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.