Coupling effect of hydrogen and strain rate on 2.25Cr1Mo0.25V steel deformed over wide strain rate ranges

To determine the cross-scale mechanism of hydrogen embrittlement, the coupling effect of hydrogen and strain rate of 2.25Cr1Mo0.25V steel plastic behavior was investigated by quasi-static and dynamic loading. It was found that the microscopic immobile dislocation density is related to the macroscopic hardening-to-softening transition (HST) during plastic deformation; therefore, the HST strain was proposed. The results indicated that the HST strain increases with increasing strain rate and hydrogen content, which implies that the dynamic recovery controlling mechanism leads to strain softening, whereas the HST strain decreases rapidly at high strain rates, which was attributed to shear localization. The HST strain was also used to determine a constant structure for each material, at which the strain rate sensitivity (SRS), activation volume, and mobile dislocation density were investigated under the theory of thermal activation. The results demonstrated that the strain rate exhibited a positive effect on the SRS, whereas hydrogen had a negative effect. The activation volume of the hydrogen-charged material was significantly higher than that of the hydrogen-free material at a low strain rate and decreased with an increase of the strain rate. The variation in the activation volume was not apparent for hydrogen-free materials or materials under high strain rates. Hydrogen determines the level of mobile dislocation density at the microscale, which controls whether hydrogen embrittlement occurs at the macroscale. 0.1 s-1 appears to be the transition strain rate for hydrogen embrittlement, which is prominent when the strain rate is less than 0.1 s-1, owing to the low level of mobile dislocations, and on the contrary, hydrogen embrittlement is slight when the strain rate is larger than 0.1 s-1.(c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.