**3.2 Effect of ball burnishing on the surface microhardness**

The effect of ball burnishing as well as the number of passes on surface microhardness Hv of the surfaces is presented in **Figure 4**. It can be remarked that all burnished surfaces show higher values of microhardness in comparison to the turned sample. This indicated the high efficiency of BB process in hardening the surface of

**Figure 3.** *3D images of (1) untreated surface and (2) burnished sample with i = 3.*

### **Figure 4.** *Microhardness values of untreated and burnished surfaces.*

316L. The impact of increasing the number of passes is manifested by a variation in the final microhardness. As a result, to one pass during ball burnishing (i = 1), an increment of approximately 90 Hv was obtained. While the increase in the number of passes to i = 2 did not really cause a remarkable change in the microhardness, the application of 3–5 passes generates a very significant hardening. The application of three passes has led to the increase in microhardness by 150 Hv while the most significant improvement in microhardness was recorded when four passes (i = 4) were applied. A similar hardening effect was caused after the application of five passes (i = 5) which is characterized by reaching a value of 593.0 Hv.

The impact of burnishing on the microhardness is interpreted by the plastic deformation which produces a structure with condensed grains and generates residual stresses loading the surface in compression. However, and contrary to the results of roughness, the increase in the number of passes does not cause any negative effect even if the ball passes 5 times successively over the same surface. Although, several works stipulate that repeating the burnishing operation several times, especially more than 3 times, destroy the surface because the surface is already saturated. We can explain our result by the high feed (0.18 mm/rev) and the low force (80 N) applied during the five passes. Indeed, since the ball is lowly loaded and moves quickly, it deforms more areas after each pass without affecting the already deformed areas. In other words, the repetitive passage of the ball over the same area will not have a detrimental effect since it does not have either the great force or the time sufficient to penetrate the surface and destroy the previously deformed layers.
