Precipitation in simultaneously nitrided and aged Mo-containing maraging steel

The excellent mechanical properties of maraging steels are ascribed to nanometer-sized intermetallics which precipitate during aging in the ductile very low carbon Ni-martensite. Their wear and fatigue properties can be improved by nitriding. The non-equilibrium precipitation reactions in Fe-Ni-Co-Mo maraging steels are studied during an aging heat treatment executed in a nitriding atmosphere. The precipitates formed during the initial stages of precipitation are characterized with transmission electron microscopy and atom probe tomography. Spherical intermetallic precipitates having a diameter of around 3 nm were detected in the aged, bulk material. These ω-type precipitates formed during the early stages of aging, have a trigonal crystal lattice and their chemical composition is close to (Fe,Ni)7Mo2. In the nitrided layer, Mo-N disc-shaped nitrides on the {100} martensitic lattice having a diameter of 3 to 4 nm were found but their exact crystal structure could not be determined with microstructural characterization techniques. Density functional theory calculations confirmed that a single layer of Mo atoms, substituting Fe on the {100} plane of the Fe-matrix, is stable and showed that the N atoms prefer to be in the Mo-layer, on the octahedral sites with Fe as nearest neighbors.