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Non-metallic inclusions in electroslag refined ingots

University of British Columbia

The objective of this research was to investigate how non-metallic inclusions (inclusions) are physically and chemically transformed, removed and controlled from electrodes to the final ESR-product. Several 1020, 4340 and rotor (Ni-Mo-V) steel electrodes were refined by two ESR-units (7.5 mm and 200 mm in mould diameter) under different slag systems. Refining of these electrodes was done under different deoxidation practices, namely pure Al, CaSi, CaSiAlBa and AISi alloys. Through this research it was found that inclusions in the electrode are physically and chemically transformed in the electrode tip by the thermal gradients. Inclusions are chemically altered by the presence of liquid slag at the liquid film and they are entirely dissolved in the matrix when the droplet is completely formed. No ingot inclusions were identifiable as of electrode origin and it is concluded that all electrode inclusions are either dissolved or removed by the slag. The effects of the slag with and without deoxidizers on the chemical composition of the liquid pool and ingot were traced during refining and hence the chemistry of inclusions was determined by extracting slag and liquid metal samples during refining. The total oxygen content was measured by the vacuum fusion technique, chemical analyses of slag by spectrophotometric techniques, electron microanalysis by SEM and EPMA and x-ray (crystallographic) analysis. The assays were used to formulate and corroborate the deoxidation and precipitation mechanisms. The chemical composition of inclusions in refined ingots are more strongly influenced by the deoxidation practice than by the electrode or the slag composition in low Si0₂ content slags. The precipitation of complex Al-Ca-Si inclusions is predictable in high silica slags (>10.0 wt%) and the most appropriate slag system to perform an efficient deoxidation is the 50 wt% CaF2, 30 wt% Al₂0₃ and 20 wt% CaO. [Figure 1] The deoxidation in ESR ingots takes place by the process of cooperative reactions between slag and deoxidizers in the following sequences: [Figure 2] An excessive deoxidation with Ca raises the Al content in the ingot according to: [Figure 3] Radial inclusion size distribution as well as dendrite arm spacings in samples extracted from liquid pool and ingots were determined. It was found that the inclusion size obeys the normal distribution and there is a normal variation of the inclusion size along radial distances. Hence the inclusion composition and size is a function of local solidification conditions and also of the local thermochemical conditions.

Text

2010-05-02

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http://hdl.handle.net/2429/24347

Materials Engineering

University of British Columbia

Graduate