Tempering behavior of a nuclear pressure vessel steel with macrosegregation

The microstructure evolution of a Nuclear Pressure Vessel Steel (SA508 Gr.3) with macrosegregation was investigated during quenching and tempering. Macrosegregation is quantitatively analyzed by EPMA, and the results show that concentration of Mn, Mo and Ni in positive segregation zone may be 0.3%-0.5% higher than in negative segregation zone. A mixture microstructure of martensite, proeutectoid ferrite, carbide-free baintie, upper bainite, M-A island has been obtained during quenching from austenitization at a rate of 45 ℃/min. Long strip shape carbides between carbide-free bainite ferrite trend to spheroidized during tempering, but they are still existing in the material. Suggestions are put forward for how to suppress the formation of carbide-free bainite and long strip shape carbides by heat treatmens. (authors)