The Foundation of Cr 7 C 3 Carbide in 25Cr20NiNbN Heat-Resistant Steel During Long-Term Service

The work was focusing on the precipitation of Cr7C3 type carbide in a 25Cr20NiNbN heat-resistant steel super heater tube after 51020 hours service. Electron backscatter diffraction (EBSD) detected the content of Cr7C3 type carbide was more than 5 at%, which was close to the amount of Cr23C6 type carbide. Transmission electron microscope (TEM) has confirmed the precipitation of Cr7C3 phase, which was distinct from the conventional aging condition. The numerical simulations showed that Cr7C3 phase has a stronger tendency to precipitate at the Σ3 twin-grain boundary, and is slightly weaker at 33.4-49.4° grain boundaries than that of Cr23C6 type carbide.