate that kinetics of carbon partitioning from martensite to austenite is controlled by carbon diffusion in martensite and it is little affected by simultaneous occurrence of bainite reaction. On the other hand martensite-austenite carbon partitioning strongly influences the bainite reaction by inhibiting the growth of bainitic ferrite.