| This study aims at clarifying microstructural factors such as tempered martensite matrix, coarse primary carbides, and fine secondary carbides, which influence thermal fatigue properties of Ni-grain cast iron roll, high Cr(Hi-Cr) cast iron roll, and high speed steel(HSS) roll. The micromechanism of fatigue process involved in crack initiation and propagation was identified by the observation of the sectioned region beneath the fracture surface and by the direct observation of fracture process using in situ SEM technique. The thermal fatigue properties were analyzed by the microstructural difference of the rolls and by the increase in tensile stress which might cause fracture when the tensile stress reached to the tensile strength of the roll. The microstructural observations indicated that coarse primary carbides acted as fatigue crack initiation sites on the specimen surface since they cleaved first to form cracks at very low stress levels. Thus, the HSS roll having the higher tensile strength and the smaller amount of primary carbides showed the better thermal fatigue properties than the Hi-Cr roll or the Ni-grain roll. In order to improve thermal fatigue properties of the rolls, it is also suggested that the homogeneous distribution of primary carbides must be achieved by the decrease in carbide segregation along the solidification cell boundary and by the optimization of the roll casting process. |
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