Hydrogen-induced Intergranular Cracking Research Articles

Quantitative analysis of local plasticity accompanying hydrogen-related fracture in low-carbon martensitic steel

In this study, the local plasticity accompanying hydrogen-related fracture in a low-carbon martensitic steel was quantitatively investigated by correlative microscopy analysis combining digital image correlation technique and electron backscatter diffraction. The results revealed that strain localization occurred mainly in the bulk of martensite blocks and at prior austenite grain boundaries, both with and without the presence of hydrogen. Moreover, hydrogen enhanced strain localization during deformation and facilitated crack nucleation and propagation. The quantitative analysis showed that the hydrogen-related quasi-cleavage cracking was closely related to the local plastic deformation, and increasing hydrogen content decreased the local strain level required for quasi-cleavage cracking. It was found that strain localization also occurred around the intergranular cracks, suggesting that hydrogen-induced intergranular cracking was not a classical simple decohesion process and strain localization was involved in the sequence of hydrogen-induced intergranular cracking. These findings shed light on the understanding of the intrinsic nature of hydrogen embrittlement.

Hydrogen-induced intergranular cracking of pure nickel under various strain rates and temperatures in gaseous hydrogen environment

Industrial-grade pure nickel, NI-201, was tensile tested in high-pressure hydrogen gas at various testing temperatures and strain rates. The degree of hydrogen embrittlement (HE) was strongly affected by the testing temperature and strain rate, and elevating the strain rate and lowering the temperature result in suppressed HE. While hydrogen trapped at GBs via local equilibrium dominated the HE degree in hydrogen-charged cases, hydrogen supply to GBs during the tests was required in gaseous hydrogen cases. Although hydrogen–dislocation interaction was one of candidates for the hydrogen supply, the lattice hydrogen, which can interact with dislocations, hardly existed and related behaviors such as increase in flow stress and serrated yielding were not observed in the gaseous hydrogen cases; therefore, the contribution of the hydrogen–dislocation interaction was speculated to be minor. Alternatively, we proposed an HE model considering hydrogen diffusion along grain boundaries (GBs) from the crack tip, which successfully demonstrated the experimental results.

Key role of plastic strain gradient in hydrogen transport in polycrystalline materials

Hydrogen transport by moving dislocations is one of the key mechanisms for hydrogen embrittlement, which is considered responsible for local hydrogen accumulation at preferential crack initiation sites. Although numerous experiments corroborate this mechanism, there are few theoretical models for it available. In the present work, a novel hydrogen transport model is developed within the coupled framework of crystal plasticity and hydrogen balance. The hydrogen transport flux in the present model is divided into two parts: the first-order hydrogen transport flux driven by plastic strain and the second-order hydrogen transport flux driven by plastic strain gradient. With the calibrated parameters by fitting experimental results, the first-order hydrogen transport flux is negligible even for nickel with a low hydrogen diffusivity. In polycrystals, due to the intrinsic plastic heterogeneity induced by grain orientation mismatch, the second-order hydrogen transport flux results in significant dynamic hydrogen segregation at grain boundaries during deformation, which is expected to drive the hydrogen-induced intergranular cracking as observed in experiments. This dynamic segregation behavior is related to the evolution rate of geometrically necessary dislocations, depending on the grain boundary characters. The roles of grain refinement in hydrogen transport are clarified with the present model. This study shows the vital role of plastic strain gradient induced by grain boundary constraint in hydrogen transport, which is essential in understanding hydrogen migration and accumulation in deformed polycrystalline metals.

Elucidating the contribution of mobile hydrogen-deformation interactions to hydrogen-induced intergranular cracking in polycrystalline nickel

Uniaxial mechanical testing conducted at room temperature (RT) and 77 K on hydrogen (H)-exposed nickel was coupled with targeted microscopy to evaluate the influence of deformation temperature, and therefore mobile H-deformation interactions, on intergranular cracking in nickel. Results from interrupted tensile tests conducted at cryogenic temperatures (77 K), where mobile H-deformation interactions are effectively precluded, and RT, where mobile H-deformation interactions are active, indicate that mobile H-deformation interactions are not an intrinsic requirement for H-induced intergranular fracture. Moreover, an evaluation of the true strain for intergranular microcrack initiation for testing conducted at RT and 77 K suggests that H which is segregated to grain boundaries prior to the onset of straining dominates the H-induced fracture process for the prescribed H concentration of 4000 appm. Finally, recent experiments suggesting that H-induced fracture is predominately driven by mobile H-deformation interactions, as well as the increased susceptibility of coherent twin boundaries to H-induced crack initiation, are re-examined in light of these new results.

