Stress-assisted Hydrogen Diffusion Research Articles

In-situ neutron imaging of delayed crack propagation of high strength martensitic steel under hydrogen embrittlement conditions

This paper presents an in-situ observation, using neutron imaging, of delayed crack propagation in a high-strength martensitic steel specimen. Delayed cracking is believed to be caused by hydrogen embrittlement occurring due to the slow diffusion and accumulation of hydrogen ahead of a crack front, causing decreased ductility and eventual cracking under constant load. The experiment involved mechanical loading of a single-edge-notch bend specimen while submerged in an electrolyte solution (H2O + 3.5% NaCl) under cathodic polarization to facilitate hydrogen ingress. Intermittent crack propagation was observed for 12 h after the environment had been removed. The stress state at each crack configuration was extracted from a three-dimensional elastic–plastic finite element simulation, which was tailored to match the quantitative information acquired from the neutron radiographs of the fracture process. To gain insight into the evolution of hydrogen concentration with crack propagation, a modeling scheme for stress-assisted hydrogen diffusion was also employed.

An adaptive phase-field simulation for hydrogen embrittlement fracture with multi-patch isogeometric method

This work proposes a phase-field framework for hydrogen-assisted failures of brittle materials using multi-patch isogeometric modeling, considering the effects of hydrogen concentration field evolution and concentration accumulation on material fracture behavior. A phase-field fracture formulation for stress-assisted hydrogen diffusion is derived within the framework of multi-patch IGA and applied to the simulation of hydrogen embrittlement fracture phenomenons in complex structural metallic materials. Nitsche’s method imposes continuous conditions to handle the coupling between interfaces, ensuring compatibility between patches and continuity of variables on the coupling edges. In response to the excessive computational burden of the phase-field method, a refinement-correction loop adaptive strategy based on combined refinement indicators is proposed to improve computational efficiency. Several numerical examples are compared with reference cases to verify the proposed model’s effectiveness, and the adaptive strategy’s performance is tested in simulating hydrogen embrittlement failure.

Effect of tempering temperature on stress-assisted hydrogen diffusion and hydrogen-induced embrittlement in a high strength low alloy steel

Hydrogen can be used as a probe to detect defects created during tensile tests. Through hydrogen diffusion behavior investigated under two loading ways in a high strength low alloy steel tempered at three different temperatures, the mechanism of embrittlement resulted from dynamic hydrogen charging was elaborated. Before yielding, the stress has no significant effect on the hydrogen diffusivity. However, near yield stress, the hydrogen diffusion varies with sustaining the loaded time, while at the plastic region, the hydrogen diffusivity returns to a relative lower and stable value. In addition, hydrogen solubility monotonously increases with elastic stress in all samples tempered at different temperatures, whereas the plastic stress can differently alter the steady state hydrogen permeation flux from sample to sample depending on the tempering temperature. Hence, all samples tempered at different temperatures can be damaged in advance by dynamic hydrogen charging but the sensitivity to hydrogen embrittlement can still be alleviated by raising tempering temperature.

Prediction of diffusion assisted hydrogen embrittlement failure in high strength martensitic steels

A stress assisted hydrogen diffusion transport model, a dislocation-density-based multiple-slip crystalline plasticity formulation, and an overlapping fracture method were used to investigate hydrogen diffusion and embrittlement in lath martensitic steels with distributions of M23C6 carbide precipitates. The formulation accounts for variant morphologies based on orientation relationships (ORs) that are uniquely inherent to lath martensitic microstructures. The interrelated effects of martensitic block and packet boundaries and carbide precipitates on hydrogen diffusion, hydrogen assisted crack nucleation and growth, are analyzed to characterize the competition between cleavage fracture and hydrogen diffusion assisted fracture along preferential microstructural fracture planes. Stresses along the three cleavage planes and the six hydrogen embrittlement fracture planes are monitored, such that crack nucleation and growth can nucleate along energetically favorable planes. High pressure gradients result in the accumulation of hydrogen, which embrittles martensite, and results in crack nucleation and growth along {110} planes. Cleavage fracture occurs along {100} planes when there is no significant hydrogen diffusion. The predictions indicate that hydrogen diffusion can suppress the emission and accumulation of dislocation density, and lead to fracture with low plastic strains.

