## 3 Results and Discussion
### Hardness Behavior  
In Figure 9, the mean microhardness profiles (HV0.05) measured from the substrate up to the tip of all heat-treated depositions are reported. In the graph, the mean microhardness profile of the specimens in the as-built condition is also shown and assumed as a reference. Concerning the specimens in the as-built condition, an average microhardness of about 392 HV0.05 was calculated within the deposited region. This value was mainly guaranteed by both the martensitic matrix and the \(\delta\)-ferrite, whose presence was due to the high cooling rate induced by the L-DED process [56; 57; 58]. A sharp microhardness decrease was then measured at the interface between the single tracks and the substrate, because of the lower microhardness of the austenitic AISI 316L steel plate (about 295 HV0.05) with respect to that of the martensitic 17-4 PH deposited track (about 400 HV0.05 in the as-built condition). Similar trends were found for the microhardness profiles measured in the heat-treated specimens, but with slight differences according to the adopted heat treatment routes. As for the specimens in the as-built condition, the profiles in Figure 9 show that the microhardness of all the heat-treated single tracks maintained a reasonable stability from the interface up to the tip of the depositions. In any case, the mean HV0.05 within the tracks was higher than that calculated in the as-built depositions; this evidence was probably due to the hardening effect provided by the fine precipitation of the Cu-rich nanometric particles or Nb carbides inside the deposited regions. At the interface between the track and the substrate, the HT1-, HT2-, and HT3-treated samples showed a steep drop in hardness of more than 100 HV0.05, until reaching mean values comparable to the microhardness of the steel plate. Local variations in HV0.05 inside the interface zone may be ascribed to the presence of the \(\delta\)-ferrite. In contrast, the HT4 and HT5 specimens displayed microhardness drops to average HV0.05 values slightly higher than those registered for the other heat treatments; due to the long-time aging treatment, a fine precipitation of M\({}_{23}\)C\({}_{6}\) carbides may occur, increasing the hardness of the AISI 316L substrate [59; 60]. Figure 8: (**a**) STEM-HAADF images of HT5-treated specimen with the dark Nb carbides and (**b**) HRTEM view of the crystal domain (in the inset, the FFT of the TEM image with the spatial frequencies of the Fe lattice). The mean Vickers microhardness \(\pm\) standard deviation of all the deposited regions is reported in Figure 10. The HT1, HT2, and HT3 routes determined a substantial increase (approximately in the range of 25-35%) in the mean microhardness of the heat-treated specimens as compared to that of the specimens in the as-built condition. In particular, direct aging performed for 3 h (corresponding to the HT3 treatment) provided the highest increase in HV0.05 (of about 35%), with an average microhardness of 523 HV0.05. These findings are in agreement with the literature, highlighting the effectiveness of direct aging in increasing the hardness of additively manufactured components [38]. Conversely, with an increase in the aging time (corresponding to the HT4 and HT5 treatments), a decrease in the mean microhardness was observed. As already mentioned in [52], with long-term aging at 480 \({}^{\circ}\)C, the CRPs start to coarsen, leading to an increase in their size and a decrease in their spatial density. However, it should be highlighted that, for longer aging times, small Nb carbides start to precipitate, thus contributing, to a minor extent, to the strengthening of the material. The HT4-treated specimens also showed a lower mean microhardness in comparison to that of the HT1-treated ones. As already discussed, long-time aging treatments (up to 100 h) performed after a complete SHT determined a coarsening effect in the Cu-rich strengthening particles. The presence of these precipitates was observed via the TEM analysis, confirming the occurrence of a coarsening effect.