The effect of water vapor on mold slag crystallization
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Wagstaff and Richards[13] reported for a SiO2 glass that the presence of H2O (g) increased the nucleation and growth rate of the crystals and suggested that the presence of water vapor in the surrounding atmosphere increased the oxygen concentration at the glass/crystal interface and weakened the network structure of the glass. Buerger[14] also showed that water increased the crystallization rate of natural glasses. He attributed this increased crystallization rate to a reduction
C. ORRLING, Graduate Fellow, and A.W. CRAMB, POSCO Professor and Co-Director, are with the Center for Iron and Steelmaking Research, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890. Manuscript submitted September 7, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
in the viscosity of the melt due to the formation of nonbridging oxygen ions in the network. Gonzales-Oliver et al.[15] found in studies of lithia-silica and soda-lime-silica glasses, which crystallize stoichiometrically, that the presence of hydroxyl ions increased the homogenous nucleation and crystallization rates. Shelby et al.[16,17] studied the effect of hydroxyl content on nucleation and crystallization phenomenon of Li2O-SiO2 glass ceramics and also found that increased hydroxyl ion content decreased the viscosity of Li2O-SiO2 melt and increased significantly the magnitude of the nucleation rate and the crystallization rate in a humid atmosphere. Thus, these previous studies in glass forming systems indicate that there is an atmospheric effect on nucleation and growth phenomena within liquid silicates. As most industrial mold slags are based upon the calcium silicate system and are easy glass formers, we decided to test these slags to determine the effect of a humid environment on the crystallization behavior of these slags. The experimental apparatus used in this study was the DHTT.[7,8,9] A schematic of the apparatus is given in Figure 1. This device allows accurate control of temperature and cooling rate (up to 250 8C/s) and the opportunity to observe the onset of precipitation as the starting liquid is transparent. The water vapor was generated by flowing argon through hot water and subsequently through a condensation spiral tube, which was surrounded by constant temperature water. The spiral tube enabled excess water vapor absorbed in carrier gas to be eliminated by condensation and allowed the humidity of the gas to be accurately set and varied. Adjustment of the temperature of both the hot water and the circulating water in the condensation spiral made it possible to control water content in gas phase. When the difference between the temperature of hot water and that in the condensation tube was less than 30 8C, the desired water vapor partial pressure in equilibrium with the temperature of the condensation spiral was achieved for a flow rate range from 30 to 140 mL/min. The slag sample was prepared by mixing individual pure oxides, which were then prefused in a platinum crucible and subsequently grinded i
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