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

Helen Kyriakopoulou,Athanasios Tazedakis,Dimitrios Manolakos,Panagiotis Karmiris-Obratański,Dimitrios Pantelis,Nikoalos Daniolos,Efthymios Dourdounis

doi:10.3390/mi11040430

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.

Highlights

Over the last 15 years, the main targeting of the petroleum and hydrocarbon industry has been to encounter the phenomenon of hydrogen embrittlement and hydrogen induced toughness drop of the oil carrying pipeline steels during their operational cycle [1–5] The presence of molecular H2S in the internal environment of pipeline steels, attributed to the oil decreased quality, results in the diffusion and entrapment of atomic hydrogen into metal structure, which leads to the reinforcement of Hydrogen Induced Cracking (HIC) phenomenon, blistering effect and development of brittle hydride phases (FeH3) [5–9].The surface micro-cracking networks have a characteristic stepwise morphology, whereas blisters depending on their growth conditions may occur locally with a dome morphology or in an extended area with elongated and continuous morphology [9–12]
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
Three micro mechanisms were proposed in the scientific field of physical metallurgy to justify the reduction of mechanical properties on pipeline steels due to the effect of atomic or molecular hydrogen, reported as Internal Pressure theory (IP), Hydrogen Enhanced Local Plasticity theory (HELP) and Hydrogen Induced Decohesion theory (HEDE)

Summary

Introduction

Over the last 15 years, the main targeting of the petroleum and hydrocarbon industry has been to encounter the phenomenon of hydrogen embrittlement and hydrogen induced toughness drop of the oil carrying pipeline steels during their operational cycle [1–5] The presence of molecular H2S in the internal environment of pipeline steels, attributed to the oil decreased quality, results in the diffusion and entrapment of atomic hydrogen into metal structure, which leads to the reinforcement of Hydrogen Induced Cracking (HIC) phenomenon, blistering effect and development of brittle hydride phases (FeH3) [5–9].The surface micro-cracking networks have a characteristic stepwise morphology, whereas blisters depending on their growth conditions may occur locally with a dome morphology or in an extended area with elongated and continuous morphology (parallel to the thermoplastic deformation axis) [9–12]. More precisely these microstructural components act like high-energy irreversible traps and lead to the supersaturation in hydrogen of the matrix material, resulting in the excess of critical stress intensity factor value and the development micro-cracking phenomenon As it can be seen from Scanning Electron Microscopy images (Figure 8), after the in situ hydrogen cathodic charging process both at 10 and 20 mA/cm[2], in the surface structure of the X65 pipeline steel was detected micro-cracking phenomenon around the interfaces of blisters and nonmetallic inclusions. River patterns, micro-voids with fish eye morphology and stepwise micro-cracking are characteristic features of the Hydrogen Embrittlement effect (HE) and are mainly attributed to the mechanisms of Hydrogen Induced Cracking (HIC), Internal Pressure theory (IP) and the development of hydride phases and blister formations

Conclusions

19. API 1104