Intergranular Hydrogen Embrittlement Research Articles

Manipulating nanostructure during wire arc additive manufacturing defeats hydrogen embrittlement

Wire arc additive manufacturing (WAAM) leads to a unique nano/microstructure. The combination of multiscale experimental and numerical analyses firstly shows that the intrinsic heat treatment during WAAM leads to an unusual depletion of Mg near the grain boundaries. During the WAAM process, a high dislocation density leads to formation of sub-grains and high-angle grain boundaries (HAGBs). It is revealed that the uniformly distributed micropores formed during the WAAM are strong hydrogen traps. The reduction in Mg content at the HAGBs, an increase in total length of HAGBs and hydrogen trapping by micropores help to defeat intergranular hydrogen embrittlement by WAAM.

Enhancing the hydrogen embrittlement resistance of 304 steel by grain boundary engineering

Abstract Grain boundary engineering has been widely applied to improve grain boundary-related properties such as intergranular corrosion, radiation damage and cracking in face-centered cubic metals with low to medium stacking fault energy. However, only a few investigations have been conducted to reduce the susceptibility to intergranular hydrogen embrittlement. Hydrogen embrittlement has severe effects on applications of engineering materials. In this study, grain boundary engineering was successfully applied to enhance the hydrogen embrittlement resistance of 304 austenitic stainless steel. Optimum grain boundary character distribution was acquired through thermomechanical treatments of 7% cold-rolling reduction and subsequent annealing at 1 0758C for 7 min, which reduced the two hydrogen embrittlement sensitivity indices, Id from 13.5 to 5.2 and Iw from 7.2 to 1.6. The effect of grain boundary character distribution comes from two aspects. On the one hand, special boundaries have some positive effects on the alleviation of hydrogen charging. Under the same charging condition, the charged hydrogen concentration decreases with increasing fraction of low-R CSL boundaries. On the other hand, special boundaries, especially R3 boundaries, could help to hinder the initiation and propagation of hydrogen- induced cracks. Therefore, the resistance of hydrogen embrittlement increases with increasing fraction of low-R CSL boundaries.

Grain-boundary modelling of hydrogen assisted intergranular stress corrosion cracking

A novel hybrid strategy for modelling intergranular hydrogen embrittlement in polycrystalline microstructures is proposed. The technique is based on a grain-boundary integral representation of the polycrystalline micro-mechanics, numerically solved by the boundary element method, coupled with an explicit finite element model of the intergranular hydrogen diffusion. The intergranular interaction between contiguous grains in the aggregate is modelled through extrinsic cohesive-frictional traction-separation laws, whose parameters depend on the concentration of intergranular hydrogen, which diffuses over the interface according to the Fick’s second law, inducing the weakening of the interface itself. The model couples the advantages of the boundary element representation of the polycrystalline micro-mechanics, namely the reduction of the mechanical degrees of freedom, with the generality of the finite element modelling of the diffusion process, which in principle allows the straightforward coupling of the interfacial effective diffusivity with other local mechanical parameters, e.g. the interfacial damage or displacement opening. Several numerical tests complete the study, showing the potential of the proposed technique.

An investigation of micro-mechanisms in hydrogen induced cracking in nickel-based superalloy 718

Hydrogen embrittlement of the nickel-iron based superalloy 718 has been investigated using slow strain rate tests for pre-charged material and also in-situ hydrogen charging during testing. Fractography analyses have been carried using scanning electron microscopy, electron back-scattering diffraction and orientation image microscopy concentrating on the influence of microstructural features and associated micro-mechanisms leading to hydrogen induced cracking and embrittlement. It was observed that hydrogen induced transgranular cracking initiates at micro-voids in the crystal lattice. Similar behaviour has been observed in multi-scale finite element chemo-mechanical numerical simulations. In contrast, hydrogen induced localized slip intergranular cracking was associated with the formation of micro-voids in intergranular regions. The effects of grain boundary and triple junction character on intergranular hydrogen embrittlement were also investigated. It was observed that low end high angle misorientations (LHAM), 15°<θ ≤ 35°, and critical high angle misorientations (CHAM), 35° < θ ≤ 50°, are preferential sites for hydrogen induced cracking. In contrast, few or no hydrogen induced cracks were observed at low angle misorientations (LAM), 0°≤ θ ≤ 15°, high end high angle misorientations (HHAM), 50°<θ ≤ 55°, or special GB misorientations (SGB), θ > 55°. Finally, the use of grain boundary engineering techniques to increase the resistance of super alloy 718 to hydrogen induced cracking and embrittlement is discussed.

