Evolution of Precipitates in Thermomechanically Strengthened P92 Steel Under Fe Ion Irradiation at 400 °C

To improve the high-temperature mechanical properties of the commercial high-chromium ferritic/martensitic (F/M) steel P92, the steel was subjected to a thermomechanical treatment (TMT) involving a warm rolling with 93 pct deformation at 650 °C and a tempering at 650 °C for 1 hour. The TMT-strengthened steel was subjected to 3.5 MeV Fe¹³⁺ ion irradiation at 400 °C to 0.68 dpa in order to study irradiation effects on the precipitates of high-chromium F/M steel after TMT. Precipitates in the TMT-strengthened steel before and after irradiation were investigated using transmission electron microscopy. Cr-rich M₂₃C₆, Nb-rich MX (NbC type) and nano-sized Cr-rich M₂X as dominant precipitate phases pre-existing in the TMT-strengthened steel were also identified in the irradiated specimens. The irradiation resulted in a partial dissolution of nano-sized Cr-rich M₂X precipitates and a coarsening of precipitates including undissolved nano-sized precipitates and relatively large-sized precipitates. After irradiation, the composition of Cr-rich M₂₃C₆ precipitates exhibited an increase in Cr and W, but a decrease in Fe. Seven types of irradiation-induced phases were identified in the irradiated specimens. They are Cr-rich M₂C and Nb-rich MX(NbN type) with a hexagonal crystal structure, Fe-rich M₃C and Fe-rich M₂C as well as Cr-rich M₂N_0.39C_0.61 with a simple orthorhombic crystal structure, Nb-rich M₆C₅ with a base-centered monoclinic crystal structure, and σ-FeCr with a simple tetragonal crystal structure. These irradiation-induced phases are also discussed.