Effect of rare earth cerium on brittleness of simulated welding heat-affected zones in a reactor pressure vessel steel

The welding coarse-grained heat-affected zones (CGHAZs) in the undoped and Ce-doped samples of SA508CL-3 reactor pressure vessel steel were simulated using a Gleeble 1500D thermomechanical simulator with a peak temperature of 1320 degrees C at the heat inputs of 30, 50 and 100 kJ/cm, respectively. The ductile-to-brittle transition temperature (DBTT) of the simulated CGHAZs was evaluated along with microstructural and microchemical characterizations. The results indicated that Ce could substantially lower the DBTT of the CGHAZs by its microstructural and microchemical effects. After the thermal cycling of welding, the microstructure in the Ce-doped samples was apparently finer than that in the undoped samples, regardless of the lath bainite obtained at the heat inputs of 30 and 50 kJ/cm or the granular bainite acquired at the heat input of 100 kJ/cm, leading to lower DBTTs for the Ce-doped samples. Moreover, grain boundary segregation of Ce occurred apparently in the Ce-doped samples and exhibited a non-equilibrium characteristic. The segregation of Ce could play an important role in lowering the DBTT of CGHAZs or toughening the CGHAZs.