Effect of hydrogen absorption on strain-induced low-cycle fatigue of low carbon steel

The effects of hydrogen have been studied on low carbon steel (JIS S10C), which contains limited amount of pearlite. Fatigue life and its strain rate dependence have been examined. The possible mechanism of hydrogen effect on those properties has been discussed. The hydrogen absorbed was 0.5–1.5 mass-ppm, according to the thermal desorption analysis (TDA). Main part of the hydrogen is dissolved in the matrix; the rest part is trapped by pearlite. This small hydrogen severely reduces the fatigue life, even in S10C steel that is considered to be less susceptible to hydrogen embrittlement. The more the hydrogen is absorbed, the less the fatigue life is. Hydrogen absorption also causes the increase in stress amplitude. Fractographic examination revealed the fracture process as the followings. The hydrogen gasifies at the interface between the non-metallic inclusion and matrix. Then the high pressure hydrogen gas atmosphere maintains hydrogen content in the matrix around inclusion. This assists the crack initiation at the interface of inclusion, and also assists the crack propagation through producing the fish-eye fracture. According to the published works on non-hydrogen charged specimens, vacancy plays an important role in both crack initiation and propagation, and determines the fatigue life. The stress amplitude is believed also influenced by the vacancy that is produced by the movement of jogged dislocation. When these vacancies are stabilized by hydrogen, fatigue life will be decreased with the increased stress amplitude. Fatigue life is decreased with slower strain rate test. Stress amplitude shows the negative dependence to the strain rate. These are claimed as the features of dynamic strain aging (DSA). Considering the above mentioned knowledge on vacancy stabilized by hydrogen, the DSA is brought about by the interaction between dislocation and vacancy-hydrogen pair.