Step cooling and isothermal aging tests at 425, 450 and 475°C, for 54000, 20000 and 5000 h, respectively, were performed on 21/4 Cr-1Mo steels with various amount (in mass pct.) of Si, Mn, Cu, Ni and impurities. The relationship between the step cooling-and the isothermal temper-embrittlement and chemical compositions was discussed.
The results are summarized as follows.
1) The shift in 50% fracture appearance transition temperature and 40ft-1b transition temperature by the step cooling treatment (ΔFATTsc, Δ40TTsc) have good correlation to a embrittling factor K2=(2Si+Mn+Cu+Ni)·X, where X=(10P+5Sb+4Sn+As)×102. Therefore, the reduction of amount of Si, Mn, Cu, Ni and impurities is effective to reduce the susceptibility to temper embrittlement.
2) The equations, which can accurately estimate the shift in 50% fracture appearance transition temperature and 40ft-lb transition temperature by isothermal temper embrittling at T°C for th (ΔFATTiT, Δ 40TTiT) from K2 and X, were derived.
3) Assuming that the life of peterochemical plants is 20 years at 425°C and the required toughness for forgings is 40TT≤10 or 38°C, chemical compositions of a 300 mm thich 21/4 Cr-1Mo steel forging shall be controlled as follows.