Analysis of life distribution by stress corrosion cracking of 3.5NiCrMoV steel forgings

The stress corrosion cracking (SCC) life time of turbine rotors in actual nuclear and fossil power plants was analyzed by a statistical method and was estimated to obey an exponential probability distribution. Then, the laboratory accelerated test condition to exhibit such an exponential probability distribution was investigated using 3.5 NiCrMoV steel forgings which are being used most widely as a material for low pressure turbine rotors. The exposure of double U-bend specimens in liquid/gas interface of undeaerated 30% NaOH aqueous solution was found to be most fitted for the purpose. Under this condition, the effects of alloying elements and mechanical strength/microstructure on SCC life were studied. The reduction of P content and/or the addition of a small amount of Nb in steel and the reduction of mechanical strength were effective to prolong the SCC life of turbine rotors. (author)