Abstract
In order to facilitate the development of CaO–Al2O3 based mould flux for casting high aluminium, non-magnetic steel, the effect of CaO/Al2O3 ratios from 0.6 to 3.2 on viscosity and crystallisation characteristics were investigated with the aid of a rotational viscometer, Fourier transform infrared spectroscopy, a single hot thermocouple technique and X-ray diffraction analysis. The results showed that, at temperatures above 1543 K (1270°C), the viscosity first decreased and then became stable with increase in the CaO/Al2O3 ratio. At temperatures below 1543 K (1270°C), the viscosity again first decreased but then increased, with the CaO/Al2O3 ratio. This viscosity behaviour can be attributed to changes in the network structure characteristics and the precipitation of solid particles within the liquid flux. Increase in the CaO/Al2O3 ratio also first inhibited and then enhanced crystallisation as demonstrated by the changes in initial crystallisation temperatures and incubation times. The X-ray diffraction results confirmed that, at both low and high CaO/Al2O3 ratios, the dominant precipitates were compounds with high melting points. On the other hand, with CaO/Al2O3 ratios in the midrange, the dominant precipitates were compounds with relatively low melting temperatures. It is concluded that mould flux with a CaO/Al2O3 ratio in the range 1.1–1.6 is the most appropriate for casting high aluminium, non-magnetic steels.
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