Strengthening of an Al-Containing Austenitic Stainless Steel at High Temperature

Three Al-containing austenitic steels with slightly different contents of Nb, V and C in the Fe-19.95Ni-14.19Cr-2.25Al-2.46Mo-1.95Mn-0.15Si-0.01B (wt.%) system were designed to study the effect of precipitations on creep/rupture resistance. After induction melting, alloys were cast into a metal mold followed by thermo-mechanical treatment. A continuous Al-rich oxide scale was formed on the surface after exposure at 800°C for 146 hrs in air. By decreasing the C content from 0.07 to 0.04%, coarse NbC precipitates in the as-cast microstructure could be removed during annealing treatment. Thermo-mechanical treatment enabled nano-scale precipitation of NbC in the alloys containing 0.04% C. Although the yield strength of the alloy with 0.07% C was relatively high at 750°C, its creep/rupture life was 164 hrs at 700°C/150 MPa. Alloys having low carbon content formed a uniform fine MC precipitation around 10–20 nm and showed a creep/rupture life between 1002 and 1530 hrs at 700°C/150 MPa. This is comparable with that of super304H tested under the same condition. Fe₂(Mo,Nb) Laves phase was found in the microstructure after creep/rupture testing. NiAl precipitated in alloys after creep/rupture testing for more than 1000 hrs. However, strengthening effect from these two phases is not obvious, indicating that nano-scale NbC precipitates are the major source of strengthening during creep/rupture at high temperature. In addition, nano-scale (Nb,V)C was found in V containing alloy corresponding to the longest creep/rupture life.