Acceleration of Nano-Sized NbC Precipitation and Improvement of Creep Resistance in Alumina-Forming Austenitic Stainless Steel via Cold Working

A feasibility study on microstructure design has been carried out to improve creep rupture life in alumina-forming austenitic (AFA) Fe-14Cr-20Ni-2.3A1 stainless steel with and without 2 wt% W addition. After creep tests under 780 degrees C/80 MPa conditions, it was observed that internal cracks initiated primarily from grain boundaries in the W-free steel while voids nucleating at primary coarse NbC carbides elongated into fractures in the W-added steel. This result indicates that the W addition shifts the fracture mode from intergranular to transgranular. Based on this observation, the fine intragranular NbC carbides, which need to precipitate as much as possible during the initial stage of creep, can be a promising metallurgical factor for enhancing the creep resistance of both the AFA steels. Cold work at 5% strain prior to creep accelerated the heterogeneous precipitation of nano-sized intragranular NbC during the early stage of creep, which was responsible for the strong hardening effect by pinning the dislocations. The creep life was found to be 3 times longer as compared to the unpre-strained AFA steels. Applying 5% cold work induces the generation of dislocations in the matrix, which can lead to the nucleation of nano-scale NbC without recrystallization. The intragranular NbC carbide showed excellent resistance to coarsening during creep: their size was as small as around 30 nm after creep rupture.