Creep properties affected by morphology of MX in high-Cr ferritic steels

MX-type carbonitrides such as (Nb, V)(C, N) are well known to be effective to improve creep properties of the heat resisting steels at elevated temperatures. The control of precipitation behavior of MX is essential to obtain the stable microstructure that is suitable for heat resisting steels in the Ultra Super Critical plants. In this study, strong relationship between creep properties and precipitation behavior of MX is investigated by using the high-Cr steels with different GIN-balance. MX is categorized to three types from their characteristic morphology, type I, II and Ill. It is found that the C/N-balance of steel is one of factors to determine which MX is dominant in the tempered steels. Presence of type III-MX that is formed by secondary precipitation of VN adhering to Nb(C, N) seems to be unfavorable to long-term creep resistance by considering relatively decreasing in distribution density of fine VN, type II-MX. Precipitation sites such as Nb(C, N), type I-MX and excess solute nitrogen promote growth of type III-MX at the tempering stage. We conclude that the control of precipitation kinetics of MX is quite important to achieve good creep properties at high temperatures.