2.1.4. Surface Roughness

In the first step, the specimens were subjected to the milling conditions indicated in Table 2 using a CNC router milling machine (LAGUN 600, MAHER HOLDING, Legutiano, Spain). The macro-texture surface parameters after the milling and burnishing processes were acquired using an Alicona microscope (InfiniteFocusSL, Bruker, Karlsruhe, Germany) and further processed with image analysis software (Mountains 5.1.1.5944, Digital Surf, Besançon, France) according to the ISO-25178-2:2016 standard [26].

**Table 2.** Initial Milling Conditions.


In order to assess the surface changes due to the burnishing process, the 3D roughness parameters were computed for each of the studied materials. Accordingly, the arithmetical mean height (Sa), root mean square height (Sq), skewness (Ssk), kurtosis (Sku), texture aspect ratio (Str), and ten-point height (S10z) were processed.

#### *2.2. Experimental Methods*

#### 2.2.1. Ball-Burnishing Process

Previous works on ferritic milled surfaces provided the path for the ball-burnishing configuration of the austenitic and martensitic microstructures [11]. Nevertheless, due to the low hardening coefficient and high hardness conferred by the martensitic matrix, UNS S46500 required a load increase in contrast to AISI 316. The ball-burnishing process was performed using a hard, metal ball with a 10 mm diameter adapted to the force transmission unit of the *Acoustomill* tool (Spanish patent number 201730385) [27]. The process setup and its descriptive scheme are shown in Figure 2.

The set assembled in the CNC router milling machine was displaced once in the perpendicular direction (x-axis) to the milling finish (z-axis) on a 10 mm × 10 mm patch for each material. The ball-burnishing operational parameters are summarized in Table 3.

**Table 3.** Ball-burnishing operational parameters.


2.2.2. Uniaxial Tensile Properties

The elastic properties were established using an ultrasonic method (Panametrics 5900 PR pulser, Olympus, Tokyo, Japan) and an oscilloscope (Hameg HM1508, RS, Corby, UK). The longitudinal plastic tensile properties were acquired using the conventional tensile test configuration (ISO 6892-1 standard) [28]. Three AISI 316 and five UNS S46500 tensile specimens fitted to the standard requirements (width = 6 mm and Lc = 34 mm) [28] were tested. The strain measurements (0.0067 s−<sup>1</sup> until the failure) were obtained by a videoextensometer device. Table 4 shows the measured mechanical properties for both materials.

**Table 4.** Mechanical properties of the AISI 316 and UNS S46500 stainless steels.

