Microstructural characterization of aging precipitation behavior of 17Cr-0.86Si-1.2Cu-0.5Nb ferritic stainless steel

Abstract

Herein, the precipitation behavior of 17Cr-0.86Si-1.2Cu-0.5Nb ferritic stainless steel during isothermal aging is studied by using scanning electron microscopy, energy dispersive spectrometer and high-resolution transmission electron microscopy. Moreover, the influence of aging temperature and time on hardness is systematically investigated. The results reveal that the Cu-rich phase mainly precipitates below 750 °C, whereas the Nb-rich phase precipitates above 750 °C. The Cu-rich phase exhibits an evolution behavior of B2-9R-FCC and the coarsening rate of Cu-rich phase increases by two orders of magnitude from 650 to 750 °C. Furthermore, the side of rod-shaped Cu-rich phase maintains incoherent interface with the matrix, however, the tip of Cu-rich phase exhibits a coherent interface with the matrix with an orientation relationship of 011¯α//1¯11Cu. The Nb-rich phase includes Fe2Nb, Fe3Nb4Si5, CrNbSi, (Fe,Cr,Si)2Nb, and Nb2C phase after thermal aging. At T < 750 °C, both sides of the Nb-rich phase provide nucleation points for the Cu-rich phase. At T = 850 °C, the tip of Cu-rich phase provides a nucleation site for Nb-rich precipitation. A coherent interface can be found between (Fe,Cr,Si)2Nb and Cu-rich phases, and the Cu-rich phase grows along (311) crystal planes of (Fe,Cr,Si)2Nb. Finally, the hardness change with aging time shows a single peak feature at T < 750 °C and a double-peak at 850 °C. The occurrence of aging peak is delayed and the peak values gradually decrease with the increase of aging temperature.