Mo precipitation in a 12Cr–9Ni–4Mo–2Cu maraging steel

A new maraging steel 1RK91, with the composition 12Cr–9Ni–4Mo–2Cu (wt.%) was investigated after heat treatments at 475°C up to 400 h using atom probe field ion microscopy (APFIM) and tomographic atom probe (TAP). According to the analytical microscopy investigations preceding this study, the high strength of the material was attributed to a new Mo-rich quasicrystalline R′ phase. Therefore the present study concerned evolution of Mo-rich precipitates. The investigation revealed that nucleation of a Mo-rich phase starts between 1 and 2 h of aging at the matrix near the Ni-rich precipitates that form earlier and at the martensite lath boundaries. Compositional changes of these two types of precipitates with aging time were followed. After 400 h of aging only a few small matrix precipitates were observed. Their structure could not be easily determined using electron diffraction because of their small size. The influence of Mo precipitates on the mechanical properties of the material is discussed.