Surface Nanocrystallization Induced By High-speed Rotation Wire-brushing Deformation And Its Effect On Plasma Nitriding

Based on the grain refinement mechanism induced by severe plastic deformation, a novel technique of high-speed rotation wire-brushing deformation (HRWD) was used to treat the surface of low carbon steel and commercially pure titanium(TA2) with the purpose of exploring the feasibilities of realizing surface nanocrystallization by HRWD. The experimental set-up was designed and rebuilt. The refined microstructure features were systematically characterized by means of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. The microhardness variation along the depth of the treated sample was examined by microhardness tester. Experimental evidences showed that after the HRWD treatment with ordinary coppered steel wire, a nanostructured surface layer of about 10-15μm in thickness was formed on low carbon steel. The mean grain size in the top surface layer was approximately 8nm. It was found that a gradient microstructure with grain size from nanoscale to microscale was obtained along the depth of its surface layer. The microhardness of nanostructured surface layer was enhanced significantly after HRWD, compared with that of the original sample, and reached 491 HV0.05. The microstructure of the surface layer was refined into the nanoscale on TA2 by HRWD. Its mean grain size in the top surface layer was up to 15nm. In the top surface nanostructured layer, the microhardness reached 489HV0.05, which was about three times that of the coarse-grained matrix. And along the depth from the top surface, the microhardness in the surface layer gradually decreased to that of the matrix.Plasma-nitriding behavior of low carbon steel and commercially pure titanium after the HRWD treatment was investigated in comparison with that in the original materials. Experimental results showed that improved nitrogen transport could be obtained by plasma nitriding after the samples were subjected to HRWD. For low carbon steel nitrided below 500℃, the surface nanocrystallized samples could produce thicker compound layer than untreated samples. With the same nitriding treatment parameters, the surface microhardness of the samples treated by HRWD, increased significantly, which nearly doubled the value of plasma-nitrided coarse-grained ones. However, after 5 hours' nitridation at 600℃, the surface nanostructured samples began to lose their good effects on the nitridation speed during the plasma nitriding. The compound layer came into existence on HRWD commercially pure titaniun nitrided at 480℃for 8 hours. Furthermore, nitrided below 600℃, the surface nanostructured TA2 could form thicker compound layer than untreated samples and raised the surface microhardness from 703HV0.05 (of nitrided coarse-grained TA2) to 909HV0.05.