The microstructures of two 9% chromium steels, P92 (30 ppm B) and B2 (100 ppm B), after heat treatment and after long-term creep deformation at 600°C were quantitatively investigated by means of transmission electron microscopy and boron trace autoradiography. The aim of the study was to show the boron distribution and identify the influence of boron on precipitation processes taking place in both steels during long-term creep exposure. The incorporation of boron into the M23C6 precipitates in both steels was demonstrated. In P92 steel (30 ppm B), boron was distributed preferentially on prior austenite grain boundaries and hardly visible on the sub-grain boundaries. In the steel B2 doped with 100 ppm B, boron was densely distributed on prior austenite grain- and sub-grain boundaries as well as within martensite laths. Quantitative TEM metallography and boron trace autoradiography investigation showed that boron retarded the growth of M23C6 by forming borocarbides M23 (C, B) 6, thereby significantly improving the creep rupture strength of boron doped 9% chromium steels.

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