Effect of heat treatment on the hydrogen enhanced fatigue crack propagation of low carbon steel S25C

The effect of heat treatment of S25C on the fatigue crack propagation rate enhanced by hydrogen was investigated under long term varying loading. The Vickers hardness was changed by heat treatment between 155 and 255. Stress hold at maximum stress was inserted in the 0.1Hz sinusoidal stress wave. The acceleration of crack propagation rate occurred irrespective of hold time in every material. However, the mean value of crack propagation rate decreased when hold time increased. The instantaneous crack propagation rate in time domain was dependent on the passage of time after the start of stress hold. The crack propagation rate was high at the beginning for several tens of second and it was reduced down to less than one hundredth after ten minutes. The time-dependent crack propagation was not maintained continuously in all materials whose Vickers hardness was between 155 and 255. As a consequence, the possibility of damage suffered by time-dependent crack growth is negligibly small even in the hardest material that can be reached by heat treatment on S25C.