Isothermal temper embrittling tests at 425, 450 and 475°C, for 54000, 20000 and 5000 h, respectively, were performed using 2 1/4 Cr-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 40 ft-lb transition temperature by isothermal embrittling at T°C for th (ΔFATTiT, Δ40 TTiT), are derived from those of step cooling embrittling (ΔFATTSC, Δ40 TTSC) and amounts of impurities defined as Peq.=(10P+5Sb+4Sn+As) ×10-1.
2) From the equations, the following facts are found. Embrittlement at 425, 450 and 475°C reach 90% saturation by aging for 380652×103, 6497×103 and 1012×103 h, respectively. Saturated isothermal embrittlement at 425, 450 and 475°C 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°C for 20 years (Δ40 TTi425×20y), which is maximum at this temperature in this study, is calculated to be 2.1Δ40 TTSC. So, 40 TTP+2.5ΔTTSC≤10 or 38°C, where 40 TTP is 40 TT after PWHT, is reasonable evaluation standard to avoid severe temper embrittlement in long time service.