Size Distribution Measurements of Ultrafine Aerosols,d p > 1.8 nm,Formed by Radiolysis in a Diameter Measurement Analyzer Aerosol Charger

A University of Vienna differential mobility analyzer-electrometer arrangement was used to measure the size distribution of charged ultrafine aerosols (d _p > 1.8 nm) generated from a N₂-SO₂-H₂O mixture by radiolysis in a bipolar diffusion charger loaded with a 2.5 mCi Am-241 α-source. Despite the short residence time in the charger, t_r < 1s, nucleation mode diameters near 2 nm were found for dry gas mixtures with water vapor concentrations < 32 ppm. At 20% relative humidity number concentrations increased and modal diameters shifted to larger sizes with increasing SO₂ concentration. The addition of trace concentrations of nitric oxide or ethanol vapor to the gas mixtures resulted in a near complete suppression of particle formation.     

Kinetics of the peritectic phase transformation: In-situ measurements and phase field modeling

An experimental study has been conducted into the role of cooling rate on the kinetics of the peritectic phase transformation in a Fe−C alloy. The interfacial growth velocities of the peritectic phase transformation were measured in situ for cooling rates of 100, 50, and 10 K/min. In-situ observations were obtained using high-temperature laser scanning confocal microscopy (HTLSCM) in a concentric solidification configuration. The experimentally measured interface velocities of the liquid/austenite (L/γ) and austenite/delta-ferrite (γ/δ) interphase boundaries were observed to increase with higher cooling rates. A unique finding of this study was that as the cooling rate increased there was a transition point where the L/γ interface propagated at a higher velocity than the γ/δ interface, contrary to the findings of previous researchers. Phase field modeling was conducted using a commercial multicomponent, multiphase package. Good correlation was obtained between model predictions and experimental observations in absolute values of interface velocities and the effect of cooling rate. Analysis of the simulated microsegregation in front of the L/γ and γ/δ interfaces as a function of cooling rate revealed the importance of solute pileup. This microsegregation plays a pivotal role in the propagation of interfaces; thus, earlier modeling work in which complete diffusion in the liquid phase was assumed cannot fully describe the rate of propagation of the L/γ and δ/γ interfaces during the course of the peritectic transformation.     

Phosphorus distribution between carbon-saturated iron at 1350 °C and lime-based slags containing Na2O and CaF2

The Corex process is capable of consistently producing hot metal with very low silicon contents (<0.1 pct), and as a consequence, its hot metal is ideally suited for the external removal of phosphorus. Various studies have shown that small additions of Na₂O significantly enhance the ability of lime-based slags to dephosphorize liquid iron. Additions of fluxes (such as CaF₂) may be required to ensure that the slags remain fluid during treatment. The aim of the present investigation was to study the dephosphorization capabilities of lime-based slags from the CaF₂-CaO-Na₂O-SiO₂ system. Phosphorus containing slag and carbon-saturated iron was equilibrated in carbon crucibles at 1350 °C under a carbon monoxide atmosphere. It was confirmed that additions of Na₂O increase the phosphate capacity of silicate and lime-based slags considerably. Additions of CaF₂ to Na₂O containing lime-based slags increase the activity coefficient of P₂O₅ and, therefore, decrease the phosphate capacity thereof. These slags have high phosphate capacities and low melting points, yielding them suitable as effective reagents for dephosphorization, and even desulphurization, of hot metal at relatively low temperatures. However, CaF₂ additions to these slags should be limited.     

Kinetics of the peritectic phase transformation: In-situ measurements and phase field modeling

An experimental study has been conducted into the role of cooling rate on the kinetics of the peritectic phase transformation in a Fe−C alloy. The interfacial growth velocities of the peritectic phase transformation were measuredin situ for cooling rates of 100, 50, and 10 K/min.In-situ observations were obtained using high-temperature laser scanning confocal microscopy (HTLSCM) in a concentric solidification configuration. The experimentally measured interface velocities of the liquid/austenite (L/γ) and austenite/delta-ferrite (γ/δ) interphase boundaries were observed to increase with higher cooling rates. A unique finding of this study was that as the cooling rate increased there was a transition point where the L/γ interface propagated at a higher velocity than the γ/δ interface, contrary to the findings of previous researchers. Phase field modeling was conducted using a commercial multicomponent, multiphase package. Good correlation was obtained between model predictions and experimental observations in absolute values of interface velocities and the effect of cooling rate. Analysis of the simulated microsegregation in front of the L/γ and γ/δ interfaces as a function of cooling rate revealed the importance of solute pileup. This microsegregation plays a pivotal role in the propagation of interfaces; thus, earlier modeling work in which complete diffusion in the liquid phase was assumed cannot fully describe the rate of propagation of the L/γ and δ/γ interfaces during the course of the peritectic transformation.     

