Roles of surface finish and hydrogen availability on the tensile properties of 1018 steel in hydrogen environments

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Virginia Polytechnic Institute and State University

Uniaxial tensile tests were performed on AISI 1018 steel specimens. Tests were conducted both in air and while the specimens were undergoing electrolytic hydrogen charging. Specimens with three different surface finishes were tested at various cathodic charging current densities. The tensile deformation was initiated only after the specimen was charged to at least 95% with hydrogen. The data presented in this study show that very low cathodic charging current densities result in larger ductility losses for specimens of all three surface finishes relative to specimens tested in air. Once this lower limit of cathodic-charging current density is reached the extent to which hydrogen induced damage is developed is primarily controlled by surface finish with cathodic charging current density playing a secondary role. Specifically, cathodic charging with hydrogen at any particular current density resulted in increased hydrogen induced damage with increased surface roughness. In addition, increased cathodic charging current density resulted in increased hydrogen induced damage within any set of specimens with a given surface roughness. Hydrogen induced damage caused the reduction in area at fracture to decrease and the extent of this damage increased as the surface roughness and the cathodic charging current density increased. The strain-to-initiate cracking decreased with increased surface roughness. Elongation to fracture was shown to be a poor measure of hydrogen induced damage and crack nucleation was shown to be easier than crack propagation. Charging of specimens to at least 95% saturation at a cathodic charging current density of 375 mA/cm² did not alter the primary fracture mode. Microvoid coalescence remained dominant although there was some evidence of quasicleavage in surface cracks. These data do not exclusively support any of the currently proposed mechanisms for hydrogen embrittlement.