A quantitative description on fracture toughness of steels in hydrogen gas

Fracture toughness or critical stress intensity factor of many steels can be reduced by hydrogen gas. In this paper, a simple quantitative model to predict the fracture toughness of steels in gaseous hydrogen is proposed. This model is based on the assumption that fracture of a cracked body occurs when the maximum principal stress ahead of the crack tip reaches the critical cohesive stress for crack initiation. The critical stress is inversely proportional to the accumulated hydrogen concentration. The notion is that the crack will initiate at the elastic-plastic boundary ahead of the crack tip when hydrogen concentration reaches a maximum value after a long-term hydrogen diffusion assisted by the hydrostatic stress. The model describes the dependence of fracture toughness on hydrogen pressure, temperature and yield strength of steels. It can be used to quantitatively predict fracture toughness of steels in hydrogen gas, particularly in high pressure. Some experimental data reported in literature were used to validate the model, and a good agreement was obtained.     

Radiation embrittlement modelling for reactor pressure vessel steels: I. Brittle fracture toughness prediction

Modelling for the irradiation effect on brittle fracture toughness of reactor pressure vessel (RPV) steel is performed on the basis of the probabilistic model for fracture toughness prediction proposed by the authors earlier. The irradiation effect on parameters controlling plastic deformation and brittle fracture of RPV steels is analyzed. The physical mechanisms are considered which control the cleavage microcrack nucleation for RPV steels in the unirradiated and irradiated states and also in state after post-irradiation annealing. Prediction of the temperature dependence of brittle fracture toughness is performed as applied to irradiated 2.5Cr–Mo–V reactor pressure vessel steel. Modelling of the fluence effect and the phosphorus and copper content effect on brittle fracture toughness is carried out. It is shown that the probabilistic model based on a new formulation for brittle fracture criterion allows the adequate modelling for the irradiation effect on fracture toughness for RPV steel. Application of alternative models is discussed for fracture toughness prediction for irradiated RPV steels.     

Effects of hydrogen on the fracture toughness of CrMo and CrMoV steels quenched and tempered at different temperatures

Tempering temperatures ranging between 500 and 720 °C were applied in order to analyse the relationship between steel microstructure and the deleterious effect of hydrogen on the fracture toughness of different CrMo and CrMoV steels. The influence of hydrogen on the fracture behaviour of the steel was investigated by means of fracture toughness tests using CT specimens thermally pre-charged with hydrogen gas.First, the specimens were pre-charged with gaseous hydrogen in a pressurized reactor at 19.5 MPa and 450 °C for 21h and elasto-plastic fracture toughness tests were performed under different displacement rates. The amount of hydrogen accumulated in the steel was subsequently determined in order to justify the fracture toughness results obtained with the different steel grades. Finally, scanning electron microscopy was employed to study both the resulting steel microstructures and the fracture micromechanisms that took place during the fracture tests.According to the results, hydrogen solubility was seen to decrease with increasing tempering temperature, due to the fact that hydrogen microstructural trapping is lower in relaxed martensitic microstructures, the strong effect of the presence of vanadium carbides also being noted in this same respect. Hydrogen embrittlement was also found to be much greater in the grades tempered at the lowest temperatures (with higher yield strength). Moreover, a change in the fracture micromechanism, from ductile (microvoid coalescence, MVC), in the absence of hydrogen, to intermediate (plasticity-related hydrogen induced cracking, PRHIC) and brittle (intergranular fracture, IG), was appreciated with the increase in the embrittlement indexes.     

Hydrogen embrittlement of structural steels: Effect of the displacement rate on the fracture toughness of high-pressure hydrogen pre-charged samples

The aim of this paper is to study the effect of the displacement rate on the fracture toughness under internal hydrogen of two different structural steels grades used in energy applications. To this end, steel specimens were pre-charged with gaseous hydrogen at 19.5 MPa and 450 °C for 21 h and then fracture toughness tests were carried out in air at room temperature. Permeation experiments were also conducted to obtain the hydrogen diffusion coefficients of the steels. It was observed that the lower the displacement rate and the higher the steel yield strength, the stronger the reduction in fracture toughness due to the presence of internal hydrogen. A change in the fracture micromechanism was also detected. All these findings were justified in terms of hydrogen diffusion and accumulation in the crack front region in the different steel specimens.     

