M5C2 Carbide Precipitates in a High-Cr Martensitic Steel

The precipitate phases in an advanced 11% Cr martensitic steel, expected to be used at 650 °C, have beeninvestigated to understand the effect of precipitates on the creep-rupture strength of the steel. M23C6 andMX precipitates were dominant phases in this steel. Needle-like precipitates with a typical length of 180nm and width of 20 nm; and metallic-element compositions of 53-74Fe, 16-26Cr, 3-18Ta, 2-8W, and 2-4Co(at%); were observed mainly within the martensite laths of the normalized-and-tempered steel. The needle-likeprecipitates have been identified as monoclinic carbide M5C2, which is not known to have been reported previouslyin high chromium steels, or in heat-resistant steels those have been normalized-and-tempered. Thisindicates that the formation of M5C2 carbides can occur in heat-resistant steels produced under appropriatetempering conditions, and that this does not require long-term isothermal aging or creep testing, in all cases.