Effect of microstructural evolution on high-temperature strength of 9Cr-3W-3Co martensitic heat resistant steel under different aging conditions

Evolution of microstructures and high-temperature strength at 650 °C of 9Cr-3W-3Co martensitic heat resistant steel after aging at 650 °C and 700 °C for different time durations have been experimentally investigated using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), field emission transmission electron microscopy (FETEM) and post-aged tensile tests. The results show that after aging at 650 °C, the high-temperature strength and the microstructures of 9Cr-3W-3Co steel keep almost stable with increasing aging time from 300 h to 3000 h. In comparison, after aging at 700 °C, there are obvious changes in the high-temperature strength and the microstructures. The strengthening mechanisms of the 9Cr-3W-3Co steel were also discussed and the athermal yield stresses were calculated. The change of the high-temperature strength is mainly affected by the evolution of dislocations and laths. The precipitates mainly act as obstacles against motion of dislocations and lath boundaries.