Microstructure and toughness of intercritically reheated heat affected zone in reactor pressure vessel steel weld

Abstract

This study is concerned with correlating the metallurgical factors and the toughness of the intercritically reheated coarse grained heat affected zone (ICCGHAZ) of multipass welded SA508–cl. 3 reactor pressure vessel steel. The influence on toughness was investigated by means of a simulated heat affected zone test. Toughness and other properties of the ICCGHAZ were strongly influenced by metallurgical factors such as high concentration of carbon martensitic islands (secondary phase, martensite–austenite constituents) formed along the prior austenite grain boundaries and martensite lath interfaces, and a softened matrix (tempered martensite or bainite). The characteristics (amount, hardness, and size) of the islands were found to be strongly correlated with both peak temperature and the cooling time of the previous pass. The toughness of the ICCGHAZ deteriorated with increasing amounts of martensite–austenite constituents, which were made more abundant by increasing the final peak temperature within the intercritical temperature range. Meanwhile, for the same intercritical peak temperature, toughness decreased with increased cooling time. When the cooling time was short, the dominant factor influencing toughness of the ICCGHAZ was the amount of martensitic islands. However, when the cooling time was long, the hardness difference between the martensitic islands and the softened matrix (tempered martensite) was found to be the dominant factor.