Research On The Effect Of High-energy Shot Peening Of 15CrMo Steel On The Properties Of Chemical Conversion Coating

The quality of conversion coating on the surface of steel is not only relation to the nature of the conversion liquid, but also the material compositon, microstructure, the surface station and so on. The surface of steel processed by high-energy shot peening possess high density of crystal defects:grain boundaries, dislocation etc. This high density crystal defect enhance the activity of atomic greatly and add the diffusion paths of atomic, the structural characteristics of high density of crystal defects obtained by shot peening is used in some surface treatment technology such as:reducing the time of nitriding, lowing the temperature of nitriding, and improving the thickness, hardness and corrosion resistance of nitrided case, but, combining this characteristics with chemical conversion film is not be reported. The object of this page is 15CrMo steel, at the first, high-energy shot peening is used to realize severe plastic deformation, metallographic analysis and X-ray diffraction technology is used to analysis the organizational characteristics of steel before and after shot peening; then, three kinds of chemical conversion film were formed on the surface of 15CrMo steel before and after high-energy shot peening, the three kinds of chemical conversion film respectively are traditional chemical phosphating, phosphating accelerated by electric field, black oxidation at room temperature. The morphology, thickness, corrosion resistance are represent to research the effect of microstructure characteristic obtained by high-energy shot peening on the properties of chemical conversion film. It turned out that:The grain boundary of ferrite on the surface of 15CrMo steel is turned fuzzy and illegible after high-energy shot peening, and the pearlite has clear deformation flow lines, bending, torsion and size reduction was produced. After high-energy shot peening, the serve plastic deformation is remarkably, the microstructures are sharply refined, XRD diffraction peak occurred obviously wide. And, after high-energy shot peening, the surface roughness is increased.Compared with the phosphating process on the surface of low carbon steel,the phosphating process of 15CrMo low alloy steel is need to heat up to 75℃ and add the content of strong oxidizer NaClO3. The crystal characteristics of phosphating film on the surface of 15CrMo steel after high-energy shot peening is not obvious, and the film cover the metal surface flocculent and density. After shot peening,the thickness of the phosphating film of steel increase markedly, from lum to 3um; after shot peening, the corrosion resistance of phosphating film is better:polarization resistance increased 2.8 times, the corrosion current density decreases by 60%, the time resistant to salt water is longer. High energy shot peening treatment significantly improved the adhesion of the subsequent composite organic coating of phosphating film on steel and it’s a litter influence on the morphology and chemical reagent of composite organic coating.Compared with the ordinary infiltrating phosphating process on the surface of 15CrMo steel, electrochemical phosphating method can be implement under normal temperature. The crystal characteristics of phosphating film on the surface of 15CrMo steel after high-energy shot peening is not obvious, and high-energy shot peening treatment could increase the thickness from 2.37um before shot peening to 3.5um. After shot peening, the corrosion resistance of phosphating film is better:polarization resistance increased 1.8 times; the corrosion current density decreases by 44%; the time resistant to salt water is longer. High energy shot peening treatment significantly improved the adhesion of the subsequent composite organic coating of phosphating film on steel and it’s a litter influence on the morphology and chemical reagent resistance of composite organic coating.After high-energy shot peening, a denser and finer black conversion film can be formed on the surface of 15CrMo steel, and the Mo in steel participate in forming film. Compared with that of the conversion film on the untreated steel, the corrosion resistance of the conversion film on the 15CrMo steel is enhanced by high-energy shot peening of the steel, as the free corrosion potential is improved by 90mV, the free corrosion current density is reduced by 60%, and the time of tolerance to salt water submersion is increased.