## 2 Materials and Methods
All the solubilization treatments were performed in a tubular Lenton LTF 12 oven (Lenton Furmaces & Ovens, Hope, Barnoldswick, UK), while the aging steps were carried out in a Remet E-79N muffle furnace (Remet di Franco Cicercchia & [PERSON], Casalecchio di Reno, Bologna). Throughout each heat treatment step, a K-type thermocouple was positioned inside the ovens and the temperature evolution was monitored and recorded using an OMEGA TC 08--Channel USB Thermocouple Data Acquisition Module (Omega, Norwalk, CT, USA) with Picolog 6 software (Pico Technology, Cambridgeshire, UK). As for the as-built specimens, the heat-treated ones were then subjected to the above-mentioned metallographic preparation and observed by using the same Leica DMi8 A optical microscope for a comprehensive analysis of their microstructural features. To detect the phase compositions at the interface between the AISI 316L substrate and the 17-4 PH deposited tracks, a D8 Bruker (Bruker, MA, USA) X-ray diffractometer (XRD) with Cu K-\(\alpha\) radiation collecting the pattern data from 20\({}^{\circ}\) to 110\({}^{\circ}\) (28 mode, 0.02\({}^{\circ}\) step size, and 1 s/step) was employed. Specifically, samples of about 8.9 \(\times\) 8 \(\times\) 2 mm\({}^{3}\) in size (including the AISI 316 substrate and the 17-4 PH clades) were analyzed. To investigate the presence of strengthening precipitates after the long-time aging treatments, one sample from each of the HT4 and HT5 specimens was prepared for observation using transmission electron microscopy (TEM). Thin specimens were fabricated from the peculiar shape of the track using the Crossbeam 340 (Carl Zeiss, Oberkochen, Germany) dual system focused ion beam--scanning electron microscope (FIB-SEM). The fabrication of the electron-transparent lamellas was performed in the upper part of the track, at a distance of about 120 \(\upmu\)m from the substrate and at an orientation for the lamella parallel to the steel substrate. The focused beam of Ga+ ions was accelerated at 30 keV for milling and at 5 keV for final polishing. The TEM analysis was carried out using an FEI Tecnai F20 (Thermo Fisher Instruments--formerly FEI, Hillsboro, OR, USA) high-resolution TEM (HRTEM) operating at 200 keV. Several imaging modes, including bright-field (BF) imaging, selected area electron diffraction (SAED), high-resolution electron microscopy (HREM), and scanning transmission imaging with high-angle annular dark-field (HAADF-STEM), were used. The average distance and the equivalent diameter of the detected strengthening particles were measured by the means of an image analysis. Vickers microhardness measurements were conducted on specimens in the as-built and PFHT conditions, following the UNI EN ISO 6507-1:2018 standard [40]. At first, three linear microhardness profiles for each deposition were acquired by a Future-Tech FM1e Vickers microindenter (Future-Tech Corp., Kawasaki, Japan) under a 50 g\({}_{\text{f}}\) load (HV0.05) with 15 s of loading time. In particular, the profiles were registered starting from the \begin{table}
\begin{tabular}{c c c c c} \hline
**Heat Treatment Route** & **Heating Ramp** & **Solution Treatment** & **Quenching** & **Aging at 480 \({}^{\circ}\)C (h)** \\ \hline HT1 & 10 \({}^{\circ}\)C/min & 1040 \({}^{\circ}\)C \(\times\) 30 min & In water & 3 \\ HT2 & NO & 1040 \({}^{\circ}\)C \(\times\) 30 min & In water & 3 \\ HT3 & NO & NO & NO & 3 \\ HT4 & 10 \({}^{\circ}\)C/min & 1040 \({}^{\circ}\)C \(\times\) 30 min & In water & 100 \\ HT5 & NO & NO & NO & 100 \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of the heat treatment routes applied in this work. substrate up to the tip of the depositions. The mean Vickers microhardness was then assessed at the center of each single track in the same conditions, as an average of ten random indentations.