溶融硝酸塩中における鋼材の腐食疲労挙動

抄録

Corrosion fatigue behaviour of plant materials such as 21/4;Cr-1Mo, SUH309 and S20C was investigated in a molten salt environment of (NaNO₂40%, NaNO₃7%, KNO₃53%) at 723K. The rotating bending fatigue strength at 4×10⁷ cycles of 21/4;Cr-1Mo steel in the molten salt environment was 8 percent lower than the fatigue limit in air. The fatigue strength at 4×10⁷ cycles of SUH309 in the molten salt environment was deteriorated by 27 percent. Corrosion fatigue cracks were initiated from corrosion pits and propagated accompanying striation predominantly in the molten salt environment. The morphology of the corrosion products on the specimen surface was examined by X ray-diffractions. The major corrosion product of these steels was Fe₃O₄. It may be concluded that the formation of corrosion pits at the initiation of fracture surface is the major cause of fatigue strength decrease of these steels in a molten salt environment containing Cl^-.
The corrosion fatigue strength of S20C steel was higher than the fatigue limit in air. This may be caused by the retardation of crack initiation from the hardened surface layer due to nitriding in a molten salt environment. Neither the initiation nor the propagation of stress corrosion cracking of these steels was observed in the molten salt environment during the initiation test up to 2880hrs and the propagation test up to 2160hrs in the stress intensity factor range K_IO of 248 and 372MPa√m, respectively.