Investigation of precipitation sequence during creep in 2.25Cr-1Mo steel

Precipitate evolutions of 2.25Cr-1Mo heat-resistant steel during creep, under stress ranges of 30–300 MPa at 500–650 °C, were investigated by using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) equipped with energy dispersive spectroscopy (EDS). As-tempered microstructure was composed of ferrite and pearlite. Major precipitates in the pearlite region were identified as M₃C, M₇C₃ and M₂₃C₆ (M = Fe, Cr, Mo) carbides. It was found that the precipitation sequence was changed according to the creep exposure temperatures and times. The composition of the carbide was shifted to the Cr-rich side as the creep exposure time at 550 °C or lower increases without changing the ferrite and pearlite structures. On the other hand, M₂C was formed in an early stage of creep at 600 °C. The amount of M₂C was decreased and M₆C was newly precipitated after a long time creep exposure at 600 °C or higher. The amount of M₃C decreased with increase in exposure time and fully disappeared even after a very short time creep exposure at temperatures higher than 600 °C. The overall precipitation sequence in the 2.25Cr-1Mo steel during creep was found to be M₃C, M₇C₃, M₂₃C₆ → M₃C, M₇C₃, M₂₃C₆, M₂C → M₇C₃, M₂₃C₆, M₆C. It was considered that the slope change in Larson-Miller Parameter (LMP) plot was attributed to the formation of Mo-rich M₂C and dissolution of M₇C₃ precipitates during creep in the 2.25Cr-1Mo steel.