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The fatigue behavior of nitrided nodular cast iron is examined in thiscontribution. Special attention is given to the quantification of the fatigue improvementand the investigation of the link between microstructure and fracture mechanisms.Rotary bending fatigue testing shows a significant increase in fatigue limit and a dualtrend in the S/N curve. Therefore, selected fracture surfaces and fatigue crack pathswere investigated to determine fracture mechanisms associated to the different fatiguelives. Factors such as structure of nodular cast iron, content of carbides, penetrationdepth and concentration of nitrogen on the boundaries of ferritic grains below the whitelayer were found relevant. An interpretative model of the fatigue response of nitridednodular cast iron is proposed.