Multiscale thermodynamic analysis on hydrogen-induced intergranular cracking in an alloy steel with segregated solutes

Abstract A multiscale analysis has been conducted on hydrogen-induced intergranular cracking at ambient temperature in medium strength (840 MPa) Ni-Cr steel with antimony, tin, and phosphorous segregation. Combining first-principles calculations and fracture mechanics experiments, a multiscale relationship between threshold stress intensity factor (K th) and cohesive energy of grain boundary (the ideal work of interfacial separation, 2γ int) was revealed. The K th was found to decrease rapidly under a certain threshold of 2γ int, where the 2γ int decreases mainly by mobile hydrogen segregation on fracture surfaces. This segregation is considered to arise during formation of the fracture surfaces under thermodynamic equilibrium in slow crack growth. The resulting strong decohesion probably makes it difficult to emit dislocations at the microcrack tip region, leading to a large reduction in stress intensity factor. Our analysis based on this mobile hydrogen decohesion demonstrates that the K th decreases dramatically within a low and narrow range of hydrogen content in iron lattice in high-strength steels.

Grain boundary migration of substitutional and interstitial atoms in α-iron

The effect of grain boundaries (GBs) on the diffusion of interstitial and substitutional atoms in α-iron was studied experimentally and theoretically. Autoradiography and the sectioning method, applied to different grain size polycrystals, prove that a higher GB area leads to a reduction of the migration of the 14C isotope. An opposite effect is observed for the 60Co isotope. Theoretical calculations of hydrogen and carbon migration in selected GBs having different disorientation angles show that, for both interstitial elements, the activation enthalpy for migration is higher in the GBs than in the bulk. Both experimental and theoretical results lead to the conclusion that GBs in α-iron act as traps for interstitial atoms and retard their diffusion. To put these results into perspective, hydrogen-induced intergranular cracking is discussed in relation to the specificity of hydrogen diffusivity along the GBs.

Fracture toughness of alloy 690 and EN52 welds in air and water

The effect of low- and high-temperature water with high hydrogen on the fracture toughness of alloy 690 and its weld, EN52, was characterized using elastic-plastic JIC methodology. While both materials display excellent fracture resistance in air and elevated-temperature (>93 °C) water, a dramatic degradation in toughness is observed in 54 °C water. The loss of toughness is associated with a hydrogen-induced intergranular cracking mechanism, where hydrogen is picked up from the water. Comparison of the cracking behavior in low-temperature water with that for hydrogen-precharged specimens tested in air indicates that the critical local hydrogen content required to cause low-temperature embrittlement is on the order of 120 to 160 ppm. Loading-rate studies show that cracking resistance is improved at rates above ∼ 1000 MPa √m/h, because there is insufficient time to produce grain-boundary embrittlement. Electron fractographic examinations were performed to correlate cracking behavior with microstructural features and operative fracture mechanisms.

Fracture toughness of alloy 600 and an EN82H weld in air and water

The fracture toughness of alloy 600 and its weld, EN82H, was characterized in 54 °C to 338 °C air and hydrogenated water. Elastic-plastic J IC testing was performed due to the inherent high toughness of these materials. Alloy 600 exhibited excellent fracture toughness under all test conditions. While EN82H welds displayed excellent toughness in air and high-temperature water, a dramatic toughness degradation occurred in water at temperatures below 149 °C. Comparison of the cracking response in low-temperature water with that for hydrogen-precharged specimens tested in air demonstrated that the loss in toughness is due to a hydrogen-induced intergranular cracking mechanism. At loading rates above ∼1000 MPa % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9vqpe0x% c9q8qqaqFn0dXdir-xcvk9pIe9q8qqaq-dir-f0-yqaqVe0xe9Fve9% Fve9qapdbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaaie% aacaWFTbaaleqaaOGaai4laiaa-Hgaaaa!3BD6! $$\sqrt m /h$$ , the toughness in low-temperature water is improved because there is insufficient time for hydrogen to embrittle grain boundaries. Electron fractographic examinations were performed to correlate macroscopic properties with key microstructural features and operative fracture mechanisms.

Effects of neutron irradiation on hydrogen-induced intergranular fracture in a low activation 9%Cr-2%W steel

Hydrogen charging changed the fracture mode in tensile tests at room temperature from ductile shear rupture to intergranular cracking, resulting in a considerable reduction of the ductility of a low activation 9%Cr-2%W martensitic steel. The critical hydrogen charging current density required to cause hydrogen-induced intergranular cracking was reduced by neutron irradiation, suggesting that neutron irradiation enhanced hydrogen-induced intergranular cracking. This hydrogen-induced intergranular cracking was not caused by irreversible damage due to hydrogen charging, since it disappeared after aging at room temperature. The recovery rate of the fracture mode from intergranular cracking to ductile rupture during aging at room temperature was reduced by irradiation. A mechanism of irradiation-induced enhancement of hydrogen embrittlement in a low activation 9%Cr-2%W martensitic steel is proposed.

