Hydrogen Embrittlement Susceptibility Of Alloy Research Articles

Characterization of intergranular phases in precipitation hardening Ni alloy UNS N07725

Elucidating the hydrogen embrittlement (HE) mechanisms of precipitation-hardened (PH) Ni-based alloys, such us Alloy 725 (UNS N07725), is of growing interest due to severe failures experienced in oil and gas service. Recent findings have linked the HE susceptibility of Alloy 725 to the coverage of the grain boundaries (GBs) by a nanoscale and brittle precipitate. However, none of the phases expected in Alloy 725 could explain the extent and the deleterious effect of the observed intergranular precipitates. This investigation successfully identified the predominant intergranular phases as M23C6 carbide and F phase—a σ-related topologically close-packed (TCP) phase not previously reported in Alloy 725—in two independent commercial Alloy 725 batches. F phase has a hexagonal P6/mmm structure with a = 12.5 Å and c = 4.6 Å and shared an orientation relationship with the γ-matrix described as 011¯γ//21¯1¯0F and (111)γ ∕ ∕ (0001)F. Due to the similarities in structure and composition, F phase could be erroneously identified as either σ phase or M23C6. F phase precipitated as approximately 20 nm thick sheets; hence, it could entirely cover the favorably oriented GBs, even at low volume fractions. Both Alloy 725 batches presented a dominant phase at GBs. In this regard, F phase was predominant in the commercial batch that was considered prone to HE in oil and gas service, while M23C6 was the main precipitate in the second independent batch. Our results suggested that the thermomechanical processing of Alloy 725 affects the extent of intergranular precipitation as well as the F phase and M23C6 relative abundance.

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

The susceptibility of age-hardened nickel-based Alloy 718 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 and the internal stress generated during cathodic charging. Pre-damages were proved to result in transgranular cracks and lead to the evident reduction of mechanical properties. In addition, the STRONG (Slip Transfer Resistance of 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.

Effect of different microstructural features on the hydrogen embrittlement susceptibility of alloy 718

Due to its high strength and acceptable corrosion resistance, Alloy 718 is a viable material for the use in subsea applications. The alloy's susceptibility to hydrogen embrittlement is however limiting its viability. In this work the effect of microstructure on the hydrogen embrittlement susceptibility of Alloy 718 was examined by the use of slow strain rate testing on three different heat treatments. Cathodic pre-charging and polarization during testing were used to introduce hydrogen into the samples. A severe reduction in the ductility of the alloy due to the precipitation phases γ″ and γ′ were found. In addition, one of the heats had a continuous film of a Nb and C rich phase on the grain boundary giving an intergranular fracture mechanism. This intergranular fracture mode was further enhanced by the presence of hydrogen. In the solution annealed condition, the presence of hydrogen reduced the dimple size on the fracture surface.

Effect of temperature on hydrogen embrittlement susceptibility of alloy 718 in Light Water Reactor environment

A 718 superalloy, presenting a standard microstructure, was mechanically tested under uniaxial tensile loading at 80 °C and 300 °C in Light Water Reactor environment after an exposure at 300 °C for 200 h. Hydrogen embrittlement mechanism was clearly observed. In order to identify the most influent metallurgical parameters on hydrogen embrittlement, three “model” microstructures were synthesized to test the efficiency of carbides, δ, γ′ and γ” precipitates to trap hydrogen at different temperatures. Results showed that γ′ and γ” played the major role on the hydrogen embrittlement susceptibility of the alloy even though carbides and δ precipitates could also act as hydrogen traps and influence the final rupture mechanism. Results also characterized the influence of temperature on the fracture modes.

Slow Strain Rate Testing for Hydrogen Embrittlement Susceptibility of Alloy 718 in Substitute Ocean Water

The hydrogen embrittlement susceptibility of near-peak-aged UNS N07718 (Alloy 718) was evaluated by performing slow strain rate tests at room temperature in air and substitute ocean water. Tests in substitute ocean water were accomplished in an environmental cell that enabled in situ cathodic charging under an applied potential of −1.1 V versus SCE. Some specimens were cathodically precharged for 4 or 16 weeks at the same potential in a 3.5 wt.% NaCl-distilled water solution at 50 °C. Unprecharged specimens tested in substitute ocean water exhibited only moderate embrittlement with plastic strain to failure decreasing by about 20% compared to unprecharged specimens tested in air. However, precharged specimens exhibited significant embrittlement with plastic strain to failure decreasing by about 70%. Test environment (air or substitute ocean water with in situ charging) and precharge time (4 or 16 weeks) had little effect on the results of the precharged specimens. Fracture surfaces of precharged specimens were typical of hydrogen embrittlement and consisted of an outer brittle ring related to the region in which hydrogen infused during precharging, a finely dimpled transition zone probably related to the region where hydrogen was drawn in by dislocation transport, and a central highly dimpled ductile region. Fracture surfaces of unprecharged specimens tested in substitute ocean water consisted of a finely dimpled outer ring and heavily dimpled central region typical of ductile fracture.

Effect of trapping and temperature on the hydrogen embrittlement susceptibility of alloy 718

Ni-based alloy 718 is widely used to manufacture structural components in the aeronautic and nuclear industries. Numerous studies have shown that alloy 718 may be sensitive to hydrogen embrittlement. In the present study, the susceptibilities of three distinct metallurgical states of alloy 718 to hydrogen embrittlement were investigated to identify both the effect of hydrogen trapping on hydrogen embrittlement and the role of temperature in the hydrogen-trapping mechanism. Cathodic charging in a molten salt bath was used to saturate the different hydrogen traps of each metallurgical state. Tensile tests at different temperatures and different strain rates were carried out to study the effect of hydrogen on mechanical properties and failure modes, in combination with hydrogen content measurements. The results demonstrated that Ni-based superalloy 718 was strongly susceptible to hydrogen embrittlement between 25°C and 300°C, and highlighted the dominant roles played by the hydrogen solubility and the hydrogen trapping on mechanical behavior and fracture modes.