Toughness evolution of 9Cr-3W-3Co martensitic heat resistant steel during long time aging

Toughness of G115 martensitic heat resistant steel after heat treatment and aging for different time at 650 °C was tested at room temperature. The corresponding microstructure was also experimentally obtained by field emission scanning electron microscope (FESEM), field emission transmission electron microscope (FETEM), X-Ray diffraction (XRD), electron back-scattered diffraction (EBSD) characterization and phase analysis. The results show that the absorbed energy of G115 steel can reach 115 J after heat treatment. After the first 300 h aging, the absorbed energy decreases drastically to just 36 J and then keeps almost stable with further increasing aging time to 8000 h. The hardness of base metal and the amount of large angle (LA) boundaries (>15°) are not the main factors dominating the toughness of G115 steel during aging process. The main reason for the change of toughness can be attributed to the precipitation of Laves phase. Since Laves phase particles are large, angular and hard, it is difficult for them to harmonize with the matrix during deformation, resulting that the crack initiation and propagation become easy and then the toughness of the steel sharply decreases. The reason why the toughness keeps nearly stable from 300 h to 8000 h can be attributed to that the negative effect of the precipitation of Laves phase and the decrease of LA boundaries is counteracted by the positive effect of the softening of metal matrix.