Grain boundary engineering for enhancing intergranular damage resistance of ferritic/martensitic steel P92

Ferritic/martensitic (F/M) steel is widely used as a structural material in thermal and nuclear power plants. However, it is susceptible to intergranular damage, which is a critical issue, under service conditions. In this study, to improve the resistance to intergranular damage of F/M steel, a thermomechanical process (TMP) was employed to achieve a grain boundary engineering (GBE) microstructure in F/M steel P92. The TMP, including cold-rolling thickness reduction of 6%, 9%, and 12%, followed by austenitization at 1323 K for 40 min and tempering at 1053 K for 45 min, was applied to the as-received (AR) P92 steel. The prior austenite grain (PAG) size, prior austenite grain boundary character distribution (GBCD), and connectivity of prior austenite grain boundaries (PAGBs) were investigated. Compared to the AR specimen, the PAG size did not change significantly. The fraction of coincident site lattice boundaries (CSLBs, 3 <= sigma <= 29) and sigma 3n boundaries along PAGBs decreased with increasing reduction ratio because the recrystallization fraction increased with increasing reduction ratio. The PAGB connectivity of the 6% deformed specimen slightly deteriorated compared with that of the AR specimen. Moreover, potentiodynamic polarization studies revealed that the intergranular damage resistance of the studied steel could be improved by increasing the fraction of CSLBs along the PAGBs, indicating that the TMP, which involves low deformation, could enhance the intergranular damage resistance.