Multi-phase modelling of intergranular hydrogen segregation/trapping for hydrogen embrittlement

Premature failure in polycrystalline materials due to hydrogen absorption affects a wide range of applications, including clean energy systems, hydrogen storage systems and rocket engines. A good understanding of the diffusion and trapping processes within such materials can inform material choices and component design to reduce the likelihood of such failures. Grain boundary segregation of hydrogen can often lead to intergranular hydrogen embrittlement (IHE). In order to understand the effects of hydrogen on intergranular and transgranular fracture in polycrystalline material it is important to first understand hydrogen diffusion and trapping in the general context of grain boundary segregation engineering (GBSE). Hydrogen diffusion is affected by local microstructural features including intergranular second phase precipitates, grain boundary (GB) thicknesses and geometrically necessary dislocation (GND) density. A multi-scale multi-phase model is presented here that has been developed to study GBSE with respect to hydrogen diffusion and IHE. The results of various multi-scale GBSE models with and without traps (including the effects of microstructure, intergranular precipitate phases and GB thickness) are compared and discussed, and the effects of microstructural parameters such as hydrogen segregation factor and GND trapping density on hydrogen diffusion are investigated.

Coupled macroscale-microscale model for hydrogen embrittlement in polycrystalline materials

Prediction of hydrogen embrittlement within a component requires the influence of several length scales to be accounted for. The loads that affect the rate of hydrogen diffusion, typically thermal and structural, derive from the macro or component scale. Micro-structural analysis has an important role to play in providing accurate estimates of the typically homogenous material characteristics employed at the component scale. This contribution considers the coupling of a micro-scale model with the component scale. A micro-scale model is employed in critical regions of the component where resolution of the heterogeneous behaviour is necessary. A tie boundary/cut boundary technique is introduced to couple the micro-scale model to the macro-scale model. The developed technique offers a computationally efficient procedure to analyse the multi-scale inter-granular hydrogen embrittlement in a polycrystalline material. This work is targeted at the prediction of hydrogen embrittlement in pulse-plated nickel and is carried out within the context of the EU FP7 MultiHy project.

A three-dimensional multiscale model of intergranular hydrogen-assisted cracking

We present a three-dimensional model of intergranular hydrogen-embrittlement (HE) that accounts for: (i) the degradation of grain-boundary strength that arises from hydrogen coverage; (ii) grain-boundary diffusion of hydrogen; and (iii) a continuum model of plastic deformation that explicitly resolves the three-dimensional polycrystalline structure of the material. The polycrystalline structure of the specimen along the crack propagation path is resolved explicitly by the computational mesh. The texture of the polycrystal is assumed to be random and the grains are elastically anisotropic and deform plastically by crystallographic slip. We use the impurity-dependent cohesive model in order to account for the embrittling of grain boundaries due to hydrogen coverage. We have carried out three-dimensional finite-element calculations of crack-growth initiation and propagation in AISI 4340 steel double-cantilever specimens in contact with an aggressive environment and compared the predicted initiation times and crack-growth curves with the experimental data. The calculated crack-growth curves exhibit a number of qualitative features that are in keeping with observation, including: an incubation time followed by a well-defined crack-growth initiation transition for sufficiently large loading; the existence of a threshold intensity factor K Iscc below which there is no crack propagation; a subsequent steeply rising part of the curve known as stage I; a plateau, or stage II, characterized by a load-insensitive crack-growth rate; and a limiting stress-intensity factor K Ic , or toughness, at which pure mechanical failure occurs. The calculated dependence of the crack-growth initiation time on applied stress-intensity factor exhibits power-law behavior and the corresponding characteristic exponents are in the ball-park of experimental observation. The stage-II calculated crack-growth rates are in good overall agreement with experimental measurements.

Hydrogen-induced delayed cracking in the AISI 301 unstable austenitic steel sheet

This work aims to study the hydrogen-induced delayed cracking related to the martensitic transformation in the AISI 301 unstable austenitic stainless steel. It happens because the metastable austenite is substantially transformed into α ′ -martensite due to the deformation occurring during the clamp-forming operation. To understand this phenomenon, morphological and chemical analyses have been made. The martensite fraction in the inside bend radius was measured by X-ray Diffraction (XRD) and the breaking patterns were observed using a Scanning Electron Microscope (SEM). The clamps breaking patterns observed after delayed cracking show a brittle intergranular fracture in the bend radius. Under laboratory conditions and after hydrogen-charged clamps, delayed cracking occurred. The main fractographic feature observed in cracked clamps is the intergranular Hydrogen Embrittlement (HE). This brittle behavior is intensified by the high heterogeneous stress state in the material and the high content of martensite.

