## 3 Results and Discussion  
### Evaluation of the Surface Integrity of the Steel Substrate  
The measured roughness values (Table 5) indicate that the rolled surface (reference sample without mechanical pre-treatment) exhibited the lowest average values for both Ra and Rz parameters. The selected process parameters for blasting and grinding technologies were considered optimal, as they produced measurable plastic deformation, which is also statistically comparable to findings in previous studies [5, 18]. Based on standard deviation values for Ra and Rz, grinding was identified as the most suitable mechanical pre-treatment, as it yielded the most homogeneous steel surface texture. This can be attributed to the consistent cutting conditions during machining. In contrast, both blasting methods exhibited higher standard deviations, primarily due to manual operation of the blasting equipment. However, these variations were not statistically significant and align with technical practice. Among the tested methods, synthetic corundum blasting produced the least homogeneous surface, which can be attributed to the aggressiveness of angular abrasives [6, 7]. Conversely, the spherical shape of steel grit particles contributed to lower roughness values compared to the other mechanical pre-treatments. A comparative analysis of Ra and Rz parameters further confirmed that the average Rz values were approximately 6.6\(\times\) (reference sample) to 8.3\(\times\) (synthetic corundum blasting) higher than Ra values. The accuracy of Rz measurements was objectively verified using metallographic specimens (Figure 6), while the appearance of individual sample types is illustrated in Figure 7.  
Figure 5: Principle of determining the percentage of surface area affected by red rust and measurement parameter settings in the ImageJ software. Microhardness measurements (HV0.1, Figure 8) confirmed the highest degree of plastic deformation in samples blasted with steel grit, with hardness increases observed up to a depth of approximately 375 \(\upmu\)m from the theoretical surface position. This finding aligns with the results of [28], which also examined the effects of blasting on the properties of carbon steel AISI 1045. In comparison, samples blasted with synthetic corundum exhibited a less pronounced effect on subsurface hardness, with hardness increases detected only up to a depth of approximately 175 \(\upmu\)m. This difference is primarily due to the higher specific density of steel grit, which enables greater kinetic energy transfer during blasting [10]. This characteristic can be particularly beneficial for blasting high-strength martensitic steels, which exhibit high toughness, as demonstrated in [29]. In that study, steel grit blasting affected material hardness only up to a depth of approximately 120 \(\upmu\)m. The final hardness values of ground samples and the reference sample were found to be equivalent, which is a direct result of the grinding process itself. Unlike blasting, grinding involves a different abrasive-substrate interaction, primarily governed by material removal rather than plastic deformation. However, both ground and reference samples exhibited a slight decrease in hardness toward the core, likely due to reduced deformation hardening in the rolled  
Figure 6: Example of Rz parameter verification on a metallographic specimen (surface blasted with steel grit). Figure 7: Appearance of test samples: (**a**) blasted with synthetic corundum, (**b**) blasted with steel grit, (**c**) ground, and (**d**) reference sample.