Improving the high-temperature creep strength of 15Cr ferritic creep-resistant steels at temperatures of 923-1023 K

The precipitation strengthening of creep-resistant 15Cr ferritic steels at temperatures of 923-1023 K was improved by nickel addition. The addition of nickel, an austenite-stabilizing element, enhanced the formation of martensitic grains in the ferritic grain boundaries and changed the precipitation behavior during creep at elevated temperatures. The precipitates produced in the creep-ruptured 15Cr steels were intermetallic compounds (Laves phase and chi-phase), carbide (Cr23C6), and Z phase (Cr(V,Nb)N). An increase in the amounts of intermetallic compounds and the preferential precipitation of Cr23C6 carbide on the grain boundaries between the ferritic and martensitic grains were accelerated by the nickel addition. The 15Cr steel after 10,000 h at 973 and 1023 K demonstrated approximately double the creep rupture strength of 9Cr steel (ASME Grade T92) with a tempered martensitic microstructure. At temperatures of up to 1023 K, the 15Cr steel had over 10 times the creep rupture lifetime of T92 steel, primarily because of increased precipitation of intermetallic compounds and Cr23C6 carbide enhanced by nickel addition. (C) 2015 Elsevier B.V. All rights reserved.