Nitriding Of The Surface Nanocrystallized Alloy Steels

The combination of the surface nanocrystallization and nitriding not only can enhance the hardness of surface, but also can improve the disadvantage of high temperature, long cycle time and high cost in traditional nitriding, there will be a wide range of applications. Low-temperature gas nitriding of the surface nanocrystallized materials has been studied systematicly at300℃～500℃. The aim of this work is to investigate the nitriding behavior on SMAT steels at higher temperature.In this work, a nanostructured surface layer was obtained on20CrMo steel,40Cr steel,42CrMo steel and0Crl8Ni9steel by means of surface mechanical attrition treatment. Nitriding of the SMAT steel samples at530℃was investigated in comparison with the coarse-grained sample by using microscope, X-ray diffraction (XRD), transmission electron microscope (TEM) and microhardness test.Experimental results showed that a nanocrystalline layer and a severely deformation layer were formed on the surface of these steels. With the increase of the distance from the top surface, the mean grain size increased and the plastic deformation decreased.The study found that the grains on the surface of SMAT samples grew up to submicron grains at530℃, but the recrystallization did not occur. The nitriding layer on the surface of the SMAT sample could be subdivided into compound layer and diffusion layer after nitriding at530℃. The hardness of the nitriding layer on SMAT samples was higher than that of original samples, and the depth of the diffusion layer was thicker than original samples. A compound layer of10μm thick was observed on the SMAT sample.Experimental results of gas nitriding at530℃showed that the higher hardness of20CrMo without compound layer was obtained in comparison with original nitriding sample by means of SMAT, but the brittle was much lower, the nitrided layer was composed of Fe2N and Fe3N phase.42CrMo and40Cr steels had similar hardness of surface of these samples, but the hardness of subsurface of42CrMo sample was much higher relative to40Cr sample; the hardness of SMAT0Cr18Ni9sample after gas nitriding was much higher compared to that in the coarse-grained counterpart nitrided under same condition reach to2000HV owing to the precipitation of Cr2N.