In this experiment, tempering time is distinctly short. And the final heat treatment process of S2-1 is the same with that of S1-4, but the impact toughness of S2-1 is much higher than that of S1-4, which indicates that segre- gation did occur at the grain boundary, but it may not be the main reason for the decrease in impact toughness. However, continuous Cr-rich carbides (M23C6) were found in Fig. 1, which is adverse on impact toughness. In the early stage of carbide precipitation, the carbides are dispersed on the matrix and the boundary. At this time, the carbides will not reduce the impact toughness of the steel. As the precipitation progresses, carbides continue to grow. Since there are many defects at the interface, it is easy to form a carbide structure continuously distributed along the interface, resulting in a decrease in impact toughness. The tempered samples were subjected to high-temperature solution treatment followed by quenched and tempered (S2), and the impact toughness was greatly improved (S2-1, S2-2, S2-3 and S2-4). This indicated that the decrease in impact toughness was caused by carbides.