Effect of electrochemical charging on the hydrogen embrittlement susceptibility of alloy 718

Abstract

The susceptibility of age-hardened nickel-based 718 superalloy to hydrogen embrittlement was studied by the controlled electrochemical charging combined with slow strain-rate tensile tests (SSRT) and advanced characterization techniques. We proposed some novel ideas of explaining hydrogen embrittlement mechanisms of the studied material in regard to two cracking morphologies: transgranular and intergranular cracking. It is for the first time to report that electrochemical charging alone could cause slip lines, surface and subsurface cracks on nickel-based superalloys. The formation of pre-damages was discussed by calculating the hydrogen concentration gradient generated during cathodic charging. Predamages were proved to result in transgranular cracks and lead to the evident reduction of mechanical properties. In addition, the STRONG (Slip Transfer Resistance of *Manuscript_clean Click here to download Manuscript: Revised Manuscript_clean_A-19-1789_R1.docx Click here to view linked References Neighbouring Grains) model was used to analyze the dependence of hydrogen-assisted intergranular cracking on the microscopic incompatibility of the grain boundaries. The results show that in the presence of hydrogen, grain boundaries with a lower dislocation slip transmission are more prone to cracking during loading and vice versa.

Alloy 718 Subsea Bolt in Relation to Surface Cracking: A Microstructural Perspective

“…This is the reason why the manufacturing route of the bolt is critical and determines the nanostructure at the extreme surface/subsurface of the alloy 718. By analogy, it has been reported [44] [49] that the machining of the tensile specimens (for hydrogen charging and slow strain rate testing) has been always performed after the heat treatment, which have resulted in different nanostructure at the surface/subsurface of the tensile specimens in comparison to that of the bulk lattice; both the number density and the length density of surface/subsurface cracks depend on the hydrogen charging time (mentioned earlier). Whereas, in the current study, the detailed examination of the outer periphery (surface) of the used subsea bolt (S7) (at thread and shank regions) revealed the Vickers hardness of 400 HV at the subsurface (370 HV at bulk) of the bolt, as shown in Table 3.…”

“…pact was less pronounced with an increase in the strength compare to that of the aged specimens (with γ"). It can still be argued that the presence of MCN at grain boundaries and within grains[17] [44] (at intersection of DSLs) is critical to HE, and additional studies are needed to clarify the exact mechanism of MCN particles on the HE of bolt. To observe the sole effect of M (C,N)s with regard to the PLC (under hydrogen charging and slow strain rate testing) study in the future, 718 variants of S1 and S3 were added and microstructurally characterized, as shown inFigure 4,Figure 6,Figure 10, andFigure 12.…”

“…Lu et al have reported[44] for alloy 718 that transgranular surface and subsurface cracks considerably increase between 80 h and 120 h of hydrogen charging; they have concluded that surface cracks induced by hydrogen charging led to the formation and propagation of transgranular cracks under loading.This observation is supported by Li et al[49] who have not observed surface cracks on 718 tensile samples that were hydrogen-charged for 2 h. Thus, it is the time-dependent process, where hydrogen evolving from CP penetrates at least to the subsurface region of the subsea bolt. We know that the concentration gradient of hydrogen (evolving from CP) varies from the surface to bulk γ lattice, and the operating service stress for bolts sometimes exceeds 70% of the yield strength.…”

An SEM compatible plasma cell for in situ studies of hydrogen-material interaction

“…Moreover, there is no risk of sample corrosion as can happen with electrochemical charging, with the possibility of sample failure. Furthermore, with cathodic charging there can be a pre-damage in the samples, which can alter the effect of H. For instance, it has been reported [47] that cathodic charging alone generated slip lines and surface and subsurface cracks. The pre-damage caused by the charging resulted finally in a reduction of yield strength and an increased embrittlement.…”

Butter-weld Interface Microstructural Analysis Employing NCS and LAMS Sections

“…The application of dissimilar welded joints is present in the power generation, petrochemical and transformation industries [1][2][3][4][5] . In the oil and gas extraction system, mainly offshore, these joints are subjected to very unfavorable working conditions, both in terms of oxidation and embrittlement and high pressures [6][7][8][9] .…”