Abstract: Back-up roller of finish rolling in 1580 production line suddenly broke during the rolling process. In order to figure out the cause of failure, microstructure, mechanical properties, fracture morphology and location of crack initiation source of the roller material were comprehensively analyzed by means of optical microscope, tensile and impact testing machine and scanning electron microscopy. The results show that core material of finish rolling support roller is nodular cast iron, mainly compounded by pearlite, rod cementite, spherical graphite and bovine eye ferrite gathered around graphite. The cleanliness of the structure is unsatisfactory, the non-metallic inclusions have the size concentrating in 10-15 μm and mainly are spherical Al₂O₃-MgO-SiO₂, Al₂O₃-MgO, Al₂O₃oxides. The fracture surface of the backup roller presents typical rolling bending fatigue fracture characteristics. The crack source area is located inside the roller and shows as multi-source cracking. The fracture surface presented typical “Fish-in-Eye” characteristics, the largest inclusion in the “Fish-in-Eye” area has a diameter of 3 mm and is spherical Al₂O₃-MgO-SiO₂. The maximum tensile stress of the crack source is 30 MPa, which reaches the fatigue strength of the material. When the crack starts around the inclusion, the maximum stress intensity factor(ΔK) at crack tip is 1.385 MPa·m^1/2, larger than the threshold value of internal crack initiation and propagation(ΔK_inc-th). Thus it is inferred that the cracks nucleate at the interface between the inclusions and the matrix. When the crack extends beyond the “Fish-in-Eye” area, the crack expands rapidly under the action of stress and converges to form a long crack, which eventually leads to the instantaneous fracture of the roller.

Key words: finishing mill back up roller, non-metallic inclusion, fatigue damage, stress intensity factor(ΔK)