The Use of the Gas Nitriding Process for the Nitridation of Powder for Laser Powder Bed Fusion

Abstract

AbstractAdditive manufacturing in the powder bed by means of laser powder bed fusion (L-PBF) combines the fast production of metallic components with complex geometrical structures with a high degree of customization and low raw material consumption. However, there is a high demand for the development of new alloys to achieve the desired properties of additively manufactured parts. Due to a lack of knowledge, only a small group of steel powders are currently available for L-PBF. As an example, Cr and CrMn steel powders with additional alloyed nitrogen could be used for L-PBF products for Ni-free medical devices. The present work investigates the possibility of alloying nitrogen to austenitic steel alloy powder and high alloyed martensitic steel powder via gas nitriding. For this purpose, an X30CrMo7-2 powder and an AISI 316L powder for additive manufacturing were arranged to form powder beds of different heights and nitrided at different temperatures, times and nitriding potentials. The nitrogen content was determined from the samples taken at different heights of the powder beds. Individual powder particles were examined by means of metallography, scanning electron microscopy and electron probe micro analysis. It is shown that the nitrogen concentration in the powder is essentially determined by the nitriding temperature. In the X30CrMo7-2 powder, nitride precipitation also occurs at high temperature and N and C gradients are present in the particles with an average nitrogen content of up to 4 mass%. In the 316L powder, a homogeneously distributed nitrogen content of 0.35 mass% can be achieved after gas nitriding at 420 °C for 24 h. The flow properties of both powders are hardly affected by gas nitriding.