Experimental Study on Hydrogen Embrittlement Behavior of X80 and X70 Pipeline Steels Evaluated by Hydrogen Permeation and Slow Strain Rate Tensile Tests

The effects of hydrogen diffusion parameters, influenced by microstructural features such as non-metallic inclusion content and ferrite grain size, on the hydrogen embrittlement behavior of X80 and X70 pipeline steels were evaluated by slow strain rate tensile tests. The microstructural characterization was done by field emission scanning electron microscopy and optical microscopy, while the hydrogen diffusion parameters and trapped hydrogen content were measured by hydrogen permeation test and thermal desorption spectroscopy, respectively. Embrittlement was also evaluated by loss of ductility and plastic strain energy from slow strain rate tensile tests. The results show that X80 steel is more resistant to hydrogen embrittlement than the X70 steels due to higher hydrogen diffusivity and lower hydrogen solubility, leading to less hydrogen trapping. The higher resistance to hydrogen embrittlement of X80 steel was related to a lower content of non-metallic inclusions. Non-metallic inclusions act as preferential sites for ductile microvoid nucleation in hydrogen-charged steels that facilitate fracture mechanism reducing the ductility and the plastic strain toughness and increasing the hydrogen embrittlement susceptibility. The ferrite grain size is related to the hydrogen diffusivity in an inverse proportion, where small grain sizes facilitate hydrogen accumulation and may cause localized plastic deformation, leading to reduced ductility and increasing the susceptibility to hydrogen embrittlement.