13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 4. The microstructure of cross-section of specimens before and after UNSM. Figure 5. The XRD pattern of Fe (211) diffraction angles for the specimens before and after UNSM. other can be observed. For the nitriding sample, a compound layer can be found on the nitriding sample surface (approximately 3 µm for N8 and approximately 8 µm for N48 sample). All the UNSMed samples can be found a grain refined layer at the edges of the cross-section. The refined layers are obvious for U1 and U2 samples approximately 40 µm for U1 specimens and approximately 70 µm for U2 specimens. However for the pre-nitriding samples, this layer is difficult to be distinguished and only some deformed grains can be found and the results show that the deformed layer induced by UNSM decreases with the increase of pre-sample surface hardness. The XRD was employed to scan the sample surface and the Fe (211) diffraction angles were scanned slowly and shown in Figure 5. Analyzing the XRD results and we can find that the Bragg diffraction peaks for all samples can be broadened by the UNSM. That means a grain refinement was induced on the sample surface according relation between grain size and the full width at half
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