Study of the effect of hydrogen charging on the tensile properties and microstructure of four variant heat treatments of nickel alloy 718

This study examined the hydrogen embrittlement sensitivity of nickel alloy 718 given four different heat treatments to obtain various microstructural states. The four heat treatments examined are the oil and gas 718 heat treatment, the aerospace 718 heat treatment, and two variant two-step heat treatments, with a difference in aging heat treatment named 718 low band and high band. Each heat treatment leads to differences in the precipitate morphologies of γ′, γ′′ and δ phases.Material characterisation and fractography of the examined heat treatments were performed using a high resolution FEG SEM. Three specimens of each condition were pre-charged with hydrogen and tensile properties were compared with those of non-charged specimens. It was observed that hydrogen embrittlement was associated with intergranular and transgranular microcrack formation, leading to an intergranular brittle fracture. δ phase may assist the intergranular crack propagation, and this was shown to be particularly true when this phase is coarse enough to produce crack initiation, but this is not the only factor determining embrittlement. Other microstructural features play a role, as does the strength of the material. The aerospace heat treatment, which gives the highest strength and ductility in the uncharged state, shows the greatest reduction in properties with hydrogen charging.     

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.     

Hydrogen embrittlement and associated surface crack growth in fine-grained equiatomic CoCrFeMnNi high-entropy alloys with different annealing temperatures evaluated by tensile testing under in situ hydrogen charging

The effect of the annealing temperature after cold rolling on hydrogen embrittlement resistance was investigated with a face-centered cubic (FCC) equiatomic CoCrFeMnNi high-entropy alloy using tensile testing under electrochemical hydrogen charging. Decreasing annealing temperature from 800 °C to 750 °C decreased grain sizes from 3.2 to 2.1 μm, and resulted in the σ phase formation. Interestingly, the specimen annealed at 800 °C, which had coarser grains, showed a lower hydrogen embrittlement susceptibility than the specimen annealed at 750 °C, although hydrogen-assisted intergranular fracture was observed in both annealing conditions. Because the interface between the FCC matrix and σ was more susceptible to hydrogen than the grain boundary, the presence of the matrix/σ interface significantly assisted hydrogen-induced mechanical degradation. In terms of intergranular cracking, crack growth occurred via small crack initiation near a larger crack tip and subsequent crack coalescence, which has been observed in various steels and FCC alloys that contained hydrogen.     

Investigation of hydrogen embrittlement properties of Ni-based alloy 718 fabricated via laser powder bed fusion

The hydrogen embrittlement behavior of heat-treated Alloy 718 fabricated by laser powder bed fusion was fundamentally investigated under electrochemical hydrogen charging. The H transitioned the fracture mode from ductile dimpled to transgranular fracture with a flat fracture surface. Crystallographic analysis showed that H promoted the dislocation slip band, and the resulting concentrated strain and H along the slip planes caused cracking regardless of the distribution of additive manufacturing (AM) microstructural features such as sub-grain boundaries. In addition, thermal desorption spectroscopy and H-permeation tests indicated that the AM microstructural features after heat treatment only slightly influenced the H trapping and diffusion.     

The hydrogen embrittlement of pure Ni fabricated by additive manufacturing

This work investigated the hydrogen embrittlement mechanism of Ni fabricated by laser-based powder bed fusion (L-PBF). In the presence of hydrogen, the L-PBFed Ni failed with a brittle mode, while its fracture surface had a “transgranular-like” appearance. This unusual fracture morphology is rooted in the special grain shape induced by the laser-based manufacturing process, and the failure process is actually predominated by the intergranular decohesion. An annealing process of the as-printed sample enhanced its elongation and mitigated the hydrogen embrittlement. The special dislocation cellular pattern formed in additive manufacturing is considered to be detrimental to hydrogen embrittlement resistance.     

The influence of intergranular and interphase boundaries and δ-ferrite volume fraction on hydrogen embrittlement of high-nitrogen steel

