Characterisation of microstructural evolutions in refractory castables by in situ high temperature ESEM

This paper deals with the characterisation of the microstructural evolutions during the first heating of two commercial refractory castables by dilatometry and by in situ high temperature environmental scanning electron microscopy (HT-ESEM) up to 1300 °C. Investigations have been made both on the castables and on their respective matrix. Effects of microstructural changes have been observed on dilatometric results. Four temperature domains could be defined, in which shrinkage is occurring. During in situ HT-ESEM observations, important microstructural changes could be seen in these four temperature domains. They deal first with microcrack initiation due to dehydration mechanisms. Microcrack opening then increases when increasing the sample temperature. An acceleration of these microcrack-opening mechanisms was observed when sintering starts in the 800–950 °C temperature range. Between 1000 and 1100 °C, liquid phases appear in the material and liquid phase sintering starts to occur. An increase of the liquid phase amount and of liquid phase sintering effects are observed when increasing temperature up to 1300 °C. Crystal growth was identified as occurring at this temperature too. Above 1100 °C, a competition between shrinkage mechanisms and expansion ones take place. The balance between these two types of mechanisms is not the same for the two studied refractory castables.