Investigation of Hydrogen Embrittlement Susceptibility and Fracture Toughness Drop after in situ Hydrogen Cathodic Charging for an X65 Pipeline Steel

Abstract

The present research focuses on the investigation of an in situ hydrogen charging effect during Crack Tip Opening Displacement testing (CTOD) on the fracture toughness properties of X65 pipeline steel. This grade of steel belongs to the broader category of High Strength Low Alloy Steels (HSLA), and its microstructure consists of equiaxed ferritic and bainitic grains with a low volume fraction of degenerated pearlite islands. The studied X65 steel specimens were extracted from pipes with 19.15 mm wall thickness. The fracture toughness parameters were determined after imposing the fatigue pre-cracked specimens on air, on a specific electrolytic cell under a slow strain rate bending loading (according to ASTM G147-98, BS7448, and ISO12135 standards). Concerning the results of this study, in the first phase the hydrogen cations’ penetration depth, the diffusion coefficient of molecular and atomic hydrogen, and the surficial density of blisters were determined. Next, the characteristic parameters related to fracture toughness (such as J, KQ, CTODel, CTODpl) were calculated by the aid of the Force-Crack Mouth Open Displacement curves and the relevant analytical equations.

The effect of hydrogen on the fracture toughness of friction‐stir welded API 5L X70 pipeline steels

“…Therefore, the hydrogen changed the fracture type from ductile to quasi‐cleavage. These results are in accordance with the results reported by other researchers 35,42,77,79,95,98 …”

“…The micro‐voids with fisheye morphology and stepwise micro‐cracking are characteristic of Hydrogen Embrittlement (HE) features. According to the internal pressure theory (IP), they are mainly attributed to hydrogen‐induced cracking (HIC) mechanisms 35 . The fracture surfaces with the quasi cleavage facets indicate the hydrogen enhanced decohesion mechanism (HEDE), 20,26,35 while the ductile fracture is contributed by the hydrogen‐enhanced localized plasticity mechanism (HELP) 21,26 .…”

“…According to the internal pressure theory (IP), they are mainly attributed to hydrogen‐induced cracking (HIC) mechanisms 35 . The fracture surfaces with the quasi cleavage facets indicate the hydrogen enhanced decohesion mechanism (HEDE), 20,26,35 while the ductile fracture is contributed by the hydrogen‐enhanced localized plasticity mechanism (HELP) 21,26 . The HEDE mechanism in fracture promotes brittle cracking due to the hydrogen aggregated at various interfaces associated with the cohesive energy reduction 11,98 .…”

“…The influence of hydrogen embrittlement on fracture toughness and fatigue life is critical for safety assessments that have been investigated in numerous studies 14–16,25,31–34 . Many reports show the hydrogen embrittlement and fracture toughness decrease in pipeline steels 15,25,31,33,35 . The effects of hydrogen on the fracture toughness of X80 pipeline steel have been studied in hydrogen pre‐charging and dynamic hydrogen charging 25 .…”

Hydrogen Permeation in X65 Steel under Cyclic Loading

“…Hydrogen assisted cracking has been extensively studied during the last 50 years, but the mechanism is not yet completely known. Several studies formulated a certain number of theories that are all supported by extensive datasets [17][18][19]. One of the most accepted theories is that of pressure, for which the cracking can be ascribed to the recombination of atomic hydrogen into defects.…”