High Performance Steels: Microstructure Development and Advanced Characterization I
Sponsored by: TMS Structural Materials Division, TMS: Steels Committee
Program Organizers: Jonah Klemm-Toole, Colorado School of Mines; Ana Araujo, Vesuvius USA; C. Tasan, Massachusetts Institute of Technology; Richard Fonda, Naval Research Laboratory; Amit Behera, QuesTek Innovations LLC; Benjamin Adam, Oregon State University; Krista Limmer, Devcom Army Research Laboratory; Kester Clarke, Los Alamos National Laboratory
Tuesday 8:00 AM
March 21, 2023
Room: Aqua F
Location: Hilton
Session Chair: Amit Behera, Questek Innovations; Benjamin Adam, Oregon State University
8:00 AM Invited
Metastability of Martensite and Bainitic Ferrite as Carbon Super-saturated Structures: Francisca Caballero1; Jonathan Poplawsky2; Esteban Urones-Garrote3; 1National Centre for Metallurgical Research (CENIM-CSIC); 2ORNL; 3Spanish National Centre for Electron Microscopy (CNME-UCM)
Both bainite and martensite reactions involve a sudden, ordered movement of iron atoms, which is accompanied by a given crystal correspondence, and a macroscopic shape strain of the transformed structure. Carbon supersaturation and tetragonality are not exclusive of martensite. A high level of carbon supersaturation in bainitic ferrite accompanied by a change in symmetry of the unit cell from cubic to tetragonal have been observed at low transformation temperatures confirming that, as martensite, bainitic ferrite grows without diffusion of carbon. It is therefore difficult to identify the morphological differences to discriminate metastable ferritic structures. In this work, comparable bainitic and martensitic structures were obtained and investigated by transmission electron microscopy (TEM) and atom probe tomography (APT) to identify all the subtle differences to discriminate both types of ferritic structures.
8:30 AM
In Situ & Post Mortem Investigations of Carbide-free Bainitic Transformations during Continuous Cooling: Cécile Rampelberg1; Guillaume Geandier1; Florimonde Lebel1; Sebastien Allain1; Julien Teixeira1; Thomas Sourmail2; 1Institut Jean Lamour; 2Ascometal
Carbide-Free Bainites are multiphase microstructures made of a lath ferritic matrix without carbide, retained austenite stabilized by carbon partitioning and martensite. We have investigated the formation of such microstructures from a fully austenitic state along different thermal treatments (isothermal holdings and continuous cooling treatments) by in situ High Energy X-Ray Diffraction (HEXRD) on synchrotron beamlines. Very precise carbon mass balances between the constituting phases have been established for the first time leading to the conclusions that the ferritic bainite is even more supersaturated in carbon than expected. The continuous cooling experiments have also proved that the bainitic transformation is highly sensitive to the transformation sequences. This result is corroborated with our post mortem Electron Back-Scattered Diffraction (SEM-EBSD) observations which reveal that microstructures after continuous cooling show very large distributions of sizes, morphologies and microtextures.
8:50 AM
Tempering of Low Carbon Martensite, Experimental Results and Model Development: Juan Macchi1; Julien Teixeira1; Guillaume Geandier1; Sabine Denis1; Frédéric Bonnet2; Sébastien Allain1; 1Institut Jean Lamour Ijl (Cnrs Umr7198); 2ArcelorMittal Research SA
As-quenched martensite has a high UTS with a relatively low maximum elongation. To customize the mechanical behavior, tempering treatments are usually applied. During these treatments, C atoms segregate from the oversaturated matrix into defects, followed by the precipitation of transition carbides (if the C content is high enough) and finishing with their dissolution and cementite formation. In the present works characterizations at different scales (Atom Probe Tomography, TEM, and in-situ High Energy XRD) were performed to determine the mentioned transitions in 0.1, 0.2, and 0.3 wt.%C steels. A class-type precipitation model has been developed considering C segregation, epsilon, and cementite precipitation. The fraction, size, and density of the carbides as well as the C concentration segregated to dislocations and matrix have been calculated for temperings at 200, 300, and 400°C. Generally good agreement is found between experimental and model results. The impact of segregation on the precipitation sequence is analyzed.
9:10 AM
A Modified Model for Predicting Retained Austenite Using Informed Compositional Modeling: Melissa Thrun1; Amy Clarke1; Kester Clarke1; 1Colorado School of Mines
The newest advanced high strength steels go through relatively complex heat treatment processes that are closely tied to the alloy’s martensite start (Ms) temperature and require rapid temperature gradients. However, predictions of Ms temperature are usually based on the bulk composition, rather than local composition, so compositional heterogeneities are not considered. Here, thermodynamic predictions are used to create a rule-of-mixtures model for martensite transformation in heterogeneous (both in grain size and composition) alloys to show the effect of compositional gradients on local and bulk Ms temperatures. Predictions of bulk Ms temperatures are then related to bulk dilatometric measurements and retained austenite models established in literature, resulting in predictions of lower retained austenite contents and a range of quench temperatures in which the maximum retained austenite content can be achieved during quenching and partitioning processing.
