Cr-Mo-Vロータ鋼の諸特性に及ぼすNiの影響

概要

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1Cr-1.25Mo-0.25V steel is widely used for high and medium pressure power generating steam turbine rotors and in general, Ni addition less than about 0.75% to this steel is allowed. On the other hand, rotor steels of much higher Ni additions are also used for turbine rotors of electric generator for ships.The effect of Ni up to 1.8% on some properties of Cr-Mo-V rotor steel changing cooling rate in hardening in 3 levels have been investigated in this paper.Results obtained will be summarized as follows.(1) Hardenability is improved by Ni addition and both ferrite nose and bainite nose are shifted to lower temperature and longer time side. The improvement of hardenability during changing of Ni content from 0.3% to 0.9% is especially remarkable.(2) In case of (550∼300°C) cooling rate in hardening of 25.3°C/min, Ni addition lowers 50% FATT and rises 2VE-20°C. 2VE+20°C after tempering and it seems that this is due to improvement of hardenability.(3) In case of (550∼300°C) cooling rate in hardening of 1.01°C/min, Ni addition rather rises 50% FATT and lowers 2VE-20°C. 2VE+20°C after tempering and this is supposed to be due to increase of retained austenite as one of the causes.(4) Rupture life (565°C, 30kgf/mm2) is reduced by Ni addition. It seems that this is because of influence of microstructure due to improved hardenability by Ni addition.
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