Influence of Tungsten and Tantalum Content on Evolution of Secondary Phases in 9Cr RAFM Steels: An Experimental and Computational Study

This paper presents the results of a systematic study on the role of alloying elements W and Ta on the microstructural evolution in 9Cr-W-Ta-V-C Reduced Activation Ferritic/Martensitic steels during long-term thermal exposure in the temperature range of 773 K to 923 K (500 °C to 650 °C). The kinetics of evolution of secondary phases like M₂₃C₆, MX, and Laves phase crucially depend upon the W and Ta content of the steel in addition to temperature and time, which has been studied in detail using analytical transmission electron microscopy as well as predictive methods. The steel with 1 wt pct W and 0.06 wt pct Ta showed slow recovery below 873 K (600 °C) and no evidence for Laves phase at any temperature. Significant change in microstructure was observed after 10,000 hours of exposure at 923 K (650 °C), while recovery at short durations was retarded by nucleation of MX precipitates. Increase in both W and Ta content of the steel enhanced the tendency for the formation of Laves phase.