Study on formation and morphology of sigma-phase in continuous annealing furnace roller

Investigations were revealed that roller fracture was caused by sigma-phase embrittlement and an increase of precipitations in the microstructure. In order to decreasing the failure, homogenization heat treatment at 1100 °C for 2 h followed by air cooling was used. In this study, optical microscopy metallography, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) examinations were performed, in order to investigation of formation and morphology of the sigma-phase before and after homogenization treatment. It was found that by transformation of delta-ferrite, first M23C6 carbides with lamellar morphology and also secondary austenite were formed and after completion of lamellar precipitation, the sigma-phase was formed at foreside of the precipitates. The formation of sigma-phase with dendritic morphology resulted in decrease in ductility and failure of the continuous annealing furnace roller. Also, homogenization heat treatment at 1100 °C for 2 h followed by air cooling can led to change in the morphology of sigma phase from dendritic structure to globular structure. Finally, it was found that the formation of sigma-phase with globular structure led to remarkable increase in the impact energy and ductility of heat resistant steels and therefore, the failure of continuous annealing furnace roller was decreased.