## 1 Introduction  
Martensitic precipitation-hardening stainless steels, such as 17-4 PH, are characterized by a low percentage of carbon and mainly contain chromium and nickel as their mayor alloying elements [1; 2; 3]. The 17-4 PH steel is intensively used in several industrial sectors, such as aeronautics and the automotive industry, for manufacturing components through conventional forming processes and welding. More recently, additive manufacturing (AM) processes [4; 5; 6], specifically, laser-direct energy deposition (L-DED) [7; 8; 9; 10] and laser-powder bed fusion (L-PBF) [11; 12; 13; 14; 15] techniques, have demonstrated several advantages in the fabrication of structural components with 17-4 PH steel. In particular, the L-DED process consists of a laser source that melts a feedstock powder material simultaneously as it is supplied through a nozzle, until the part is built. The high energy density provided by the laser induces an ultra-high cooling rate, considerably impacting the microstructure and the mechanical properties of the deposited part [16]. It is well-known that post-fabrication heat treatments (PFHTs) become crucial to enhance the properties of AM components [17; 18; 19; 20]. The strengthening of 17-4 PH steel results from a combination of the martensitic matrix and the precipitation of nanometric particles, mainly Cu-rich precipitates (CRPs) and fine Nb carbides [21]. The precipitation of these particles is usually obtained by the H900 heat treatment, which involves a solution treatment followed by aging at 480 \({}^{\circ}\)C [22; 23]. In the literature, the H900 heat treatment is the most widely studied, but other post-fabrication heat treatment routes, such as the only solution treatment [19; 24; 25; 26], solution treatment + aging [26; 27; 28; 29], the only aging treatment [26; 30; 31; 32], and solution treatment + over-aging [27; 33], are discussed. In the study performed by [PERSON] et al. in 2015 [34], the effects of a double solution treatment on the microstructural features and hardness of wrought 17-4 PH steel samples were investigated. Specifically, this treatment consisted of a double solution treatment at 1028 \({}^{\circ}\)C for 1 h, followed by aging at 540 \({}^{\circ}\)C for 4 h. The heat-treated samples revealed the full presence of lathy martensite with irregular grain growth, indicating the complete transformation of the retained austenite. Unfortunately, the aging parameters promoted a drop in the material hardness due to the long-time aging duration. These results are similar to those reported by [PERSON] et al. in [33]. In this work, welded 17-4 PH steel samples were aged at temperatures between 480 \({}^{\circ}\)C and 620 \({}^{\circ}\)C and for holding times from 1 h to 4 h. The authors further confirmed a hardness decrease in samples treated at 540 \({}^{\circ}\)C and 620 \({}^{\circ}\)C due to the over-aging phenomenon. Concerning the precipitation of strengthening particles, it was also emphasized that, at 620 \({}^{\circ}\)C, the precipitation of a significant amount of CRP and carbide particles may occur, but these precipitates became coarser when increasing the holding time. Another study on the influence of aging parameters on the microstructure of a 17-4 PH bar was conducted by [PERSON] et al. [35]. The authors performed a traditional solution treatment at 1040 \({}^{\circ}\)C for 30 min, followed by quenching in iced water and subsequent aging at different temperatures (470 \({}^{\circ}\)C, 510 \({}^{\circ}\)C, and 600 \({}^{\circ}\)C) for durations ranging from 5 min to 8 h. The authors noted the presence of CRPs in the microstructure above 470 \({}^{\circ}\)C, but when the samples were heat-treated at 510 \({}^{\circ}\)C for 2 h, the precipitation of M\({}_{23}\)C\({}_{6}\) arose, enhancing their hardness behavior. Nevertheless, at 510 \({}^{\circ}\)C, an increase in the aging time from 2 h to 8 h, as well as an aging treatment carried out at 600 \({}^{\circ}\)C for 1 h, resulted in the coarsening of these precipitates, causing a critical hardness drop.