Effects of nitrogen on precipitation and tensile behaviors of 25Cr-20Ni austenitic stainless steels at elevated temperatures

摘要

To understand the effects of nitrogen on precipitation and tensile behaviors at elevated temperatures, 25Cr-20Ni austenitic stainless steels with different nitrogen contents (0.065 wt %, 0.13 wt % and 0.17 wt %) were tensioned from room temperature to 1000 degrees C. Results show that the elongation and area reduction change differently with temperature increasing from 800 degrees C to 1000 degrees C, which are caused by different fracture modes. The necking concentrates in a small zone in the tensile samples of 0.17 wt % N steel, while the necking deformation occurs in all gauge length in specimens of other two steels. The local necking in 0.17 wt % N steel is induced by the enhanced recrystallization softening effect during tensile deformation. As the dislocation recovery are retarded by more solute nitrogen atoms and nitrides in 0.17 wt % N steel, the dislocation density difference between deformed grains and recrystallized grains turns larger, which results in a higher recrystallization rate and enhanced softening effect in further deformation. Additionally, the content of M23C6 precipitates unexpectedly increases with the increasing nitrogen after tensile test at 800 degrees C. This is because the increasing nitrogen promotes the formation of NbCrN in higher nitrogen steels during solution heat treatment, instead of Nb(C, N) in lower nitrogen steels. The precipitation of NbCrN releases carbon into austenitic matrix, leading to the acceleration of M23C6 precipitation during tensile test at 800 degrees C.