Hiroshi Ohba

Effect of Loads on Refractoriness-under-Load of Magnesia Bricks and the Change in Microstructures under Loads

Refractoriness-under-load measurements were carried out on two kinds of burnt magnesia bricks under the load ranging from 1 to 30kg/cm2 and the effect of load on the temperature of deformation and failure was investigated. It was estimated that the mechanism of deformation and failure under loads below about 10kg/cm2 is different from that above. The mechanism will be classified into two types, ie, lower-temperature-higher load type and higher temperaturelower load type. It was found that the liquid formation will play an important role in the process of deformation and failure in higher temperaturelower load type, and that the failure under the standard load 2kg/cm2 will take place when the liquid amounts to about 15%.The deformation and failure under loads was found to have such relation with the microstructure of bricks that it proceeds with the change in matrix part in brick. Some points to be improved in R-u-L test were proposed.

Sintering of Magnesia Brick an Influence of Various Factors on its Contraction Rate

Magnesia bricks were fired at 1600° to 1700°C and changes in their properties, especially the volume contraction due to sintering and the grain growth of periclase, were examined. Effects of temperature and firing time on the rate of sintering contraction were investigated.The volume change of refractories in reheating has been examined usually by the repeat contraction measurements. In this investigation, the reheat linear change of magnesia bricks was considered as a kind of rate process including sintering reaction.The reheat linear changes for two kinds of burnt magnesia bricks from sea-water magnesia were measured at 1600° to 1700°C, and the correlation among reheat change, temperature and reheating time was analysed. It was found that the repeat contraction (S) will be approximately expressed by the following equation including reheating temperature (T), time (t) and activation energy for sintering contraction (Es);S=ks√texp(-Es/RT)This correlation may be applied to the estimation of brick contraction in furnace linings during service and also to adjust the firing condition for making bricks.Reheating promotes the grain growth of periclase in magnesia bricks approximately proportionally to reheating temperatures, where silicates will retard the grain growth of periclase.The standardized methods for reheat change measurements prevailing at present will not be always fit to bricks of these grades. The authors propose an example of optimum condition for reheat change test: 10hrs reheating at 1700°C.