Steel Product Metallurgy and Applications: Atomic, Nanoscale, and Mesoscale Microstructure Characterization
Program Organizers: Amy Clarke, Los Alamos National Laboratory
Monday 2:00 PM
October 8, 2012
Room: Room 406
Location: David L. Lawrence Convention Ctr
Session Chair: Kester Clarke, Los Alamos National Laboratory; Roger Pradhan, Mittal Steel USA
2:00 PM
Characterization of Precipitation and Segregation Behavior in a Novel Ultra High Strength Steel with XRD, TEM and 3D Atom Probe: Matthew Hartshorne1; Paul Novotny2; Michael Schmidt2; Dieter Isheim3; David Seidman3; Mitra Taheri1; 1Drexel University; 2Carpenter Technology Corporation; 3Northwestern University
Ultra High Strength (UHS) steels used in high performance applications typically require large concentrations of alloying elements to achieve the required strength and toughness. The Carpenter Technology Corporation has developed a new family of UHS steels with comparable mechanical properties to current alloys, yet possessing markedly reduced levels of a wider variety of alloying elements. The toughening mechanisms of this system are not well understood, necessitating a detailed investigation of its microstructural, precipitation and solid solution behavior. Experiments will be presented detailing precipitate type and location within the microstructure by utilizing XRD and high resolution TEM. These experiments are coupled with 3D atom probe studies, which allows for the analysis of solute segregation at grain, lath and precipitate boundaries. The combination of these techniques provides a more comprehensive understanding of the role and interactions of each of the alloying elements in this new alloy.
2:20 PM
Atomic Scale Investigation on Precipitates in High Carbon Bearing Steel 100Cr6
: Wenwen Song1; Ulrich Prahl1; Pyuck-Pa Choi2; Wolfgang Bleck1; Dierk Raabe2; 1RWTH Aachen University; 2Max-Planck-Institut Für Eisenforschung GmbH
Given that controlling the precipitates in bearing steels plays a very important role in enhancing their wear resistance and fatigue properties, in this work Transmission Electron Microscopy (TEM) and Local field Electrode Atom Probe (LEAP) were applied to study carbides in high carbon bearing steel 100Cr6 with bainite matrix. TEM investigation proves the spheroidized carbides in 100Cr6 are (Fe, Cr)3C. 3 Dimensional Atom Probe analyses show concentration and distribution of elements, i.e. C, Si, Mn, Cr, Fe in carbides and the results further reveal that spheroidized (Fe, Cr)3C with a mean Cr content of 10 at% shows a 10-15nm pure cementite layer on its surface after the followed bainitizing process. Moreover, results also show that during higher bainitizing temperature, the second phases within bainite are mostly Θ-phase, while in lower temperature ε-phase is indicated. The influences of interstitials and alloy elements on precipitates and the property enhancements through precipitates are further discussed.
2:40 PM
Control of Carbide Distributions by Modifying Heating Rates in Induction Tempering Treatments: Cristina Revilla1; Pello Uranga1; Jose M. Rodriguez-Ibabe1; Beatriz López1; 1CEIT and Tecnun
Microstructures produced by quenching and tempering treatments are characterized by carbide particles distributed in a steel matrix. In conventional tempering, the treatment parameters are selected in order to obtain predefined strength requirements. Sometimes a proper control of the coarse carbide sizes and volume fractions becomes difficult, aspect that becomes relevant when high toughness requirements are necessary. Induction tempering appears as a procedure able to modulate the characteristics of the distribution of coarse grain boundary carbides. This study focuses on the influence of different heating rates, ranging from 1 to 300şC/s, on the nucleation and growth of carbides for the case of a low alloy 0.4%C steel. The study analyses differences between carbides nucleated at high angle grain boundaries and those formed in the matrix and their evolution as a function of tempering time. Quantitative microstructural measurements including EBSD analysis have been done, correlating carbide nucleation steps with matrix recovery levels.
