Abstract
Medium-Mn Quenching & Partitioning (Q&P) steels have been recently considered as potential candidates for the 3rd generation advanced high-strength steels. The processing of these steels aims to induce the partitioning of substitutional alloying elements from martensite to austenite during an isothermal treatment at high temperature, where the diffusivity of substitutional alloying elements is sufficiently high. In this way, austenite increases its concentration of austenite-stabilising elements and thus its thermal stability. The present study aims to investigate the microstructural evolution during high temperature partitioning treatments in a medium-Mn steel and the possible occurrence of additional phase transformations that may compete with the process of atomic partitioning between martensite and austenite. Q&P routes in which the partitioning steps take place in the range of 400 °C - 600 °C for times up to 3600 s were investigated. The final microstructures display an increased fraction of retained austenite with increasing holding times during partitioning at 400 °C, while at higher partitioning temperatures, 450 °C - 600 °C, leads to cementite precipitation in austenite films and pearlite formation in blocky austenite, resulting in a decrease of the fraction of retained austenite with the holding time. This observation is supported with theoretical calculations of the volume change, suggesting that for maximising the fraction of retained austenite, short holding times are preferred during partitioning at high temperatures. Observations from the current study reveal that the successful application of high-temperature partitioning treatments in medium-Mn steels requires microstructure design strategies to minimize or suppress competitive reactions.
Highlights
The quenching and partitioning (Q&P) process, proposed by Speer and co-workers [1], has been considered as one of the most promising heat treatments for the production of third generation advanced high strength steels (AHSSs) with exceptional combination of strength and ductility
The microstructural evolution during the different applied Quenching & Partitioning (Q&P) heat treatments is evaluated based on the dilatometry measurements, X-ray diffraction analysis, magnetisation measurements and microstructural observations
The volume fraction of martensite formed at the quenching temperature was determined by analysing the dilatometry response of the as-quenched specimen austenitized at 950 °C for 120 s and directly quenched to room temperature, as Fig. 2a shows
Summary
The quenching and partitioning (Q&P) process, proposed by Speer and co-workers [1], has been considered as one of the most promising heat treatments for the production of third generation advanced high strength steels (AHSSs) with exceptional combination of strength and ductility. The carbon enrichment of the austenite leads to its stabilization at room temperature. If part of the austenite is insufficiently enriched with carbon, this may transform into fresh martensite (M2) during the final quench to room temperature [2,3,4]. Speer et al [4] proposed the constrained carbon equilibrium (CCE) model to describe the thermodynamics of the carbon partitioning process. The CCE model is characterized by two assumptions: a) the carbon partitioning from martensite to austenite is finalized when the chemical potential of carbon in both phases is equal and b) the austenite/martensite interface is immobile during the partitioning step as the number of iron atoms in each phase are conserved. Most studies are concentrated on studying the stabilization of austenite by carbon [2,5,6,7,8]
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