Abstract
In order to optimize the tempering process of boron bearing steel 25CrMoNbB, the effect of tempering temperature and holding time on martensitic microstructure, precipitation and mechanical properties has been studied. Thermo-Calc software was used to carry out thermodynamic calculation of the equilibrium precipitation. The precipitates growth behaviors were simulated using the program package diffusion-controlled transformations (DICTRA). Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAD) and energy dispersive spectrometer (EDS) were used to analyze microstructure and precipitates. B distribution was observed by secondary ion mass spectroscopy (SIMS). Electron back-scattered diffraction (EBSD) and TEM were used to analyze the martensitic microstructure evolution during tempering. The results showed that the B-bearing precipitates had a significant effect on the impact toughness and little effect on the tensile strength. The B atoms segregating at prior austenite grain boundaries (PAG) during austenitizing process promoted precipitation of M2B during tempering. The M2B precipitates mainly contained Fe, Mo, Cr and B elements and significantly reduced the toughness. Therefore, B content in 25CrMoNbB steel should be controlled below 20ppm. In addition, the tempering temperature and holding time should also be optimized to reduce the detrimental effect of M2B on the impact toughness. In as-tempered condition, besides M2B precipitates, M23C6, M7C3, and MC carbides were identified. M7C3 and MC carbides tended to precipitate in the matrix, whereas M23C6 tended to precipitate at PAG and sub-grain boundaries. Although carbides were coarsening with increasing tempering temperature or holding time, the coarsening of martensitic blocks and laths has reduced their detrimental effect on the toughness.
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