The main factor that results in aging embrittlement of 9Cr3W3Co steel was investigated through microstructure observation using scanning electron microscopy (SEM). The amount of each precipitates under equilibrium state in 9Cr3W3Co steel with different W content (W content is 2.36%, 2.63%, 2.96% and 3.11%, mass fraction) was calculated using Thermo-Calc software. The microstructure evolution during aging was studied by transmission electron microscopy (TEM), small angle X-ray scattering (SAXS) and phase analysis method. The results show that the rapid drop of impact toughness after aging for 100h is caused by the formation of Laves phase. The mass fraction of Laves phase at equilibrium state is mainly determined by the concentration of tungsten. The steel with lower tungsten content exhibits higher impact toughness after aging for 8000h,meanwhile with the smallest size and the lowest coarsening rate of Laves phase. Thus, the aging embrittlement of 9Cr3W3Co steel can be successfully suppressed by the reduction of tungsten content.
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