Abstract
In this study, a possibility of application of advanced 9%Cr steel containing 130 ppm boron for boiler components utilized at around 650 °C to higher temperature steam turbine rotor materials has been investigated by means of reduction in silicon promoting macro-segregation in the case of large size ingots, using laboratory heats. Tempered martensitic microstructure without proeutectoid ferrite in all steels studied is obtained even at the center position of a turbine rotor having a barrel diameter of 1.2 m despite lower amounts of nitrogen and silicon. The strength at room temperature is almost the same level of practical high Cr steels such as X13CrMoCoVNbNB 9-2-1 for ultrasuper critical steam turbine rotors. The toughness is sufficient for high temperature rotors in comparison with CrMoV steels utilized as sub-critical high pressure steam turbine components. The creep rupture strength of the steels is higher than that of the conventional 9-12Cr steels used at about 630 °C. The creep rupture strength of 9%Cr steel containing 130 ppm B, 95 ppm N, 0.07 % Si and 0.05 % Mn is the highest in the steels examined, and it is therefore a candidate steel for high temperature turbine rotors utilized at more than 630 °C. Co-precipitation of M23C6 carbides and Laves phase is observed around the prior austenite grain boundaries after the heat treatments and the restraint of the carbide growth is also observed during creep exposure. An improvement in creep strength of the steels is presumed to have the relevance to the stabilization of the martensitic lath microstructure in the vicinity of those boundaries by such precipitates.