Effect of thermal aging at 550 °C on microstructure and mechanical properties of a 9Cr martensitic steel with high silicon content

Effect of thermal aging at 550 °C on microstructure and mechanical properties of a 9Cr martensitic steel with high silicon content (named as HSi steel) was investigated by field emission scanning electron microscopy (FESEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM) and tensile and impact test. During aging, the martensitic matrix (including lath size, effective grain size and high angle boundaries) basically keeps stable. The M23C6 in HSi steel gradually coarsens with increasing aging time and the morphology of M23C6 on grain boundary tends to be chain-shaped, and then gradually becomes discrete again. Meanwhile, the Laves phase will precipitate adjacent to M23C6 due to a Si-rich layer around M23C6 and coarsens quickly before aging for 3000 h. With further increasing aging time, Laves phase is stable with a size of about 136 nm. The mechanical properties test results show that yield strength and elongation have little change during aging. However, the impact energy of HSi steel drops obviously after precipitation of Laves phase and the impact fracture presents cleavage fracture morphology. When the aging time is over 3000 h, the coarsening rate of Laves phase is very low and the HSi steel exhibits a relatively stable impact energy about 113 J. After aging up to 10,000 h, HSi steel still has excellent comprehensive mechanical properties.