Experiments have shown that fibrous alumina-bonded insulation, fixtures and setters can be successfully used in a hydrogen sintering furnace without adverse effects on weight loss or shrinkage.
An alumina-bonded refractory shape. Photo courtesy of Zircar Ceramics, Inc., Florida, N.Y.
Silica-bonded insulation materials are known to give superior performance in oxidizing and neutral environments, while alumina-bonded types have classically been used as thermal insulation, fixtures and setters in applications where reduction by aggressive furnace atmospheres is encountered. One such aggressive reducing atmosphere is hydrogen, a common cover gas in furnaces for sintering powdered metal parts and some technical ceramics. In hydrogen gas atmospheres, silica—a common binder that imparts high temperature stability and increased mechanical strength—is attacked, which causes the silica to dissociate and volatilize and results in premature failure of the refractory. However, the effects of hydrogen gas on alumina are not generally known.
To determine the stability of alumina-bonded refractories in a hydrogen atmosphere, researchers tested multiple types of fibrous alumina-bonded insulation materials, including premium and specialty high-density grades. The results of these experiments can help manufacturers of technical ceramics and sintered metal components choose the best insulation for their sintering atmosphere, thereby increasing the life of their refractories and improving the efficiency of their furnaces.
Cubes of alumina- and silica-bonded insulation, roughly 1 in. thick per side, were measured and weighed. They were fired at 1450C in a model 1725 HTF box furnace, manufactured by CM Furnaces, Inc. The furnace was purged with 15 scfh hydrogen gas with a dew point of <40C, and was then heated at a rate of 200C/hour with soak times of 1, 2, 10 and 50 hours. The samples were removed after each soak, and were subsequently measured and weighed. Weight loss and thickness shrinkage were calculated using the data obtained from the experiments. Shrinkage amounts in length and width were averaged to obtain the test results. The materials tested are described in Table 1.
Figure 1. Weight loss vs. time at 1450C in hydrogen (all types).
As seen in Figure 1, both the premium and economy grades of silica-bonded insulation (SALI and AL 25/1700, respectively) began exhibiting weight loss almost immediately. The premium-grade, mid-density silica-bonded insulation reached 18% weight loss after 10 hours, and nearly 30% weight loss after 30 hours. The weight loss of the economy-grade, mid-density silica-bonded insulation was slightly lower but was still considerably higher than the weight loss of all of the alumina-bonded insulation types.
Figure 2. Weight loss vs. time at 1450C in hydrogen (alumina-bonded insulation).
Figure 2 compares the weight loss results of the various types of alumina-bonded insulation. The premium-grade, low-density alumina-bonded insulation and the economy-grade, mid-density alumina-bonded insulation (ZAL-15AA and ECO-20AA, respectively) showed similar results, with both types losing approximately 5% of their weight after 10 hours and 6.5% after 50 hours. The premium-grade, high-density alumina-bonded insulation (ZAL-45AA) lost just 3.5% of its weight after 20 hours and stayed at that level for the remainder of the test. The specially prepared, high-density alumina-bonded insulation (ZAL-60AA) exhibited the best results, losing only 3% of its weight over the entire test.
Figure 3. Shrinkage results in length and width at 1450C in hydrogen (all types).
Researchers also measured the thermal shrinkage in length, width and thickness for all of the insulation types tested. In all cases, the thermal shrinkage was inversely proportional to density, independent of the bond type. As seen in Figure 3, both types of silica-bonded insulation exhibited length and width shrinkage in the median range. The premium-grade, low-density alumina-bonded insulation showed the highest overall length and width shrinkage, while the specially prepared, high-density alumina-bonded insulation exhibited the lowest length and width shrinkage.
Figure 4. Shrinkage results in thickness at 1450C in hydrogen (all types).
Figure 4 shows the thickness shrinkage of the various insulation types. The premium-grade, mid-density silica-bonded insulation actually exhibited the least thickness shrinkage, showing just 2% shrinkage over the entire test. However, due to the insulation’s high level of weight loss, its performance even at a slightly greater thickness would be poor. The premium-grade, high-density alumina-bonded insulation and the specially prepared, high-density alumina-bonded insulation both exhibited thickness shrinkage in the median range, well within acceptable limits given their low weight loss figures.
All of the alumina-bonded materials showed significantly less weight loss after exposure to hydrogen gas at 1450C compared to the silica-bonded types. Although the silica-bonded materials did not experience a great amount of shrinkage, they both exhibited significant weight loss, proving that they are unsuitable for use in a hydrogen atmosphere. The premium-grade, high-density and specially prepared, high-density grades of fibrous alumina-bonded insulation exhibited the least weight loss and thermal shrinkage of all of the specimens tested. Based on these results, researchers determined that these two types of insulation are best suited for use as thermal insulation, fixtures and setters in furnaces with hydrogen atmospheres.
The data presented in this article was collected by CM Furnaces, Inc. (http://www.cmfurnaces.com
) and provided to ZIRCAR Ceramics, Inc. by Donald T. Whychell Sr. (e-mail firstname.lastname@example.org
), CM Furnaces’ director of Research and Development.
For More Information
For more information about alumina-bonded insulation, contact ZIRCAR Ceramics, Inc., 100 N. Main St., PO Box 519, Florida, NY 10921-0519; (845) 651-6600; fax (845) 651-0441; e-mail email@example.com
; or visit http://www.zircarceramics.com