# Effect of Surface Micro-Hardness Change in Multistep
## 3 Experimental Results and Analysis
### Surface Wear Microtopography  
Figure 10 shows that the surface wear microtopography was observed after 15 min in the two-step machining. Figure 11a,b show the surface wear microtopography observed after 15 min and 30 min in the three-step machining, respectively. Additionally, Figure 12a,b show the wear loss measured after 15 min and 30 min in the two-step machining, respectively. It could be seen from Figures 10 and 12 that within the roughing cutting speed parameters, when the roughing cutting speed was 50 m/min, the surface wear spalling  
area was relatively minimal and the wear loss was the least. On the contrary, when the roughing cutting speed was 70 m/min, the surface wear spalling area was the largest and the wear loss was the most. As the roughing depth of the cut increased, the surface wear spalling area was more severe and the wear loss was greater. It could be seen from Figure 11 that, as the friction time increased, the surface wear spalling area significantly enlarged, and the wear loss was more serious. When the cutting speed was 70 m/min or the cutting depth was 1.5 mm, it could be found from Figure 10a,b that severe spalling appeared on the surface by the amplification of local area, and wear fracture striations occurred due to spalling.  
Although the finishing cutting conditions and the process of the friction experiment were the same, the surface wear spalling and wear loss were significantly different. The main reason was that the change of roughing surface layer micro-hardness has an effect on the finishing surface micro-hardness to some extent [22], which led to differences in finishing surface micro-hardness in the local area. The greater the difference of micro-hardness in the local area was, the greater the difference of plastic deformation of the material was. The surface layer subjected to alternating contact stress was more prone to cracks during the friction process, which in turn peeled off. Therefore, the surface wear spalling was different, the wear loss was different. Additionally, the surface wear spalling and wear loss were positively correlated with the standard deviation of micro-hardness.  
Figure 13 showed the subsurface wear microtopography under different friction times. It was found from Figure 13 that the cracks occurred in the subsurface layer, and even  
Figure 11: The surface wear microtopography under different friction times in the three-step machining. (Roughing cutting parameters: \(v=50\) m/min,\(f=0.25\) mm/r, and \(a_{p}=1.0\) mm; Semi-finishing cutting parameters: \(v=80\) m/min,\(f=0.15\) mm/r, and \(a_{p}=0.3\)mm). (a) 15 min. (b) 30 min.  
Figure 10: The surface wear microtopography observed after 15 min in the two-step cutting condition. (**a**) \(f=0.25\) mm/r, \(a_{p}=1\) mm. (**b**) \(v=50\) m/min,\(f=0.25\) mm/r.  
spalling pits appeared. The longer the friction time was, the more severe the surface wear spalling area was. This was mainly because there was a change in the finishing surface micro-hardness in the unit feed along the feed direction. The greater the standard deviation was, the greater the difference of plastic deformation of the material was. At the same time, since the alternating contact stress during the friction process was quite high, cracks appeared in the weak area of surface layer. As the experiment progressed, the crack spread and peeled off, and even the spalling pits occurred due to cyclic impact spalling.