Evaluation of Strength and Fracture Toughness of Ferritic High Strength Steels Under Hydrogen Environments

The susceptibility of high strength ferritic steels to hydrogen-assisted fracture in hydrogen gas is usually evaluated by mechanical testing in high-pressure hydrogen gas or testing in air after pre-charging the specimens with hydrogen. We have used this second methodology, conventionally known as internal hydrogen. Samples were pre-charged in an autoclave under 195 bar of pure hydrogen at 450ºC for 21 hours.
Different chromium-molybdenum steels submitted to diverse quenching and tempering heat treatments were employed. Diverse specimens were also used: small cylindrical samples to measure hydrogen contents and the kinetics of hydrogen egression at room temperature, tensile specimens, notched tensile specimens with a sharp notch, and also compact fracture toughness specimens. Fractographic examination in SEM was finally performed in order to know the way hydrogen modify fracture micromechanisms.
The presence of hydrogen barely affects the conventional tensile properties of the steels, but it clearly alters their notched tensile strength and fracture toughness. This is due to the strong effect that stress triaxiality (dependent also on the steel yield strength) has on the accumulation of hydrogen on the notch/crack front region, being the displacement rate used in the test another important variable to be controlled, due to its influence on hydrogen diffusion to the embrittled process zone. Moreover, the modification of fracture micromechanisms was finally determined, being ductile (initiation, growth and coalescence of microvoids) in the absence of hydrogen and brittle and intergranular under the material conditions of maximum embrittlement.