Effects of normalizing temperature on microstructure and high-temperature properties of 2.25Cr-1Mo-0.25V bainitic steel

Abstract

Abstract In 2.2Cr-1Mo-0.25V bainitic steels, the effect of the initial microstructure on carbide precipitation has not yet been elucidated. Therefore, in this work, the effect of normalizing temperature on the microstructure evolution and corresponding high-temperature strength change of bainitic steels was investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) using extra-thick hydrogenation reactor steel as the assessed material, and the precipitation strengthening mechanism was discussed. When the normalizing temperature increased from 900 °C to 990 °C, the high-temperature yield strength increased from 451 to 475 MPa and the average absorbed Charpy impact energy at −30 °C decreased from 93 to 41 J. Therefore, the optimal normalizing temperature was 960 °C. The volume fraction of MC carbides with almost no change in average diameter increased from 0.21% to 0.44%, and the dispersed strengthening of carbide was significantly enhanced. The increase in normalizing temperature increased the solid solution of alloying elements within the bainite matrix, which increased the precipitation of MC carbides during the tempering process. In addition, the lamellar M-A constituents in the initial microstructure decomposed to form MC carbides.