Størkning av 75\% ferrosilisium : forløp, struktur og styrke

Målsettingen med oppgaven har vært å komme frem til praktiske størkningsmodeller som beskriver størkning av 75% ferrosilisium utstøpt under forskjellige betingelser samt å beskrive de resulterende egenskaper hos materialet. Avhandlingen består av fire hoveddeler. I stor grad er disse bygd opp omkring de samme forsøk utført ved Thamshavn Verk og ved NTH. Første del omhandler temperatur-tid resultater. Disse resultatene benyttes til beskrivelse av størkningsforløp ved ulik avkjølingshastigheter. Avkjølingshastigheten gjennom to-faseområdet er variert fra 02 °C/min til 1170 °C/min. Avkjøling gjennom to-faseområdet, eutektisk størkning og eutektoid omvandling i fast fase er beskrevet. Den eutektoide omvandlingen kan deles i to reaksjonstrinn. Trinn i skjer ved varierende avkjølingshastigheter. Denne reaksjonen kan forårsake ødeleggelse av strukturen. Trinn 2, som gir likevektsfase, skjer bare etter varmebehandling over lengre tid. Eksistensen av deltrinn i og 2 er påvist gjennom målinger. Annen del omhandler segregering i 75% ferrosilisium. Det er målt variasjoner i analyse innen en blokk fra 65.7% Si til 83.2% Si. Segregering er delt i fem ulike typer. Forskjellen i segregering ved vertikal størkning nedover og ved vertikal størkning oppover er beskrevet. En enkel modell for segregering ved størkning nedover er laget. Tredje del omhandler numerisk beregning av avkjølingsforløpet for tre situasjoner. Forutsetningene for beregningene samt de termofysiske størrelser er vurdert. Entalpi for størkning og avkjøling av ferrosilisium er beregnet. Varmeovergang til omgivelser er kontrollerende for avkjølingshastigheten. Beregningsresultatene sammenlignes med målte verdier. En viktig modelleringsfaktor er varmeovergang fra metall til kokille. Den representeres i modellen av et skikt med endelig tykkelse og med lav varmeledningsevne. Fjerde del omhandler struktur og styrkemålinger. Kornstørrelse er bestemt som funksjon av lokal størkningshastighet. Kornstørrelse er beregnet ved teorier fra Kattamis og Flemings. Forventet kornstørrelse gjennom tre støp er beregnet ved bruk av avkjølingshastighet lokalt gjennom støpet kombinert med en empirisk sammenheng mellom kornstørrelse og størkningstid. Videre er trykkstyrke funnet som funksjon av kornstørrelse. Slitasjestyrke (abrasjonsstyrke) avhenger av kornstørrelse og kjemisk analyse.The aim of the work is to carry out a fundamental study on the solidification of 75% ferrosilicon cast under different conditions. It is attempted to predict resulting properties of the alloy such as gram size and segregation. This research work is divided in four main topics. Each topic starts with a theoretical introduction. Then the experimental work and the results are presented. The first part presents temperature-time results from eleven experiment. The accuracy of the measurements is evaluated. The results are used to describe solidification of different casting methods. The cooling rate through the mushy (two-phase) zone is varied from 0.2 °C/min to 1170 °C/min. Eutectic solidification and solid state reaction are discussed. An important result is the recognition that the solid state reaction ( consist of two reactions. The first reaction seems to be harmful to the structure, Normally the reaction only reach the equilibrium-state after a prolonged heat treatment. The second part of the thesis report macro-segregation in cast ferrosilicon which have a vertical heat flux. The Silicon-content in one cast have been found to vary from 65.7% Si to 83.2% Si. The difference between segregation in top and bottom cooled solidification is described. A simple model for segregation with solidification from the top is presented. The third part consist of numerical calculations of the temperature-time histories of different positions in the ingots. Three cases have been computed and measured. Heat transfer from the surface of the block controls the cooling rate. Modelling results for one and three dimensional calculation are given. The heat transfer resistance from the metal to the mould is an important factor in the models. The last part of the thesis reports results concerning structure and strength of 75% ferrosiicon, The gram diameter for 75% ferrosilicon are measured and computed with a grain-coarsening model based on the theory of Kattamis and Flemings. The gram diameter (in i m) is given as a function of the local solidification time (in seconds) as: Gram diameter = 31.5 (Solidification time) 1”3 Abrasion strength and compression strength are measured. Small gram size and high Silicon-content seems to favour high strength of the solid ferrosilicon.dr.ing.dr.ing

