## 4 Analysis of Surface Morphology and Surface Roughness after Grind-Hardening  
### Surface Morphology Analysis  
The surface morphology after grind-hardening was observed using an OLYMPUS-DSX500 optical digital microscope (Figure 1). As shown in Figures 2-5, the specimen surface after grind-hardening is not smooth and the grinding texture is coarse, with many defects and damage, such as scratches and bonding.  
In the cutting-in area of the grind-hardened surface, the grinding pattern is clear, and many grinding defects, such as bonding and breakage, can be observed. As the abrasive grains on the grinding wheel were sharp after dressing, their cutting effect was strong. As a result, they left clearer patterns in the cutting-in area. Due to the large grinding thickness,  
Figure 4: Two-dimensional view of the surface morphology of sample No. 7 (20\(\times\)). (**a**) cutting-in area; (**b**) middle area; (**c**) cutting-out area.  
Figure 5: Three-dimensional view of the surface morphology of sample No. 7. (**a**) cutting-in area; (**b**) middle area; (**c**) cutting-out area.  
Figure 3: Three-dimensional view of the surface morphology of sample No. 5. (**a**) cutting-in area; (**b**) middle area; (**c**) cutting-out area.  
Figure 2: Two-dimensional view of the surface morphology of sample No. 5 (20\(\times\)). (**a**) cutting-in area; (**b**) middle area; (**c**) cutting-out area.  
the abrasive grains directly entered the cutting stage as soon as they came into contact with the workpiece (skipping the slipping and plowing), and the chips could not be discharged in time. The combined action of excessive grinding force and grinding temperature caused a series of physical and chemical reactions of the dislodged abrasive grains and chips, which adhered to the surface of the workpiece, resulting in bonding and burn.  
In the middle area there is a section of a relatively stable and flat surface with localized flow coating and pits. As the grinding wheel passed through the cutting-in area, the abrasive grains produced a certain degree of dullness and had no time to self-sharpen. Under high temperatures and pressure, the air holes of the grinding wheel were blocked by the chips that were not discharged in time, rendering the grinding wheel surface smooth. At this time, the metal in the vicinity of the grinding arc was in a plastic flow state due to the high temperature. Thus, the \"smooth\" state of the wheel was \"copied\" onto the grinding surface, creating a relatively flat machined area. Compared with the cutting-in area, the grinding temperature in the middle area increased further, and traces of flow coating of the metal material appeared in some areas. Meanwhile, as the hardness of the metal material reduced with the increasing temperature in the grinding area, and the chipped abrasive grains were not discharged in time, the grinding wheel \"pressed\" some of the chipped abrasive grains into the grinding area, creating craters.  
In the cutting-out area, numerous microcracks, breakages and folds appeared. The grinding wheel was worn badly and the air holes were clogged severely, resulting in further increases in the grinding force and grinding temperature. In the final grinding area, as part of the grinding wheel that left the grinding surface, the metal grinding rate decreased suddenly. Consequently, the grinding force dropped sharply, causing a sudden increase in the unbalanced vibration of the grinding wheel and workpiece. When the grinding wheel was completely off the workpiece, the temperature dropped sharply, leading to the initiation and extension of microcracks. Microcracks are observed in both the longitudinal and transverse directions and are longer in the longitudinal direction (workpiece feed direction).