Mechanism of thermal shock damage of Mo–W–ZrO2 cermet sensor for molten steel temperature caused by water vapor during sintering

Abstract

Mo–W–ZrO2 cermet has been used in molten steel temperature sensors for thin-walled structures with fast responses. However, thermal shock damage often occurs when it is dipped into molten steel because of the presence of water vapor during sintering; this study investigates this underlying mechanism. The results reveal that water vapor decreases the perimeter to area ratio of the metal phase from 1 (sintered in the absence of water vapor) to 0.92. This implies that the metal phase tends to agglomerate and coarsen, thus resulting in a decrease in the bridge toughening performance of the metal phase. Moreover, water vapor causes segregation of metal elements in the metal phase, which may reduce their strength and further lead to a decrease in the toughening performance. As the water vapor is removed, the cermet sensor is able to withstand the severe thermal shock test of dipping it into molten steel for temperature measurement. Furthermore, the relationship between the thermal shock damage and the evolution of the microstructure and composition of the cermet is clarified; this may have implications for optimizing the thermal shock resistance of other cermets.