High-nitrogen steel has high strength, good corrosion resistance, and low cost. However, its difficulty in processing hinders its application. In this study, selective laser melting (SLM) is used to manufacture high-nitrogen stainless steel, which is suitable for manufacturing complex components and ensuring good mechanical properties. This study focuses on the effects of the laser volume energy density on the nitrogen content, relative density, phase composition, microstructure, and mechanical properties of manufactured samples.

In this study, the self-made high-nitrogen steel powder is used, and a vacuum heating furnace is used to dehydrate the powder before the experiment. Three laser parameters are designed for additive experiments. The nitrogen content and relative density of the specimens are measured after grinding and polishing. Phase analysis is performed using X-ray diffraction (XRD). The tensile strength is measured using a universal testing machine and the fracture morphologies of the tensile specimens are observed using scanning electron microscopy. The relationship between the microstructure and mechanical properties is studied. In addition, the phase compositions of the sample before and after a tensile test are measured using high-energy synchrotron XRD to analyze the strengthening mechanism.

The study shows that, when the laser volume energy density increases from 190.5 J·mm^-3 to 285.7 J·mm^-3, the tensile properties first increase and then decrease (Fig. 9). When the laser power is 200 W, the laser scanning speed is 400 mm/s, and the laser volume energy density is 238.1 J·mm^-3, the specimen exhibits the best mechanical properties with the tensile strength of 1275.0 MPa and elongation of 14.7%. The nitrogen content(mass fraction) of the specimen reaches 0.76% (Table 3), and the relative density of the specimen is 99.8% (Fig. 6). An appropriate laser volume energy density can improve the relative density and increase the solubility of nitrogen in the material. Supersaturated nitrogen plays an important role in solid solution strengthening in the austenite lattice, which then improves the tensile strength and elongation of the additive-manufactured components. Plastic transformation triggers the deformation-induced ferrite transformation (DIFT) effect. The transformation of austenite to ferrite enhances the strength and toughness of the material.

In this study, high-nitrogen stainless steel is manufactured via SLM. The microstructure primarily comprises austenite with a small amount of ferrite. After process optimization, with a laser power of 200 W and laser scanning speed of 400 mm/s, the manufactured parts demonstrate the highest relative density of 99.8%. They show excellent mechanical properties, with tensile strength of 1275 MPa and elongation of 14.7%. Microstructural studies show that the laser volume energy density affects the nitrogen content of the specimen. An increase in the austenite content leads to an improvement in the mechanical properties. Simultaneously, Cr₂N precipitates remain in the sample at low volume energy densities. When the volume energy density is high, numerous circular pores appear in the sample, which cause premature failure during the tensile test. Our study shows that high-nitrogen stainless steel components with high relative density and excellent mechanical properties can be obtained by optimizing the laser volume energy density, which leads to the development of SLM for high-nitrogen steel.