Application of intercritical heat treatment to improve toughness of SA508 C1.3 reactor pressure vessel steel

The new heat treatment processes for manufacturing high toughness SA508 C1.3 steels have been developed by application of intercritical heat treatment (IHT). In the new heat treatment processes, the IHT is added between the quenching and the tempering of the conventional heat treatment process. The application of IHT resulted in the increase of ductility and upper shelf energy and in the decrease of strength and ductile-to-brittic transition temperature (DBTT). The modilication of tempering conditions reduced the loss of strength reslilling from the THT. The beneficial effects from the IHT were consistently maintained in spite of the changes of heating and cooling rates. Additionally, the cause of the increase in toughness was investigated in relation to the microstructural change. The IHT produces a composite structure of hard tempered martensite and soft double-tempered bainite. More sub-grain boundaries are conlained in the composite structure, and these act as obstacles lo cleavage pi-opagaton. Furthermore. the coarse and long carbides are much spheroidized and inter-carbide distance becomes longer by IHT. High toughness is measured when the microstructure contains 20-60% tempered martensite. It was concluded that the spheroidized carbides with longer inter-particle distance arc one of important causes for high toughness. © 1999 Elsevier Science S.A. All rights reserved.