Multi-scale simulation of hydrogen influenced critical stress intensity in high Co–Ni secondary hardening steel

Abstract Hydrogen embrittlement was an important and long-standing problem in the fields of steels, especially ultra-high strength steels. In order to simulate the ability of hydrogen embrittlement resistance for high Co–Ni secondary hardening steels, a multi-scale simulation method with four steps was used to calculate the critical stress intensity (K IC ) and hydrogen influenced critical stress intensity (K ISCC ). For the four steps: the atomic scale and nm scale simulation were mainly used to simulate the effect of stress-assisted hydrogen diffusion at the crack tip; the μm scale simulation was used to handle the effect of microstructure; the cm simulation was used to analyze the size effect. As the effect of hydrogen concentration at the crack tip, the simulation results of critical cohesive strength of the Fe(110) at the crack tip decreased by 82.3%. The μm scale simulation showed the improvement of fracture toughness with the help of austenite layer between martensite laths. Compared with the mechanical properties of 300 M and AerMet100 steels, the accuracy of this simulation method was proved.

Hydrogen Diffusion in Metals Assisted by Stress: 2D Numerical Modelling and Analysis of Directionality

Hydrogen diffusion within a metal or alloy is conditioned by the stress-strain state therein. For that reason it is feasible to consider that hydrogen diffuses in the material obeying a Fick type diffusion law including an additional term to account for the effect of the stress state represented by the hydrostatic stress. In this paper the hydrogen transport by diffusion in metals is modelled in notched specimens where loading generates a triaxiality stress state. To this end, two different approaches of stress-assisted hydrogen diffusion, one-dimensional (1D) and two-dimensional (2D), were compared in the vicinity of the notch tip in four notched specimens with diverse triaxiality level at two different loading rates. The obtained results show that the 2D approach predicts lower values of hydrogen concentration than the 1D approach, so that a loss of directionality of hydrogen diffusion, depending on both notch geometry parameters (radius and depth) and loading rate, appears when a 2D approach is considered.

Optimization of the simulation of stress-assisted hydrogen diffusion for studies of hydrogen embrittlement of notched bars

The stress-strain assisted hydrogen diffusion in metals under variable loading is concerned as a key element of elucidation of hydrogen embrittlement (HE). The suitability of simplified treatments of hydrogen diffusion in notched solids under monotonic loading is addressed comparing various 1D and 2D modeling approaches with the purpose to assess if generated approximate solutions can provide acceptable results along the diffusion depth towards prospective rupture sites, so that quite more expensive simulations may be eluded. For different geometry-and-loading cases, respective time-depth domains are revealed where certain simplified procedures can be fairly suitable to carry out calculations of metal hydrogenation for the purposes of HE analysis and control, while the choice of the optimum strategy for the stress-strain assisted diffusion simulations in notched members is case- and purpose-dependent.

Application of hydrogen influenced cohesive laws in the prediction of hydrogen induced stress cracking in 25%Cr duplex stainless steel

Cohesive zone finite element modeling is applied in the simulation of hydrogen induced stress cracking in 25%Cr duplex stainless steel. Hydrogen influence is implemented in linear and polynomial cohesive laws. Suitability of the laws in prediction of hydrogen induced stress cracking is investigated by applying models of U and V-notched tensile specimens representing a 25%Cr duplex stainless steel component submerged in sea water under cathodic protection (CP). Fracture prediction is performed by a three step procedure; elastic plastic stress analysis, stress assisted hydrogen diffusion and cohesive stress analysis. Local cohesive stress fields as well as the time to fracture initiation are investigated as a function of the shape of the traction separation laws and the element size for three levels of tensile stress. Simulated results are also compared with results from laboratory tensile tests and discussed with respect to the suitability of describing fracture initiation and fracture mechanism of the steel. The results show that the polynomial law and a mesh size of 0.5 μm gives the most accurate description of the local cohesive stress field. The simulated time to fracture is closest to laboratory test results for stresses of 0.85–0.9 times the yield strength.