Equilibrium and growth characteristics of hydrogen-induced intergranular cracking in phosphorus-doped and high purity steels

By means of fracture mechanics analyses, acoustic emission techniques, and fracture surface analyses by scanning Auger microscopy and X-rays, it was determined how segregated phosphorus, yield strength and grain size affect the equilibrium and growth characteristics of hydrogen-induced intergranular cracking in high strength steels. The effect of yield strength on the threshold stress intensity was found to be greater than those of phosphorus segregation and grain size. The intergranular phosphorus segregation greatly accelerated the growth rate of hydrogen-induced intergranular cracking and caused a large number of acoustic signals to be emitted during the crack growth. The crack growth rate increased in a steel with segregated phosphorus and slightly decreased in high purity steels, where only occasional acoustic emissions were measured during the cracking process, by increasing grain size. Fracture surface analyses indicated more featureless intergranular fracture facets and higher levels of residual strain in the lattice adjacent to the fracture surface in the phosphorus-doped steel than in the high purity steels. These results suggest that while in steels with segregated impurities the macrocrack tends to grow by discretely rapid formation of intergranular microcracking which gives rise to dislocation emissions at the growing crack tip, in high purity steels slow growth of intergranular microcracking proceeds which is accompanied by either the absence or substantial mitigation of dislocation generation at the crack tip.

A microscopic model of hydrogen-induced intergranular cracking—II. Steady state crack growth

A steady state model of hydrogen-induced intergranular crack growth has been developed. It is assumed that the steady state growth of an intergranular crack, screened by dislocations, is controlled by the grain boundary and crack surface diffusion processes of hydrogen. A series of second-order differential equations controlling the diffusion processes of hydrogen in the intergranular crack regions is solved numerically to determine the hydrogen distribution along a steadily moving crack and thereby the change of the ideal work of fracture. The relationship of the stress intensity required for the crack growth to the crack velocity is established as a function of several values of the grain boundary and crack surface diffusivities, the crack length, and the grain size. It is found that while the susceptibility to hydrogen-induced intergranular crack growth increases with increasing hydrogen diffusivities, it decreases with increasing crack length and grain size. The effect of grain size on the growth characteristics of hydrogen-induced intergranular cracking are discussed in terms of the direction change of the hydrogen flux along the moving crack.

A microscopic model of hydrogen-induced intergranular cracking—I. Equilibrium crack growth

A microscopic model of hydrogen-induced integranular cracking is developed using the premise that a crack tip maintains a local equilibrium condition. It is assumed that the local equilibrium condition at the crack tip is achieved by the presence of screening dislocations. The embrittlement mechanism is a reduction of the surface and grain boundary energies due to the segregation of hydrogen. The equilibrium segregation of hydrogen in the intergranular crack region is examined and the change of the ideal work of slow and fast intergranular fracture induced by the hydrogen segregation is determined using a thermodynamic theory. In order to examine the effect of hydrogen on equilibrium crack growth, the local and total energy criteria of a dislocation-screened crack are taken into account. It is shown, using a dislocation-screened crack model, that while it is difficult for fast intergranular fracture to be induced by hydrogen, at low levels of applied stress intensity the segregation of hydrogen to crack surfaces readily facilitates slow crack growth along grain boundaries. The mechanisms of hydrogen-induced intergranular cracking are discussed in terms of comparison of the present result with that obtained from the model of a plastically deformed crack.

Solute segregation and hydrogen-induced intergranular fracture in an alloy steel

Hydrogen-induced intergranular fracture of laboratory heats of a 3.5 Ni-1.7 Cr steel doped with P, Sn, or Sb and having a yield strength of 840 MPa and a prior austenite grain size of 120 μm has been compared with that of an undoped steel at a hydrogen pressure of 0.17 MPa (1.68 atm). The intergranular concentrations of the impurities were controlled by varying the time of aging at 480 °C. Cracking of the undoped steel tested in hydrogen occurred along martensitic lath boundaries at high stresses. However, the susceptibility of the doped steels to hydrogen-induced intergranular cracking increased precipitously with impurity concentration. The susceptibility was measured in terms of the threshold stress intensity Kthfor the first detectable crack extension in precracked specimens and in terms of the threshold stress σth for microcrack formation in notched specimens. A comparison between the intergranular strength in hydrogen and in air revealed that absorption of hydrogen produced a profound intergranular weakening when the grain boundaries contained even a small amount of a segregated embrittling element. The relative embrittling potencies of P, Sn, and Sb in hydrogen gas were the same as in air. The combined effects of hydrogen and the impurities in reducing intergranular cohesion are discussed in terms of a newly proposed dynamic model which takes into account the accumulation of hydrogen ahead of a moving microcrack.

Dynamic model of hydrogen-induced intergranular cracking

Dynamic model of hydrogen-induced intergranular cracking

The role of grain boundary solutes and structure on the yielding and intergranular cracking of iron

The effect of interstitial solute concentration and partitioning, and grain boundary extrinsic structure on the magnitude of the Petch parameters and frequency of hydrogen-induced intergranular cracking was systematically investigated in both decarburized iron and Fe-0.15 pct Ti. It was established that grain boundary interstitial solutes increase the value of the petch slope and the presence of these solutes in the lattice increases the value of the “friction” stress. Their presence in the grain boundary also increases the frequency of intergranular cracking. This is not true for extrinsic grain boundary ledges, whose presence does not appear to effect either the Petch parameters or hydrogen-induced cracking. The observed results were rationalized by the behavior of interstitial solute complexes at the boundary and the ability of such complexes to react with hydrogen.