Effect of strain rate on acoustic emission during hydrogen assisted cracking in high carbon steel

Hydrogen charging of a high carbon spring steel exerts a profound effect on the ductility and damage accumulation during three-point-bending testing. Since the acoustic emission (AE) technique reflects perhaps in the best way microfracturing dynamics, it is found particularly useful for real-time capturing of hydrogen assisted cracking and overall in situ tracing of damage evolution during deformation. In the present work the propensity for hydrogen assisted cracking, its mechanisms and strain rate dependence are investigated using integral characterization of AE paired with microstructure investigations. Analysis of experimental data suggests that two competitive mechanisms operate during three-point bending deformation of the high carbon steel: (i) highly localized plastic deformation and (ii) brittle intercrystalline and transcrystalline cracking promoted by hydrogen influence. The synergistic interplay between these two mechanisms is discussed.