Effect of Hydrogen on the Apparent Yield Stress—Research on the Cause of Hydrogen Induced Delayed Plasticity

The effect of hydrogen on the apparent yield stress, which is a necessary exterior stress to cause the local macroscopic plastic deformation, was investigated for various carbon and low alloy steels with a wide range of tensile strength. Smooth tensile, bent beam, and precracked WOL specimens were used. The results show that the effect of hydrogen on the yield strength of a smooth tensile specimen is insignificant. However, for the smooth bent beam or precracked specimens with a stress gradient, hydrogen decreases the apparent yield stress considerably if the strength of steel and the amount of hydrogen entering into the specimen exceed critical values. This is the cause of hydrogen induced delayed plastic deformation and then hydrogen induced delayed cracking. The apparent yield stress of the charged specimen decreases with increasing tensile strength and the amount of hydrogen in the steel. It depends evidently on the strain rate and test temperature and is reversible, but is not related to the prestrain condition. The variation of the apparent yield stress of the charged specimens with the test temperature is not monotonous, there is a minimum near room temperature. According to these results, a possible mechanism of hydrogen induced apparent yield stress reduction has been suggested.