Effect of minor rare earth cerium addition on the hot ductility of a reactor pressure vessel steel

Abstract

The hot ductility of the specimens for a reactor pressure vessel steel (SA508-III), undoped and doped with minor rare earth Ce, is studied through hot tensile tests by means of a Gleeble machine in conjunction with scanning electron microscopy, optical microscopy, and field emission gun scanning transmission electron microscopy. Both the undoped and Ce-doped specimens are heated at 1300 °C and then cooled down to different test temperatures between 650 and 1050 °C, followed by tensile deformation up to fracture. As demonstrated by the results, for the undoped steel, there is a wide and deep hot ductility trough and the bottom of the trough is flat and wide (700–850 °C), being only about 60% in reduction of area (RA). However, for the Ce-doped steel, the hot ductility trough becomes considerably narrower and shallower. In addition, the flat and wide bottom of the trough disappears. Overall, the Ce-doped specimens exhibit a much higher hot ductility than the undoped ones, being over 70%RA at temperatures above 800 °C. Therefore, the minor addition of rare earth Ce can considerably enhance the hot ductility of the steel. It may be expected that this Ce-doped steel will have a high performance in hot working. As shown by microanalysis, the segregation of Ce to austenite grain boundaries occurs, which could play an important role in the enhancement of ductility in the high-temperature single austenite range (above 700 °C). Moreover, the Ce-induced grain refinement may also contribute to the hot ductility improvement. Keywords: Reactor pressure vessel steel, Hot ductility, Rare earths, Grain boundary segregation, Dynamic recrystallization