## 3 Results and Discussion
### Martensite Analysis  
Martensite needles characterization is very important due to their influence on the mechanical properties of the final component, that can be eventually correlated to the process environment [36; 55]. Acicular width was chosen therefore as the most suitable and representative property to compare, and the resulting distributions for all the analysed samples are reported in Figure 7. The exclusive use of shielding gas as a protective mean leads to the formation of a very fine martensitic microstructure, characterized by a mean width of 0.69 \(\upmu\)m. This value is sensibly lower when compared to acicular widths of 1.83 \(\upmu\)m and 1.60 \(\upmu\)m of SE-CB and SE-HB samples, respectively. Although, the difference in microstructure between ArS (acicular \(\alpha^{\prime}\) and lamellar \(\alpha+\beta\)) and the other samples (acicular \(\alpha^{\prime}\)) must be taken into account, as it certainly influences martensitic width and causes the shift of the ArS curve towards lower values. As mentioned before, microstructural variations are caused by the different deposition environments used. Hence, this shift is related to interstitials pick-up and unrelated to thermal factors. Instead, the dissimilar width values measured for the samples built using a sealed environment might be related to the different cooling rate that defines the deposition process in the case of platform heating. Conduction is in fact the most influencing heat dissipation phenomenon occurring during the process [37; 53]. The cooling rate, which greatly influences the final material microstructure [36; 37], is then directly dependent on the temperature difference between the melt pool and the baseplate. As a consequence of that, a variation in the building platform temperature leads to a different cooling rate during the deposition, which eventually results in different microstructural features.  
Although this seems to be in contradiction to the results obtained from the \(\alpha^{\prime}\) width evaluation in which SE-HB samples were characterized by a finer martensitic microstructure (Figure 7), even if SE-CB samples underwent a faster cooling. This phenomenon might be explained considering that the martensitic size is the result of an athermal growth. In fact, since the same type of microstructure (completely martensitic) is involved, the SE-HB sample is characterized by larger grains, hence by a lower M\({}_{\text{S}}\) and elastic strain energy (\(\Delta g_{el}\)), as demonstrated by previous studies [56; 57]. \(\Delta g_{el}\) is a thermodynamic property which is roughly correlated to dislocation density [58]. [PERSON] et al. [59] used this parameter to describe the change in free Gibbs energy \(\Delta G\) in a transformation that leads to the formation of a martensitic product, by approximating it as an elliptic inclusion in the \(\beta\) phase:  
\[\Delta G=2\pi r^{2}\gamma-\frac{4}{3}\pi r^{2}c(\Delta g_{ch}-\Delta g_{el}) \tag{4}\]  
where \(c\) is half the thickness of the inclusion, \(r\) is the radius, \(\Delta g_{ch}\) is the chemical driving force per volume unit. The \(\Delta G\) value is in fact a balance between this last term and the elastic strain energy [56]. It has been assessed that a reduction in elastic strain energy corresponds to finer \(\alpha^{\prime}\) needles [57], as confirmed by experimental data.  
From a phenomenological point of view, a slower cooling induces less stress-related defects in the sample, such as dislocations, that favour nucleation and growth of martensite needles [36; 42]. It is then reasonable to assume that in SE-CB samples martensite is bigger in size, as confirmed by experimental data. This correlation between microstructural features size and thermal history of the process demonstrates a behaviour for martensite completely opposite to the more stable microstructures (\(\alpha+\beta\)), such as the aforementioned Widmanstatten [60], which formation is diffusion-related instead.