Sub critical crack growth in hydrogen assisted cracking of cold drawn eutectoid steel

The fracture behaviour of eutectoid cold drawn steel wires under constant load in hydrogen charged condition was evaluated. Hydrogen charging was obtained by dipping steel wires in ammonium thiocyanate solution. Sub-critical crack growth was monitored by means of Acoustic emission (AE) technique. Fractographic analysis revealed a mixed mode crack propagation (mode I and mode II) characterized by a multi-terrace appearance of the surface fracture. A modification of the macroscopic mechanical behaviour of the steel was also evidenced by micro hardness measurements. A simplified stress-assisted hydrogen diffusion model was used to interpret experimental observations and to estimate a theoretical crack propagation rate. Such a value was in accordance with that obtained from the analysis of AE data.