Numerical modeling of nitrogen transport and loss during laser powder bed fusion of high nitrogen steels

High Nitrogen Steels represent a highly promising material class for Laser Powder Bed Fusion, but their processing is challenged by significant nitrogen loss due to a drop in solubility upon melting. To systematically investigate the underlying degassing kinetics and nitrogen transport mechanisms, this study establishes a high-fidelity computational fluid dynamics model utilizing the Volume of Fluid method. The continuous species conservation framework accounts for convection-dominant transport, integrating a temperature-dependent nitrogen solubility limit and an interfacial mass transfer sink term at the free melt pool surface. The numerical model is successfully calibrated and validated against single-track experiments on Fe-15Cr-0.3C-N steel substrates across varying surface energy densities, demonstrating good agreement with both experimental melt pool geometries and a spatial nitrogen concentration profile measured via Wavelength Dispersive X-ray Spectrometry. The results reveal that the final nitrogen distribution across the melt pool depth is fundamentally governed by intense keyhole degassing coupled with complex convective transport. Marangoni convection drives nitrogen depleted melt backwards, inducing steep surface concentration gradients, while recirculation vortices transport the degassed melt deeper into the substrate. Using a novel Nitrogen Retention Index, parametric analysis demonstrates that higher surface energy densities prolong active surface degassing, thereby intensifying nitrogen loss.