3:00 PM
Optimization and Characterization of a Quenching and Partitioning Heat Treatment on a Low Carbon Steel: Dorien De Knijf1; Roumen Petrov1; Leo Kestens1; Cecilia Fojer2; 1Ghent University; 2OCAS
Restrictions on fuel consumption and safety in the automotive industry have stimulated the development of new advanced high strength steels. A new way of achieving a favourable compromise between strength and ductility is by a Quenching and Partitioning heat treatment. This method produces a martensitic microstructure with a considerable amount of retained austenite. A low carbon steel was designed based on thermodynamic calculations whereby 3 wt. % Mn is added to suppress bainite formation and 1.5 wt. % Si to avoid cementite precipitation. Optimized quenching and partitioning cycles are performed in the thermo mechanical simulator Gleeble©. The microstructural changes are identified by optical microscopy, scanning electron microscopy, X-ray diffraction, EBSD study and high resolution TEM analysis. Tensile tests are carried out to study the obtained mechanical properties which are finally correlated with the annealing parameters and microstructures in terms of optimal austenite fraction and distribution.
3:20 PM
The Effect of Silicon on the ε --> θ Transformation in Ultra-Strong Spring Steels: Bij-Na Kim1; David San Martín2; Jesús Chao2; Pedro E.J. Rivera-Díaz-del-Castillo1; 1University of Cambridge; 2Centro Nacional de Investigaciones Metalurgicas (CENIM-CSIC)
Tempered martensitic steels have shown considerable strengthening by controlled nanoprecipitation of the transitional ε-carbide, a precursor to cementite. Initial studies have been carried out in a spring steel model alloy of composition Fe-0.55C-2.2Si wt.%. Carbon was chosen as a carbide former, and silicon because of its inhibiting effect in the cementite transformation from epsilon. Tempering was carried out in order to understand the transformation mechanism. The classical nucleation theory was used in order to understand the carbide transition. Both formation and misfit strain energies have been modelled in order to explain precipitation behaviour. Mechanical tests have shown that in the transformation regime there was a loss in strength and an increase in ductility. Albeit the apparent simple alloy composition, yield strengths above 2.4 GPa were obtained. Analysis of the microstructure was carried out in a systematic manner in order to statistically distinguish precipitation behaviour during tempering.
3:40 PM Break
4:00 PM
A Study on the Microstructual Evolution and Phase Transformation of Lightweight Duplex Steel: Jai-hyun Kwak1; Kyoo-young Lee1; Kwang-geun Chin1; 1POSCO
Recently, duplex lightweight steels containing a low manganese and high aluminium content have been designed in POSCO as a new class of AHSS for the purpose of the specific weight reduction without decreasing the mechanical properties and without the expensive alloy elements. The mechanical properties of the alloys depend on the microstructure and chemical composition of nano-sized κ-carbide. However, the decomposition of metastable austenite into ferrite and κ-carbide has not been studied in detail, in particular at the atomic scale. Therefore, In this study, phase transformation and microstructual evolution of lightweight duplex steel have been characterized using XRD, STEM and APT in conjunction with thermodynamic descriptions.
4:20 PM
Three-Dimensional Boundary Networks in a Grain Boundary Engineered Stainless Steel: Alexis Lewis1; David Rowenhorst1; 1Naval Research Laboratory
Grain boundary engineering (GBE) has been applied to many FCC materials, resulting in vast improvements in a number of boundary-controlled properties, including diffusional creep and stress corrosion cracking. In this work, the microstructure of an austenitic stainless steel (316L) was reconstructed in three dimensions to characterize the 3D networks of grain boundaries, including both general high-angle boundaries and special coincident site lattice (CSL) boundaries. The microstructure is characterized using serial sectioning and EBSD, and the connectivity of the grain boundary networks is measured, for both an as-received specimen, and one which has been subjected to Grain Boundary Engineering to increase the population of special CSL boundaries. The 3D connectivity of the boundary networks is quantified and correlated to simulated materials response.
