# Tensile Properties of As-Built 18 Ni300 Maraging Steel Produced by DED
## 3 Results and Discussion
### Mechanical Properties Affected by Microstructure  
The graph illustrated in Figure 11 presents the hardness distribution along the cross section of the printed wall from the substrate to the topmost region, a hardness of \(392\pm 21\) HV0.3. The presence of smaller hardness values in the bottom regions of the wall, though having less and smaller porosities, can be attributed to a partial annealing happening in those zones during the wall printing process. The heat released by the molten material can act as a heat source affecting the hardness of the material beneath. A similar effect was distinguished in a previous study revealing that the heat generated during consecutive layers changed the microstructure and the hardness of the heat affected zone [35]. The porosity was quantified through optical images shown in Figure 12a,b with the use of the ImageJ software, yielding results of 99.26% and 99.71% for the bottom and middle sample, respectively. This is consistent with the hardness evolution along the building direction, as there is no significant increase or decrease in microhardness values above or below the standard deviation. It is important to mention that the obtained porosities are consistent with components manufactured through DED [40].  
Figure 9: Horizontal specimen: SEM images of the: (**a**) fracture surface; (**b**) dimples and microporosities observed; (**c**,**d**) optical microscopy images of the lateral side, etched surfaces.  
Several researchers have sought to establish relationships between steel's hardness and its true yield or true tensile strength [44; 45; 46; 47]. [PERSON] et al. [47] developed a relationship between a steel's hardness and ultimate tensile strength, as shown in Equation (2),  
\[\sigma_{uts}=\frac{H}{2.9}\left(\frac{n}{0.217}\right)^{n} \tag{2}\]  
in which \(H\) is the hardness expressed in N mm\({}^{-2}\), and \(n\) is the strain hardening exponent. The hardening exponent \(n\) for both horizontal and vertical directions was computed by curve fitting the strain hardening equation proposed by [PERSON] [48] to the plastic portion of the tensile curve of the test specimens, for both directions. The applied curve-fitting method consisted of the least squares method. [PERSON]'s equation is shown in Equation (3),  
\[\sigma=K\epsilon^{n} \tag{3}\]  
in which the \(K\) is the strength coefficient. The computed values of \(n\) for the horizontal and vertical direction yields 0.1365 and 0.1108, respectively. The ultimate tensile strength predicted by Equation (2) thus results in 1244 and 1230 MPa for the horizontal and vertical specimens, respectively. There is, therefore, strong agreement between the predicted ultimate tensile strength of the deposited structure and the tested specimens, following [PERSON]'s expression, as the average true tensile strength of the specimens are 1279 and 1261 MPa, resulting in a deviation of 2.74 and 2.43% for the horizontal and vertical specimens, respectively, in relation to the predicted values.  
Regarding the deviation observed in the yield strength of the horizontal specimen, Table 4, this wide dispersion can be influenced by the distribution of microstructure, e.g., dendritic/grain size affected by the remelting/reheating of the consecutive printed lines/layers as shown in Figure 12c. Moreover, as seen in Figure 12a,b, the effective  
Figure 10: Vertical specimen: SEM images of the: (**a**) fracture surface (**b**) dimples and microporosities observed; (**c,d**) optical microscopy images of the lateral side, etched surfaces.  
gauge length of the tensile specimen in the vertical specimen seems to be constituted by a more homogeneous microstructure, excluding microporosities, than that of the horizontal specimen.