Effect of Co addition on microstructure in high Cr ferritic steels

Co is one of the interesting alloying elements in advanced high Cr ferritic steels used to improve their creep properties at elevated temperatures. However, it has been reported that Co bearing tends toward rapid deterioration in creep property at long-term testing. In this study, microstructures were studied in detail by comparing two kinds of 9 % Cr ferritic steels with 3 % Co and Co free for better understanding difference in those creep behaviors. Creep property of the steel with 3 % Cc was superior to that of the steel without Co within this study, as many researchers pointed out. Adding Co certainly suppressed delta-ferrite formation and there was large difference in prior-austenite grain size between two steels after tempering. In addition to this macroscopic difference, there was also remarkable change in precipitation behavior between them. It was found that heterogeneous precipitation of inter-metallic compounds and MX type carbonitrides was observed in Co free steel, while any extreme localization of such precipitation were not seen in 3 % Co steel. It was deduced that such microstructural difference in connection with 45-ferrite formation between two steels was an important factor to understand better high temperature creep properties in 3 % Co steel rather than those in Co free steel.