A hydrogen embrittlement mechanism for sensitized types 304, 316 and 310 austenitic stainless steels in boiling saturated magnesium chloride solutions

We have already proposed a mechanism for intergranular hydrogen embrittlement (IG-HE) for solution annealed austenitic stainless steels (types 304, 316 and 310) in HCl solutions and in boiling saturated magnesium chloride solutions. The proposed IG-HE mechanism was based on martensite transformation, hydrogen-enhanced local plasticity (HELP), grain boundary sliding (GBS). Recently, it was reported that the fracture susceptibility and fracture mode for sensitized steels in boiling saturated magnesium chloride solution under an open-circuit condition were significantly different from those observed for solution annealed steels. In the present paper, the hydrogen embrittlement behavior of sensitized types 304, 316 and 310 in boiling saturated magnesium chloride solutions was explained in more details in terms of an inhibiting effect of chloride ions, martensite transformation, Cr depletion, HELP, the degree of corrosiveness through the comparison with those for the solution annealed steels. Furthermore, a transgranular HE (TG-HE) cracking mode that was not observed for the solution annealed steels was discussed as well as IG-HE. Then a TG-HE mechanism for sensitized austenitic stainless steels was proposed, while the IG-HE mechanism for solution annealed austenitic stainless steels which was discussed in details was applied to IG-HE of sensitized austenitic stainless steels. It was also pointed out that the occurrence of both TG-HE and IG-HE was explained with an identical concept.

Grain-boundary engineering markedly reduces susceptibility to intergranular hydrogen embrittlement in metallic materials

The feasibility of using “grain-boundary engineering” techniques to reduce the susceptibility of a metallic material to intergranular embrittlement in the presence of hydrogen is examined. Using thermomechanical processing, the fraction of “special” grain boundaries was increased from 46% to 75% (by length) in commercially pure nickel samples. In the presence of hydrogen concentrations between 1200 and 3400 appm, the high special fraction microstructure showed almost double the tensile ductility; also, the proportion of intergranular fracture was significantly lower and the J c fracture toughness values were some 20–30% higher in comparison with the low special fraction microstructure. We attribute the reduction in the severity of hydrogen-induced intergranular embrittlement to the higher fraction of special grain boundaries, where the degree of hydrogen segregation at these boundaries is reduced.

A new aspect on intergranular hydrogen embrittlement mechanism of solution annealed types 304, 316 and 310 austenitic stainless steels

The objective of this paper is to propose a new intergranular hydrogen embrittlement mechanism of solution annealed austenitic stainless steels (types 304, 316 and 310) on the basis of the results already reported. An intergranular hydrogen embrittlement (IG-HE) took place for type 316 at potentials less noble than the open-circuit potential in a HCl solution, and for types 304 and 316 at a lower test temperature under an open-circuit condition in saturated boiling magnesium chloride solutions by using a constant load method, while type 310 suffered only a transgranular stress corrosion cracking (TG-SCC) in both solutions under the same experimental conditions, but not IG-HE. In addition, TG-SCC occurred for types 304 and 316 under an open-circuit condition in the HCl solution irrespective of test temperature and in saturated boiling magnesium chloride solutions at higher test temperatures. Thus, the occurrence of IG-HE depended upon the material and test temperature. The new IG-HE mechanism was developed that explains the results obtained in terms of martensite transformation, hydrogen-enhanced local plasticity (HELP), grain boundary sliding (GBS) and so on.

The effect of Li, He and Ca on grain boundary cohesive strength in Ni

Boron is added to nickel-base superalloys such as Alloy X-750 in order to enhance high temperature strength and ductility so that the alloy may be more easily hot worked[1]. Boron additions also have been shown to ameliorate intergranular hydrogen embrittlement in nickel[2], and to improve the high temperature resistance of Alloy X-750 to aqueous stress corrosion cracking (SCC) in the absence of irradiation[3]. Recent quantum mechanical calculations demonstrate that boron strengthens grain boundaries in pure nickel[4], and may contribute to the observed benefits of boron on workability and fracture resistance of nickel alloys. Alloy X-750 exhibits greater susceptibility to intergranular stress corrosion cracking (IGSCC) when irradiated[5], and it has been proposed that the presence of grain boundary helium and/or lithium is responsible. Arguments have been advanced that helium embrittlement of the grain boundaries is primarily responsible for the greater observed susceptibility to IGSCC in irradiated X-750[1]. Alternatively, it has been proposed that lithium promotes IGSCC either by entering the water at the crack tip and lowering the local pH, or by inducing a restructuring of the grain boundary itself[1]. Direct embrittlement of grain boundaries by lithium also has been investigated by ion bombardment in Nimonic PE16, illustrating that under certain conditions lithium can produce degrees of embrittlement in nickel comparable to that produced by helium[6]. It is important to understand the relative roles of these species in grain boundary embrittlement in nickel alloys so that better predictive abilities and mitigation strategies can be developed. Toward that end, quantum mechanical calculations have been performed to investigate the influence of isolated lithium and helium atoms on the cohesive strength of an ideal grain boundary in pure nickel.

