Precipitation of Cu- and Nb-rich phases and its strengthening effect in 17Cr ferritic stainless steel during high-temperature creep process

In the present study, the evolution and strengthening effect of precipitates in 17Cr ferrite stainless steel during the creep process are investigated. The precipitates are characterized by scanning electron microscopy, energy dispersive spectroscopy and transmission electron microscopy. Moreover, the interactions between the precipitates and the grain boundary and dislocation are quantitatively analysed. The results show that Cu- and Nb-rich phases are formed during the creep process. The stress concentration at the tip of the precipitates, the stacking fault at the precipitate edge and the internal dislocation can promote the nucleation of the Nb-rich phase. The Nb-rich precipitates can provide the driving force for the nucleation of Cu-rich particles through stacking fault, dislocation entanglement and internal dislocation. At 700 °C, as the creep time increases, the pinning effect of precipitates on the grain boundary increases linearly, while the pinning effect on the dislocation decreases. As the creep temperature increases, a peak pinning effect on the grain boundary is observed at 750 °C, and the pinning effect of the precipitates on dislocation decreases gradually inside the grain. The formation of dislocation networks at grain boundaries can delay pinning effect reduction of the Nb-rich phase, and the main blocking mechanism of precipitates on dislocation movement is bypassing. The precipitation strengthening effects of Cu- and Nb-rich phases are also compared with each other.