Effects of high pressure hydrogen environment on the mechanical properties, especially on the Low Cycle Fatigue behavior at room temperature in quenched and tempered SCM 4 steel have been investigated. The decrease in peak stress with the number of cycle under hydrogen environment compared to that tested in air was not remarkable. The softening and the strength-differential effects were also almost the same between air and hydrogen environment. However the fatigue life in hydrogen was reduced significantly. Also fatigue ductility coefficient, $_εf$, and the reversal transition cycle, 2Nc.t, were decreased from 68% and 2500 cycles in air to 28% and 1700 cycles in $PH_2$=7 Mpa, respectively. This means the strain amplitude is very sensitive to environment. As the hydrogen pressure, $PH_2$, increases, the critical fatigue life, Nc, is reduced to a certain limit value. It shows that there is no direct relationship between Nc and $\sqrt{PH_2}$, although the meaning of Nc is obscure.
The modes of crack-propagation and failure were investigated. In air, fatigue crack propagated according to "the Laird's Model", which showed the plastic blunting at the crack-tip. The specimen failed, at first, perpendicular to the stress axis and finally to the maximum shear stress plane. Whereas under hydrogen environment, crack propagated as the quasi-cleavage type in which micro-void coalescence, tongue formation and secondary cleavage had been shown. And it failed perpendicular to the stress axis.
The results of this study may be useful for the prediction of the fatigue life under hydrogen environment and may indicate that classical hydrogen embrittlement results from a combination of two effect, the decohesion theory and the planor pressure theory.
