Effects of the contents of Al, Ti, W and Y2O3 on long-term thermal ageing behavior of 15Cr ODS ferritic steels

Abstract

To study the long-term thermal ageing behavior of 15Cr–2W–0.2Ti–0.25Y2O3 (15Cr) and 15Cr–4.5Al–0.5Ti–0.5Y2O3 (15Cr–5Al) ODS ferritic steels, which have been isothermally aged at 475 °C for 9000 h, the age-hardening behavior have been studied by tensile test, and the resultant nanoscale structure and chemistry have been characterized by atom probe tomography (APT), transmission electron microscopy (TEM) and energy-filtered transmission electron microscopy (EFTEM). Thermal age-hardening is much more significant in 15Cr–5Al ODS steel, relative to 15Cr ODS steel. The α′ phases are finer and less developed in 15Cr ODS steel, relative to 15Cr–5Al ODS steel. The (Ti, Al)-enriched β′ phases precipitated in 15Cr–5Al ODS steel during the ageing. Ti partitions predominantly into α′ phases in 15Cr ODS steel whereas Ti partitioning into α phases is much stronger, relative to α′ phases, in 15Cr–5Al ODS steel. The number density of core/shell structured oxide particles is much higher for 15Cr ODS steel, relative to 15Cr–5Al ODS steel. It is considered that the formation of core/shell structured oxide particles should retard α–α′ phase separation. The age-hardening of 15Cr ODS steel was well predicted by the dislocation shearing mechanism of α′ phases, while the more significant thermal age-hardening in 15Cr–5Al ODS steel is attributed to the strengthening of not only α and α′ phases but also the additional β’ phases. 15Cr ODS steel was found to be more thermally stable than 15Cr–5Al ODS steel under the present ageing conditions.