## 3 Results and Discussion
### Experimental Results
Approximately 40% more hardness was measured following the SP procedure. This can be explained by an increase in plastic deformation leading to an increase in hardness and an increase in Almen intensity. At 24-26 A Almen intensity, AISI 1020 material's maximum hardness value (240.7 Vickers) was recorded. Following the SP procedure, the hardness value rose by around 20%. The AISI 4140 material's maximum hardness value, 347.3 Vickers value, was found at 24-26 A Almen intensity. The AISI 4140 material's hardness value rose by about 40% following the SP procedure. Figure 2: The hardness values of the specimens shot-pened at various Almen intensities: (**a**) AISI 8620; (**b**) AISIS 1020; (**c**) AISI 4140; (**d**) AISI 5140. Figure 3 displays the hardness values of the samples subjected to vibratory SP at varying times and ball diameters, along with the SP procedure. The highest hardness value of AISI 8620 material was found to have a 268 Vickers value in the sample with a ball diameter of 4 mm and a processing time of 60 min. It is clearly seen in Figure 3 that hardness increases with an increase in the processing time if the ball diameter remains constant. This is because longer processing times allow for the better development of nano-thin layers on the part's surface. The AISI 1020 material's maximum hardness value was discovered to be 276.10 Vickers. Similarly to AISI 8620 material, 4 mm ball diameter, and 60 min of processing time were found to yield the maximum hardness value in AISI 1020 material. The AISI 4140 material has a maximum hardness rating of 363 Vickers. Similarly to AISI 8620 material, samples treated with a ball diameter of 4 mm and a processing duration of 60 min yielded the highest hardness value in AISI 4140 material. It is known that materials with nano-sized grains have better mechanical properties than materials with micron size. In order to obtain nano-sized grains, methods are applied by creating excessive plastic deformation without changing the chemical structure. The SP method is used in some fields for this purpose. The purpose of this study was to use SP to boost the material surface's useful residual stress profile and stress value. In order to achieve this, various process parameters were used to assess the residual stress of the samples that underwent vibratory SP (ball diameter and processing time) and SP (Almen intensity). Table 6 lists the residual stress levels for each sample group. It is evident from the measurements that the shot-peened surfaces have developed compressive residual  
Figure 3: Hardness values of the specimens treated with VSP using various process parameters: (**a**) AISI 8620; (**b**) AISIS 1020; (**c**) AISI 4140; (**d**) AISI 5140.  
stress. When the residual stress results measured from samples peened at different Almen intensities were examined, it was determined that the highest value was obtained at 24-26 A intensities. Also, it was seen by analyzing the data that as Almen intensity grew, so did the compressive residual stress area. In the examinations, it was determined that compressive residual stress areas increased with the increase in ball diameter in the samples applied with the VSP method. Figure 4 compares the residual stress values that were discovered on the material surfaces following the application of the SP procedure to the specimens. As seen in Figure 4, the residual stress values increased with increasing the Almen intensity from 11-14 A to 24-26 A. The samples to which the VSP method was applied were examined, and it was determined that the residual stress values increased as the processing time and ball diameter increased. Figure 5 shows optical microscope pictures of the SP samples. It is anticipated that the grain will be thinned as shot-peened materials' surfaces are plastically deformed. The shot-peened samples showed evidence of both grain thinning and a plastically deformed layer following plastic deformation, as shown in Figure 6. The surface quality of the SP-treated samples was lower than the VSP-treated samples.