INFLUENCE OF MICROSTRUCTURE ON HIGH TEMPERATURE STRENGTH OF MAGNESIA-CHROME BRICK

By changing the content of the components in the refractory bricks, the high temperature strength and microstructure are studied, and the results show that the high temperature strength can be effectively improved by controlling the secondary spinel
1 Introduction
Magnesia-chrome bricks have good corrosion resistance and high high temperature strength, and have been widely used in secondary refining equipment such as RH and AOD.
It is understood that the structure and amount of secondary spinel generated during the sintering process have a great influence on improving the high temperature strength of magnesia-chrome bricks. It is generally believed that this secondary spinel is formed and grown by the liquid phase sintering containing SiO2 and CaO, so it is not only affected by the main components MgO and Cr2O3, but also by the secondary components such as CaO, SiO2, Al2O3, Fe2O3. .
By changing the ratio of the raw materials used and the content of the auxiliary components, the samples were prepared, the high temperature strength and microstructure were studied, and the influencing factors for improving the high temperature strength were discussed. The report is as follows.
2. Test sample
Semi-rebonded magnesia-chrome bricks were prepared by changing the composition ratio of the raw materials below 1 mm, which were used as test samples. Sample A is the basic sample. Samples B and C reduce the fused magnesia-chromium and increase the ratio of chromium oxide to fused magnesium, mainly reducing the secondary components of Al2O3+Fe2O3. In samples D and E, fused magnesia-chromium was reduced to increase the proportion of chrome ore and reduce the proportion of fused magnesia, so the content of Al2O3+Fe2O3, SiO2 and Cr2O3 increased slightly.
These test samples were molded into 150mm×75mm×50mm under the same molding conditions, and fired at ultra-high temperature in a tunnel kiln.
3. Test results
Samples B and C have low porosity and high bulk density, and the compressive strength tends to decrease, but the high temperature strength increases. For samples D and E, the physical properties and strength remain basically constant, and the high temperature strength increases with the increase of chrome ore.
The distribution of Fe and Cr was found in the bright part of the SEM image. It is presumed that the distribution of Al is basically the same as the distribution of Cr and Fe phases. In all sample impurities, Al2O3 and Fe2O3 together with Cr2O3 constitute the spinel phase of Mg(Cr, Al, Fe)2O4. The difference between the samples is that, compared with the sample A, the sample C has more Cr and less Fe content. The sample E has a large Fe component and a small Cr component.
Regarding the distribution of Si and Ca, in all the samples, a phase in which Si, Ca, and Mg overlapped in the vicinity of the secondary spinel existed. According to the point analysis results, it is presumed that CaMgSiO4 and Mg2SiO4 in the superimposed phase are present in a weight ratio of 80:20. Samples D and E were found to have less Ca and more Mg, and the ratio of the phases was CaMg2SiO4:Mg2SiO4=16:84. Compared with sample A, Si and Ca are widely distributed.
4. Analysis
The high temperature strength of the samples B and C added with chromium oxide is considered to be due to the fact that the secondary spinel is rich in Cr. It is presumed that samples B, C, Al2O3 and Fe2O3 decreased, and the amount of Cr2O3 in the secondary spinel increased. As shown in Figure 2 and Figure 3, MgCr2O4 is difficult to dissolve into CaMgSiO4 at high temperature, so it is generally believed that the dissolution of secondary spinel is inhibited, and high temperature strength is shown.
In the samples D and E to which chrome ore was added, a phase with a higher melting point was precipitated due to an increase in the amount of SiO 2 . As a result, it is presumed that the amount of the liquid phase produced at high temperature is reduced, the dissolution of the secondary spinel is suppressed, and the high temperature strength is maintained.
5 Conclusion
The high temperature strength and microstructure of the samples with the content of auxiliary components in the bricks were changed. According to the test results, it is speculated that the secondary spinel did not dissolve and disappear, so it was able to maintain a high high temperature strength.
Based on the results of this study, it is expected that materials with improved high temperature strength will be developed to improve the service life of the kiln.