Failure analysis of bullnose refractory in reheating furnace of Hot Strip Mill

•A systematic work has been done to study the failure of refractory in reheating furnace of Hot Strip Mill at Tata Steel.•Alumina–silica based castable in bullnose structure of reheating furnace was undergoing sagging.•Alkali corrosion was identified as the reason of failure.•Addition of 15% SiC in the existing composition showed better corrosion resistance serving as a potential solution.•The reason for such effect was attributed to the formation of SiO2 on the surface of silicon carbide particles.

This paper presents the failure analysis of aluminosilicate based castable used in the bullnose structure of the recuperative type reheating furnace of Hot Strip Mill, where there is a greater wear with respect to the other areas in the furnace. The wear of the refractory was envisaged after a major shutdown that took place at Hot Strip Mill of Tata Steel. The various characterisation tests yielded that there is a reaction between alumina and alkali oxide (sodium oxide) from the mould powder in the presence of silica forming sodian anorthite which undergoes volume expansion and thus generates stresses in the pre-assembled bullnose blocks. These developed stresses were leading to the problem of sagging of the refractory in the reheating furnace. The source of alkali was identified as mould powder along with mill scale getting deposited on the refractory through hot air acting as a carrier and causing reaction. As it was considered as the major problem in the reheating furnace, an elaborate work was done on providing a viable solution which entails study of effect of addition of silicon carbide in alumina based refractory mix.