Grain refinement's effect on hydrogen embrittlement of 304 austenitic stainless steel: A comparative investigation of hydrogen in-situ charging vs. pre-charging.

Controversy surrounds the potential of grain refinement in suppressing metal hydrogen embrittlement (HE). The literature presents two methods for introducing hydrogen into metals: hydrogen pre-charging (HPC) and hydrogen in-situ charging (HISC). This study compares the effect of grain refinement on HE under electrochemical HPC and HISC conditions, focusing on type 304 austenitic stainless steel with a grain size range of 0.5–35 μm. HE behaviors were evaluated through tensile testing and fracture morphology analyses, while hydrogen diffusion and concentration were examined using hydrogen uptake amount measurements and hydrogen permeation tests, complemented by finite element (FE) simulations. Results indicate that coupling with plastic deformation significantly accelerated hydrogen transport in the HISC condition, leading to higher apparent hydrogen diffusivity compared to the HPC condition, with grain boundaries (GBs) act as hydrogen traps. Under the HISC, grain refinement reduced HE susceptibility by decreasing the apparent diffusivity due to an increased trapping effect of GBs and suppressing intergranular fracture by reducing the hydrogen amount distributed to each GB (hydrogen occupancy of the GBs). However, grain refinement to below 1 μm increased HE under the HPC condition. Hydrogen permeation tests showed that under the HPC, grain refinement slightly decreased hydrogen diffusivity. In the ultrafine-grained specimen, as GBs trapped more hydrogen atoms, they not only provided hydrogen to stress-concentrated lattices and newly-formed dislocations by detrapping but also maintained a higher hydrogen occupancy of the GBs, thus increasing HE. It is concluded that whether grain refinement suppresses HE depends primarily on the resulting hydrogen occupancy of the GBs, which is influenced by the charging conditions rather than the quantity of traps. • A comparison of effect of grain size on HE in 304 steel is conducted under in-situ H charging and pre-charging. • Coupling with plastic deformation accelerates H transport in in-situ charging condition. • Grain refinement reduces HE in in-situ charging, but it slightly decreases and then increases HE in pre-charging. • The former is attributed to decreased H diffusivity and reduced H occupancy of grain boundaries (GBs). • The latter is due to that H-filled GBs act as H reservoirs, providing H to stress concentrations and dislocations. [ABSTRACT FROM AUTHOR]