Motions of alloying additions in the CAS steelmaking operations

Water model studies in a pilot scale ladle (D = 1.12 m andL = 0.93 m) were carried out to investigate the subsurface motion of both buoyant and sinking additions during the CAS (com-position adjustment by sealed argon bubbling systems) alloy addition procedure in steelmaking. This technique involves placing a refractory baffle around a rising gas/liquid plume within a stirred ladle of steel. Alloy additions are then made by projecting them into the slag-free region of steel within the baffled region. It was found that such particles while moving through the upwelling two-phase plume region can experience a significant reduction in drag forces, causing buoyant particles to penetrate more deeply than anticipated for a homogeneous fluid. Therefore, considering reduced drag on particles penetrating the upwelling gas liquid plume region, predictions were made for the trajectories of spherical-shaped particles using Newton’s law of motion. Predictions were in very reasonable agreement with those measured. Incorporating the velocity fields in industrial size vessels already reported by the present authors, trajectories of spherical-shaped additions (diameter ∼ 80 mm) in a 150-ton ladle during CAS operations were then predicted. The industrial implications of such trajectories, together with the alloy’s dissolution and dispersion behavior, were also analyzed. Finally, advantages of the CAS alloy addition procedure over conventional methods, in terms of the recovery rates of buoyant additions, are discussed.     

Electro-absorption in a polydiacetylene

The electro-absorption in a DCH-polydiacetylene has been investigated both experimentally and theoretically. In addition to the red Stark shift of the exciton, a significant feature in the difference spectrum is observed at a higher energy. Within an interacting electron model (extended Peierls-Hubbard model), the roles played by the dipole-allowed one-photon states and the dipole-forbidden two-photon states are studied in detail. It is found that the high-energy signals in the difference spectrum are due to the strong field-induced mixing of the one-photon state at the edge of a continuum structure, or ‘conduction band’ in the language of band theory, with its neighboring two-photon states. Contributions to the electro-absorption from the one- and two-photon states cancel deep within the band. Three-photon resonance due to the band edge is expected in third-harmonic generation (THG) and has been seen in both a polydiacetylene and in polysilanes.     

Bus transfer practices at nuclear plants

The dominant trend in US nuclear plants is to use sequential (break-before-make) fast bus transfers. This study indicates that there have been at least 54 bus transfer failures at US nuclear plants between 1985 and 1989. The authors analyze the lessons learned from these failures and compare the alternate designs and practices that can improve bus transfer practices in the United States. Although the current rate of bus transfer failures is not alarming it can be substantially reduced by incorporating some of the features that are described     

Measurement and computation of drag forces in thermogravimetric studies

The upward drag force experienced by a hanging assembly in a thermogravimetric setup due to flowing gas has been measured using an electrobalance for two different gases, viz., argon and carbon dioxide, over a temperature range of 1000 °C to 1250 °C and at a gas flow rate of 1.42×10⁻⁶ to 3.33×10⁻⁶ m³/s (STP). Analytical estimates, assuming the assembly to be a solid sphere of equivalent surface area, predicted drag forces about two orders of magnitude lower than the corresponding experimental values. This discrepancy was resolved by numerical simulation employing the commercial computational fluid-dynamics (CFD) package called “FLUENT” and by employing the exact geometry of the reactor and assembly, with experimental and computed values differing by about 20 pct, on average. The duration of the initial transient flow upon starting the gas flow was very small (less than 5 seconds).     

Microstructural characterization of amorphous and nanocrystalline boron nitride prepared by high-energy ball milling

Microstructural parameters like crystallite size, lattice strain, stacking faults and dislocation density were evaluated from the X-ray diffraction data of boron nitride (BN) powder milled in a high-energy vibrational ball mill for different length of time (2-120 h), using different model based approaches like Scherrer analysis, integral breadth method, Williamson-Hall technique and modified Rietveld technique. From diffraction line-broadening analysis of the successive patterns of BN with varying milling time, it was observed that overall line broadening was an operative cause for crystallite size reduction at lower milling time (∼5 h), whereas lattice strains were the prominent cause of line broadening at higher milling times (>19 h). For intermediate milling time (7-19 h), both crystallite size and lattice strain influence the profile broadening although their relative contribution vary with milling time. Microstructural information showed that after long time milling (>19 h) BN becomes mixture of nanocrystalline and amorphous BN. The accumulations of defects cause this crystalline to amorphous transition. It has been found that twin fault (β′) and deformation fault (α) significantly contributed to BN powder as synthesized by a high-energy ball-milling technique. Present study consider only three ball-milled (0, 2 and 3 h) BN powder for faults calculation because fault effected reflections (1 0 1, 1 0 2, 1 0 3) disappear with milling time (>3 h). The morphology and particle size of the BN powders before and after ball milling were also observed in a field emission scanning electron microscope (FESEM).     

Heat Flow and Solidification Modeling of Industrial Scale,Ingot Casting Operation

A model, based on the concept of effective thermal conductivity, was developed to study thermal fields and the resultant solidification behavior of large, round, industrial size ingots. In this, flow and turbulence phenomena during mold filling as well as subsequent solidification were not modeled explicitly but their influence was accounted for by artificially raising the thermal conductivity of solidifying steel. Thus, a conduction like equation embodying a conjugate approach was applied to simultaneously predict the evolution of temperature fields in the mold as well as in the solidifying ingot following teeming. Prior to comparing model predictions against industrial scale measurements, sensitivity of calculations to grid size, time step height, convergence criterion etc. were rigorously assessed. Similarly, modeling of interfacial resistance, chemical reactions and heat effects in the hot top as well as their influence on predicted results were evaluated computationally. Embodying mixed thermal boundary conditions (free convection + radiation) at the mold wall, temperature fields during solidification of two different industrial large ingots were predicted numerically. Parallely, mold wall temperature was monitored as a function of time and surface temperature of ingot was measured at the instant of mold stripping using hand held, radiation pyrometers. Incorporating relevant operating conditions (viz., mold dimensions and size, ingot and hot top dimensions and material, initial mold and liquid temperature etc.) into the calculation scheme, predictions were made via a computational procedure developed in-house and results thus obtained were compared against equivalent industrial scale measurements. Very reasonable agreement between the two was demonstrated.     

Valence bond theory of narrow-band conductors

A diagrammatic valence bond (VB) theory is introduced for solids with one valence state per site, Coulomb intersite interactions, and nearest-neighbor electron transfers leading to a bandwidth 4 |t|. The VB method is applied to partly-filled regular segregated stacks of acceptors (A) and donors (D), which occur in all π-molecular organic conductors. The dimensions of the site representation for N_e = γN electrons or holes on N >N_e sites are found, each VB assignment is represented by a diagram, and the interconnection of these many-electron site functions due to electron transfers is also obtained diagrammatically. The resulting configuration interaction (CI) between all orbital assignments is solved exactly in subspaces of total spin, S. The complete thermodynamics of N_e = 4 electrons on chains of N = 4, 5, 6, and 7 sites are found for the Hubbard choice of on-site repulsion, U. The VB method extends the computation of the susceptibility χ(T) of partly-filled Hubbard models to the previously inaccessible regime U 4|t| which, however, is suggested by experiment. Comparison with absolute χ(T) data for TTF-TCNQ is slightly improved for U 4|t| = 0.56 eV. Extensions of the VB method to more realistic potentials, to other data, and to N → ∞ extrapolations are discussed.