Ebrahim Karimi-Sibaki

A Dynamic Mesh-Based Approach to Model Melting and Shape of an ESR Electrode

This paper presents a numerical method to investigate the shape of tip and melt rate of an electrode during electroslag remelting process. The interactions between flow, temperature, and electromagnetic fields are taken into account. A dynamic mesh-based approach is employed to model the dynamic formation of the shape of electrode tip. The effect of slag properties such as thermal and electrical conductivities on the melt rate and electrode immersion depth is discussed. The thermal conductivity of slag has a dominant influence on the heat transfer in the system, hence on melt rate of electrode. The melt rate decreases with increasing thermal conductivity of slag. The electrical conductivity of slag governs the electric current path that in turn influences flow and temperature fields. The melting of electrode is a quite unstable process due to the complex interaction between the melt rate, immersion depth, and shape of electrode tip. Therefore, a numerical adaptation of electrode position in the slag has been implemented in order to achieve steady state melting. In fact, the melt rate, immersion depth, and shape of electrode tip are interdependent parameters of process. The generated power in the system is found to be dependent on both immersion depth and shape of electrode tip. In other words, the same amount of power was generated for the systems where the shapes of tip and immersion depth were different. Furthermore, it was observed that the shape of electrode tip is very similar for the systems running with the same ratio of power generation to melt rate. Comparison between simulations and experimental results was made to verify the numerical model.

Influence of Crystal Morphological Parameters on the Solidification of ESR Ingot

Electroslag remelting (ESR) is an advanced process to produce high quality steel. During the ESR process, the steel electrode is melted and then solidified directionally in a water-cooled mold. The quality of the ingot is strongly dependent on the shape of melt pool, i.e. the depth and thickness of mushy zone, which is in turn influenced by the bulk and interdendritic flow. Here, we perform a numerical study to investigate the effect of crystal morphological parameter such as primary dendrite arm spacing on the solidification of the ESR ingot ( 750 mm). The crystal morphology is dominantly columnar and dendritic, thus a mixture enthalpy-based solidification model is used. Accordingly the mushy zone is considered as a porous media where the interdendritic flow is calculated based on the permeability. The permeability is determined as function of the liquid fraction and primary dendrite arm spacing according to Heinrich and Poirier [Comptes Rendus Mecanique, 2004, pp. 429-44]. The modeling results were verified against experimental results.

An attempt to model electrode change during the ESR process

The electrode change technology is used to produce very large heavy ingots in which a number of electrodes are remelted one after another during the ESR process. Preparing the new electrode for remelting requires a certain period of time when the electric current is stopped (power off). Here, CFD simulation is used to study the behavior of a large scale ESR process during the electrode change (power off). Firstly, the electromagnetic, temperature, and turbulent flow fields in the process before electrode change are modelled. Mold current and thermal effect due to shrinkage of ingot is considered in the model. Then, a transient simulation is performed and the response of the system to the power off is continuously tracked. It is observed that the pool profile of ingot is preserved before and after electrode change. Details of the flow and temperature distributions during electrode change are presented in the paper.

Process Simulation for the Metallurgical Industry: New Insights into Invisible Phenomena

In order to demonstrate how advanced process simulation can help to understand metallurgical process details and thus to improve industrial productivity, a number of examples are shown and discussed. The paper covers recent simulation results gained at the Chair of Simulation and Modeling of Metallurgical Processes, namely (i) the flow and shell formation in thin slap casting of steel, (ii) multiphase flow and magneto-hydrodynamic during Electro-Slag-Remelting, (iii) mold filling, surface wave dissipation and solidification during horizontal centrifugal casting of rolls, and (iv) forced and natural convection during electro-refining of copper in an industrial-size tankhouse cell.ZusammenfassungIn dieser Arbeit wird anhand von vier Beispielen gezeigt, wie fortschrittliche Prozesssimulationen helfen können, metallurgische Prozessdetails zu verstehen und somit die industrielle Produktivität zu erhöhen. Die Beispiele stammen aus laufenden Forschungsarbeiten des Lehrstuhls für Simulation und Modellierung metallurgischer Prozesse. Es werden i) Strömungen und Erstarrung beim Dünnbrammengießen von Stahl, ii) Mehrphasenströmung und Magnetohydrodynamik beim Elektroschlackeumschmelzen, iii) Formfüllung, Bewegung von Oberflächenwellen und Erstarrung beim horizontalen Schleuderguss von Großwalzen, und iv) erzwungene und natürliche Strömung in industriellen Aggregaten bei der Elektroraffinationselektrolyse von Kupfer behandelt.

Transient melting of an ESR electrode

Melting parameters of ESR process such as melt rate and immersion depth of electrode are of great importance. In this paper, a dynamic mesh based simulation framework is proposed to model melt rate and shape of electrode during the ESR process. Coupling interactions between turbulent flow, temperature, and electromagnetic fields are fully considered. The model is computationally efficient, and enables us to directly calculate melting parameters. Furthermore, dynamic change of electrode shape by melting can be captured. It is necessary to control the feeding velocity of electrode due to melting instabilities in the ESR process. As such, a numerical control is implemented based on the immersion depth of electrode to achieve the steady state in the simulation. Furthermore, the modeling result is evaluated against an experiment.

