STEEL FASTENERS FAILURE BY HYDROGEN EMBRITTLEMENT

ABSTRACT: his paper details the analysis carried out on two failed bolts to find out the reason for failure, using macrofractography, optical metallography and scanning electron microscope (SEM) te t chniques. Usual measures to preven such failures are also referred. Similar fractures occurred at th r e head to shank transition of the bolts during fastening, o afterwards, while waiting for delivery. Since failure occurred in a region where foreseen stresses were quite below yield strength and there was no evidence of mechanical deformation, embrittlement was suspected. SEM observation of the fracture surface crack initiation region showed areas of inte r rgranular crack propagation indicating brittleness, and highe magnification observation showed hydrogen embrittlement features f. The bolts, which microstructure was typically that o hardened and tempered steel, had been zinc electroplated and it is well known that hydrogen release concurs with zinc deposition. Moreover, the diffusion and accumulation of the hydrogen in metals is fav oured by cold working, as is the case of the head to shank transition region