Material performance of age-hardened beryllium–copper alloy,CDA-C17200, in a high-pressure,gaseous hydrogen environment

In order to develop safer and more energy-efficient, hydrogen pre-cooling systems for use in hydrogen refueling stations, it is necessary to identify a high-strength metallic material with greater thermal conductivity and lower susceptibility to hydrogen embrittlement, as compared with ordinary, stable austenitic stainless steels. To accomplish this task, the hydrogen compatibility of a precipitation-hardened, high-strength, copper-based alloy was investigated by slow-strain-rate tensile (SSRT), fatigue-life, fatigue-crack-growth (FCG) and fracture toughness tests in 115-MPa hydrogen gas at room temperature. The hydrogen solubility and diffusivity of the alloy were also determined. The hydrogen solubility of the alloy was two or three orders of magnitude lower than that of austenitic stainless steels. The alloy also demonstrated absolutely no hydrogen-induced degradation of its strength properties, a factor which could contribute to the reduction of costs related to the construction and maintenance of hydrogen refueling stations, owing to the downsizing and improved cooling performance of the pre-cooling systems.     

Numerical simulation of the hydrogen trapping effect on crack propagation in API 5CT P110 steel under cathodic overprotection

The possibility of the development of hydrogen embrittlement processes increases in cathodic protection systems when cathodic overprotection occurs, and large amounts of hydrogen are produced. Additionally, the hydrogen embrittlement susceptibility of steel depends on solubility, diffusivity, and hydrogen trapping. This paper presents a numerical simulation of the reversible and irreversible hydrogen trapping effects on crack propagation in API 5CT P110 steel using a model based on a synthesis of fracture mechanics and continuum damage mechanics, in which the trapping term of the diffusion equation was replaced by the trapping terms of other more complete model. The simulation was performed with using a C(T) specimen loaded in the tensile opening mode, in the linear elastic regime, in plane strain state, under the action of a static mechanical loading and the effect of hydrogen. The simulations showed that the material degradation ahead of the crack tip increases with increases in hydrogen concentration at the crack tip due to the hydrogen trapping effect. Furthermore, the process of onset and crack growth in material with irreversible traps is slower than that in material with reversible traps. These results are consistent with macroscopic observations of the trapping effect, providing a better understanding of the hydrogen embrittlement in structural steels.     

Simulation of JR-curves for reactor pressure vessels steels on the basis of a ductile fracture model

A procedure for the prediction of J_R-curves for reactor pressure vessels (RPVs) steels in the initial and embrittled states is presented. Prediction of the J_R-curves is performed over the ductile fracture temperature range on the basis of a ductile fracture model. A procedure for the determination of ductile fracture model parameters based on tests of smooth and notched cylindrical specimens is proposed. The stress and strain fields near the stationary and growing crack tip are analyzed by FEM. Approximate analytical solutions for stress and strain fields near a growing crack tip are proposed. Comparison of the predicted J_R-curves and experimental 2T–CT data for the initial and embrittled RPV 2Cr–Ni–Mo–V steels for WWER-1000 is performed.     

Investigating the influence mechanism of hydrogen partial pressure on fracture toughness and fatigue life by in-situ hydrogen permeation

In this work, we investigate the influence mechanism of hydrogen partial pressure on fracture toughness and fatigue life of a high strength pipeline steel. Both fracture toughness test and fatigue life test are carried out under different hydrogen partial pressure. The experimental results show that with the increasing of hydrogen partial pressure, fracture toughness and fatigue life decrease and the decrease trends gradually flatten out. Hydrogen has a larger effect on fatigue life than fracture toughness. Only 3% hydrogen gas can cause a 67.7% decrease of fatigue life. The in-situ hydrogen permeation test is performed respectively in 2 MPa, 5 MPa and 8 MPa hydrogen partial pressure. With the increasing of hydrogen partial pressure, the increase trend of hydrogen permeation current gradually tends to be gentle, which indicates that the hydrogen atoms entering into the material gradually become saturated. This result can be used to clarify the influence mechanism of hydrogen partial pressure on fracture toughness and fatigue life.     

A new engineering method for prediction of the fracture toughness temperature dependence for RPV steels

In order to predict the temperature dependence of fracture toughness for RPV steels with various degrees of embrittlement, up to extremely high levels, an engineering method is proposed which is similar to the Master Curve concept and named the Unified Curve concept. Wide verification of the Unified Curve concept is performed and comparison of the Master and Unified Curve concepts is carried out. It is shown that the Master Curve concept is a partial case of the Unified Curve concept.     

