Hydrogen enhanced fatigue crack propagation of bainitic and tempered martensitic steels

The hydrogen-enhanced fatigue crack propagation (FCP) of bainitic and tempered martensitic structures in AISI 4130 steel has been evaluated. Three bainitic structures involved in the study were obtained by isothermal transformation at 330°C, 370°C and 470°C. By selecting appropriate tempering temperatures, three tempered martensitic specimens with yield strengths corresponding to those of the bainitic structures, were prepared. For the bainitic specimens, their hydrogen-assisted FCP rates and fracture mode were found to be similar despite the fact that they have entirely different tensile strengths and hydrogen permeation behavior. The hydrogen-assisted FCP rate for the tempered martensites increased significantly with decreasing tempering temperature. By comparing FCP rates in bainitic and tempered martensitic structures for AISI 4130 steel, it is clear that the effect of microstructure was the dominant factor in this steel-environment system.