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 q uenching and the tempering of the conventional heat treatment process. The application of IHT resulted in the increase of ductility and upper shelf en ergy and in the decrease of strength and ductile-to-brittle transition temp erature (DBTT). The modification of tempering conditions reduced the loss o f strength resulting from the IHT. The beneficial effects from the IHT were consistently maintained in spite of the changes of heating and cooling rat es. Additionally, the cause of the increase in toughness was investigated i n relation to the microstructural change. The IHT produces a composite stru cture of hard tempered martensite and soft double-tempered bainite. More su b-grain boundaries are contained in the composite structure, and these act as obstacles to cleavage propagaton. Furthermore, the coarse and long carbi des are much spheroidized and inter-carbide distance becomes longer by IHT. High toughness is measured when the microstructure contains 20-60% tempere d martensite. It was concluded that the spheroidized carbides with longer i nter-particle distance are one of important causes for high toughness. (C) 1999 Elsevier Science S.A. All rights reserved.