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

Abstract

Precipitate evolutions of 2.25Cr-1Mo heat-resistant steel during creep, under stress ranges of 30-300 MPa at 500-650 degrees 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 degrees 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 degrees C. The amount of M2C was decreased and M6C was newly precipitated after a long time creep exposure at 600 degrees 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 degrees 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.