The (love & hate) role of entropy in process metallurgy

Process metallurgy is the basis for the production, refining and recycling of metals and is based on knowledge of transport phenomena, thermodynamics and reaction kinetics, and of their interaction in high-temperature, heterogeneous metallurgical processes. The entropy concept is crucial in describing such systems, but, because entropy is not directly observable, some effort is required to grasp the role of entropy in process metallurgy. In this paper, we will give some examples of how entropy has a positive effect on efforts to reach the process objectives in some cases, while in other cases, entropy acts in contradiction to the desired results. In order to do this, it is necessary to have a closer look at both the entropy concept itself as well as at other functions like free energy and exergy since they encompass entropy. The chosen case is the production of silicon. It is the huge entropy change in the process that is utilized. The case is not chosen arbitrary. Indeed, it is the authors’ strong belief that silicon will be one of the foundations for the environmental and energy future planned for in the “Paris-agreement”. We will also explore relatively recent research in physics and thermodynamics that led to the description of the concepts like “dissipative systems and structures”. Dissipative systems are thermodynamically open systems, operating out of, and often far from thermodynamic equilibrium and exhibit dynamical regimes that are in some sense in a reproducible self-organized steady state. Such structures can arise almost everywhere provided this structure, feeding on low entropy resources, dissipates entropy generated in the form of heat and waste material in parallel with the wanted products/results. Examples range from metallurgical processes to the emergence of industrial symbiosis

Effect of different SiO2 polymorphs on the reaction between SiO2 and SiC in Si production

This work investigates the phase transformations in silica (SiO2) during heating to a target temperature between 1700 °C and 1900 °C and the effect of SiO2 polymorphs on the reduction reaction 2SiO2 + SiC = 3SiO + CO in silicon production. Different heating rates up to target temperature have been used to achieve the different compositions of quartz, amorphous silica and cristobalite. The different heating rates had a minor effect on the final composition, and longer time at temperatures > 1400 °C were necessary to achieve greater variations in the final composition. Heating above the melting temperature gave more amorphous silica and less cristobalite, as amorphous silica also may form from β-cristobalite. Isothermal furnace experiments were conducted to study the extent of the reduction reaction. This study did not find any significant difference in the effects of quartz, amorphous silica or cristobalite. Increased temperature from 1700 °C to 1900 °C increased the reaction rate

Effect of different SiO2 polymorphs on the reaction between SiO2 and SiC in Si production

This work investigates the phase transformations in silica (SiO2) during heating to a target temperature between 1700 °C and 1900 °C and the effect of SiO2 polymorphs on the reduction reaction 2SiO2 + SiC = 3SiO + CO in silicon production. Different heating rates up to target temperature have been used to achieve the different compositions of quartz, amorphous silica and cristobalite. The different heating rates had a minor effect on the final composition, and longer time at temperatures > 1400 °C were necessary to achieve greater variations in the final composition. Heating above the melting temperature gave more amorphous silica and less cristobalite, as amorphous silica also may form from β-cristobalite. Isothermal furnace experiments were conducted to study the extent of the reduction reaction. This study did not find any significant difference in the effects of quartz, amorphous silica or cristobalite. Increased temperature from 1700 °C to 1900 °C increased the reaction rate.publishedVersio

Effect of different SiO2 polymorphs on the reaction between SiO2 and SiC in Si production

This work investigates the phase transformations in silica (SiO2) during heating to a target temperature between 1700 °C and 1900 °C and the effect of SiO2 polymorphs on the reduction reaction 2SiO2 + SiC = 3SiO + CO in silicon production. Different heating rates up to target temperature have been used to achieve the different compositions of quartz, amorphous silica and cristobalite. The different heating rates had a minor effect on the final composition, and longer time at temperatures > 1400 °C were necessary to achieve greater variations in the final composition. Heating above the melting temperature gave more amorphous silica and less cristobalite, as amorphous silica also may form from β-cristobalite. Isothermal furnace experiments were conducted to study the extent of the reduction reaction. This study did not find any significant difference in the effects of quartz, amorphous silica or cristobalite. Increased temperature from 1700 °C to 1900 °C increased the reaction rate