Relationship between carbon segregation and the carbide precipitation along grain boundary based on the structural unit model

Intragranular embrittlement is key to the weakening of properties of ultra-supercritical applied materials as HR3C heat-resistant steel in service and is considered to be related to the structure of grain boundary and the precipitation of carbides. In this work, the structural unit model was applied to study the structure of grain boundary and carbide formation with molecular dynamics simulation, and then, the precipitation tendency of carbides was analyzed through the structural similarity between grain boundary and carbide by using radial distribution function of the metal atom. At the same time, an electron backscatter diffraction study on serviced and treated HR3C heat-resistant steel was used for experimental exploration. Results show that the structural similarity between grain boundary and carbide depends on the grain boundary angle, wherein the structural similarity between grain boundary and carbide is higher for the grain boundary angle of 20–50°, while it is lower at the grain boundary angle close to 60° or smaller than 20°. The structure of Cr23C6 and Cr7C3 carbides is more similar to the local structure of grain boundary than that of Cr3C2 carbide. The grain boundary angle study on HR3C heat-resistant steel shows that the high content of 20–50° grain boundary indeed increases the precipitation of carbides, which is proposed to reduce the proportion of 20–50° grain boundaries by a proper procedure of cold deformation + heat treatment as 20% + 900 °C/6 h.