Hydrogen Diffusion and Trapping in Low-Alloy Tempered Martensitic Steels

Structural characterization of ten low-alloy tempered martensitic steels of varied composition (C, Cr, Mo, Mn, and V contents) and tempering temperature was performed to question the impact of microstructural features on hydrogen state. Thermal desorption spectroscopy and electrochemical permeation data for each alloy were acquired and interpreted in view of hydrogen diffusion/trapping models. This large database provided precise information regarding solubility, diffusion coefficient, activation energies for diffusion and trapping, hydrogen distribution into lattice, and reversible and irreversible trap sites. The results reveal a tendency for the apparent diffusion coefficient to decrease with increasing yield strength, mainly related to the density of trap sites rather than lattice diffusion. Estimates of trapping at dislocation core could explain the irreversible trapping in the six steels with sub-surface hydrogen concentration smaller than 1.5 wppm. For the four steels with higher solubility, it was calculated the superabundant vacancies concentration necessary to justify the amount of trapping sites. The steel with the highest Mo and V contents presented superior solubility of trapped hydrogen which was related to its precipitation of few nanometers in size. It was considered irreversible trapping at carbon vacancies as well as reversible trapping at elastic strain fields around the detected MC carbides.