Effect of precipitation behavior on creep strength of 15%Cr ferritic steels at high temperature between 923 and 1023 K

Abstract

The effect of precipitation strengthening on the high-temperature creep strength of heat-resistant 15Cr steels with dual phases of ferrite and martensite was investigated. The creep rupture lifetime of the 15Cr steel at temperatures up to 1023K was 10 times longer than that of conventional 9Cr steel (ASME Grade T92) with a tempered martensitic microstructure. The creep rupture strength of the 15Cr steel after 10,000h at 973 and 1023K was approximately two times higher than that of the T92 steel. The precipitates identified in the creep ruptured 15Cr steels were intermetallic compounds (Laves phase and χ-phase) and carbide (Cr23C6). In the 15Cr steels creep ruptured at high temperatures of 973K and above, the Laves phase was precipitated in both ferritic and martensitic grains, and Cr23C6 carbide was precipitated at the grain boundaries. The 15Cr steel exhibited longer creep rupture lifetime than 9Cr steel with a tempered martensitic microstructure strengthened by carbides and carbonitrides, due to the precipitation of Laves phase and Cr23C6.