During dendritic solidification of castings and ingots, a number of processes take place simultaneously within the semisolid region. These include crystallization, solute redistribution, ripening, interdendritic fluid flow, and solid movement. The dendritic structure which forms is greatly affected by convection during the early stages of solidification. In the limit of vigorous convection and slow cooling, grains become spheroidal. Alloys with this microstructure possess rheological properties in the semisolid state which are quite different from those of dendritic alloys. They behave thixotropically, and viscosity can be varied over a wide range, depending on processing conditions. The metal structure and its rheological properties are retained after solidification and partial remelting. The semisolid alloys can be formed in new ways, broadly termed «semisolid metal (SSM) forming processes». Some of these are now employed commercially to produce metal components and are also used to produce metal-matrix composites

Behavior of metal alloys in the semisolid state

Dendrite growth at the limit of stability: tip radius and spacing

Steel slag is a byproduct from either the conversion of iron to steel in a basic oxygen furnace, or the melting of scrap to make steel in an electric arc furnace. This paper reviews the production, processing, and characteristics of steel slag, and its use as a cementing component in different cementing systems. The chemical composition and cooling of molten steel slag have a great effect on the physical and chemical properties of solidified steel slag. Steel slag with high basicity and being cooled properly can exhibit cementing property. Ground steel slag has been used in several different cementing systems. The use of steel slag in these cementing systems results in some advantages over conventional cements. At the moment, most steel slag is being used as unbound aggregate for ashphalt concrete pavement in many countries. However, the use of steel slag as a cementing component should be given a priority from technical, economical, and environmental considerations.

Steel Slag—Its Production, Processing, Characteristics, and Cementitious Properties

Steel slag is a byproduct of the steelmaking and steel refining processes. This paper provides an overview of the different types of steel slag that are generated from basic-oxygen-furnace (BOF) steelmaking, electric-arc-furnace (EAF) steelmaking, and ladlefurnace steel refining processes. The mineralogical and morphological properties of BOF and electric-arc-furnace-ladle [EAF(L)] slag samples generated from two steel plants in Indiana were determined through X-Ray Diffraction (XRD) analyses and Scanning Electron Microscopy (SEM) studies. The XRD patterns of both BOF and EAF(L) slag samples were very complex, with several overlapping peaks resulting from the many minerals present in these samples. The XRD analyses indicated the presence of free MgO and CaO in both the BOF and EAF(L) slag samples. SEM micrographs showed that the majority of the sand-size steel slag particles had subangular to angular shapes. Very rough surface textures with distinct crystal structures were observed on the sandsize particles of BOF and EAF(L) slag samples under SEM. The characteristics of the steel slag samples considered in this study are discussed in the context of a detailed review of steel slag properties.

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Chemical, mineralogical, and morphological properties of steel slag

Unidimensional solute redistribution during freezing is modelled for the important case of complete diffusion in the liquid and incomplete back-diffusion in the solid by the Brody and Flemings analysis. The treatment, however, does not conserve solute and leads to predictions which are incorrect unless solid state diffusion is very limited. This paper outlines the origin of this effect and presents a modification to the model in which the predicted behavior is quantitatively correct in the important limits. Application of the modified analysis is shown to be simple and to lead to realistic descriptions of the solidification behavior in mushy freezing alloys. As an example, the model is applied to two steels solidifying over a range of cooling rates and the results indicate that such systems will in many cases be quite close to thermodynamic equilibrium with respect to carbon redistribution.

Solute redistribution during solidification with rapid solid state diffusion

Steady state directional freezing experiments have been performed with two steels containing 0.59 and 1.48 pct carbon. Primary and secondary arm spacings were directly measured. In addition, average primary arm spacings were computed from the number of arms present on the observed area using the model of a hexagonal arrangement. The latter method seems to be more objective and reproducible than the line counting method. Arm spacings λ were related by the empirical equation λ =c RmGn to growth rateR and temperature gradientG. For primary arms, the exponentsm andn were different, whereas for secondary arms they were almost identical. Some consideration is given to dendrite spacings in ingot solidified steel, where under parabolic growth conditions thermal gradients and growth velocity are coupled by heat flow. Hence, a single variable may be used if the boundary condition for heat flow remains the same. Using the present results the laws describing dendrite spacings as a function of local solidification time are derived and compared with previous data available in the literature.

Dendrite morphology of steady state unidirectionally solidified steel

For the modelling of strains und stresses arising during solidification data on the mechanical and thermal properties of the steel are needed. Data on density and on related properties of iron and iron-carbon alloys in the literature were evaluated giving the thermal contraction as a function of temperature and carbon content

Thermomechanical properties of iron and iron-carbon alloys : density and thermal contraction

A model is presented to compute the three-dimensional flow field in rotational electromagnetic stirring of round strands. The model involves the solution of the Maxwell equations, the Navier-Stokes equations, and the transport equations for the turbulence characteristicsk ande. For the limiting case of one-dimensional stirring, the computations were checked with experiments using mercury as the fluid. Several sets of computations were carried out to determine the influence of stirrer position, stirrer length, and electromagnetic parameters on the flow field in continuous casting of steel strands.

Rotational electromagnetic stirring in continuous casting of round strands

Radial and axial profiles of gas concentration and bubble frequency have been measured for vertical gas bubble jets in the systems air/water, helium/water, and nitrogen/mercury using an electroresistivity probe. Gas velocities have been determined in the air/water system. All radial profiles were close to Gaussian. An integral model was applied to calculate axial distributions theoretically. Entrainment coefficients were determined for the experimental conditions. Axial profiles were correlated also in nondimensional representations. The bubble frequencies were used to compute local and average values of bubble size. Jet expansion and bubble size were found to depend considerably on the physical properties of the system.

Characteristics of round vertical gas bubble jets

The entrainment phenomena occurring at liquid/liquid interfaces during passage of single bubbles have been investigated using the systems water/cyclohexane, mercury/water, and mercury/silicon oil. Measurements were performed on the increase of the continuous interfacial area, the residence time of the bubble at the interface, the length of the liquid jet of the lower phase drawn upwards, the number and size of droplets of the lower phase in the upper phase, the residence time of the droplets and their velocity of fall. Dimensionless correlations were derived for some of these quantities, and some consideration is given of the application of the data in mass transfer models.

Klaus Schwerdtfeger

Papers

Dendrite morphology of steady state unidirectionally solidified steel

Thermomechanical properties of iron and iron-carbon alloys : density and thermal contraction

Rotational electromagnetic stirring in continuous casting of round strands

Characteristics of round vertical gas bubble jets

Characteristics of Entrainment at Liquid/Liquid Interfaces due to Rising Bubbles