Environment-Assisted Intergranular Cracking

Intergranular separation in polycrys-talline materials involves breaking metallic bonds along grain boundaries in response to stress. The surfaces created in this manner expose the grain facets on either side of the original boundary, as shown in Figure 1. This mode of fracture often occurs at much lower fracture stress and energy than cracking by ductile processes through the interior of grains. The exposure of specific materials to certain environments and stress can promote this low-energy, intergranular mode of separation, even when fracture of the same material in vacuum would occur along a ductile transgranu-lar path. Three types of environment-assisted intergranular cracking can occur in a wide variety of alloy/environment systems: intergranular stress-corrosion cracking (IGSCC), intergranular hydrogen embrittlement, and intergranular liquid-metal embrittlement.Figure 1 shows an example of IGSCC. This type of cracking is a pervasive problem in many technological applications, leading to extensive repairs, loss of service function, and safety concerns. IGSCC occurs in the weld-heat-affected zones of stainless-steel pipes in high-purity primary coolant waters within nuclear power plants, and in nickel-based alloys utilized as heat-exchanger tubing when exposed to the high-purity primary as well as secondary coolant waters in power plants. It is also seen in Al-based alloys used for fuselage skins and structural components in military and commercial aircraft when exposed to humid atmospheric conditions. Ferrous alloys used in the oil and gas industry are also susceptible. For instance, IGSCC of mild steels used in buried gas-transmission pipelines is a widespread international problem, leading to explosions when leaking natural gas ignites.

On the intergranular hydrogen embrittlement mechanism of Al-Li alloys

An investigation is conducted of the stress-assisted grain-boundary diffusion of hydrogen in Al-Li alloys, with a view to the characterization of the intergranular embrittlement mechanism. A comparison of theoretical predictions with experimental observations indicates that intergranular hydrogen embrittlement involves the nucleation of an incremental crack at the edge of the plastic zone that extends not only backwards, to join the original crack, but also forwards: in a region that is stressed above a critical level and has a rather elevated hydrogen concentration. Crack jump distance predictions are in excellent agreement with experimental observations; ligament fracture is the step controlling the hydrogen-embrittlement fracture process. 17 refs.

Application of molecular dynamics to the study of hydrogen embrittlement in Ni-Cr-Fe alloys.

Molecular dynamics and the embedded-atom method (EAM) have been applied to the study of intergranular hydrogen embrittlement in Ni-based alloys. A new technique was developed for constructing EAM functions from experimental data on binary systems. The technique allows one to express experimental quantities of the binary alloy in terms of EAM functions for each constituent element, and then to fine tune the parameters for one of the elements to fit the experimental data. The method was used to construct EAM functions for H and for the Ni-Cr-Fe system. The functions were then used in dynamic simulations of grain boundaries to study the effect of H on their stress-strain--fracture behavior. The simulations suggest that hydrogen-embrittlement susceptibility is strongly linked to grain-boundary structure. Boundaries possessing large gaps and structural irregularities are affected adversely, while boundaries lacking these defects are resistant to embrittlement. The results of the study are discussed in terms of the influence of the simulation methodology on the observed grain-boundary fracture behavior of Ni-Cr-Fe alloys as a function of boundary type, boundary structure, and hydrogen content.

Grain Boundary Composition Effects on Environmentally Induced Cracking of Engineering Materials

Abstract There is now clear evidence that intergranular stress corrosion cracking (IGSCC) and hydrogen embrittlement (HE) of engineering materials depend on grain boundary composition. Two examples are used to illustrate this interrelationship: (1) stress corrosion cracking (SCC) of austenitic stainless steel (SS) in high temperature water and (2) HE of NiCrMoV rotor steels in acidic solutions. Grain boundary compositions are characterized by analytical electron microscopy and scanning Auger electron spectroscopy. Environmental cracking susceptibility is indicated by slow strain rate tests (SSRTs) in appropriate aqueous environments. Direct correlations are documented between measured grain boundary compositions and environmental cracking. The ability to establish these types of correlations enables diagnosis of embrittlement susceptibility and helps identify methods to control or eliminate embrittlement by bulk chemistry or processing modifications.

