3.4.1. 20kH13 (AISI 420) Anticorrosion Steel

The roughness parameter *Ra* of the samples produced by laser powder bed fusion method from 20kH13 (AISI 420) steel was 7.24 ± 0.19 μm. The topology of the surface is shown in Figure 6. The measured results of roughness parameters (*Ra*, *Rz*, *Rtm*) are presented in Table 5; the obtained topology is shown in Figure 7 (visible area is 3.8 μm × 3.8 μm). The obtained profiles are presented in Figure 8. The untreated surface has a non-uniform topography with many inclusions formed due to metal splashing in the molten pool. The surface acquires a relatively regular structure and is characterized by the absence of inclusions after cavitation-abrasive finishing.

**Table 5.** The roughness parameters (*Ra*, *Rz*, *Rtm*) of the 20kH13 (AISI 420) steel sample surfaces before and after cavitation-abrasive finishing (after Gaussian regression).


**Figure 6.** Surface topology of the samples (optical microscopy, 300×): (**a**) after production; (**b**) after cavitation-abrasive finishing.

**Figure 7.** Surface topology of the samples (atomic force microscopy): (**a**) after production; (**b**) after cavitation-abrasive finishing.

 **Figure 8.** Surface profile of the samples: (**a**) after production; (**b**) after cavitation-abrasive finishing.

It is correlated to the distribution of cavitation activity, which depends on the treated surface's microgeometry parameters. Working fluids, which are cavitation bubbles and abrasive particles, have the most significant effect in places of most significant irregularities. In this case, the abrasive particles have damping functions, i.e., take on the energy arising from the collapse of cavitation bubbles, and the deformation of the surface is carried out due to the impact of the abrasive and not due to the effect of cumulative streams, which can lead to erosion. Thus, the applied cavitation-abrasive finishing is erosion-free and leads to surface smoothing.

It is quantitatively expressed in a decrease in the roughness parameters (*Ra*, *Rz*, *Rtm*) by 30%–60%, decreasing the average pitch of irregularities by more than two times. Increasing the processing time (over 120 s) does not increase the effect further.

### 3.4.2. 12kH18N9T (AISI 321) Anticorrosion Steel

The roughness parameter *Ra* of the samples produced by laser powder bed fusion method from 12kH18N9T (AISI 321) steel varied in the range of 8.5–14.1 μm. The obtained topology is shown in Figure 9; measuring results are presented in Table 6. 3D-profiles of the surfaces are presented in Figure 10. The roughness parameter *Ra* for the walls of after the vibratory tumbling was reduced by more than two times—from 14.1 to 5.0 μm.

**Table 6.** The roughness parameters (*Ra*, *Rz*) of the 12kH18N9T (AISI 321) steel surfaces before and after vibratory tumbling (before Gaussian regression).


**Figure 9.** SEM-image in secondary electrons of the surface produced from 12kH18N9T (AISI 321) anticorrosion steel powder: (**a**) after laser powder bed fusion method, 500×; (**b**) after vibratory tumbling, 502×.

**Figure 10.** Surface topology of the samples (profilometry): (**a**) after production; (**b**) after vibration tumbling.

The surface longitudinal and traverse profiles on the area of 500 μm × 500 μm after Gaussian regression are presented in Figures 11 and 12. The measured roughness parameters *Ra* and *Rz* after regression are:


In the first case, the peaks of roughness profile (Figures 11b and 12b a peak of 12 μm) are associated with the presence of the unmelted granules on top of the surface that has metallurgical contact with the built sample. For the samples after vibratory tumbling, this type of morphology is absent; the surface retains a less pronounced wave character, without peaks, but with wells from the used abrasive, which are evenly distributed and regularized (Figure 11d).

**Figure 11.** Longitudinal waviness and roughness profile after Gaussian regression (profilometry): (**a**) waviness after laser powder bed fusion method; (**b**) roughness profile after laser powder bed fusion method; (**c**) waviness after vibratory tumbling; (**d**) roughness profile after vibratory tumbling.

**Figure 12.** Transverse waviness and roughness profile after Gaussian regression (profilometry): (**a**) waviness after laser powder bed fusion method; (**b**) roughness profile after laser powder bed fusion method; (**c**) waviness after vibratory tumbling; (**d**) roughness profile after vibratory tumbling.

### *3.5. Heat Treatment, Hardness and Wear Resistance*

The effect of subsequent heat treatment on the hardness of the 20kH13 (AISI 420) steel samples produced by laser powder bed fusion and traditional casting are presented in Table 7. The measured microhardness of the samples correlated to the data obtained for Rockwell hardness. The results of studies on water-jet wear of the laser powder bed fused samples and cast samples after combined quenching and low tempering are presented in Figures 13–15.

**Figure 13.** Profiles and microphotographs of the wells of the 20kH13 (AISI 420) steel samples after 3 min of test time (abrasion wear analyzer): (**a**) profile of the LPBF produced sample; (**b**) well image of the LPBF-produced sample; (**c**) profile of the cast sample treated by quenching with low tempering; (**d**) well image of the cast sample treated by quenching with low tempering.

**Figure 14.** Dependence of the wear rate on the friction path of the 20kH13 (AISI 420) steel samples.

**Figure 15.** Histogram of volumetric losses during abrasive wear of steel 20kH13 (AISI 420), 9 min of wear time.

**Table 7.** Rockwell hardness of the samples made of the 20kH13 (AISI 420) steel in raw state after various types of post-processing.


1 Provided for two different directions—along *Z*-axis and *X*- and *Y*-axes for the samples produced by laser powder bed fusion due to known anisotropy (orthotropy) of the properties and one value HRC for cast samples due to known isotropy of the properties.
