*3.2. Hardness and Toughness*

The hardness and toughness values of the tested Charpy samples under different heat-treated conditions are presented in Figure 6. The results indicate that the as-printed state exhibits the lowest hardness and the highest toughness. The average hardness of the as-printed samples is insufficient for most engineering applications. However, the advantage of this alloy lies in its ability to be hardened by heat treatment. Prior to the aging heat treatment, annealing is performed. This heat treatment aims to maintain all the alloying elements in a solid solution state achieved by rapid cooling from the austenitic phase. Typically, this is the softest state of the material. However, as demonstrated by the results, the hardness of the annealed samples is higher than that of the as-printed state. According to the ASTM A693 standard, the maximum hardness is obtained in the annealed state, with a Rockwell hardness of 38 (HRC) or a corresponding Vickers hardness of 348 (HV). Our measured value of 314 (HV10) slightly falls below this maximum value. In practical terms, the hardest state of this alloy can be achieved by the H900 aging treatment. In this state, the hardness values usually range between 388 HV and 458 HV [26–28]. In our case, the average hardness of the aged samples is close to the maximum value of 445 (HV10). Regarding the measured impact energy (IE) values, an opposite trend can be observed.

This observed trend can be attributed to the fact that harder samples have a higher susceptibility to fracture when subjected to high-impact forces, resulting in lower impact energy (IE) values. The as-printed state exhibits the highest IE value of 22 Joules. However, after the annealing process, this value decreases to 9 joules. The lowest IE value of 6 joules was measured in the H900 state. In comparison to the results reported in the literature and the standard for the CP 17-4PH alloy, this impact energy is relatively low. The presence of unmelted areas within the printed microstructure (Figure 7) is undoubtedly the reason behind this discrepancy. These areas significantly reduce the toughness of the material [31].

**Figure 6.** Hardness and impact energy at different heat-treated states [26–30]. Values from the same fields were marked with rectangle and circles.

**Figure 7.** SEM picture on a broken surface of a Charpy specimen.
