Refractoriness and microstructure of 10%Cr-2%Mo type steel with vanadium and niobium additions

The results are reported of a study into the effect of carbon, chromium, molybdenum, vanadium and niobium contents and of microstructure on refractory properties of a new type steel used for boiler tubes and steam generator tubes of fast reactors. At 600 degC, the steel of the 0.05% C, 9 to 10% Cr, 2% Mo, 0.1% V, 0.05% Nb type shows excellent refractoriness which combines with a satisfactory level of creep plasticity and good weldability. The steel microstructure is of the two-phase type, characterized by the presence of tempered martensite and delta ferrite. After long-term exposure to temperature of 500 to 700 degC, the basic precipitates present in the steel include the M23C6 and M6C carbides and the Laves Fe2Mo phase. Chromium, molybdenum, vanadium, and niobium additions at optimal levels lead to solid solution reinforcement and to precipitation hardening. (B.S.)