Advanced High-Strength Steels: Impact of Solutes
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Tilmann Hickel, Max-Planck-Institut fuer Eisenforschung GmbH; Wolfgang Bleck, RWTH Aachen; Amy Clarke, Colorado School of Mines ; Young-Kook Lee, Yonsei University; Matthias Militzer, The University of British Columbia

Tuesday 2:00 PM
February 28, 2017
Room: 17A
Location: San Diego Convention Ctr

Session Chair: Tilmann Hickel, Max-Planck-Institut für Eisenforschung; Mohamed Goune, ICMCB-Bordeaux1

2:00 PM  Invited
New Insights into H Trapping and Diffusion in Steel Microstructures Obtained from Atomistic Simulations: Matous Mrovec1; Davide Di Stefano2; Christian Elsässer2; Roman Nazarov3; Tilmann Hickel4; Jörg Neugebauer4; 1ICAMS, Ruhr University Bochum, Germany; 2Fraunhofer IWM; 3Lawrence Livermore National Laboratory; 4Max Planck Institute for Iron Research
    A correct description of hydrogen diffusion and trapping is prerequisite for understanding the phenomenon of hydrogen embrittlement. Nevertheless, knowledge of microscopic diffusion and trapping processes in complex microstructures of modern materials, such as advanced high-strength steels, is still limited. We have recently explored the interaction of H with various microstructural features found in modern steels using accurate first-principles calculations based on density functional theory. The investigated cases include point defects, dislocations and grain boundaries as well as interfaces between the Fe matrix and various carbide precipitates. We computed both H diffusion barriers and segregation energies in the vicinity of these defects and examined structural, chemical and mechanical aspects of the H-defect interactions. This analysis enabled us to gain new insights into microscopic mechanisms of hydrogen diffusion and trapping that can be transferred to phenomenological models and theories as well as help to interpret experimental observations.

2:20 PM  
Hydrogen Solubility near Surfaces and Interfaces: Robert Spatschek1; Giorgia Gobbi2; Claas Hueter1; Aurab Chakrabarty3; Ugur Aydin4; Steffen Brinckmann4; Joerg Neugebauer4; 1Forschungszentrum Juelich; 2Politecnico di Milano; 3Texas A&M University at Qatar; 4Max-Planck-Institut fuer Eisenforschung GmbH
    Hydrogen embrittlement is a major issue especially for high strength steels. Different mechanisms are proposed as reasons for this kind of material failure. We pursue here the formation of hydrides on a theoretical basis, using scale bridging descriptions. In particular, we investigate phase separation including elastic coherency effects in the bulk and at surfaces and find a reduction of the solubility limit in the presence of free surfaces. This mechanism favours phase separation near free surfaces even in the absence of external stresses. The theoretical framework is applied to different metals and compared to experimental findings. Ultimately, a reduction of the solubility limit by up to two orders of magnitude at room temperature in the presence of free surfaces can occur, which may favour hydrogen embrittlement.

2:40 PM  
Ab Initio Calculations of Solute Effects on the Lattice Parameters and Elastic Constants of Fe Phases: Michael Fellinger1; Louis Hector Jr.2; Dallas Trinkle1; 1University of Illinois at Urbana-Champaign; 2General Motors
    Integrated computational materials engineering of third-generation steels requires a multi-scale approach that passes first principles data to meso-scale (e.g. microstructural) models. Here, density functional theory is used to compute Al, B, Cu, Mn, Si, C, and N solute effects on the lattice and elastic constants of BCC, BCT, and FCC Fe. We propose a solute strain misfit tensor to quantify the solute dependence of the lattice parameters, and the strain contributions to changes in elastic constants. We also compute the effects of changes in chemical bonding due to solutes on the elastic constants. Computing the two elastic constant contributions separately is computationally efficient, and their sum agrees with costlier direct calculations that encompass both effects. The computed data estimates solute-induced changes in mechanical properties like strength and ductility, and serves to increase the predictive capabilities of phase field and crystal plasticity simulations.

3:00 PM  
Tempering Reactions in Martensitic Stainless Steels Studied by Dilatometry and Correlative Magnetic Saturation Measurements: Qiuliang Huang1; Olena Volkova1; Horst Biermann1; Javad Mola1; 1Technische Universität Bergakademie Freiberg
    The influence of Si and Mn addition on tempering reactions in a Fe–13Cr–0.47C martensitic stainless steel was studied by dilatometry and magnetic saturation measurements. For this purpose, apparent coefficient of thermal expansion (CTE) during continuous dilatometry heating was correlated with a novel method of studying tempering reactions based on the magnetic saturation measurement after each tempering step. The formation of paraequilibrium transition carbides and cementite in dilatometry was associated with a decrease in the apparent CTE. In magnetic saturation measurements, on the other hand, the formation of cementite had a demagnetizing effect. Both methods clearly confirmed the suppressing effect of Si on the cementite formation. Mn too weakly opposed the formation of cementite. Saturation magnetization of all steels increased gradually in the approximate temperature range of 400 °C-600 °C which was attributed to the partitioning of substitutional alloying elements between the martensitic matrix and the pre-existing precipitates.

