高強度鋼の応力腐食割れき裂発生に及ぼす微小振動応力の影響

Abstract

Stress corrosion Cracking (SCC) initiation behavior was examined on a high-strength low alloy steel sensitive to hydrogen embrittlement (HE) type SCC under a sustained load with vibratory stresses of small amplitude superimposed in an open circuit and cathodically charged conditions in 3.5%NaCl solution. In an open circuit condition, time to crack initiation under a sustained load with superimposed vibratory stress range greater than 74 MPa is shorter than that of static SCC under a sustained load, resulting in lower 600 ks strength than that of static SCC σSCC. At a stress range Δσ smaller than 74 MPa, however, 600 ks dynamic SCC strength hardly decreases from σSCC. In case of σmax>σSCC SCC cracks are always initiated at corrosion pits formed on specimen surfaces irrespective of superimposed vibratory stress range. On the other hand, at σmax<σSCC and Δσ>74 MPa, the superimposed vibratory stresses promote stress assisted crack-like dissolution preceded by a corrosion pit. Dynamic SCC crack is initiated at the bottom of crack-like portion, thereby reducing the dynamic SCC strength. K*ISCC calculated from the sizes of corrosion pits and crack-like portion is roughly equivalent to KISCC obtained from a long sharp one.When a cathodic potential is applied to specimens, corrosion pits or stress assisted crack-like dissolution preceded by corrosion pits, are no more generated, and no dependence of superimposed vibratory stresses of small amplitude on 600 ks SCC strength is shown. In this situation hydrogen occulusion is much more enhanced than that in an open circuit, and SCC crack is initiated at the interior of the specimen where hydrostatic pressure reaches the maximum.