We study the effect of grain size of austenitic and ferritic phases and volume fraction of δ-ferrite, which were obtained in different solution-treatment regimes (at 1050, 1100, 1150 and 1200 °C), on hydrogen embrittlement of high-nitrogen steel (HNS). The amount of dissolved hydrogen is similar for the specimens with different densities of interphase (γ-austenite/δ-ferrite) and intergranular (γ-austenite/γ-austenite, δ-ferrite/δ-ferrite) boundaries. Despite, the susceptibility of the specimens to hydrogen embrittlement, depth of the hydrogen-assisted surface layers, hydrogen transport during tensile tests and mechanisms of the hydrogen-induced brittle fracture all depend on grain size and ferrite content. The highest hydrogen embrittlement index I_H = 32%, the widest hydrogen-affected layer and a pronounced solid-solution hardening by hydrogen atoms is typical of the specimens with the lowest fraction of the boundaries. Even though fast hydrogen transport via coarse ferritic grains provides longer diffusion paths during H-changing, the width of the H-affected surface layer in the dual-phase structure of the HNS specimens is mainly determined by the hydrogen diffusivity in austenite. In tension, hydrogen transport with dislocations increases with the decrease in density of boundaries due to the longer dislocation free path, but stress-assisted diffusion transport does not depend on grain size and ferrite fraction. The contribution from intergranular fracture increases with an increase in the density of intergranular and interphase boundaries.     

Hydrogen embrittlement associated with strain localization in a precipitation-hardened Fe–Mn–Al–C light weight austenitic steel

Hydrogen embrittlement of a precipitation-hardened Fe–26Mn–11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance.     

On the role of nitrogen on hydrogen environment embrittlement of high-interstitial austenitic CrMnC(N) steels

This work investigates the susceptibility of high-interstitial CrMn austenitic stainless steel CN0.96 to hydrogen environment embrittlement. In this context, an N-free model alloy of CN0.96 steel was designed, produced, and characterized. Both steels were subjected to tensile tests in air and in a high-pressure hydrogen gas atmosphere.Both steels undergo severe hydrogen embrittlement. The CN0.96 steel shows trans- and intergranular failure in hydrogen, whereas the N-free model alloy shows exclusively intergranular failure. The different failure modes could be related to different deformation modes that are induced by the presence or absence of N, respectively. In the CN0.96 steel, N promotes planar dislocation slip. Due to the absence of N in the model alloy, localized slip is less pronounced and mechanical twinning is a more preferred deformation mechanism. The embrittlement of the model alloy could therefore be related to mechanisms that are known from hydrogen embrittlement of twinning-induced plasticity steels.     

Effect of specific microstructures on hydrogen embrittlement susceptibility of a modified AISI 4130 steel

The focus of this study is to analyze hydrogen embrittlement susceptibility of a modified AISI 4130 steel by means of incremental step loading tests. Three different microstructures with a hardness of 40 HRC were analyzed: martensite with large and small prior austenite grains and dual-phase (martensite/ferrite). According to the results, the dual-phase microstructure presented the lowest hydrogen embrittlement susceptibility and martensite with large prior austenite grains, the highest. This behavior was attributed to the lower fraction of high-angle boundaries presented by the martensite with large prior austenite grains, which led to a higher diffusible hydrogen content. Moreover, the ferrite local deformation in the dual-phase microstructure enhanced its hydrogen embrittlement resistance by lowering the stress concentration. A synergic effect of decohesion and localized plasticity was identified on the hydrogen induced fracture of the tested microstructures leading to an intergranular + quasi-cleavage fracture in the martensite and quasi-cleavage in the dual-phase microstructure.     

Strain rate sensitivity of hydrogen-assisted ε-martensitic transformation and associated hydrogen embrittlement in high-Mn steel

This study presents the degree of promotion of deformation-induced γ?ε martensitic transformation by hydrogen charging and the associated fracture behavior at various strain rates ranging from 10^?5 to 10^?2 s^?1. A decrease in the strain rate from 10^?2 to 10^?5 s^?1 promotes the deformation-induced γ?ε martensitic transformation regardless of hydrogen charging (65%→82% in area fraction for uncharged specimens, 68%→84% for hydrogen-charged specimens). Hydrogen charging, which provides 11.7 mass ppm hydrogen concentration, further promotes the γ?ε martensitic transformation. However, the degree of promotion of the transformation by hydrogen is insensitive to the strain rate. Corresponding to the promotion of the γ?ε martensitic transformation, the hydrogen embrittlement susceptibility increases with a decrease in the strain rate. For instance, the elongation of the hydrogen-charged specimens decreases from 36 to 32% by decreasing strain rate from 10^?2 to 10^?5 s^?1. Hydrogen uptake deteriorates the resistance to crack initiation and propagation. Furthermore, the primary effect of the decrease in strain rate on hydrogen embrittlement is the acceleration of the crack propagation. In addition to the promotion of γ?ε martensitic transformation, a decrease in the strain rate in the presence of hydrogen may cause hydrogen localization at the crack tip, which assists brittle-like martensite cracking.