Abstract
To improve the fatigue reliability for SCM440 Steel by an environmentally sound surface modification the influence of the modification on fracture mechanism was studied experimentally. The fatigue properties of SCM440 Steel are found to be obviously improved by a Radical Nitriding and TiCN coating, so that they can no longer be estimated by using a concept of "S-N Globe" Which has successfully been applied to many surface-treated materials. The detailed observation have revealed that fatigue crank initiation in a duplex surface treatment specimen occurs in a hardened region near surface not in a subsurface region with virgin microstructure. The fatigue crack was induced by slip in the ferritic matrix that was restricted by the TiCN layer. And the fatigue crack propagation in the hardened region was restricted by compressive residual stress. Such a crack initiation mechanism is different from that previously observed in other surface treated materials, explaining the reason why the S-N Globe concept can not be applied to the present case.
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