UNDERSTANDING OF 9-12% CR STEELS ON A NANO- AND SUBNANO-SCALE - THE SECRET OF TAF

Other conference contribution, 2010

The old Japanese high-boron 10.5% Cr martensitic test steel TAF, has still unsurpassed long-term creep strength at high temperatures amongst 9-12% chromium steels. Within the framework of the European COST 501 project, a series of very similar test materials were produced, and one of these has now been investigated by atom probe tomography. It contains 300 ppm B and was studied in the as-tempered condition. In-situ lift out technique was employed to pinpoint M23C6 precipitates located in vicinity and away from the prior austenite grain boundaries (PAGB). It was found that the B concentration is as high as 2 at.% in a M23C6 precipitate located in vicinity of a PAGB. In addition the concentration increases towards the centre of the precipitate. For M23C6 located more than10 micrometers away from the PAGB the B concentration is less than 1 at.%. The concentration is usually slightly higher towards the interface of M23C6/matrix. This phenomenon is discussed in the light of the B diffusion process. Disk-like small, ~ 10 nm, MX precipitates with a very high number density were observed. Some of the precipitates, which contain relatively high C, contain high amount of Nb as well. Trace of B was found in many of these precipitates. The secret of TAF appears to be two unusually dense precipitate families, both stabilized by boron.