Creep behaviors of Fe-18Ni-12Cr based alumina-forming austenitic steels with ultralow carbon

Two kinds of Fe-18Ni-12Cr-based alumina-forming austenitic (AFA) steels with ultralow carbon content were prepared by vacuum induction melting. Creep rupture tests were carried out with the applied stress range between 80 and 180 MPa at the target application temperature of 700 degrees C. Microstructure evolution was characterized using glancing X-ray diffraction, scanning electron microscopy equipped with a detector of electron backscatter diffraction and transmission electron microscope. Compared with 18-12-Al, 18-12-AlNb exhibited superior creep properties including ten times longer creep rupture life (t(rup)) and one magnitude smaller minimum creep rate ((epsilon) over dot(min)). For example, the (epsilon) over dot(min) at the applied stress of 140 MPa was 4.4 x 10(-5) and 2.1 x 10(-4) h(-1) for 18-12-AlNb and 18-12-Al, respectively. The strengthening effect of B2-NiAl would weaken at 700 degrees C due to its ductile-to-brittle transition. The addition of Nb not only promoted the formation of Laves-Fe-2(Mo,Nb), but also resulted in formation of nanoscale NbC. The GB Laves-Fe-2(Mo,Nb) did not impair the creep ductility by strengthening GBs, and all the creep ruptured AFA steels exhibited typical ductile fracture mode. The GB Laves-Fe-2(Mo,Nb) and NbC precipitates contributed to the superior creep resistance of 18-12-AlNb.