Residual stresses, microstructure and fatigue behaviour of carburized layers before and after shot peening

The investigations were made on steel type 20HNM, used for carburizing. The specimens were carburized at different time periods as to obtain differentiated thicknesses of diffusion layers. The carburized layers of thickness HV550 from 0.6 to 1.3 mm were pneumatically shot peened with steel shot 0.8 mm (S280) and the time of exposition of tile shot peened surface varied. The value and distribution of residual stresses at carburized layers as well as carburized and shot peened ones were determined. The measurements were done by destructive method. There were recorded the strains of the flat sample after the release of residual stresses in result of removing, by etching, a thin layer of the material. The fatigue characteristics were determined, for the preset technological parameters, at four-point bending. It was proved that the increase in fatigue strength was the result of the changes of value and distribution of compressive residual stresses caused by structural changes which appear at carburized layer after shot peening. By means of Mossbauer spectroscopy, the changes of retained austenite at carburized samples, which appear as the result of surface plastic strain caused by shot peening, were determined. The results of the investigation should be used in technology of hardening of highly loaded gears.