Effects of solute Nb atoms and Nb precipitates on isothermal transformation kinetics from austenite to ferrite

Nb is a very important micro-alloying element in low-carbon steels, for grain size refinement and precipitation strengthening, and even a low content of Nb can result in a significant effect on phase transformation kinetics from austenite to ferrite. Solute Nb atoms and Nb precipitates may have different effects on transformation behaviors, and these effects have not yet been fully characterized. This paper examines in detail the effects of solute Nb atoms and Nb precipitates on isothermal transformation kinetics from austenite to ferrite. The mechanisms of the effects have been analyzed using various microscopy techniques. Many solute Nb atoms were found to be segregated at the austenite/ferrite interface and apply a solute drag effect. It has been found that solute Nb atoms have a retardation effect on ferrite nucleation rate and ferrite grain growth rate. The particle pinning effect caused by Nb precipitates is much weaker than the solute drag effect

Spectroscopic Coronal Observations during the Total Solar Eclipse of 11 July 2010

The flash spectrum of the solar chromosphere and corona was measured with a slitless spectrograph before, after, and during the totality of the solar eclipse, of 11 July 2010, at Easter Island, Chile. This eclipse took place at the beginning of the Solar Cycle 24, after an extended minimum of solar activity. The spectra taken during the eclipse show a different intensity ratio of the red and green coronal lines compared with those taken during the total solar eclipse of 1 August 2008, which took place towards the end of the Solar Cycle 23. The characteristic coronal forbidden emission line of forbidden Fe XIV (5303 {\AA}) was observed on the east and west solar limbs in four areas relatively symmetrically located with respect to the solar rotation axis. Subtraction of the continuum flash-spectrum background led to the identification of several extremely weak emission lines, including forbidden Ca XV (5694 {\AA}), which is normally detected only in regions of very high excitation, e.g., during flares or above large sunspots. The height of the chromosphere was measured spectrophotometrically, using spectral lines from light elements and compared with the equivalent height of the lower chromosphere measured using spectral lines from heavy elements.Comment: 14 pages, 8 figures, 1 table; Solar Physics, 2012, Februar

Microstructure Control of Fire-resistant, Low-alloy Steel; An in-situ 3D X-ray Diffraction and A Small-angle X-ray Scattering Study

