Mechanism for H-shaped precipitate formation in 1.25Cr-0.5Mo steel

Needle-like M₂C precipitates with a hexagonal close-packed (HCP) structure, which are referred to as Mo₂C, are well known to be responsible for the precipitation strengthening of low Cr heat-resistant steel. This paper reports H-shaped precipitation of M₂C on CrN in a 1.25Cr-0.5Mo ferritic steel aged at 550 °C for 5000 h and explains the precipitation mechanism. Mo diffuses into CrN from the ferrite matrix during the aging. Chemical elemental mapping in transmission electron microscopy (TEM) shows that Mo-enriched regions in CrN act as nucleation sites of M₂C. Furthermore, the thermodynamic calculation indicates that the solubility of Mo in CrN is higher than that in the matrix at 550 °C. The precipitation of M₂C on CrN seems to reduce the misfit strain energy between the ferrite matrix and CrN during long-term aging. High resolution transmission electron microscopy (HRTEM) reveals that M₂C has a specific orientation relationship with the ferrite matrix as well as CrN. Only M₂C precipitates meeting the orientation relationship with the matrix tend to grow dominantly in one direction, which seems to be responsible for the H-shaped precipitation.