Effects of nitriding on fatigue strength of quenched and tempered steel: role of interstitial nitrogen

This paper describes an investigation into the effects of nitriding, denitr iding, and renitriding, on the rotating bending fatigue strength of notched specimens of the quenched and tempered steels 24CrMo13 and 42CrMo4. The ef fects of the different treatments were characterised by microstructural inv estigation, hardness measurements, determination of the nitrogen and carbon contents in the diffusion layer, and determination of surface macrostresse s. Nitriding induced a doubling of the fatigue limit. Denitriding only part ly removed the effect of the nitriding treatment, because the strength incr ease caused by precipitation of alloying element nitrides remained unchange d, and only the effect of interstitially solved nitrogen disappeared. For n itrided and renitrided specimens an approximately linear relationship was f ound between the surface hardness and the fatigue strength, Denitriding cau sed a larger decrease in the fatigue strength than could be explained by th e 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 thu s far. MST/4598.