Evaluation of fracture toughness based on results of instrumented Charpy tests

The modified Gurson model is used to determine micromechanical toughness parameters of a weld material from the irradiation surveillance of a reactor pressure vessel. These parameters were used to numerically simulate static and dynamic fracture mechanics tests for the determination of J-resistance curves. The results were confirmed by means of experiments with irradiated subsized specimens of the original material. The essential steps of the analysis—the consideration of rate-dependent stress-strain curves and the determination of the material parameters, as well as the numerical modelling of the boundary conditions at the supports of the Charpy specimen—were validated by comparing analyses and experiments with a representative unirradiated submerged are weld material. The initiation values of the J-resistance curves converted into pseudo-plane strain fracture toughness values K_Ic allowed the conservative adjustment of the ASME reference fracture toughness curve.     

Radiation embrittlement modelling for reactor pressure vessel steels: II. Ductile fracture toughness prediction

Modelling for the irradiation effect on ductile fracture toughness of reactor pressure vessel steels (RPV) is performed on the basis of ductile fracture criterion proposed earlier by the authors. The irradiation effect on mechanisms controlling ductile fracture is considered from a physical viewpoint. Modelling of the irradiation effect is carried out on the critical strain for smooth cylindrical specimens and on the local critical strain for cracked specimens. On the basis of the performed studies a scheme that allows an evaluation of the upper shelf level of the K_IC(T) curve for irradiated RPV steels is proposed.     

Hydrogen effect on a low carbon ferritic-bainitic pipeline steel

Hydrogen effect on an API 5L X65 low carbon ferritic-bainitic steel is investigated, by evaluating the fracture toughness parameters in air and in hydrogen environment. The hydrogen environment is manifested by in situ hydrogen charging of the X65 steel, using the electrolytic solution NS4, which simulates the electrolyte trapped between the pipeline steel and the coating in a buried pipeline. The fracture toughness results of the X65 are compared to two other pipeline steels with different microstructures, namely an X52 and an X70, possessing a banded ferritic-pearlitic and banded ferritic-mixed bainitic-pearlitic microstructure, respectively. The X65 steel exhibits significant reduction of fracture toughness parameter J₀ integral due to hydrogen charging and insignificant variation of fracture toughness parameter K_Q. Comparing the three steels, the lowest reduction of J₀ integral due to hydrogen charging, is met on the X52 and the highest in the X65.     

An upper-shelf fracture toughness master curve for ferritic steels

When assessing the resistance of a steel structure to failure under load, fracture toughness is a key input variable. Since it is usually not possible to establish a priori if the anticipated service temperature (or temperatures) is in the fracture mode transition or on the upper shelf, it is useful to know the temperature dependence of fracture toughness in both the transition and on the upper shelf. In the transition, the master curve proposed by Wallin defines both the variation of the median value of fracture toughness with temperature and the scatter of fracture toughness about this median value. However, Wallin's master curve does not quantify fracture toughness on the upper shelf. In this paper we assemble a database of upper shelf fracture toughness data (J_Ic) for ferritic steels. These data demonstrate that the temperature dependence of upper J_Ic is consistent for all ferritic steels contained in the database and has the same form as the temperature dependence of the flow strength anticipated from dislocation mechanics considerations. This similarity between the temperature dependence of flow strength and the temperature dependence of J_Ic is physically expected because both directly depend upon the energy required for dislocation mobility in the ferrite matrix. Both the empirically derived model and the physical basis for the model are described.     

Notched-tensile properties under high-pressure gaseous hydrogen: Comparison of pipeline steel X70 and austenitic stainless type 304L, 316L steels

The effect of high-pressure gaseous H₂ on the fracture behavior of pipeline steel X70 and austenitic stainless steel type 304L and 316L was investigated by means of notched-tensile tests at 10 MPa H₂ gas and various test speed. The notch tensile strength of pipeline X70 steel and austenitic stainless steels were degraded by gaseous H₂, and the deterioration was accompanied by noticeable changes in fracture morphology. The loss of notch tensile strength of type 316L and X70 steels was comparable, but type 304L was more susceptible to hydrogen embrittlement than the others. In the X70 steel, hydrogen embrittlement increased as test speed decreased until the test speed reached 1.2 × 10^?3 mm/s, but the effect of test speed was not significant in 304L and 316L steels.     

Study on fracture strain of Cr–Mo steel in high pressure hydrogen

Cr–Mo steel is often used as the material of the hydrogen storage vessel, but its ductility can be deteriorated by high pressure hydrogen, which makes it possible that the local area of strain concentration on the hydrogen storage vessel made of Cr–Mo steel may fail due to excessive plastic deformation. The limit criterion of local strain established according to the study of the fracture strain is the basis for local failure assessment of the vessel. However, the correlation between the fracture strain and the stress state of Cr–Mo steel in high pressure hydrogen is still unclear, so the limit criterion of local strain for hydrogen storage vessel made of Cr–Mo steel has not been established. In this paper, the slow strain rate tensile test (SSRT) of notched specimens with different notch sizes was carried out in air, 45 MPa hydrogen and 100 MPa hydrogen, respectively. Based on the test results, the whole process from tensile to fracture of the specimens was simulated by finite element method. The distribution of stress triaxiality and plastic strain during the tensile process was analyzed, and the correlations between the stress triaxiality and the fracture strain in different environments were obtained. Finally, the limit criterion of local strain for local failure assessment of 4130X hydrogen storage vessel was established.     

