Relationship Between the Activation Energy of Hydrogen Trap Site and a Decrease in Fatigue Strength by Absorption of Irreversible Hydrogen in Cold-Drawn High Strength Steels

Abstract

Fatigue tests have been conducted to investigate the influence of irreversible hydrogen and its activation energy for hydrogen desorption from trap site on the fatigue strength for cold-drawn high strength steels with different activation energy, but having the same chemical composition. Specimens were cathodically hydrogen charged, and internal hydrogen states were changed as follows : (a) non-charged sample and (b) the one that contained only irreversible hydrogen. Since the fatigue crack was initiated at internal inclusion or microstructural inhomogeneity, the fatigue strength was discussed based on the stress intensity factor calculated from stress and defect size. In the case of the sample with higher activation energy, 71.4 kJ/mol, irreversible hydrogen had no influence on the fatigue strength. In this case, the fracture surfaces of the sample with and without irreversible hydrogen were the same. However, for the sample with lower activation energy, 60.3 kJ/ mol, the fatigue strength of the specimen having only irreversible hydrogen decreased compared with non-charged specimens. The fracture surface of the sample with irreversible hydrogen was brittle compared with that of non-charged sample. This indicates that the influence of irreversible hydrogen on the fatigue strength tends to become small with increasing activation energy for irreversible hydrogen desorption.