Currently, NiCrMoV Steels have been widelyused for low pressure steam turbine rotor shafts infossil power generation plants. However, NiCrMoVSteels have a strong potential to temper embrittlementand the temper embrittlement problem of the turbinematerial has been one of the largest concerns for thesafe operation of power plants. On the other hamd, theinlet steam temperature of LP turbines isprogressively increasing in the newly designed powerplants such as an ultra super critical unit. In theseapplications, the temper embrittlementsusceptibility of the LP turbine material is acritical problem. As a solution to the temperembrittlement problem, superclean material and itsproduction technologies have been established andseveral superclean rotors are already in operation inJapan. In the present paper, the results of isothermalageing tests up to 100,000h, by using LP turbinematerials made around 15 years ago, are introduced.The effect of chemical and impurity contents on thetemper embrittlement behaviour is discussed based onthe data. The results show that the comparativelysmall J-factor material still has a significantmaterial should be used for NiCrMoV rotor forgings forhigher temperature application.
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