A numerical study on electrochemical transport of ions in calcium fluoride slag

Electrically resistive CaF 2-based slags are widely used in electroslag remelting (ESR) process to generate Joule heat for the melting of electrode. The electric current is conducted by ions (electrolyte) such as Ca +2 or F -, thus it is necessary to establish electrochemical models to study electrical behavior of slag. This paper presents a numerical model on electrochemical transport of ions in an arbitrary symmetrical (ZZ) and non-symmetrical (CaF2) stagnant electrolytes blocked by two parallel, planar electrodes. The dimensionless Poisson-Nernst-Planck (PNP) equations are solved to model electro-migration and diffusion of ions. The ions are considered to be inert that no Faradic reactions occur. Spatial variations of concentrations of ions, charge density and electric potential across the electrolyte are analyzed. It is shown that the applied potential has significant influence on the system response. At high applied voltage, the anodic potential drop near the electrode is significantly larger than cathodic potential drop in fully dissociated CaF2 electrolyte.

A (non-)hydrostatic free-surface numerical model for two-layer flows

A semi-implicit (non-)hydrostatic free-surface numerical model for two layer flows is derived from the Navier–Stokes equations by applying kinematic boundary conditions at moving interfaces and by decomposing the pressure into the hydrostatic and the hydrodynamic part. When the latter is ignored, the algorithm conveniently transforms into a solver for a hydrostatic flow. In addition, when the vertical grid spacing is larger than the layer depths, the algorithm naturally degenerates into a solver for the shallow water equations. In this paper, the presented numerical model is developed for the horizontal centrifugal casting, a metallurgical process, in which a liquid metal is poured into a horizontally rotating cylindrical mold. The centrifugal force pushes the liquid metal toward the mold wall resulting in a formation of a sleeve with a uniform thickness. The mold gradually extracts the sensible and the latent heat from the sleeve, which eventually becomes solid. Often a second layer of another material is introduced during the solidification of the first layer. The proposed free-surface model is therefore coupled with the heat advection-diffusion equation with a stiff latent heat source term representing the solidification. The numerical results show a good agreement with measurements of temperatures performed in the plant. A validation of the proposed model is also provided with the help of using other numerical techniques such as the approximate Riemann solver for the two layer shallow water equations and the volume of fluid method.

Advanced Process Simulation of Solidification and Melting

At some stage in the production of every metal part or product, the metal material has been melted and solidified to form the primary or final shape as well as the as-cast structure. Quantitative prediction and control of solidification and melting has been, and remains, the most critical issue in the metallurgical industry. Example questions currently raised by the Austrian metallurgical industry, which is one of the key economic sectors of this country, are as follows: in thin-slab casting, how does the solid shell form and interact with turbulent flow? How can the electro-slag-remelting (ESR) process be better understood and controlled (stabilized)? How can metallurgical imperfections (macrosegregation, porosity, non-metallic inclusion, surface crack, etc.) in castings be predicted and minimized? Therefore, a Christian-Doppler laboratory—Advanced Process Simulation of Solidification and Melting was established in July 2011 with the final goal to address the questions mentioned above. This article reports on some progresses.ZusammenfassungSo gut wie jeder metallische Werkstoff wurde im Laufe seines Herstellprozesses ein- oder mehrmals er- bzw. umgeschmolzen und anschließend erstarrt. Dabei bildete sich ein Gussgefüge mit charakteristischen Gefügemerkmalen (Korngröße, Textur, Lunker, Poren, Seigerung, usw.), welche die Gebrauchseigenschaften des Produkts wesentlich beeinflussen. Quantitative Prognosen zur Kontrolle von Schmelz- und Erstarrungsvorgängen sind deshalb in der gesamten metallurgischen Industrie von entscheidender Bedeutung. Beispielsweise werden folgende Fragen von der österreichischen metallurgischen Industrie, welche als einer der ökonomischen Schlüsselsektoren des Landes gilt, aufgeworfen. Wie bildet sich die Strangschale von Strangguss im Kokillenbereich und wie interagiert diese mit der turbulenten Strömung? Wie kann der Elektro-Schlacke-Umschmelzen (ESU) Prozess besser verstanden und gesteuert (stabilisiert) werden? Wie können die metallurgisch unerwünschten Imperfektionen in den Gussteilen vorhergesagt und minimiert werden? Zur Klärung dieser Fragen wurde im Juli 2011 ein Christian Doppler Labor für Prozesssimulation von Erstarrungs- und Umschmelzvorgängen eingerichtet. Über diesbezügliche Forschungsfortschritte wird im Rahmen dieses Artikels berichtet.