Research and demonstration on hydrogen compatibility of pipelines: a review of current status and challenges

Hydrogen transportation by pipelines gradually becomes a critical engineering route in the worldwide adaptation of hydrogen as a form of clean energy. However, due to the hydrogen embrittlement effect, the compatibility of linepipe steels and associated welds with hydrogen is a major concern when designing hydrogen-carrying pipelines. When hydrogen enters the steels, their ductility, fracture resistance, and fatigue properties can be adversely altered. This paper reviews the status of several demonstration projects for natural gas-hydrogen blending and pure hydrogen transportation, the pipeline materials used and their operating parameters. This paper also compares the current standards of materials specifications for hydrogen pipeline systems from different parts of the world. The hydrogen compatibility and tolerance of varying grades of linepipe steels and the relevant testing methods for assessing the compatibility are then discussed, and the conservatism or the inadequacies of the test conditions of the current standards are pointed out for future improvement.     

Development of Prometey local approach and analysis of physical and mechanical aspects of brittle fracture of RPV steels

In the present paper, the probabilistic model for brittle fracture toughness prediction proposed by the authors earlier and known as the Prometey model, is formulated with two stochastic parameters—the critical stress for microcrack nucleation σ_d and the critical stress for microcrack propagation S_C The stochastic dependence of the critical stress for microcrack propagation S_C is experimentally studied for an RPV steel. The Prometey model with two stochastic parameters is applied to model the effect of irradiation on the fracture toughness transition curve and to predict the loss of constraint associated with shallow cracks.     

Prediction of brittle fracture of RPV steels under complex loading on the basis of a local probabilistic approach

A new local criterion of brittle fracture used to produce a probabilistic model for the temperature dependence of fracture toughness has been modified for non-isothermal and non-monotonic loading. An approach is proposed which allows calculation of the brittle fracture probability for cracked components under complex loading typical, for example, of that in pressurised thermal shock of a reactor pressure vessel. The proposed approach has been verified by comparison of calculated and experimental results for various warm pre-stressing conditions in reactor pressure vessel steels.     

Prediction of cleavage fracture from non-sharp defects using the Weibull stress based toughness scaling model

The structural integrity assessment of engineering plant is based on the principles of fracture mechanics that assume defects to be sharp cracks. Whilst conservative, this assumption may be overly conservative in some situations, e.g. for defects that are non-sharp. This paper describes the prediction of cleavage fracture initiation from blunt notches of varying root radii using the Weibull stress based toughness scaling model. Failure predictions are compared with the results of experiments performed on single edge notch bend SE(B) specimens containing both cracks and notches of varying notch root radii. Cleavage initiation sites were located close to the peak tensile stress ahead of the notch, implying that a tensile stress criterion is the main controlling factor for cleavage fracture. The cleavage fracture predictions from the toughness scaling model correlate well with the experimental data; but care needs to be taken to ensure that calibration of the Weibull parameters references fracture toughness data from constraint levels that span that of the defect of interest. This ensures the model interpolates between the constraint states used for calibration, rather than extrapolating outside the range of applicability.     

Atomistic simulation study of the grain-size effect on hydrogen embrittlement of nanograined Fe

Although hydrogen-induced fracture at grain boundaries has been widely studied and several mechanisms have been proposed, few studies of nanograined materials have been conducted, especially for grain sizes below the critical size for the inverse Hall-Petch relation. In this research work, molecular dynamics (MD) simulations are performed to investigate the hydrogen segregation and hydrogen embrittlement mechanism in polycrystalline Fe models. When the same concentration of H atoms is added, the H segregation ratio in the model with the smallest grain size is the highest observed herein, showing the high hydrogen trapping ability of small-grain Fe, while the H concentration at the grain boundaries (GBs) is, on the contrary, the lowest. Uniaxial tensile test simulations demonstrate that as the grain size decreases, the models show an increased resistance to hydrogen embrittlement, and for small-grain models (d < 10 nm), the GB-related deformation modes dominate the plastic deformation, where the segregated H mainly influences the toughness by inhibiting GB-related processes.