Simultaneous ageing and plasma nitriding of grade 300 maraging steel: How working pressure determines the effective nitrogen diffusion into narrow cavities

Abstract Maraging steels combine excellent mechanical properties at medium-high temperatures with good fracture toughness, but they usually require the aid of thermochemical treatments to enhance superficial properties such as resistance to abrasive wear and corrosion resistance. In particular, plasma nitriding, due to its duration and temperature, which are compatible with the age-hardening cycles of maraging steels, permits the realization of both processes simultaneously. In this work, simultaneous ageing and arc-activated plasma nitriding of maraging steel grade 300 were studied as a function of the working pressure of the gases (N 2 and Ar) present in the vacuum reactor chamber. In addition, the influence of the operating pressure was established by analysing the capacity of the plasma to penetrate in 2.5 mm in diameter blind holes. In-depth chemical composition, hardness and micro-hardness profiles, X-Ray diffraction, roughness, tribological characterization and corrosion tests were carried out to compare maraging steel samples treated in one simultaneous plasma nitriding and ageing process and in two-step processes. Metallographic tests and nanoindentation measurements were performed to study how the plasma penetrates and diffuses into the blind holes and how the nitride case layer thickness formed at the inner surfaces of the holes changes as a function of the distance to the surface for various working pressures.