Influence of Irreversible Hydrogen on the Fatigue Strength in Cold Drawn High Strength Steel

Abstract

Slow strain rate tensile (SSRT) tests and fatigue tests were conducted to investigate the influence of irreversible hydrogen on the quasi static SSRT strength and fatigue strength for cold drawn eutectoid steels which were cathodically hydrogen charged. Internal hydrogen states were changed as follows: (a) virgin sample, (b) the sample that contained both diffusible and irreversible hydrogen, and (c) the one that contained only irreversible hydrogen. The SSRT strength properties of only irreversible hydrogen contained sample were the same as that of virgin sample, whereas those of the sample having both diffusible and irreversible hydrogen were smaller than the others. This indicates that only the diffusible hydrogen caused hydrogen embrittlement under the quasi-static loading. Contrary to this, the fatigue strength of the sample having both diffusible and irreversible hydrogen was lower than that of virgin sample. However, the influence of irreversible hydrogen on the fatigue strength was unclear since S-N curves had a large scatter. This is due to the fact that the fatigue crack was initiated at internal inclusion. Therefore, the fatigue strength was discussed based on the stress intensity factor, ∆K inc , calculated from stress and inclusion size. ∆K inc giving the same N f decreased in the order of virgin sample, irreversible hydrogen charged sample and the sample having diffusible and irreversible hydrogen, indicating irreversible hydrogen caused the fatigue strength degradation.

Fatigue crack growth acceleration caused by irreversible hydrogen desorption in high-strength steel and its mechanical condition

“…Although irreversible hydrogen does not influence the hydrogen embrittlement under static or quasi-static loading, the fatigue strength is decreased by irreversible hydrogen [13]. Moreover, da/dN under the irreversible hydrogen charged condition was determined by K eff , as shown earlier.…”

“…The fracture surface shows a sharp edge and quasi-cleavage cracking whose morphology is dependent on the pearlitic colony. This fracture morphology is a characteristic of the hydrogen embrittlement fracture in pearlitic steel [13,16,29]. In the higher K eff region, however, a ductile fracture surface was observed (see Fig.…”

“…Irreversible hydrogen decreases the fatigue strength of a smooth sample [13,16]. Moreover, irreversible hydrogen desorption is related to the intensity of the stress field, as noted above.…”

“…The former can rapidly diffuse to a stress concentration area at room temperature and causes hydrogen embrittlement [1,[9][10][11]. On the other hand, irreversible hydrogen cannot diffuse at room temperature and does not influence the mechanical properties of steel because it is strongly trapped at the trap site [11][12][13]. Thus, it is expected that the generation of irreversible trap sites using microstructural control would be an effective way to improve the resistance to hydrogen embrittlement [14,15].…”

“…However, we have already clarified that irreversible hydrogen decreases the fatigue strength in cold-drawn high-strength steels [13,16]. This implies that the generation of irreversible trap sites would not be a perfect solution for hydrogen embrittlement.…”