Evaluation of mechanical properties of an experimental microalloyed steel subjected to tempering heat treatment and its effect on hydrogen embrittlement

The aim of this study is to evaluate the hydrogen effect on the mechanical properties of experimental microalloyed steel subjected to post-tempering treatments at different temperatures (200-600 °C) and residence times (1-10 min). The results obtained that at higher hydrogen concentrations in the un-tempered steel condition their strength showed from 700 to 900 MPa and the ductility was from 9 to 11% elongation, which represents the highest embrittlement index (%EI), this results clearly presents that the un-tempered conditions are more sensitive to hydrogen embrittlement (HE). The tempered steels showed remarkable variations on the same properties ranging from 650 to 850 MPa, possibly due to the presence of microalloying elements, co-precipitation and dislocation movements. Permeability tests were carried out to evaluate the microalloyed steel hydrogen trapping and diffusion rate under the same post-tempering conditions. The diffusion coefficient (Deff) was higher for the steel in its initial condition showing values of 2.8 × 10−6 cm2/s; and the lowest Deff values was presented at 200 °C-1 min, its value was 1 × 10−6 cm2/s. The apparent hydrogen concentration (Capp) was higher at the tempering condition of 200 °C-3 min, presenting values of 1.95 ppm H, and the lowest Capp value was obtained at 400 °C-10 min (0.76 ppm H). The steel samples with lower Capp showed a lower effect in their ultimate tensile strength (UTS) and yield strength (YS) values, this means that the hydrogen traps are less massive.