Sub critical crack growth in hydrogen assisted cracking of cold drawn eutectoid steel

The fracture behaviour of eutectoid cold drawn steel wires under constant load in hydrogen charged condition was evaluated. Hydrogen charging was obtained by dipping steel wires in ammonium thiocyanate solution. Sub-critical crack growth was monitored by means of Acoustic emission (AE) technique. Fractographic analysis revealed a mixed mode crack propagation (mode I and mode II) characterized by a multi-terrace appearance of the surface fracture. A modification of the macroscopic mechanical behaviour of the steel was also evidenced by micro hardness measurements. A simplified stress-assisted hydrogen diffusion model was used to interpret experimental observations and to estimate a theoretical crack propagation rate. Such a value was in accordance with that obtained from the analysis of AE data.

On the Effect of Hydrogen on the Fracture Toughness of Steel

A model is developed to quantify the effect of hydrogen on the critical stress intensity factor or fracture toughness of steels. The stress-assisted hydrogen diffusion model proposed by Liu (1970) is assumed and combined with the elastic stress field around the crack tip for quantifying the hydrogen concentration at the crack tip. Introducing a fracture criterion as the critical hydrogen concentration at a critical distance ahead of the crack tip, this model is successfully applied to the interpretation of hydrogen embrittlement behavior in a piping material. Experimental data at constant temperature were used to validate the model. With further development, the model has the potential to predict fracture toughness values at temperatures other than the test temperature.

Effects of hydrogen on the mechanical properties of Ti–Al–Zr alloy

The distribution and structure of hydrides in the alloy and the effects of hydrogen on the mechanical properties of the alloy have been studied using electron microscope and mechanical test systems. The results indicated that the hydrides in Ti–Al–Zr alloy formed as platelets and had the fcc δ-structure with lattice parameter a = 0.438nm. The alloy showed different sensitivities to the hydrogen embrittlement in tensile tests and impact tests. Hydrogen and formed hydrides induced hardening and caused slight loss in the ductility examined in the tensile tests. However, the impact toughness decreased dramatically with increasing hydrogen concentrations, indicated that the critical hydrogen concentration required to embrittle the alloy might be lower than 700wppm. The difference in ductility measured in the two test methods were thought being resulted from the stress assisted hydrogen diffusion and the hydride formation in the impact specimen and in the necking area of a tensile specimen. The results have been discussed extensively.

Hydrogen embrittlement of a V4Cr4Ti alloy evaluated by different test methods

The hydrogen embrittlement behavior of a V4Cr4Ti alloy (NIFS-Heat 2) has been studied using tensile tests, impact tests and J integral tests. Hydrogen in the alloy was high up to 310 wppm by exposures in a hydrogen atmosphere. The alloy showed different sensitivities to the hydrogen embrittlement in the tests. Hydrogen-induced solid solution hardening occured but caused slight loss in the ductility of the alloy before 215 wppm H in the tensile test. However, the impact toughness, J 1c and the tearing modulus ( T mat) decreased largely with increasing hydrogen concentration, indicated that the critical hydrogen concentration required to embrittle the alloy might be lower than 130 wppm. The fracture characteristics varied a lot for the different specimens. All these differences were thought being resulted from the stress concentration and the stress assisted hydrogen diffusion in the compact tension specimen and in the necking area of a tensile specimen.

Numerical modelling of hydrogen embrittlement of cylindrical bars with residual stress fields

A diffusion-based numerical model was developed to predict the life of cylindrical structural elements suffering hydrogen embrittlement in the presence of residual stress fields. The formulation of the model is based on stress-assisted hydrogen diffusion from the external environment into the metallic material and on a fracture criterion including both the stress level and a critical hydrogen concentration at the fracture point. The model is able to elucidate the effect on hydrogen embrittlement of residual stress fields which are frequently introduced into the engineering components during the manufacturing process.

Finite-element modeling of stress-assisted hydrogen diffusion in 316L stainless steel

We present a numerical investigation of stress-assisted hydrogen diffusion in AISI 316L stainless steel by means of a diffusion software assembled to a finite-element elastoplastic code. Notched cylindrical bars with different radii of the notches are simulated and the effect of the stress field on the process of hydrogen transport is analyzed. The numerical results show that, regardless of the stress field, hydrogen penetration depths always remain smaller than 0.1 mm for simulated durations of the tests of about two months. If the effect of hydrogen damage is represented by a crack originating from the root of the notch, then we observe a decrease in the load-bearing capacity of the bars, although the contours of hydrogen concentration are remarkably similar to those found in bars without cracks.