# An Advanced DEM-FEM Method for Herringbone Gear in Shot Peening
## 4 Results and Discussion
### Effect of Shot Process Parameters on Surface Integrity Parameters
#### 4.3.3 Effect of Shot Hardness  
The study investigated the effects of S110 high-hardness and S110 low-hardness cast steel shot, as well as S170 high-hardness and S170 low-hardness cast steel shot, on the surface integrity of herringbone gears at various shot peening velocities. Due to the limited impact of the hardness parameter, a shot peening process variable, on shot impact information within the discrete element software, the impact counts on the gear surfaces for S110 high-hardness versus S110 low-hardness and S170 high-hardness versus S170 low-hardness were found to be consistent. The distinction between high-hardness and low-hardness shots was incorporated into the finite element calculations, with yield strengths of 2100 MPa and 1600 MPa, respectively, for high-hardness and low-hardness shots.  
Figure 27 shows the impact of shot hardness on the surface topography. Shot peening with low-hardness shots still revealed a significant initial grinding topography, whereas  
Figure 26: Effect of shot velocity on residual stress.  
Figure 25: Effect of shot velocity on surface topography (**a**) Shot velocity 60 m/s, (**b**) Shot velocity 80 m/s, (**c**) Shot velocity 100 m/s.  
the initial grinding topography at the pitch circle was largely covered after peening with high-hardness shots. The pit size of low-hardness shots was smaller than the pit radius of high-hardness shots, indicating that the pit radius on the gear surface increases with the increase in shot hardness. The surface roughness Sa for S110 high-hardness shots was 0.523 \(\upmu\)m, and for S110 low-hardness shots, it was 0.316 \(\upmu\)m. For S170 high-hardness shots, the surface roughness Sa was 0.721 \(\upmu\)m, and for S170 low-hardness shots, it was 0.339 \(\upmu\)m. The results indicate that as the shot hardness increases, the surface roughness also increases. This is because, at lower shot hardness, the shot peening process induces greater plastic deformation in the shots, while the plastic deformation on the target surface is relatively minor.  
Figure 28a shows the residual stress after impacts from S110 high-hardness and S110 low-hardness shots. With increasing hardness, the surface residual stress of the herringbone gear increased from \(-\)662.04 MPa to \(-\)874.41 MPa, a rise of 32.1%. The maximum residual compressive stress after shot peening with S110 low-hardness shots was \(-\)1000 MPa, compared to \(-\)1350 MPa with S110 high-hardness shots, corresponding to an increase of 35% in maximum residual compressive stress with higher shot hardness. Additionally, the position of the maximum residual compressive stress shifted from a depth of 15 \(\upmu\)m to 30 \(\upmu\)m, indicating a significant influence of shot hardness on the location of maximum residual compressive stress. The depth of influence of residual compressive stress for S110 low-hardness shots was 60 \(\upmu\)m, whereas for S110 high-hardness shots, it was 70 \(\upmu\)m, demonstrating that the depth of influence of residual compressive stress increased with shot hardness.  
Figure 28b displays the effect of S170 high-hardness and S170 low-hardness shots on residual stress. As the shot hardness increased, the surface residual stress of the herringbone gear increased from \(-\)542.63 MPa to \(-\)760.25 MPa, and the maximum residual compressive stress increased from \(-\)895.37 MPa to \(-\)1307.36 MPa. The position of maximum residual  
Figure 27: Effect of shot hardness on surface morphology. (**a**) S110 high hardness, (**b**) S110 low hardness, (**c**) S170 high hardness, (**d**) S170 low hardness.  
compressive stress also shifted from 30 \(\upmu\)m to 45 \(\upmu\)m, and the depth of influence of residual compressive stress increased from 80 \(\upmu\)m to 100 \(\upmu\)m. This indicates that higher shot hardness introduces deeper residual compressive stress, resulting in greater surface residual compressive stress and a higher maximum residual compressive stress.