NOTCH SEVERITY EFFECT ON HYDROGEN EMBRITTLEMENT OF TYPE 4340-STEEL

Hydrogen embrittlement of Type 4340 steel has been investigated by str aining round-notched specimens in 10(5) Pa hydrogen atmosphere at a co nstent cross-head spead of 1.4x10(-4) mm/s. The circumferentially notc hed specimens exhibited a significant embrittlement when their mechani cal behaviour in hydrogen was compared with that in air. Although the effect of notch depth on fracture strength in air is negligible, an in crease in the depth of notch increase susceptibility to embrittlement when testing in gaseous hydrogen. However, analysis of the effects is complicated by the facts that (i) the specimens show some degree of no tch severity even when strained in air and (ii) the behaviour is compl icated by the localised plastic deformation that may occur for relativ ely shallow notches. Such effects are eliminated at high stress concen tration factors, so there is a systematic loss in fracture stress in h ydrogen as the notch sensitivity increases from K-t=2.6 to 5.7 (where a 87% reduction of fracture stress occurs) but a relatively stable val ue is then reached even for very severe notching by fatigue pre-cracki ng. Whether or not the effect is due to increasing concentration of hy drogen in the triaxial stress region ahead of the notch, there is no d oubt that increasing the stress concentration makes hydrogen more effe ctive as an embrittlement agent.