Quenching and Partitioning of Martensite and Other Advancements in Steels: Session 2
Program Organizers: Emmanuel De Moor, Colorado School of Mines

Tuesday 4:00 PM
July 11, 2017
Room: Comiskey
Location: Hyatt Regency Chicago

Session Chair: Sébastien Allain, Institut Jean Lamour

4:00 PM  
In situ Synchrotron Study on Q&P Steels Involving Tetragonal Martensite: Bij-Na Kim1; Jilt Sietsma1; Maria Santofimia1; 1TU Delft
    Recent literature shows that a complex carbon interplay exists in Q&P steels, where several processes compete for the carbon in martensite. In order to elucidate these processes, Q&P heat treatments were performed and characterised in situ using high energy synchrotron radiation on a model alloy Fe-1C-1Mn-2Si (wt.%). Given the high carbon content, ferrite tetragonality is clearly detected in martensite. The observed reduction in tetragonality during partitioning is attributed to the changes occurring in the microstructure. In the early stages, there is strong evidence that carbon is predominantly consumed in carbide precipitation. Furthermore, as martensite forms during quenching, a contraction in the austenite lattice parameter is observed. According to literature this is due to the hydrostatic compressive strain in the untransformed austenite induced by the martensitic transformation. The effect of the strain in estimating the carbon content in austenite by changes in the austenite lattice parameter during partitioning is discussed.

4:20 PM  
Multi-probe Tracking of Nano-scale Tempering Reactions in Low-carbon Lath Martensite: Lutz Morsdorf1; Elena Emelina1; Dirk Ponge1; Dierk Raabe1; Cem Tasan2; 1MPIE; 2MIT
     Tempering of as-quenched microstructures in high strength steels is a widely applied process to overcome martensite’s inherent brittleness. Thereby, the final strength-ductility properties rely to a large extent on the atomic-scale redistribution of carbon. Though tempering reactions such as carbon segregation, formation of Cottrell atmospheres and carbide precipitation have been studied for years, the exact pathway of carbon redistribution out of the supersaturated bcc/bct lattice is still a matter of debate. In this study we investigate carbon diffusion and carbide precipitation kinetics starting from an as-quenched, yet autotempered state. A three-step methodology consisting of electron channeling contrast imaging, transmission electron microscopy and atom probe tomography, all carried out in an (quasi-) in-situ way, allows for direct observation of the microstructure evolution. The results indicate that pipe diffusion and subsequent carbide nucleation at dislocations is the dominating mechanism, which is in agreement with theoretical considerations and simulation works in literature.

4:40 PM  
​Non Equilibrium Thermodynamics of Quench and Partition Steels: Amit Behera1; Greg Olson2; 1Northwestern University; 2Northwestern university
    Quench and Partitioning (Q&P) is a novel concept that promises to take the third generation of advanced high strength steels (AHSS) to better combinations of strength and ductility while reducing the overall manufacturing costs. Thermodynamics based genomic design approach supported by advanced microstructural characterization techniques would help design optimized processing routes and new steel compositions without the need for prolonged experimental studies. The current work puts effort to understand and model non-equilibrium thermodynamics behind the different phase transformations occurring during the Q&P cycle. Experimental measurements are used to quantify stored energy for displacive transformations. Models based on the thermodynamic and kinetic aspects of these transformations are developed and simultaneously used to design optimized processing cycles and new alloy compositions. The thermodynamic and kinetic simulations with use of commercially available software such as ThermoCalc, DICTRA are calibrated and compared to highly accurate experimental measurements using electron microscopy and 3-Dimensional Atom Probe.

4:55 PM  
Impact of the Prior Austenite Grain Size on the Martensitic Transformation and Consequences During the Q&P Route: Carola Celada Casero1; Jilt Sietsma1; Maria J. Santofimia1; 1Delft University of Technology
    Generally, the impact of the Prior Austenite Grain Size (PAGS) is not considered when designing Quenching & Partitioning (Q&P) processing routes. In this work, austenitization temperatures in the range 900-1100 °C were applied to a cold-rolled 0.2C-3.5Mn-1.5Si (wt. %) steel to generate different PAGSs. It was observed that decreasing PAGSs lead to a reduction of the martensite start temperature (MS). Consequently, under fixed Q&P conditions, decreasing the PAGS involves the formation of a smaller volume fractions of prior athermal martensite (fM1). In order to study the influence of the PAGS on the phase sizes and distributions but excluding the effect on the MS temperature, the quenching temperature (TQ) has been selected for each grain size to obtain comparable values of fM1. The results of this investigation have allowed for a better understanding of how the microstructure scale and distribution influences the mechanisms leading to microstructural changes during the Q&P processing.

5:15 PM  
Reversed Austenite for Enhancing Ductility of Martensitic Stainless Steel: Sebastian Dieck1; Thorsten Halle1; 1Otto-von-Guericke University
    The quenching and partitioning (Q+P) heat treatment enables a higher deformability of high strength martensitic steels. Therefore it is necessary to have some metastable austenite in the microstructure, which transforms in martensite during plastic deformation (TRIP-effect). This condition is guaranteed by the quenching and the additional partitioning treatment. Due to local carbon diffusion retained austenite is stabilized and a partial reversion of austenite from martensite occurs. The Q+P heat treatment was investigated for the martensitic stainless steel 1.4034 (X46Cr13) concerning the influence of partitioning time. In line with these efforts metallographic, XRD- and EBSD-measurements were performed to characterize the microstructural evolution. The mechanical experiments included mechanical testing with different strain rates. The reversion of austenite by the partitioning treatment could be detected with EBSD- and XRD-measurements. Furthermore the results of the mechanical testing showed improved values of strength and deformability as a consequence of the Q+P heat treatment.