Isothermal aging tests at 375, 400 and 450°C for 1000020000 hs were performed on 3.5NiCrMoV steel forgings for low pressure turbine rotors with various amount of impruities, Si and Mn, in the Mn range of less than about 0.30 mass %. The results are summarized as follows.
1) The shift in 50% fracture appearance transition temperature by isothermal temper embrittlement at T°C for t h (ΔFATTiT) decreases with decrease in amount of impurities, defined as X = (10P + 5Sb + 4Sn +As) × 102, Si, Mn, cooling rate during quenching and aging temperature. When X is less than 2.1, no temper embrittlement occurs, regardless of Si and Mn contents, but when X is greater than 4.1, embrittlement occurs according to Si and Mn contents. However, with X = 10.7, some embrittlement is observed even at 0.02% Si and 0.04% Mn in the aging tests at 450°C for 10000 hs. Therefore, reduction of amount of impurities is important to reduce susceptibility to temper embrittlement.
2) When X is 4.1 to 7.0, the equation, which can accurately estimate ΔFATTiT from K = (Si + Mn)·X, X and the cooling rate, is derived.
3) When K is less than 0.78 (X ≤6.0, Si≤ 0.05% and Mn ≤ 0.08%), maximum isothermal temper embrittlements at 375450°C for 20 years are calculated to be 24°C at surface and 16°C at center of a 1800 mm diameter rotor forging, and the fairly good toughness is estimated to be obtained even after using for a long term.