Failure analysis of a furnace roller in continuous annealing line after long-term service at high temperatures

Abstract

The rollers used to convey steel strips in a continuous-annealing furnace were deformed after about 12 years of service, resulting in an unevenness of the rolled strips. In this study, failure causes of the furnace rollers were investigated in terms of degradation of microstructure and mechanical properties of the roller material. After a long period of high temperature service, the microstructure of the material has changed significantly. A large number of network-like carbides precipitated at the grain boundaries, and these carbides have been greatly coarsened. It was also found that a great number of σ-phase have formed on the coarsened carbides, and that the volume fraction of the σ-phase on the grain boundaries has exceeded 50%. In addition, creep cavities have been found in the grain boundaries. Especially in regions with dense grain boundaries, the cavities formed at the intersection of grain boundaries have been linked into chains, which tend to develop into cracks. The grain boundaries of the furnace roller material after service has been severely embrittled. Both tensile and impact fracture surfaces show brittle intergranular fracture characteristics. Compared to the unused material, the tensile strength and impact toughness of the failed roller decreased significantly due to the deterioration of the microstructure. Because of the degraded mechanical properties, the rollers are prone to deformation under the service loads, thus cannot guarantee the normal production of strip steels. The results of this study provide a reference for evaluating the remaining life of furnace rollers after long-term service at elevated temperatures.