Abstract
This paper describes an investigation into the effects of nitriding, denitriding, and renitriding, on the rotating bending fatigue strength of notched specimens of the quenched and tempered steels 24CrMo13 and 42CrMo4. The effects of the different treatments were characterised by microstructural investigation, hardness measurements, determination of the nitrogen and carbon contents in the diffusion layer, and determination of surface macrostresses. Nitriding induced a doubling of the fatigue limit. Denitriding only partly removed the effect of the nitriding treatment, because the strength increase caused by precipitation of alloying element nitrides remained unchanged, and only the effect of interstitially solved nitrogen disappeared. For nitrided and renitrided specimens an approximately linear relationship was found between the surface hardness and the fatigue strength. Denitriding caused a larger decrease in the fatigue strength than could be explained by the loss of surface hardness. The change in macrostresses from compressive to tensile also played a part. The role of interstitially dissolved nitrogen in determining the fatigue strength was found to be larger than assumed thus far.
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