Fe ion irradiation effects on the precipitates of P92 steel after thermomechanical treatment

Abstract

Although thermomechanical treatment (TMT) can significantly improve the high temperature strength of high Cr F/M steels being material candidates for Generation-IV reactors, the change of precipitates in TMT-strengthened steels under irradiation remains unknown. To understand this, high Cr F/M steel P92 was subjected to an optimized TMT process, and which was irradiated with 3.5 MeV Fe13+ ions at 700 °C to 0.37 and 0.75 dpa, while precipitates in the unirradiated and irradiated steels were observed and analyzed using transmission electron microscopy. After irradiation, overall, precipitates number reduced, large precipitates increased but nano-sized precipitates uniformly distributed throughout the matrix, and precipitates distribution changed from densely networked to isolated dispersion distribution. The change of precipitate composition after irradiation varied with precipitate phase type, mainly, the composition of M23C6 precipitates was unchanged; Cr decreased, and V increased in Cr-rich M2X precipitates, along with a significant decrease in Cr/V ratio; Nb increased, and V decreased in Nb-rich MX-I precipitates, along with an obvious increase in Nb/V ratio; Nb increased, Cr significantly decreased in Nb-rich MX-II precipitates, along with a striking increase in Nb/Cr ratio. During irradiation, the obvious coarsening of Cr-rich M23C6 and M2X precipitates occurred mainly due to the irradiation assisted diffusion, and densely networked Cr-rich M2X precipitates partially dissolved, leading to a decrease in precipitates number. The compositional changes of the precipitates during irradiation were also discussed.