New insights into the improvement of hydrogen embrittlement resistance of heat-treated carbon steels by shot peening

The effectiveness of shot peening in suppressing hydrogen embrittlement (HE) of the heat-treated steels with different strength levels, 790 MPa (115 ksi) and 930 MPa (135 ksi), was comprehensively investigated. A plastically deformed layer on the surface facilitated an increased number of dislocations and refined grain morphology. This hindered hydrogen transportation, as confirmed by the results of electrochemical permeation, exhibiting a decrease in the effective diffusion coefficient up to 47 %. The trapping behaviour of the steels scrutinized through Thermal Desorption Spectroscopy (TDS) proposed that dislocations are primary traps. Along with this, residual compressive stresses (RCS) were introduced into the materials, reaching a maximum of −650 MPa and a depth of 250 μm. This prevented fracture of the steels under constant load in a plastic regime (1.05xYS) and 120 bar H ₂ environment. Slow Strain Rate Tensile (SSRT) tests indicated superior mechanical properties of the shot-peened steels under electrochemical charging, reducing HE susceptibility by 15 %. Fracture morphology confirmed the protective nature of the plastically deformed layer, highlighting a higher ductility of the fracture. RCS has been indicated as a determining factor in suppressing HE by shot peening, regardless of the strength level of the steel.