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The initiation and propagation of thermal fatigue cracks in gray cast iron and vemicular graphite cast iron were investigated by Uddeholm method to reveal the complex thermal fatigue behaviors of cast iron. Differences of thermal fatigue behaviors of gray cast iron and vemicular graphite cast iron were observed and analyzed. It is found that the observed differences are related to the combination of graphite morphology and the oxidization of matrix. More oxidized matrix is observed in gray cast iron due to its large specific surface area. The brittle oxidized matrix facilitates the propagation of microcracks along the oxidization layer. By contrast, the radial microcracks are formed in vermicular graphite at the edge of graphite due to fewer oxidization layers. It indicates that the thermal fatigue resistance of gray cast iron is dominated by graphite content and morphology while that of vermicular graphite cast iron strongly relates to the strength of the matrix. [ABSTRACT FROM AUTHOR]