Effects of grain-boundary segregation and precipitation on the hydrogen susceptibility of nickel

The straining electrode tensile test was employed to study hydrogen embrittlement in three grades of nickel. Aging at intermediate temperatures (400 to 800 °C) after solution annealing caused sulfur segregation to grain boundaries in the materials. This sulfur segregation was found to influence intergranular hydrogen embrittlement, and the effect was most prominent when the surface supply of hydrogen was low and when the grain boundaries were free of graphite precipitates. Aging also induced graphite precipitation at the grain boundaries of one grade of nickel (Nickel 200). The presence of these grain-boundary particles reduced susceptibility to hydrogen embrittlement, but at the same time caused a form of aging embrittlement. The effect of the second-phase precipitate, when present, masked the effect of sulfur segregation on hydrogen susceptibility.

Factors affecting the intergranular hydrogen embrittlement of Co 3Ti

The influence of intergranular hydrogen embrittlement on the mechanical behavior and fracture behavior of Co 3Ti and 0.1 wt% B-doped Co 3Ti compounds were studied. The significant environmental effect was investigated at ambient temperatures. The elongation and tensile strength showed lower values in the sequence of vacuum, air and hydrogen conditions though the yield strength was insensitive to the environment. The remarkable strain rate dependence was also investigated. As the strain rate decreased, the elongation and tensile strength decreased in a sigmoidal form and again the yield strength remained constant. The decreases of the elongation and tensile strength were investigated with decreasing test temperature below 400°C at the lower strain rate. These embrittlements were severer in the nearstoichiometric alloys of Co-23 at.% Ti than in the off-stoichiometric alloys of Co-21 at.% Ti. The addition of 0.1 wt% B did not actually affect the mechanical behavior of undoped Co 3Ti compounds. Fracture behavior was fairly consistent with the mechanical behavior; there was a change in fracture mode from the transgranular fracture mode, through a mixed mode, to the intergranular fracture mode as the strain rate decreases, by the alternation from vacuum to hydrogen conditions and as the Co atoms become excess. The reduction of ductility and tensile strength occurred in the present Co 3Ti compound was suggested to be due to hydrogen-assisted intergranular embrittlement. The dynamic and atomistic mechanism by which the cohesive strength of grain boundary was affected by the hydrogen was proposed as the probable explanation.

The effect of stress state on the hydrogen embrittlement of nickel

The ductility of nickel sheet subjected to in situ cathodic hydrogen charging has been investigated over a range of multiaxial stress states including uniaxial, plane-strain, and equibiaxial tension. The data show that the extent of ductility loss due to the presence of hydrogen increases as the stress state tends from uniaxial to equibiaxial tension. In all instances, the hydrogen embrittlement is characterized by intergranular fracture with failure occurring due to microcrack formation, microcrack link-up, and macrocrack growth. The increased susceptibility of nickel to intergranular hydrogen embrittlement with increasing biaxiality of stress state is shown to be a consequence of an enhanced rate of the link-up of strain-induced intergranular microcracks.

Effect of phosphorus segregation and Ni2Cr formation on hydrogen embrittlement in 70Ni–30Cr alloys

The effects of aging at 773 K on hydrogen embrittlement in Ni–30Cr (wt-%) alloys having two levels of P have been investigated by considering the grain-boundary segregation of impurity atoms and the Ni2 Cr ordered-phase formation. Aging at 773K suppressed intergraular fracture and reduced the susceptibility to hydrogen embrittlement in the low-P alloy. Such behaviour can be explained in terms of the grain-boundary strengthening caused by the segregation of C atoms. During aging at 773 K, the Ni2Cr ordered phase formed and the deformation mode changed from wavy slips to coplanar slip with paired dislocations, and then to coplanar slip with microtwins. In the low-P alloy, this change of deformation mode induced step-like cracks which may have occurred by the separation of either the {111} slip planes or the microtwin interfaces. In the high-P alloy, aging for short times caused C segregation to the grain boundaries which suppressed intergranular fracture. However, aging for longer times induced drastic intergranular hydrogen embrittlement because of the grain-boundary segregation of P atoms, which offset the effect of the boundary strengthening caused by C atoms.MST/177