2.2 Subsurface properties 2.2.1. Nano crystallization Figure 3. Comparison of grain size before and after UNSM treatment for SAE52100 bearing steel specimen. Figure 3 shows modification of the grain structure in the subsurface of specimen made of SAE52100 bearing steel. It can be seen that strikes induced grain size refinement from 2 µm to 0.4 µm. Figure 4 shows grain size in the subsurface of UNSM generated nanoskin of SUS 304 steel. The grain was refined almost 1/1000 [8, 13]. Depth, μm Grain size, nm 5 30 10 36 20 42 30 45 Figure 4. TEM and XRD analysis and calculation of SUS 304 steel grain size of after UNSM 2.2.2. Increase of top surface and subsurface hardness SKD61 SAE52110 Figure 5. Microhardness depth profile of SKD61 and SAE52110 steels treated with UNSM 650 700 750 800 850 900 0 0.5 1 1.5 2 2.5 3 3.5 Hardening Depth[mm] Hv[300g] UNSM V i
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