**3. Results**

#### *3.1. Microstructural Study and XRD Analysis*

The microstructural study revealed, in general, the apparition of precipitates that showed a distribution mainly oriented. Then, two typologies of phases were clearly differenced, a darker area named "matrix", and plenty of light grey precipitates. In Figure 9, the SEM images showed such precipitates, as well as their special orientation in the microstructure of a specimen fabricated under air and argon conditions during the PTA process without thermal treatment (named "as-built"), and placed at the position 1.2 (according to Figure 7) in both Hastelloy C-22 walls.

**Figure 9.** CBS-SEM images of Hastelloy C-22 specimens manufactured without thermal treatment (**<sup>a</sup>**,**b**), and in air (**<sup>c</sup>**,**d**) argon.

As can be seen in Figure 9a,c and Figure 10, at low magnifications (100×) the grain distribution could be clearly identified, regardless of the thermal treatment. Therefore, their origin and distribution could be attributed to possible segregation phenomena during the additive process. Residual porosity could be observed in the specimens, even after the heat treatments.

**Figure 10.** CBS-SEM images of Hastelloy C-22 specimen manufactured in air with thermal treatment, (**a**) thermal treatment 1 (TT1) (rapid air cooling (RAC)) and (**b**) TT2 water quenching (WQ).

Subsequent test by EDS analysis confirmed the compositional differences between the darker regions (matrix) and the light grey precipitates, observed in Figure 9. The representative spots checked, in addition to the results of the EDS analysis, are summarized in Figure 11 and Table 4. The elements analysis in light grey spots resulted in regions rich mainly in Mo, and in which the percentage in weight of Cr and Ni elements were lower than in the area considered as the matrix in the Hastelloy C-22. That suggested a possible diffusion of the Mo atoms, mainly during the layer depositions, as well as decrement of other characteristic elements of the Hastelloy C-22, as Ni and Cr. The mapping test (Figure 12) confirmed the elements' compositional variations occurred in the alloy, caused by the manufacturing process. Similar results were obtained in analysis carried out in other specimens processed in argon and thermal treated.

**Figure 11.** SEM image of the Hastelloy C-22 specimen manufactured under air without thermal treatment and EDS analysis (analyzed specimen 1.2).

**Table 4.** EDS composition for the Hastelloy C-22 specimen manufactured under air without thermal treatment.


**Figure 12.** Mapping of specimen in air without thermal treatment (analyzed specimen 1.2): CBS-SEM (upper-left), Si, Mo, Cr, Mn, Fe, Ni, Cu.

The XRD analyses were performed, and the patterns of specimens produced under air are shown in Figure 13 and the ones manufactured in argon atmosphere are illustrated in Figure 14. The pattern of specimens without thermal treatment processing in air showed the highest peaks in comparison with the observed peaks in the pattern of the rest of the specimens under air or argon independently. This result suggested that the intensity of these peaks was related to the high crystallinity that this specimen could present. On the contrary, the alloy peaks belonging to the specimen processed under argon without any thermal treatment showed low intensity; note, in that regards, the processing conditions modified the characteristics on the crystallinity of the specimens. The pattern of specimens treated by cool down by water quenching (WQ = TT2) after the heat treatment showed almost the same tendency of the alloy peaks, independently of the processing conditions.

**Figure 14.** Pattern diffraction of specimens produced in argon conditions.

As it can easily appreciate, there were significant differences in the patterns of specimens whose thermal treatment finalized by rapid air cooling (TT1). This cooling process presented the lowest cooling rate, promoting in this stage singular variations in the crystallinity and phases presented in the specimens. Hence, in the framework of the thermal treatments, significant variations were found in the patterns of specimens where the crystallinity could be affected by the circumstances related to the cooling rate, independently of the air or argon fabrication conditions. Possible phases could be forming since peaks of these phases as FeNi and carbides could be matched to the main alloy peak. However, they were probably overlapped under the alloy peak. For that reason, the mentioned peaks were not marked in Figures 13 and 14.

#### *3.2. Mechanical Properties and Wear Behavior*

In this section, the mechanical properties obtained by tensile tests were studied and evaluated. The values of the most representative mechanical properties are summarized in Tables 5 and 6, being Young's modulus (E), yield strength (YS), ultimate tensile strength (UTS), and deformation. Three

specimens were tested for each category, which means three specimens without treatment, three su ffered a cooling down by rapid air cooling (TT1), and three after treatment by cool down by water quenching (TT2). When the empirical values of mechanical properties were compared to the theoretical ones, slight di fferences were observed [26]. The Hastelloy C-22 walls produced by the PTA technology, in average, (Tables 5 and 6) did not reach the mechanical properties that indicate the standard reference, the elongation properties, in particular, independently of the processing conditions and the thermal treatment [27]. This behavior is characteristic of the additive manufacturing process itself. While the YS values were closer to the minimum reference values (310 MPa) and UTS values were slightly lower than the reference ones (690 MPa), the elongation results were very low, not achieving even one-third or one-fourth of the standard value (reference value 45% of elongation). These results were similar with other ones obtained using additive manufacturing techniques [28]. Moreover, after performing the di fferent thermal treatments (TT1 and TT2), the mechanical properties did not undergo some significant improvements. Although UTS properties in specimens fabricated in air and after TT1 and TT2 slightly improved on average, 660 MPa and 644 MPa, respectively, the elongation receded from the reference visibly below 11% and 9.2% in each case. Regarding the cooling rates referred to TT1 and TT2, the results suggested that the low cooling rate (TT1 = RAC) improved more the UTS than using a high cooling rate (TT2 = WQ) (see Table 5). In specimens processed in argon conditions, the mechanical properties showed almost the same trend, a ffecting the highest cooling rate (TT2 = WQ) to the elongation of the specimens, on average below 8.5%.


**Table 5.** Mechanical and physical properties of the specimens produced in air atmosphere condition.

**Table 6.** Mechanical and physical properties of the specimens produced in argon atmosphere condition.


Comparing the manufacturing atmosphere conditions, the alloy presented almost similar mechanical properties for both conditions in average values. While the wall built in air condition showed

very low UTS values in specimens extracted on the top layer, the wall built under argon presented more homogeneity related to their mechanical properties, independently of the extracted location.

The obtained results after the tribological characterization were evaluated in order to determinate if there were correlations to the mechanical properties. In Figure 15, the friction coefficient of the tested specimens vs. distance is represented. This makes it possible to make a rough comparison of their friction behavior, according to the processing conditions and thermal treatment employed. Notwithstanding that the coefficient did not show significant differences among them, there was a slight improvement in the friction coefficient in specimens fabricated in air. Furthermore, the tendency of the friction coefficient in specimens cooled by RAC (TT1) suggested that this treatment produced a reduction of the friction coefficient with respect to the specimens tested as-built (without thermal treatment). Therefore, in some sense it might be said that this was a suitable treatment to decrease the friction coefficient. Regarding the effect of the TT2 in the friction coefficient, there were no major influences of such treatment in the friction coefficient, if specimens without treatment and specimens after TT1 were compared.

In the context of the mechanical properties and the friction coefficients, while the TT1 was considered a treatment that could cause embrittlement of the specimens, its effect promoted better friction behavior, in particular in specimens produced under air. In specimens produced under argon conditions, by contrast, a clear trend could not be appreciated easily.

**Figure 15.** Friction coefficient vs. distance (m).
