2 1/4Cr-1Mo および 3Cr-1Mo 鋼の長時間恒温焼もどし脆化量の推定

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Isothermal temper embrittling tests at 425, 450 and 475℃, for 54000, 20000 and 5000h, respectively, were performed using 2 1/4Cr-1Mo and 3Cr-1Mo steels with various amounts of Si, Mn, Cu, Ni and impurities. Relationship between long time isothermal embrittlement and step cooling embrittlement is discussed. The results are summarized as follows. 1) The equations, which can accurately estimate the shift in 50% fracture appearance transition temperature and the 40ft-lb transition temperature by isothermal embrittling at T℃ for th (△FATT_<iT>, △40TT_<iT>), are derived from those of step cooling embrittling (△FATT_<SC>, △40TT_<SC>) and amounts of impurities defined as P_<eq.>=(10P+5Sb+4Sn+As)×10^<-1>. 2) From the equations, the following facts are found. Embrittlement at 425, 450 and 475℃ reach 90% saturation by aging for 380∿652×10^3, 64∿97×10^3 and 10∿12×10^3h, respectively. Saturated isothermal embrittlement at 425, 450 and 475℃ are 2.44, 1.98 and 1.51 times the step cooling embrittlement, respectively. Embrittling rate is slower and saturated embrittlement is greater at lower aging temperatures. 3) Isothermal temper embrittlement at 425℃ for 20 years (△40TT_<i425×20y>), which is maximum at this temperature in this study, is calculated to be 2.1△40TT_<SC>. So, 40TT_P+2.5△TT_<SC>≦10 or 38℃, where 40TT_P is 40TT after PWHT, is reasonable evaluation standard to avoid severe temper embrittlement in long time service.
社団法人日本鉄鋼協会の論文
1992-02-01

2 1/4Cr-1Mo および 3Cr-1Mo 鋼の長時間恒温焼もどし脆化量の推定

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