A new process route for the additive manufacturing of a high nitrogen containing martensitic stainless steel - A feasibility study

Abstract

High-nitrogen martensitic stainless steels, such as X30CrMoN15 (0.3 to 0.5 mass% nitrogen), exhibit an excellent combination of strength and corrosion resistance, making them well-suited for applications in the medical technology and aerospace industry. The qualification of these steels for additive manufacturing (AM) could generate new application areas where AM, due to its process-specific advantages, could offer added value compared to conventional manufacturing methods. However, the laser powder bed fusion (PBF-LB/M) of high-nitrogen alloyed steels is challenging due to the high tendency for gas pore formation, resulting from the limited nitrogen solubility in the steel melt. In this work, a new process route for AM of a high nitrogen containing X50CrMoV15 martensitic stainless steel is presented, which consists of a process combination of powder nitriding, PBF-LB/M and subsequent hot isostatic pressing (HIP) with integrated quenching. Gas nitriding is used to achieve a nitrogen content in the starting powder that exceeds the maximum solubility in the melt. Although the nitrogen content decreases during the PBF-LB/M process, the high solidification and cooling rates prevent the melt from reaching equilibrium nitrogen levels, resulting in a nitrogen content above the solubility limit in the final PBF-LB/M state. The pores formed during the process are closed through HIP, which also allows hardening via integrated gas quenching. With an additional cryogenic treatment, the process produces a fully dense steel with 75% martensitic structure and 0.246 mass% nitrogen. Further optimization opportunities have been identified and are discussed.