Secondary precipitation of vanadium carbides during creep of ferritic 9-12% Cr steels

An experimental proof was given of precipitation of very fine dispersions of vanadium carbides and nitrides in the creep of ferritic steels containing ca. 9% chromium. In addition to M23C6 carbides, these carbides and nitrides contribute to the dispersion hardening of said steels used for the manufacture of fast reactor steam generators. Steel specimens containing 9.1 to 9.5% chromium and 0.1 or 0.3% vanadium were studied. The basic heat treatment involved 950 degC/0.5 h hardening in the air and 750 degC/24 h tempering in the air. Creep tests were performed at a temperature of 575 degC and stress of 70 to 160 MPa over 900 to 73,000 hours. Selection electron radiograms were obtained of the individual components in the replicas of all specimens. Fine platelet-type precipitates were observed in addition to M23C6, the main precipitation phases in martensite and bainite. In part of the precipitates, the measurement of the lattice parameter showed the structure to be VCsub(0.93). During creep, precipitates occur of a composition of VNsub(0.63). The measured volume densities and mean sizes of the precipitates are tabulated. (A.K.)