## 4 Conclusions  
In the present study, a novel nickel-free austenitic high-nitrogen stainless steel was processed via metal binder jetting. Microstructure and tensile strength were analyzed in the as-sintered condition and after post-densification by HIP. The main results can be summarized as follows:  
* The porosity after sintering was 2.8 vol.-% at maximum, which means a relative density of at least 97.2% could be achieved by sintering. Subsequent densification by HIP was able to reliably density almost all pores and leads to a relative density in the order of 100%. This represents a crucial accomplishment to enable application within the medical or watch and jewelry market. Most of the visual areas of components are polished to mirror class finish, which is enabled by full pore densification using HIP. Figure 14: EDX spectrum taken punctually on a non-metallic inclusion on the fracture surface of a tensile specimen in the HIP condition. Figure 13: Secondary electron images of the fracture surfaces of tensile specimens tested in the (**a**,**b**) as-sintered and (**c**,**d**) after HIP condition. Non-metallic inclusions are visible in (**b**,**d**). About 0.7 vol.% non-metallic inclusions were detected in the condition after HIP. In the as-sintered condition, it was not possible to distinguish pores and inclusions. The inclusions were determined as Si-Mn-Al-rich oxides, where only aluminum appears to be an impurity. These inclusions of approximately 3 \(\upmu\)m size are found in both the cross-sections and the fracture surfaces. * In the as-sintered condition, about 10 vol.-% delta ferrite were detected, after HIP only vol.-2%. This is in good qualitative agreement with the delta ferrite content in thermodynamic simulations due to the lower HIP temperature compared to the sintering temperature. * Thermodynamic equilibrium calculations suggest susceptibility for sigma phase and chromium nitrides formation. None of these phases could be clearly identified. However, seams around the delta ferrite might indicate sigma phase due to increased molybdenum content. * HIP post-densification improved both yield strength and tensile strength by about 10%. The increase in elongation at break due to HIP post-densification is in the order of 50%. The hardness also increases due to the densification of residual porosity. It can also be assumed that the cyclic strength benefits even more from post-densification, which is to be investigated by fatigue tests in the future. This would also allow an assessment of the detrimental nature of the inclusions. If delta ferrite and sigma phase are not intentional, it is necessary to perform a heat treatment for this material to set the desired microstructure and avoid sigma phase and nitrides. Based on the thermodynamic calculations, heat treatment at 980 \({}^{\circ}\)C followed by quenching is recommended to obtain a fully austenitic microstructure. This temperature is about 150 \({}^{\circ}\)C below the HIP temperature. It would be feasible to lower the HIP temperature to 980 \({}^{\circ}\)C while simultaneously increasing the HIP pressure to 150-200 MPa. The authors assume that such HIP parameters can also be used to achieve adequate densification while maintaining a 100% austenite microstructure. If rapid cooling in the HIP system is technically possible, this will enable proper integration of the optimal heat treatment into a HIP cycle. Consequently, the setting of an optimum microstructure by a (pressure-assisted) heat treatment following the sintering process is an interesting and important research question to be investigated next. A.K. : conceptualization, methodology, visualization, validation, writing--original draft, supervision; F.R. : investigation, visualization, formal analysis, writing--original draft; S.H. ([PERSON]): methodology, visualization, writing--review and editing; P.K. : conceptualization, investigation, resources, writing--original draft; S.H. ([PERSON]): conceptualization, supervision, resources, writing--review and editing; C.B. : resources.