Study of Nb non-equilibrium segregation at prior austenite grain boundary in welding using atom probe tomography and modeling

The non-equilibrium segregation of Nb at prior austenite grain boundary (PAGB) induced by welding is experimentally investigated by means of atom probe tomography (APT). The Nb concentration at PAGB appeared to be 0.115 at.% for experimental steel containing 0.06 at.% Nb at the simulated welding thermal cycle for 10 s from 1320 to 950 °C. The critical time tc for achieving maximum segregation is calculated to be 565 s for the experimental steel. For the first time, the diffusion coefficient of Nb in desegregation process is calculated for the temperature of Ti = 950 °C using non-equilibrium segregation model and APT experimental result and is confirmed to be 9.23 × 10−11 m2/s. Based on it, the kinetics of Nb non-equilibrium segregation at the PAGBs in welding process is outlined, and the segregation process and desegregation process are discussed. The Nb segregation at PAGB is also predicted over the t8/5 range of 3–600 s. The critical t8/5, at which the highest Nb concentration of 0.144 at.% at PAGB is predicted to be achieved, is ~ 29 s.