High-temperature exposure of the high-strength 18Ni-300 maraging steel manufactured by laser powder bed fusion: oxidation, structure and mechanical changes

Abstract

The present work describes the effect of long-term (8 weeks) high-temperature oxidation (500 °C) on the formation of an oxide layer as well as on the microstructure and mechanical properties of the 3D-printed 18Ni-300 maraging steel. For this purpose, samples produced by additive manufacturing in the as-built and the as-built + solution annealed and aging treated states were used. The as-built + solution annealed and aging treated material was found to be more prone to oxide layer formation due to a homogeneously distributed Ni3Mo intermetallic phase in the material matrix compared to the as-built material. The 8 weeks long exposure to a temperature of 500 °C has caused the formation of a thick oxide layer that exhibited a very bad adhesion with the metal matrix/oxide. The X-ray diffraction analysis confirmed the formation of a layer with a complex phase composition: martensite, austenite, Fe2O3, and Fe3O4. Moreover, the presence of CoFe2O4 was determined on the thin outer oxide layer using X-ray photoelectron spectroscopy. The phenomenon of over-aging was found to be the most significant after the first week of high-temperature oxidation. Then, a negligible change in the microhardness was observed throughout the entire experiment. X-ray diffraction analysis and energy dispersive spectroscopy confirmed the phase composition of the alloy corresponding to 75% of martensite + 25% of austenite as well as the change of Ni3Mo precipitate to Ni3(Mo, Ti) type after the long-term oxidation.