Application of acoustic emission method for investigation of hydrogen embrittlement mechanism in the low-carbon steel

•The drop of ductility of the low-carbon steel is caused by action of mobile hydrogen.•The nucleation and growth of the “fisheyes” defects occurs during the necking stage.•No acoustic emission has been detected in relation to the formation of “fisheyes”.•Formation of “fisheyes” cannot be explained by a decohesion mechanism. The hydrogen embrittlement (HE) phenomenon in the electrolytically charged mild low-carbon steel was investigated during tensile testing coupled with in situ acoustic emission (AE) measurements and post mortem fractographic analysis. It was demonstrated that the ductility reduction correlated with the formation of HE-induced quasi-cleavage regions known as “fisheyes” which formed during the necking stage under the influence of mobile hydrogen. Based on the AE analysis we conclude that formation of “fisheyes” in the mild steel under load cannot be explained by a decohesion mechanism and the alternative mechanisms are discussed.