The detrimental effects of a few parts per million of hydrogen in steel at temperatures below about 400°F have long been recognized. Common experience of such effects include reduction of ductility and reduction of fracture strength. More recently there has been added some realization of accelerated subcritical crack propagation when steel is exposed to hydrogen environments. All these effects are important to the application of pressure vessels for high-pressure hydrogen, and for vessels exposed to corrosive media capable of charging steels with hydrogen. From the materials properties standpoint, damage is much more apt to occur as the hardness or the strength level increases. Thus, the problem of damage by hydrogen is particularly important in the application of higher strength steels, which have the most favorable strength-to-weight ratio for pressure vessels.

Although the detrimental effects of hydrogen are generally recognized, the significance of many of the variables has not been quantitatively established in terms that allow direct application to pressure vessel design or use. Only in a general way are the effect of notches or other stress raisers known. The primary objectives of research programs should be to furnish quantitative information on limiting values for mechanical properties and environmental factors so as to insure against low stress failures in environments capable of supplying hydrogen to the vessel steel.

The interpretation of the literature and experiences examined in the report suggests that the predominant mode of failure in the higher strength steels is a relatively brittle, quasi-cleavage fracture which results from propagation of microcracks, and that this mode may extend into the realm of the lower strength steels also. Research should be aimed toward establishing limiting conditions with respect to strength levels, hydrogen content or effective hydrogen pressure, and temperature for the change over from this quasi-cleavage fracture behavior to one of ductile rupture.