congresos y reuniones científicas
"Evaluation of the in-service degradation of alumina-magnesia-graphite refractories used in the steelmaking industry"
Congreso; 59th International Colloquium on Refractories; 2016
The increasing need to achieve longer lifetimes of refractory materials and to reduce their environmental impact has made the study of degradation mechanisms in service an essential issue. In this work, the study of the degradation of an alumina-magnesia-graphite (Al2O3-MgO-C; AMC) brick after use at the bottom of a steelmaking ladle (post-mortem) was performed. To analyze the degradation of the refractory, the followings techniques were employed: scan electronic microscopy coupled with X-Ray dispersive energy spectroscopy (SEM/EDS), differential thermal (DTA) and thermalgravimetric (TGA) analyses and measurements of density and porosity. Post-mortem brick was previously cut in order to obtain samples at different distance from the hot face; the dimension of specimens were approximately 20x30x25 mm3 (Figure 1).Figure 1. Samples of the post-mortem brick.Additionally, the characterization of an original AMC brick before use (unused) using different analysis techniques (XRD, DTA/TGA, gravimetry, measurements of density and porosity and SEM/EDS) was carried out. Some results are shown in Table 1. The qualities of alumina aggregates as observed by SEM are mainly tabular and brown electrofused (brown corundum) ones. Also, sintered magnesia was found as aggregates, but not as fine particles in the matrix. The antioxidants, as detected by SEM/EDS and XRD, are aluminum and silicon (Figure 2). According to the manufacturer data sheet, the organic binder is a phenolic resin. The content of carboneous components, determined by gravimetry of thermally treated samples, are 4.2 wt.% of resin (2.3 wt.% of volatiles and 1.9 wt.% of glassy-carbon) and 1.9 wt.% of graphite.Besides, the characterization of the slag in contact with the brick was performed. The basicity of the slag (CaO/SiO2 weight ratio) was 10.6. The slag viscosity was calculated using the Urbain model and the chemical composition obtaining by X-Ray fluorescence (XRF): 1.84 and 1.15 poise at 1600 and 1700ÂșC, respectively.Table 1. Density, porosity and carbon content (by TGA) of unused and post-mortem AMC materials. (ρg: global density; ρpic: real density; A: apparent porosity; V: true porosity; C: close porosity).Figure 2. SEM/EDS image of unused AMC brick. (EF: electrofused brown alumina; M: sintered magnesium; Al: aluminum; Si: silicon; G: graphite).The analyses by XRD and SEM/EDS (Figure 3) techniques showed the presence of phases in the post-mortem material which were formed by reaction of the AMC refractory with the liquid slag, such as calcium aluminates and MgAl2O4 spinel (MA). Moreover, it was determined that aluminum and silicon were completely reacted in the zone next to the hot face of the brick. In Figure 3, it is observed that the melted steel penetrated the brick. The formation of CA, CA2, CA6, C12A7 and spinel MA was determined by XRD. These results were confirmed by the SEM/EDS analysis. These experimental evidences point out that the corrosion of alumina and magnesia particles by the slag produced phases similar to those found in laboratory test of similar refractory materials and slag composition.Figura 3. SEM/EDS images of post-mortem brick. (A: attacked alumina aggregate).It was observed that the post-mortem samples closer to the hot face have higher global porosity due to the formation of volatiles during the pyrolysis of resin and the oxidation of graphite. However, the increase of porosity in samples further away from the hot face was less and an increase of solid real density was observed. This was attributed to the pyrolysis of resin, which is a low density compound. The termogravimetric analysis showed that the resin completely pyrolised in service, but some carbon present as glassy-carbon and/or graphite didn?t oxidize.