マルテンサイト系3%Ni-Cr-Mo-V鋼の焼もどし後の強さと焼もどし脆化感受性との関係について

抄録

The effect of strength levels obtained by changing tempering conditions of quenched martensitic 3% Ni-Cr-Mo-V steel on susceptibility to temper embrittlement was examined by applying isothermal embrittling treatment. In addition, the fractured surfaces of Charpy impact specimens tested at different temperatures were also investigated in detail by using a scanning electron microscope.
The main results obtained are as follows:
(1) The temper embrittling rate and amount designated by the shift of FATT during embrittlement treatment of martensitic steels tempered for 6 hours clearly depend on tempering temperatures of 600°C, 630°C and 660°C after quenching. The steel tempered at a higher temperature tends to be embrittled more rapidly to a larger extent. The apparent activation energies for embrittlement depend on the tempering temperature; this implies that the microstructure affects segregation of embrittling elements to grain boundaries.
(2) Intercrystalline fracture dominates brittle fracture in the vicinity of transition temperature region, although it decreases rapidly with decrease in the testing temperature. With the temperature decreasing, the intercrystallinely fractured region in a Charpy specimen appears only near the bottom of notch, which may indicate that the fracture mode changes with crack velocity as well as with difference between cleavage and grain boundary fracture strength.