| dc.description.abstract | The ever-growing competition in the bulk material industry is increasingly a battle between branches of materials producers rather than within each branch. In this climate, an enduring competitiveness relies on continous improvements of both the products and the processes that creates them. In this situation, an increasingly detailed knowledge of the materials involved and their behaviour at different temperatures is needed in order to find potential keys to further improvements.
The very general idea behind the present work was to investigate the thermochemistry of Mn – rich liquid alloys belonging to the Mn-Fe-C-O quarternary system, which is relevant for the ferromanganese producers and their customers. A comprehensive literature study undertaken as part of this work, has revealed deficiencies in the knowledge about the more fundamental (binary) sub-systems, and even for elemental Mn some key physio-chemical properties have either not been determined, or existing data have not been evaluated optimally.
Besides the major literature evaluation, a large fraction of the present work has been devoted to the construction and building of suitable laboratory equipment, and the development (or adaption) of experimental methods for gathering experimental information, as well as theoretical methods for optimal calculations based on this information in combination with literature data.
The main part of the equipment was a compact MoSi2 – Al2O3 vacuum-furnace, constructed and built to serve the need for metallurgical investigations under (mainly) carbon-free conditions at temperatures up to 1650 ºC. The specially adapted experimental techniques are related to ceramics sintering, EMF measurements, melt sampling, alloy heat treatment, quenching, temperature calibration, and thermal analysis. The technical accomplishments includes guidelines for material selection.
The theoretical accomplishments include equations describing mixing energies of ordered liquids, procedures to correct measurement errors due to Mn vapourisation, and an equation that promotes the use of the meta-stable transformation temperatures of Mn to obtain improved assessments/calculations of the thermal properties of manganese.
The liquidus boundaries of the binary subsystems Fe-C and Mn-C and parts of Mn-O were determined by solubility measurements and thermal analysis. Regarding the binary phase diagrams, moderate changes are proposed for Fe-C, major changes are required for Mn-C, while dramatic changes are suggested for Mn-MnO. For the latter system the solubilities are found to be much larger than previously known, and agree best (qualitatively) with the earliest of previous diagrams (Benedicks et Loefquist 1930). In light of present results for Mn – C and Fe – C, the four transition elements Mn, Fe, Co and Ni have been grouped according to their behaviour towards carbon. It appears that at graphite saturation the binary systems Mn – C and Fe – C form one group that behaves distinctly different from Co – C and Ni – C. A limited part of the Fe-Mn diagram has also been discussed, and here a new eutecticum is suggested.
Some ternary and quarternary liquidus surfaces were determined, in accordance with the initial ideas for this work, and the corresponding phase diagrams have been outlined. The solubility of graphite in Fe-Mn C was established and described empirically for all compositions and temperatures up to above 2000 ºC. The agreement with results for the binary sub-systems were very satisfying. The solubility of MnO in liquid Fe-Mn-C was established in the Mn – rich range, equilibrium oxygen content decreases with increasing levels of Fe as well as C.
Another initial idea was to determine of the activity relationships in liquid Mn – C alloys. These relations have been investigated briefly. Present data and literature data agree very well when skew quasi chemical equations were used for the optimisations.
For elemental manganese, a number of stable and meta-stable transformation temperatures for the five condensed allotropes have been re-determined. It has been demonstrated that below 300 ºC β-Mn is meta-stable and does not transform readily to α-Mn. Even at 400 ºC the process takes several minutes. The heat of melting of manganese is proposed to be 8.5 ± 1.5 kJ/mol, significantly below all commonly tabulated values, but in excellent agreement with data from Wust et al. (1918). | nb_NO |
