Hydrogen-assisted fatigue crack propagation in a pure BCC iron. Part I: Intergranular crack propagation at relatively low stress intensities

¹ Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
² Research Fellow of the Japan Society for the Promotion of Science, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
³ Department of Physics, Centre for Materials Science and Nanotechnology, University of Oslo, PO Box 1048 - Blindern, NO-0316 Oslo, Norway
⁴ Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
⁵ Research Center for Hydrogen Industrial Use and Storage (HYDROGENIUS), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
⁶ International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University,744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
⁷ AIST-Kyushu University Hydrogen Materials Laboratory (HydroMate),744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
⁸ Department of Mechanical Engineering, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan
⁹ SINTEF Materials and Chemistry, PO Box 124 - Blindern, NO-0314 Oslo, Norway

^* Corresponding author: yuhei.ogawa.bz@gmail.com

Abstract

The role of hydrogen on intergranular (IG) fracture in hydrogen-assisted fatigue crack growth (HAFCG) of a pure iron at low stress intensity was discussed in terms of the microscopic deformation structures near crack propagation paths. The main cause of IG fracture was assumed to be the hydrogen-enhanced dislocation structure evolution and subsequent microvoids formation along the grain boundaries. Additionally, the impact of such IG cracking on the macroscopic FCG rate was evaluated according to the dependency of IG fracture propensity on the hydrogen gas pressure. It was first demonstrated that the increased hydrogen pressure results in the larger area fraction of IG and corresponding faster FCG rate. Moreover, gaseous hydrogen environment also had a positive influence on the FCG rate due to the absence of oxygen and water vapor. The macroscopic crack propagation rate was controlled by the competition process of said positive and negative effects.