3:20 PM  
Atomic Scale Study of Boron Non-equilibrium Segregation and Precipitation at Prior Austenite Grain Boundary in High Strength Steels: Gregory da Rosa1; Philippe Maugis1; Josée Drillet2; Veronique Hebert2; Nathalie Valle3; Khalid Hoummada1; 1Aix-Marseille Université, CNRS, IM2NP; 2ArcelorMittal Maizières Research SA; 3 Luxembourg Institute of Science and Technology
    It has been proposed that boron tends to segregate at prior austenite grain boundaries (PAGB) in low carbon steels, decreasing their interface energy. In this way, boron retards the austenite-to-ferrite transformation; it means an improvement on the hardenability. It is generally accepted that B segregation is controlled by formation and diffusion of vacancy-solute complex (non-equilibrium segregation, NES) toward PAGB during cooling. However, as consequence of this segregation, B can also precipitates at these PAGB’s, losing its effectiveness. Therefore, B segregation has to be controlled. These behaviors were studied by Atom probe tomography and NanoSIMS in a model alloy of high strength steels. The couple of these techniques shows that precipitation takes place after boron NES exceeds a certain level.

3:40 PM Break

4:00 PM  
Influence of Microalloying Elements Ti and Nb in Solid Solution and as Precipitates during Annealing of Advanced High-strength Steels: Marion Bellavoine1; Myriam Dumont1; Josée Drillet2; Véronique Hebert2; Philippe Maugis1; 1IM2NP; 2ArcelorMittal Research SA
    Microalloying elements Ti and Nb are commonly added to high-strength steels as they can provide efficient means for additional strengthening due to grain refinement and precipitation strengthening mechanisms. In the form of solute elements or as nano-scaled carbonitride precipitates, Ti and Nb are also expected to have a significant effect on the microstructural changes during annealing. The present work investigates the separate influence of Ti and Nb on precipitation, ferrite recrystallization and austenite formation during annealing of various cold-rolled Dual Phase steel grades. Qualitative and quantitative data regarding these mechanisms are obtained by matrix dissolution techniques, SEM-FEG and TEM. Coupling models for non-isothermal precipitation, recrystallization and austenite formation allows obtaining good agreement to experimental results together with additional conclusions regarding the influence of microalloying elements. One of these conclusions is regarding the comparative effect of Ti and Nb in the form of solute elements and precipitates on delaying ferrite recrystallization.

4:20 PM  
Low Alloy High Strength Martensitic Nitrogen Steel: John Chinella1; 1U.S. Army Research Laboratory
    This presentation describes Fe-Cr-Ni-W-C-N-V high strength steels designed and fabricated for enhanced damage tolerance and environmental durability. Material designs and characteristics are described by Thermo-Calc software. The design objectives are high strength, low alloy martensitic steels with a short range order structure scale, by nitrogen alloying, specifically to better resist dynamic high pressure loading effects of early onset of failures possible by thermodynamic instabilities, softening of plastic flow, plastic flow instabilities, e.g. shear bands, or decohesion or shear failure along grain boundaries or precipitates. Thermo-Calc phase versus temperature property diagrams at ambient or elevated pressures reveal hardening and toughening phases of low enthalpy, high strength, and thermodynamic stability relative to the matrix at elevated temperature and/or pressure. At elevated pressures there is an absence of microstructure reversion to austenite that may promote softening. At hardening temperatures, there is an absence of phases which may contribute weak or coarse constituent carbides.

4:40 PM  
Effects of Aluminum Addition on Warm Ductility and Microstructure in Mn-rich Steels: Guan-Ju Cheng1; Chun-Te Wu1; Delphic Chen2; Ching-Yuan Huang2; Hung-Wei Yen1; 1National Taiwan University; 2China Steel Corporation
    The current work investigated the effects of aluminum on microstructure evolution during warm deformation in a strong but ductile Mn-rich steel. The cold-rolled steel was deformed by uniaxial tensile during inter-critical annealing at 600°C to 720°C. The microstructure was characterized by using transmission electron microscopy, X-ray diffraction and transmission Kikuchi diffraction. It was found that austenite transformation enhances the warm ductility of these steels. Besides, addition of aluminum changes the amount of austenite when inter-critical annealing and the stability of austenite after cooling. Both effects are crucial in designing strong but ductile steels for warm processing.

5:00 PM  
The Role of Copper in Microstructures and Mechanical Properties of Laser-welded Fe-19Ni-3Mo-1.5Ti Maraging Steel Joint: Kun Li1; Jiguo Shan1; Peng Wen1; Aiping Wu1; Chunxu Wang2; Zhiling Tian2; 1Tsinghua University; 2Central Iron & Steel Research Institute
     Maraging steel welded joints with high strength and toughness were successfully obtained. The element of Cu has a significant effect on precipitates and reverted austenite, by influencing the element diffusion and phase transformation. The element of Cu in the weld metal brings out a lot of ε-Cu precipitate during the aging period. The amount of ε-Cu precipitate increases gradually with the increase of the aging temperature, which is beneficial to the strength, yet harmful to the toughness. Meanwhile,the addition of Cu changes the diffusion kinetic condition to promote the diffusion of Ni, resulting in the decrease of segregation of elements. The reverted austenite in grain boundaries which has an adverse effect on the toughness has been correspondingly reduced. Besides, Cu makes the thermodynamic condition different, with a lower critical driving force of phase transformation, to cause the increase of reverted austenite in the matrix.