Structural Evolution of 10% Cr-3% Co Steel Microalloyed with Re and Cu during Creep at 923 K

Structural evolution of the tempered lath martensite of the 10% Cr-3% Co steel microalloyed with rhenium and copper with a low nitrogen content and a high boron content during creep at 923 K was investigated for the purpose of establishment of the rison of decrease in creep resistance of this steel under the low applied stress. The tempered martensite lath structure of 10% Cr-3% Co steel with an average lath size of 370 nm and a high dislocation number density of 2 x 10(14) m(-2) was observed after normalizing at 1323 K with the following tempering at 1043 K for 3 h. The structure was stabilized by M23C6 carbides, M6C carbides, and NbX carbonitrides. During long-term creep, the lath structure strongly experienced an evolution: the width of the martensitic laths increased significantly, dislocation density decreased, the Laves phase and Cu-enriched particles remarkably coarsen. Such structural evolution correlates with an appearance of creep strength breakdown on curves "Applied stress vs. Time to failure" and "Minimum creep rate vs. Applied stress". Significant coarsening of the Laves phase particles and Cu-enriched particles via formation of the large particles with sizes more than 250 nm along high-angle boundaries and full dissolution of the fine particles with sizes less than 50 nm along low-angle boundaries of martensite laths is considered to be the main cause of degradation of the creep resistance of the steel studied.