Preliminary creep testing of the alumina-forming austenitic stainless steel Fe-20Cr-30Ni-2Nb-5Al

The alumina-forming austenitic stainless steel Fe-20Cr-30Ni-2 Nb-5Al was given a solutionizing anneal at 1250 °C followed by anneals at 800 °C for 0, 2.4, 24, 240 h to produce B2 (ordered b.c.c.) and Laves phase precipitates of different sizes with different extents of grain boundary coverage. Both tensile tests and constant-stress (43 MPa) creep tests were performed on the heat-treated materials and on the as-cast alloy at 760 °C. The precipitates grew during the creep testing. In addition, L12 (ordered f.c.c.) precipitates nucleated and grew during the creep testing to similar particle sizes after 500 h independent of the prior heat treatment at 800 °C. The specimens given the shortest heat treatment of 2.4 h, which had the smallest initial particle sizes, showed both the highest yield strength and the smallest creep strain after 500 h. The extent of grain boundary coverage by precipitates did not appear to affect the creep rates. No grain boundary cracking or precipitate cracking was found in the heat-treated specimens after creep testing, but the as-cast material failed around 600 h.