Austenite Grain Growth and the Surface Quality of Continuously Cast Steel

Austenite grain growth does not only play an important role in determining the mechanical properties of steel, but certain surface defects encountered in the continuous casting industry have also been attributed to the formation of large austenite grains. Earlier research has seen innovative experimentation, the development of metallographic techniques to determine austenite grain size and the building of mathematical models to simulate the conditions pertaining to austenite grain growth during the continuous casting of steel. Oscillation marks and depressions in the meniscus region of the continuously casting mold lead to retarded cooling of the strand surface, which in turn results in the formation of coarse austenite grains, but little is known about the mechanism and rate of formation of these large austenite grains. Relevant earlier research will be briefly reviewed to put into context our recent in situ observations of the delta-ferrite to austenite phase transition. We have confirmed earlier evidence that very large delta-ferrite grains are formed very quickly in the single-phase region and that these large delta-ferrite grains are transformed to large austenite grains at low cooling rates. At the higher cooling rates relevant to the early stages of the solidification of steel in a continuously cast mold, delta-ferrite transforms to austenite by an apparently massive type of transformation mechanism. Large austenite grains then form very quickly from this massive type of microstructure and on further cooling, austenite transforms to thin ferrite allotriomorphs on austenite grain boundaries, followed by Widmanstätten plate growth, with almost no regard to the cooling rate. This observation is important because it is now well established that the presence of a thin ferrite film on austenite grain boundaries is the main cause of reduction in hot ductility. Moreover, this reduction in ductility is exacerbated by the presence of large austenite grains.     

Numerical and experimental estimation of thermomechanical fields heterogeneity at the grain scale of 316L stainless steel

Polycrystalline metallic materials are made of an aggregate of grains more or less well oriented with respect to the loading axis. During mechanical loading, the diversity of grain orientations leads to heterogeneous deformation and it is well known that most of the plastic work generated during the deformation process reappears in the form of heat whereas a certain proportion remains latent in the material and is associated with microstructure changes. To access the local stored energy, experimental and numerical energy balances are needed at a suitable scale. In this way experiments have already been done in-house on 316L stainless steel to monitor the evolution of temperature and deformation fields at the microstructural scale. The aim of the present study is now to develop a simplified numerical model, based on experimental observations and able to simulate the thermomechanical behavior, in order to provide a first assessment of the stored energy level and heterogeneity. Our model renders well the average thermomechanical behavior of the specimen and reveals a high heterogeneity of the stored energy at the microstructure scale. Better knowledge of the stored energy at this scale should improve our understanding of strain localization mechanisms.     

Evaluating the Mobility of Nanorods in Electric Fields

The mobility of a nonspherical particle is a function of both particle shape and orientation. In turn, the higher magnitude of electric field causes nonspherical particles to align more along the field direction, increasing their mobility or decreasing their mobility diameter. In previous works, Li et al. developed a general theory for the orientation-averaged mobility and the dynamic shape factor applicable to any axially symmetric particles in an electric field, and applied it to the specific cases of nanowires and doublets of spheres. In this work, the theory for a nanowire is compared with experimental results of gold nanorods with known shape determined by TEM images. We compare the experimental measured mobility sizes with the theoretical predicted mobility in the continuum, free molecular, and the transition regime. The mobility size shift trends in the electric fields based on our model, expressed both in the free molecular regime and in the transition regime, are in good agreement with the experimental results. For rods of dimension: width d_r = 17 nm and length L_r = 270 nm, where one length scale is smaller than the mean free path and one larger, the results clearly show that the flow regime of a slender rod is mostly controlled by the diameter of the rod (i.e., the smallest dimension). In this case, the free molecule transport properties best represented our nanorod. Combining both theory and experiment we show how, by evaluating the mobility as a function of applied electric field, we can extract both rod length and diameter.

Copyright 2013 American Association for Aerosol Research