The effect of stress concentration on hydrogen embrittlement of a low alloy steel

The hydrogen embrittlement of a quenched and tempered low alloy steel has been investigated by straining notched specimens in 1 bar hydrogen atmosphere at 1.3 x 10(-4) mm/s. The circumferentially notched specimens exhibited a significant embrittlement when their mechanical behaviour in hydrogen was compared with that in air. Although the effect of notch depth on fracture strength in air is negligible, an increase in the depth of notch increases susceptibility to embrittlement when testing in gaseous hydrogen. Analysis of the effects is complicated by the facts that: (i) the specimens show some degree of notch sensitivity even when strained in air, and (ii) localised plastic deformation may occur with relatively shallow notches. Such effects are eliminated at high stress concentration factors, where there is a systematic loss in fracture stress in hydrogen as the notch severity increases from K-t = 2.6 to 5.7 (where a 87% reduction of fracture stress occurs), but a relatively stable value is then reached even for very severe notching by fatigue pre-cracking. Whether or not the effect is due to increasing concentration of hydrogen in the triaxial stress region ahead of the notch, there is no doubt that increasing the stress concentration makes hydrogen more effective as an embrittling agent. On the other hand, it is also clear that severe embrittlement can be introduced with relatively low nominal stress concentration factors by increasing the depth of relatively blunt notches, and this is believed to be due to the greater volume of material then affected.