Determination of hydrogen diffusibility and embrittlement susceptibility of high-strength steel evaluated at different temperatures based on the local equilibrium theory

The effects of diffusible or non-diffusible hydrogen on hydrogen trapping states and hydrogen embrittlement susceptibility of cold-drawn pearlitic steel have been quantitatively evaluated at temperatures of 30 °C and 240 °C based on Oriani's local equilibrium theory. Hydrogen absorbed in cold-drawn pearlitic steel had two trapping states; Peak 1 H desorbed below 200 °C and Peak 2 H desorbed above 200 °C as determined by thermal desorption analysis. Peak 1 H with a binding energy between hydrogen and a trapping site ( E b ) of 56 kJ·mol −1 was diffusible and harmful to hydrogen embrittlement at 30 °C. That was because the E b of Peak 1 H was lower than that of Oriani's local equilibrium condition of 69 kJ·mol −1 at 30 °C. In contrast, Peak 2 H with an E b of 76 kJ·mol −1 was non-diffusible and innocuous. That was because the E b of Peak 2 H was higher than 69 kJ·mol −1 at 30 °C. However, Peak 2 H was diffusible and harmful to hydrogen embrittlement at 240 °C. That was because the E b of Peak 2 H was lower than that of the local equilibrium condition of 117 kJ·mol −1 at 240 °C. These findings indicate that hydrogen diffusibility and embrittlement at a certain temperature can be determined by the local equilibrium condition. Hydrogen embrittlement at 240 °C was presumably caused by solid-solution hydrogen because hydrogen was not trapped at dislocations and grain boundaries, and vacancy-type defects formed by plastic strain were annihilated at 240 °C.