The Nature and Features of the Fracture Surface in the Hydrogen-Embrittled Low Alloy Steel with Ultrafine-Grained Microstructure

It has been shown recently that hydrogen embrittlement in severely deformed low alloy steel results in the formation of specific tearing morphology on the fracture surface. The origin of such a fracture surface has not yet been fully understood. This paper reports the results of fractographic examination combined with electron backscatter diffraction study of the microstructure underlying the fracture surface in ultrafine-grained low alloy steel 09G2S produced by equal channel angular pressing. Tensile tests are carried out on hydrogen-free and cathodically hydrogen-charged specimens. It is found that all hydrogen-charged specimens exhibit tearing fracture morphology and numerous secondary cracks. The metallographic images obtained by scanning electron microscopy and electron backscatter diffraction clearly show that both hydrogen-induced cracking and hydrogen-assisted cracking in the ultrafine-grained steel occur primarily along hydrogen-embrittled high- and low-angle ultrafine grain boundaries and dislocation cell walls, resulting in the formation of tearing morphology on the fracture surface. The mechanism governing the formation of specific morphological elements on the fracture surface is proposed and discussed.