9:30 AM Break
9:50 AM
A New Approach for Optimizing Heat Treatment Parameters in Q&P Steel: Casey Gilliams1; Kip Findley1; John Speer1; 1Colorado School Of Mines
Quenched and partitioned (Q&P) steels are designed to maintain strength and improve formability while utilizing leaner alloyed steels through the development of martensitic microstructures with retained austenite. Literature regarding Q&P steels primarily focus on maximizing retained austenite volume fractions rather than considering the influence of chemical and morphological characteristics of the retained austenite. In this work, consideration of retained austenite characteristics and the influence of prior processing on the heat treating response and resulting microstructures of Q&P steels are investigated in a 0.17C-2.8Mn-1.5Si steel through dilation simulations. A new methodology for optimizing Q&P heat treatment parameters utilizing fixed austenitizing conditions at a fixed partitioning temperature, involving careful monitoring of secondary martensite formation during final quenching, was introduced. Findings suggest that for a fixed composition, application of a general Q&P heat treatment results in similar microstructures and retained austenite volume fractions, independent of prior processing.
10:10 AM
Semi In-situ Observation of Micro-crack Formation in Dual-phase Steels: Hung-Wei Yen1; Ming-Yu Tseng1; Yi-Fan Hu1; Kuo-Cheng Yang2; Kangying Zhu3; 1National Taiwan University; 2China Steel Corporation; 3ArcelorMittal Maizières
This work presents a prototype of semi in-situ tensile apparatus (SISTA), which enables semi in-situ observation on materials deformation under scanning electron microscope. SISTA is applied to study micro-crack formation in DP780 and DP980 dual-phase steels in tension by using electron backscattering diffraction and electron channeling contrast image. The results show that DP980 steel tends to form micro-cracks from ferrite grain surrounded by martensite. In contrast, DP780 steel tends to form micro-cracks at the vicinity of martensite/ferrite interface. The related mechanisms of micro-cracking are discussed in the view of deformation behaviors in two DP steels. This study provides insights into materials failure in forming and materials design of DP steel.
10:30 AM
Nucleation during Static Recrystallization of Austenite - A Combined Experimental and Modeling Approach: Pablo Garcia Chao1; Vitesh Shah2; Jonathan Joe Eipe1; Jesus Galan-Lopez1; Monika Krugla3; Winfried Kranendonk3; Jilt Sietsma1; Cornelis Bos3; Sven Erik Offerman1; 1Delft University of Technology; 2Max-Planck-Institut für Eisenforschung; 3Tata Steel, Research & Development
The implementation of new steels with improved performance can be compromised by the time-consuming setups typically required in the actual production line. To minimize the number of trials needed, models comprising realistic microstructural topology represent a more robust approach. Following this line, this study intends to predict the evolution of microstructure during static recrystallization (SRX) of austenite using a 3D cellular automaton (3D-CA). Prediction of nucleation through a physics-based sub-model is emphasized, since present recrystallization models are found to lack accuracy when topology is incorporated. The role of the oriented early growth of recrystallized grains in the competition between potential nucleation sites is also considered. Additionally, the competition between the various nucleation sites (namely, multiple grain junctions and grain boundaries) is investigated via electron backscatter diffraction (EBSD) experiments performed in a Fe-Ni alloy. The mechanism of nucleation at multiple grain junctions is also discussed in the light of experimental evidence.
10:50 AM
Numerical Investigations of Interface-controlled Phase Transformations during Intercritical Annealing of DP Steels: Clelia Couchet1; Julien Teixeira1; Sébastien Allain1; Guillaume Geandier1; Frédéric Bonnet2; 1Institut Jean Lamour-Ijl (Cnrs Umr 7198); 2ArcelorMittal Maizières Research
Austenite formation kinetics during intercritical annealing of various steels have been simulated using DICTRA/Thermo-Calc software. Carbide dissolution, austenite islands growth and interaction were investigated in a ternary Fe-C-Mn considering several geometries inspired by our SEM studies. The interface conditions were studied in multi-constituent systems (up to quinary). In particular, carbides were found to dissolve rapidly at the Partition-No partition Transition Temperature (PNTT). A regime transition from No Partitioning Local Equilibrium (NPLE) to Partitioning Local Equilibrium (PLE) occurs at the beginning of the holding stage. Austenite formation kinetics was found to be highly sensitive to substitutional partitioning during the holding stage. By considering silicon in simulations, silicon partitioning in ferrite strongly slows the austenite formation kinetics, making the simulated austenite fraction closer to the fraction determined from High Energy X-Ray Diffraction experiments.