Hydrogen effect on notch toughness of an X70 pipe steel with different loading rates and notch orientations: Experiment and Modelling

In this work, the hydrogen effects on the notch toughness properties of an X70 pipe steel were investigated using ex-situ Charpy test at impact rate (5.1 m/s) and in-situ three-point bend test at quasi-static rates (2 × 10−8−2 × 10 − 6 m/s ). Hydrogen was introduced into the specimens using an electrolytic charging method and the total hydrogen content was estimated using LECO analysis. Finite element (FE) simulations were performed to deduce the actual hydrogen content under the hydrogen charging and toughness testing conditions, considering the hydrogen egress during the specimen transfer processes. Charpy specimens with two different notch orientations, i.e., through-thickness notch (TTN) and surface-notch (SN) were tested. The load–deflection curves showed that the slow loading rate resulted in significantly reduced maximum loads and failure deflections. The ratios of absorbed energy for hydrogen-charged specimen to that for uncharged specimen were used as an index of hydrogen embrittlement (HE) susceptibility. The lower the ratio, the more severe the hydrogen effects. At the impact rate, the hydrogen effect on Charpy absorbed energy (CVN) was small (i.e., 24 ∼ 27% reduction). Under the slow-rate three-point bend test conditions, a much more severe hydrogen effect was observed (i.e., 68 ∼ 95% reduction in the absorbed energy). The TTN specimens showed lower fracture energies and higher HE susceptibility than the SN specimens. A stress-coupled diffusion model was utilized to analyse the hydrogen distribution and hydrostatic stress–strain fields ahead of the notch tip in the Charpy specimens, providing mechanistic interpretation of the fracture process and HE behaviour of the steel.