Microstructural evolution and the effect on strength of G115 steel after long-term creep at 923 K

Abstract

The microstructural evolution and the effect on the strength of G115 steel after creep under 923 K and 130 MPa for up to 7288 h were systematically investigated using OM, SEM and TEM. The results show that the variation tendency of total calculated strengthening contribution is in a good agreement with the measured hardness of G115 steel after creep at 923 K. A new nucleation mechanism of Laves phase nucleated adjacent to Cu-rich phase in the grain interiors is found in G115 steel during creep at 923 K. Compared with Laves phase at grain boundaries and martensitic lath boundaries, the growth velocity of Laves phase nucleated adjacent to Cu-rich phase is lower. Laves phase and Cu-rich phase play a vital role in the strength of G115 steel during creep at 923 K in addition to M23C6 carbides. Not only Laves phase and Cu-rich phase in the grain interiors help to increase the strength by enhancing the dislocation strengthening and precipitation strengthening, but also Laves phase and Cu-rich phase affect the strength variation by influencing the precipitation strengthening obviously. In contrast, Laves phase at martensitic lath boundaries is easy to coarsen, which may be unfavorable to the strength of G115 steel due to the weakened martensitic lath strengthening. In order to increase the beneficial effect of Laves phase and Cu-rich phase, it is necessary to increase the volume fraction of Cu-rich phase and Laves phase in the grain interiors, and control the growth of Laves phase. Increasing the amount of Cu-rich phase can promote the precipitation of Laves phase with low growth rate by providing more nucleation sites for Laves phase in the grain interiors.