Abstract
Thermal embrittlement of high-chromium ferritic and martensitic steels at intermediate temperatures has been investigated in PM2000 (Fe20Cr) steel exposed to 475 °C by various non-destructive techniques, including magnetic Barkhausen noise (MBN), positron annihilation spectroscopy (PAS) and Mössbauer spectroscopy (MS). These techniques were used in a complementary manner to characterize the phase separation of the Fe-Cr ferrite phase into Fe-rich (α) and Cr-rich ferrite (α') phases, the phenomenon known as 475 °C embrittlement. Together with a reference steel T91, samples were subsequently isothermally annealed up to 750 h. While the MS technique clearly confirmed the formation of bcc Cr-rich precipitates in PM2000 steel with annealing time, the MBN and PAS techniques only indicated the formation of new pinning sites for magnetic domains and trapping sites for positrons in PM2000, up to 500 h annealing time, respectively. This observation was attributed to the precipitation process of Al-rich oxides in the oxide-dispersion strengthened PM2000 steel. This process, competitive to the chromium-rich α' phase precipitation, was found to be dominating the micromagnetic and positron-annihilation characteristics during the early stages of the annealing.
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