13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- maximum (FWHM). With FWHM results of all samples it can be found that though the nitrides within the sample surface zone and the XRD intensity of all un-UNSMed samples is different, the values of FWHM are close. And the degrees of the change are different. It is easy to find that the N48 samples were broadened easily compared with the un-treated and N8 samples. In many references [6-8], the hard phase can enhance the ability of the refinement of ferrite as different mechanism. So we can see that with the growth of nitrides concentration, harder sample surface can be obtained. More easy grain refinement can be found the harder sample, but thinner grain refined layer would be obtained as the reason of the surface with high hardness hindering the transfer of the plastic deformation energy produced by UNSM. Figure 6. The microhardness of cross-section of specimens before and after UNSM. The microhardness results of the all samples along the cross-section were shown in Figure 6. After UNSM treatment not only was surface hardness improved but also a gradual enhancement in the sub-surface region also can be observed. For the UNSM samples, the microhardness in the top nanostructured layer is 372 Hv for U1 sample and 418 Hv for U2 sample. For the N8 specimens, the surface hardness increased from 443 HV to approximately 540 HV after U1 treatment and to 560 HV after U2 treatment. These values were even higher than that of the N48 samples without UNSM treatment. For the N48 specimens, the surface hardness increased from 510 HV to 630 HV after U1 treatment and to 650 HV after U2 treatment. For all UNSMed samples the top surface hardness and the depths of effect increase with more strike number. It also can be found that the depths of effect regions are different with the change of samples. After the UNSM treatment, the materials surface was subjected to a severe plastic deformation induce a grain refinement. With the UNSM treatment, increased number of grain boundaries as the refined grains and generation of high density dislocation with strain hardening restrict the dislocation motion and render the material harder and stronger. According the microstructure results, we can also infer that the concentration of nitrides has a close connection with the hardening depth. Zhou [6] has found that compared with the strain-induced grain refinement in the pure Fe under the same SMAT processing, not only was refinement of ferrite much facilitated by the presence of dispersed cementite particles but also a thicker nanostructured layer can be obtained. The hard phase has the ability of transfer energy during S2PD treatment. For soft surface materials, as the amplitude of 30 µm during UNSM processing energy of UNSM can not transfer too far as the absorption of energy by the top surface to have a plastic deformation. With the increase of nitrides concentration the hard phase of nitrides can transfer the energy to the deeper region. However when the surface hardness is too high, it is difficult to have a plastic deformation in the top surface and only induce a refinement of grains. So with just enough nitrides concentration, a deeper hardening layer can be produced. The top surface residual stress was measured and shown in Figure 7. Before UNSM treatment un-treated and plasma nitriding samples all have a compressive residual stress on the surface
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