The research presented in this thesis aims at deepening our understanding of the formation of the microstructure of steel during thermal processing in order to control the microstructure and thereby improve the fire-resistance of low-alloy steel. The strength of steel decreases during a fire mainly due to the following changes in the microstructure: 1) increased dislocation motion at elevated temperatures, facilitating plastic deformation, 2) coarsening of the microstructure by grain growth and coarsening of precipitates, which reduce the pinning effect on dislocations, 3) the phase transformation from ferrite to austenite can result in a coarser-grained single-phase structure, and 4) grain boundary sliding. Therefore it is very important to understand the kinetics for the formation of precipitates, dislocation structures, and grains and sub-grains during the austenite-to-ferrite phase transformation during thermal processing. In-situ characterization of these microstructural features is possible by using large-scale synchrotron radiation facilities. The precipitate size distribution evolution during thermal processing can be studied in-situ by small-angle x-ray scattering (SAXS). The nucleation and growth of grains during solid-state phase transformations can be studied in-situ by means of three-dimensional x-ray diffraction (3DXRD) microscopy. The evolution of the dislocation structure during thermal processing can be studied by combining SAXS and 3DXRD. Chapter 2 introduces the basic concepts of microstructures and mechanisms to strengthen low-alloy steel at elevated temperatures. Firstly, the different types of dislocations and slip mechanisms in steel are reviewed. Secondly, the main characteristics of low and high-angle grain boundaries are presented. Thirdly, the main mechanisms for the loss of strength of steel at high temperature are discussed briefly. Lastly, a review of the literature for improving the fire-resistance of steel is presented. Chapter 3 introduces the theory of small-angle X-ray scattering (SAXS) and 3 Dimensional X-ray diffraction (3DXRD) microscopy using synchrotron radiation for microstructural analysis. In the section related to SAXS, the experimental method and data analysis strategies which are used for measuring the characteristics of precipitates and dislocation walls are described in detail. In the section related to 3DXRD, the experimental method, the pre-processing steps for the data analysis, and the 3DXRD data analysis procedure applied for finding grain characteristics are described. Chapter 4 presents the evolution of the size distribution of the (Fe,Cr)-carbides and dislocation structures in steel during a heat treatment as investigated by in-situ SAXS using synchrotron radiation with a new data analysis strategy. The size distribution of the (Fe,Cr)-carbides during heat treatments is determined from the isotropic component of the SAXS patterns. Bright-field transmission electron microscopy (BF-TEM) and high-resolution transmission electron microscopy (HR-TEM) reveal the nearly spherical morphology of the precipitates. Additional measurements have been carried out on a single crystal of ferrite containing (Fe,Cr)-carbides by combining 3DXRD and SAXS during rotation of the specimen at room temperature in order to understand the origin of the streaks in the SAXS-pattern. From simulations based on the theory of SAXS from dislocations we derive that the measured streaks, in the 2D SAXS-patterns including the spottiness, correspond to a dislocation structure of symmetric low-angle tilt boundaries, which in turn corresponds to the crystallographic orientation gradient in the single crystal of ferrite as measured by 3DXRD. Chapter 5 presents the effect of niobium and the grain boundary density on the fire-resistance of Fe-C-Mn steel. Two steels are used: Fe-C-Mn steel and Fe-C-Mn-Nb steel with an atomic ratio Nb/C=1.3. Two different sets of heat treatments are carried out before the fire-test. The first set of heat-treatments consists of heating the steel to 1100°C to bring all niobium in solid solution and austenitizing before continuous cooling at three different cooling rates during the austenite-to-ferrite phase transformation. The second set of heat treatments consists of heating the steel to bring all niobium in solid solution and austenitizing, rapid cooling to room temperature, reheating to 600?C, and annealing for different times before rapid cooling to room temperature. The fire-tests are carried out on specimens by applying a force equal to 60% of the room temperature yield strength at elevated temperature using a Gleeble thermo-mechanical simulator. In this study two techniques are used for analysing the microstructure: 1) Electron Back Scattered Diffraction (EBSD) and, 2) Optical microscopy. In this chapter we show that the low-angle and high-angle grain-boundary densities increase with increasing cooling rate during the austenite to ferrite phase transformation for the niobium containing steel, whereas total grain boundary density decreases with increasing the annealing time at 600°C. We show that the addition of 0.10 wt.% niobium to Fe-C-Mn steel increases the failure temperature of steel by 92°C during a fire-resistance test. Moreover, we demonstrate that the failure temperature increases linearly up to an additional 45°C with increasing grain boundary density from 0.06 to 0.64 ?m-1 for the niobium-containing steel. In Chapter 6 the effect of niobium in solid solution and NbC-precipitates on the austenite-to-ferrite phase transformation kinetics is investigated. In order to separate the effects of NbC-precipitates and Nb in solid solution on the phase transformation kinetics, we use three high-purity Fe-C-Mn steels with different niobium concentrations and one without niobium. Three-dimensional x-ray diffraction (3DXRD) microscopy is used at a third-generation synchrotron radiation facility to study in-situ and simultaneously the nucleation and growth of individual ferrite grains in the bulk of steel during the austenite/ferrite phase transformation. The measured nucleation rate is compared to the classical nucleation theory (CNT) to determine the nucleation parameters. The effects of NbC-precipitates and the concentration of Nb in solid solution on the incubation time, frequency factor, and the activation energy for ferrite nucleation are quantified. The experimentally measured nucleation start temperature is 40-115°C lower than predicted by Thermo-calc for the four alloys under ortho-equilibrium conditions, depending on the concentration of niobium. The frequency factor for nucleation is found to decrease exponentially with increasing concentration of Nb in solid-solution. The ?-parameter, which contains information about the shape of the nucleus and the interfacial energies that are involved in the nucleation process, increases with increasing concentration of Nb in solid solution. The dependence of ?-parameter on the concentration of Nb in solid solution is best described by a pill-box type of geometry of the nucleus and a square root dependence of the ?/?-grain boundary energy with the Nb concentration in solid solution. The ratio of the density of ferrite grains to the density of austenite grains decreases by more than 25% due to the formation of NbC-precipitates, which can be interpreted as reduction of nucleation probability of potential nucleation sites for ferrite, i.e. grain corners, by NbC-precipitates. The ratio of the density of ferrite grains to the density of austenite grains does not depend significantly on the amount of niobium in solid solution. The delay in the start of the transformation as a function of the concentration of niobium in solid solution is found to depend on three factors: 1) the segregation of Nb to the ?/?-grain boundaries, which increases the activation energy for nucleation, 2) reduction of nucleation probability of potential nucleation sites by NbC-precipitates, and 3) diffusion of Nb-atoms from the ?/?-grain boundaries back into the matrix during the nucleation. The drag-effect of niobium in solid-solution on the growth of individual ferrite grains is quantified for different Nb-concentrations. In this work we follow the ‘dissipation’-approach initially developed by Hillert and Sundman et al. and further developed by Odqvist et al. By measuring the velocity of the interface and estimating the chemical driving force, the pressure due to curvature, and the pinning pressure, we determined the dissipation of Gibbs energy caused by the diffusion of the solute atoms being dragged along with the migrating interface, ?Gdiff (J/mol), as a function of the Nb-concentration in solid solution. The drag effect increases with increasing concentration of Nb. Chapter 7 presents an in-situ study in which the partial 3D microstructure of the austenite and ferrite phases before and after the transformation are characterized and in which the nucleation rate of ferrite is measured in an Fe-C-Mn alloy. Separating the experimentally observed ferrite nucleation rate in the bulk and in the surface region of the sample shows that the maximum nucleation rate is reached at lower temperature in the surface region than in the bulk. The activation energy for the nucleation of ferrite in the surface region is higher than in the bulk. An explanation for this could be the application of a thin nickel coating at the surface of specimen, which might result in interfaces between austenite and nickel grains that have lower energy than grain boundaries between austenite grains in the bulk. Future work could focus on deepening our understanding of the effect of low-angle grain boundaries on the fire-resistance of steel containing precipitates. Hitherto, it was assumed that low-angle grain boundaries do not contribute substantially to the strength of steel. However, this research demonstrates that low-angle grain boundaries do contribute substantially to the strength of steel at room temperature and they improve the fire-resistance. How does the interaction between a dislocation and a low-angle grain boundary lead to strengthening of the steel? How do the low-angle grain boundaries form during the austenite-to-ferrite phase transformation in niobium containing steel? What are the prerequisites for the formation of low-angle boundaries? Would the low-angle boundaries also form in steel containing different alloying elements than niobium? Which methods can be used to introduce an even higher density of grain boundaries? From a more application oriented point of view, future research could focus on testing the fire-resistant steel that is presented in chapter 5 under more realistic fire-conditions. Fire-tests could be performed in tensile-mode rather than compression-mode. Moreover, the creep-resistance of this alloy could be investigated to explore its potential applications in gas turbines for power generation and steel-pipes for the transport of hot-gasses from sea to land. An investigation into the low-temperature properties of the steel would also be very interesting in order to test the suitability of the steel for arctic conditions.Materials Science & EngineeringMechanical, Maritime and Materials Engineerin