4:40 PM
A Study on Goss Texture Evolution in Silicon Steel: Jae Kyoum Kim1; Dong Nyung Lee2; Kyu Seok Han3; Chan Hee Han3; Yang Mo Koo1; 1Pohang University of Science and Technology; 2Seoul National University; 3POSCO
Grain-oriented silicon steel which is made by complex process has a strong Goss {110}<001> texture. The Goss texture which is one of shear deformation textures develops in the surface layers of hot rolled steel sheets. In this study, a Fe-3%Si alloy sheet with shear deformation textures was cold rolled up to 85% reduction and annealed at different temperatures. The evolution of microstructure and texture during cold rolling and annealing was investigated by using X-ray diffraction and electron backscatter diffraction (EBSD) technique. The <110>//RD fiber and {111}<112> were major components of the cold rolling texture. After primary recrystallization the Goss texture strongly developed.
5:00 PM
Surface Segregation of Phosphorus and Its Influence on the Texture Evolution of Si Steel: Jun-young Park1; D.T Alnahyan1; B.J. Lee2; Nack J. Kim1; 1Graduate Institute of Ferrous Technology, POSTECH; 2Department of Materials Science and Engineering, POSTECH
The control of texture is of utmost importance in electrical steels for improving their magnetic properties and there have been numerous studies on modifying the texture of the electrical steels. However, the effect of surface segregation on the texture development in electrical steels has not been clearly understood yet. In this research, the surface segregation of phosphorus in pure Fe and Si steel has been investigated, with particular emphasis on its effect on texture evolution. The specimens were subjected to various thermo mechanical treatments consisting of hot rolling, cold rolling, and annealing. The microstructure has been analyzed by 3DAP, XRD, and EBSD. The relationship between the surface segregation and the texture development in pure Fe and Si steel will be discussed along with their magnetic properties.
5:20 PM
Influence of Stacking-Fault Energy on the Deformation Mechanisms in High Mn Austenitic Steels: Dean Pierce1; James Bentley2; Jose Jimenez3; James Wittig1; 1Vanderbilt Univsersity; 2Microscopy and Microanalytical Sciences; 3Centro Nacional de Investigaciones Metalurgicas (CSIC)
Fe-Mn based austenitic steels have great potential for automotive applications owing to exceptional ductility (>70% fracture elongation in tension), strain hardening behavior, and toughness. These outstanding mechanical properties result from competing deformation mechanisms that include dislocation glide, martensitic transformation-induced plasticity (TRIP), and twinning-induced plasticity (TWIP). The active mechanism is strongly influenced by the stacking-fault energy which in turn is controlled by composition and temperature. This paper presents some of the first experimental SFE measurements for Fe-xMn-3Al-3Si (x=22, 25, 28 wt%) alloys using weak-beam dark-field imaging of partial-dislocation separation. Correlation between SFE and deformation behavior shows that the 22%Mn alloy with a SFE of ~11 mJ m-2 results in TRIP, 25%Mn with SFE ~16 mJ m-2 exhibits both TRIP and TWIP, and the 28%Mn alloy with SFE ~32 mJ m-2 deforms predominately by TWIP. The occurrence of both deformation mechanisms in the 25%Mn alloy produced the greatest ductility (>90%).
5:40 PM
Microstructural Evolution in TWIP Steels during Wire Drawing: Joong-Ki Hwang1; Il-cheol Yi1; Il-Heon Son2; Byoungkoo Kim1; Nack J. Kim1; 1Graduate Institute of Ferrous Technology, POSTECH; 2Wire Rod Research Group, Technical Research Laboratories, POSCO
Effect of wire drawing on the microstructural evolution of Fe-Mn-Al-C TWIP steel has been investigated. The results obtained by wire drawing have been compared with those by uniaxial tension and cold rolling. It has been shown that the evolution of texture and twins is dependent on the imposed stress state during straining. Analyses of texture in drawn wire show that a mixture of <111> and <100> fiber texture is developed; however, the distribution of <111> and <100> fibers is inhomogeneous along the radial direction of wire due to uneven shear strain. The thickness of deformation twins in drawn wire is in the range of 100nm to several micrometers, which is smaller than the ones formed by uniaxial tension and cold rolling. It has also been shown that the morphology of the twins depends on the stress state although the twinning mechanism does not appear to depend on the stress state.