CB2 cast steel is a 9%~12%cr martensitic heat-resistant steel with a composition of 0.13c-9cr-1.5mo-1co-vnbnb, which is obtained by reducing C, adding CO and adjusting the content of N and B on the basis of B2 cast steel in cost522 R & D plan. Its alloying principle is as follows:
1) The Cr content is as high as 9%~12% to improve its ability to resist steam oxidation;
2) Reduce Ni content as much as possible and improve creep strength;
3) Adding 1% CO can strengthen the solid solution, compensate the Ni content and inhibit δ Ferrite precipitation;
4) Adding 0.002%-0.018% B, on the one hand, element B plays a role of interstitial solid solution strengthening, and at the same time, B can inhibit the size coarsening of M23C6 carbides and stabilize M23C6 carbides, so as to improve the structural stability and high temperature endurance strength of the cast steel.
The microstructure of CB2 cast steel after performance heat treatment is tempered martensite. Under transmission electron microscope, the microstructure is composed of martensite laths, high-density dislocations and fine precipitates. The precipitates are a large number of M23C6 carbides and a small amount of MX carbon and nitrogen compounds, of which M23C6 phase is in the shape of a short rod, mainly distributed along the original austenite grain boundary and martensite lath boundary, and a small amount of MX phase is in the form of circular particles, mainly dispersed in the matrix, as shown in Figure 2. After 13926h high temperature creep test at 650 ℃, CB2 cast steel precipitated a large amount of Laves phase at the grain boundary and lath boundary. In the process of high temperature creep, Laves phase begins to precipitate within hundreds of hours. At the initial stage of Laves phase formation, it can play a role of precipitation strengthening due to its small size; As Laves phase increases in size with the increase of creep time, and its growth will continue to consume Mo in the matrix, so as to reduce the solid solution strengthening effect of alloy elements on the matrix. Because Laves phase is a brittle phase, the dispersion strengthening effect disappears after size coarsening, and with size coarsening, it will have an adverse impact on the creep strength at high temperature. MX carbon nitrogen compounds rich in V and Nb are extremely stable and not easy to coarsen, which are important strengthening phases to ensure the long-term stability of CB2 cast steel structure. M23C6 carbide in CB2 cast steel has better dimensional stability due to the addition of B element, and grows at a very low rate under the action of high temperature and stress, which effectively hinders the dislocation movement and the growth of subgrain, thus improving the structural stability and high temperature creep strength of CB2 cast steel. The pinning of dislocation and sub grain boundary by a large number of M23C6 carbides is considered to be one of the most important strengthening mechanisms in CB2 cast steel.