Mechanical properties and thermal stability of nanostructured ODS RAF steels

The main goal of the research reported in this paper was to evaluate the microstructure and mechanical properties of an oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel after a long-term and high temperature annealing. The mechanically alloyed ODS RAF steel powder with a composition of Fe–14Cr–2W–0.3Ti–0.3Y2O3 was consolidated by hot isostatic pressing at 1150 °C for 3 h followed by a high speed hydrostatic extrusion at 900 °C. The samples were subsequently exposed to various heat treatments, such as annealing at 850 °C–1350 °C for 1 h in argon and to a long-term isothermal annealing at 750 °C up to 10 000 h in air. The results indicate that due to the Zener pinning of the nanoparticles at the grain boundaries, the ODS RAF steel is thermally stable up to high annealing temperature of 1250 °C. However, annealing at 1050 °C applied after the hydrostatic extrusion of the steel significantly improves its ductility. Further increase in the temperature up to 1350 °C resulted in nanoparticles coarsening (size ∼8.0 nm, number density 2.5 × 023 m−3) accompanied by an increase in the volume fraction of the coarse grains and a reduction of microhardness from 460 to 330 HV0.1. Long-term annealing at 750 °C up to 10 000 h brought about a systematic loss of both strength and ductility. However, the microhardness increases up to 500 HV0.1. TEM observations revealed that the grain size after ageing remains almost unchanged. On the other hand, the number of dislocation free larger grains and of larger precipitates with a size of ∼25 nm located at grain boundaries significantly increase.