TEM evaluation of steel nanocrystalline surfaces obtained by severe shot peening

TEM analysis was performed in this study to analyze microstructural changes of a quenched and tempered 39NiCrMo3 low alloy steel as a function of the kinetic energy induced by means of different shot peening treatments: conventional (CSP) and severe (SSP). Residual stresses and microhardness evolution in depth were also determined by means of an X-ray diffractometer and microhardness tester, respectively. Surface roughness depending on the applied shot peening treatment was also analyzed. TEM analysis reveals that plastic deformation induced by the CSP treatment promotes ferrite amorphization and the reduction of carbides grain size to nanometric scale. On the other hand, after applying SSP treatments, (Fe,Cr)(23)C-6 nanocrystals are nucleated from the amorphous matrix previously created. Accordingly, the amorphous matrix obtained in the earliest stages of the SP treatment seems to contribute to promote new nanostructures at the end of the SSP. Regarding the mechanical features evolution, the maximum compression stress value was similar in all the shot peening treatments, in the range of -450 MPa, while the thickness of the affected layer by the compressive residual stresses increased when SSP treatments were applied. Microhardness measurements evidenced a notable hardening in the upper layer of the material, increasing with the coverage level. The thickness of the affected layer in depth was around 200 mu m, for all the series. The influence of the CSP and the SSP on the steel hardening was also corroborated by FWHM (Full Width at Half Maximum) measurements.