Investigation Of Microstructure And Property Of The Low Carbon Steel After Nanocrystallization

Most failures of materials occur on surface, including fatigue fracture, corrosion and wear etc., which are very sensitive to microstructure and properties of the surface. As a result, the surface modification of engineering materials is found to process more and more industrial applications. Surface nanocrystallization is a new concept proposed in recent years. It uses methods such as severe plastic deformation (SPD) to generate a nanostructured surface layer, which can improve the overall properties and behaviors of materials. In this article, a nanostructured surface 1ayer was fabricated on the low carbon steel by means of surface shot peening treatment.By using optical microscopy, X-ray diffraction (XRD), transmission electronic microscopy (TEM), scanning electronic microscope(SEM), microhardness testing machine to investigate: (1) the layers of severe plastic deformation with nanocrystalline structures were gotten on low carbon steel after shot peening treatment; (2) effect on properties of low carbon steel after shot peening treatment; (3) effect on friction and wear properties of low carbon steel by means of surface nanocrystallization. In the last part the anomalous response of low carbon steel plate during surface nanocrystallization is studied using Finite Element Analysis method. And how the thickness of the test sample and impact load affects the anomalous response is studied.After the surface shot peening treatment, a nanocrystallization surface layer was formed, the average grain size of which is 20-30nm, and the grain size increases with the increase of depths to the surface.The microhardness of nanostructured surface is enhanced significantly after the shot peening treatment compared with that of the original sample. Base on the comparison of the microhardness, surface roughness and corrosion of the sample, the results that the best time of nanocrystallization for low carbon steel is 90 min is given.After nanocrystallization the wear resistance of low carbon steel in low and medium load is improved. And the friction coefficient is reduced significantly. When the load increases to 55N, the wear resistance decreased slightly compared with the original sample. As the load increases, the main wear mechanism changes from abrasive wear to fatigue wear. Surface nanocrystallization helps weaken the fatigue wear effect of low carbon steel.The possibility that the anomalous response of low carbon steel plate could appear during Surface nanocrystallization process using SMAT method is proved by the finite element analysis results.