Crack defect formation during manufacture of fused cast alumina refractories
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cast refractories are widely used in the glass industry for furnace linings owing to their superior corrosion/ erosion resistance. During the manufacturing process, these refractories develop a variety of defects that are comparable to those found in the metal castings. The most important of these defects include cracks arising from excessive thermal stresses or strains and porosity caused by the volumetric contraction associated with solidification. Under the extreme conditions existing in modern glass furnaces, these defects can lead to excessive chemical attack with negative implications for glass quality. On-going efforts to improve glass quality while at the same time reducing operation and maintenance costs have led to a need for higher quality fused cast refractories free from crack defects. This investigation focused on the development of a mathematical model capable of predicting the thermal and stress/ strain histories in fused cast ␣ -alumina refractories during the casting process. The model was developed using the commercial finite element software package, ABAQUS,* *ABAQUS is a trademark of Hibbit, Karlsson & Sorenson, Inc., Pawtucket, RI.
with enhancements included through user-written subroutines. The ultimate goal of this work is to understand the mechanism(s) leading to the formation of cracks. II. BACKGROUND/LITERATURE REVIEW Monofrax Inc. (Falconer, NY) manufactures various types of fused cast ceramic components for different applications. DOMINIC AU, Process Design Engineer, is with Titan Steel and Wire, Surrey, BC, Canada V3V 2R8. STEVE COCKCROFT, Head and Associate Professor, and DAAN MAIJER, Assistant Professor, are with the Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Contact e-mail: [email protected] Manuscript submitted July 30, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fused cast ␣ -alumina, sold under the trade name Monofrax-M, is one class of product, which is used in glass furnace superstructures as crown blocks. The crown blocks are manufactured by first melting a fusion of oxides, typically Al2O3-3.8 wt pct Na2O and other minor oxides, in an electric arc furnace and then pouring the resulting liquid into a graphite mold.[1] After several minutes, the graphite mold is removed. The refractory block is then picked up with a crane and transported a short distance and placed into an annealing bin (box), and covered with insulating annealing sand (alumina ore). The refractory block may take a few weeks to cool after which it is inspected, cut to size and shipped to a customer. These refractory blocks typically solidify with a ␣ -alumina outer region, which is surrounding a  -alumina core—refer to Figures 1 and 2. The  -alumina core forms due to rejection of Na2O, a  stabilizer, at the solid/liquid interface which accumulates in the center of the block. From a manufacturing viewpoint, cracking is a common defect found in large refractory blocks. Cracks have been found in all the castings that have been cut open
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