Evolution of surface morphology and microstructure in reactor pressure vessel studs during triaxial rolling

Reactor pressure vessel (RPV) studs act as sealing fasteners connecting the top cover and cylinder of RPVs. The failure of the external threads of RPV studs negatively affects the operation and maintenance of nuclear islands. In this paper, triaxial rolling technology was applied to the production of RPV studs for the first time, and the process parameters were optimized based on finite element simulations and rolling tests. The effects of triaxial rolling on the surface morphology and microstructure of the external threads of an RPV stud were examined. Scanning electron microscope (SEM), transmission electron microscope (TEM), and electron back-scattered diffraction (EBSD) were employed to explore the grain refinement mechanism of the external threads of the RPV stud induced by severe plastic deformation during the triaxial rolling process. An RPV stud assembly test platform was used to conduct a 1:1 installation and tensile pre-tightening test on the external threads of the RPV stud processed by turning and rolling. It was found that the surface roughness of the RPV stud processed by triaxial rolling was less than 0.2 μm, which was 88% lower than that of the RPV stud processed by turning. A large number of ultra-fine grains appeared at the grain boundary of lamellar ferrite, forming a chain structure, and the proportion of ultra-fine grains on the thread root surface reached 63.5%. The microhardness at the thread root was 385 HV, which was 16% higher than that of the matrix. Under the same load, the RPV stud processed by triaxial rolling had more stable mounting torque and less tensile elongation than the RPV stud processed by turning, manifesting better assembly performance.