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.     

Effect of hydrogen on the tensile properties of 42CrMo4 steel quenched and tempered at different temperatures

The influence of hydrogen on the mechanical behaviour of a 42CrMo4 tempered martensitic steel was investigated by means of tensile tests on both smooth and circumferentially-notched round-bar specimens pre-charged with gaseous hydrogen in a pressurized reactor.Hydrogen solubility was seen to decrease with increasing tempering temperature. Moreover, hydrogen embrittlement measured in notched specimens was much greater in the grades with higher hardness, tempered at the lowest temperatures, where a change in the fracture micromechanism from ductile in the absence of hydrogen to intermediate and brittle in the presence of hydrogen was clearly observed. Results were discussed through FEM simulations of local stresses acting on the process zone.     

Hydrogen embrittlement of the coarse grain heat affected zone of a quenched and tempered 42CrMo4 steel

The coarse grain heat affected zone (CG-HAZ) of welds produced in a quenched and tempered 42CrMo4 steel was simulated by means of a laboratory heat treatment consisting in austenitizing at 1200 °C for 20 min, oil quenching and finally applying a post weld heat treatment at 700 °C for 2 h (similar to the tempering treatment previously applied to the base steel). A tempered martensite microstructure with a prior austenite grain size of 150 μm and a hardness of 230 HV, similar to the aforementioned CG-HAZ weld region, was produced. The effect of the prior austenite grain size on the hydrogen embrittlement (HE) behaviour of the steel was studied comparing this coarse-grained microstructure with that of the fine-grained base steel, with a prior austenite grain size of 20 μm.The specimens used in this study were charged with hydrogen gas in a reactor at 19.5 MPa and 450 °C for 21 h. Cylindrical specimens were used to determine hydrogen uptake and hydrogen desorption behaviour. Smooth and notched tensile specimens tested under different displacement rates were also used to evaluate HE.Embrittlement indexes, EI, were generally quite low in the case of hydrogen pre-charged tensile tests performed on smooth tensile specimens. However, very significant embrittlement indexes were obtained with notched tensile specimens. It was observed that these indexes always increase as the applied displacement rate decreases. Moreover, hydrogen embrittlement indexes also increase with increasing prior austenite grain size. In fact, the embrittlement index related to the reduction in area, EI_(RA), reached values of over 20% and 50% for the fine and coarse grain size steels, respectively, when tested under the lowest displacement rates (0.002 mm/min).A comprehensive fractographic analysis was performed and the main operative failure micromechanisms due to the presence of internal hydrogen were determined at different test displacement rates. While microvoids coalescence (MVC) was found to be the typical ductile failure micromechanism in the absence of hydrogen in the two steels, brittle decohesion mechanisms (carbide-matrix interface decohesion, CMD, and martensitic lath interface decohesion, MLD) were observed under internal hydrogen. Intergranular fracture (IG) was also found to be operative in the case of the coarse-grained steel tested under the lowest displacement rate, in which hydrogen accumulation in the process zone ahead of the notch tip is maximal.     

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.     

Hydrogen embrittlement and segregation of impurities in type 10MnNi2Mo pressure vessel steel

This paper summarizes the findings of a study in which the brittle fracture resistance and hydrogen embrittlement resistance of a 10MnNi2Mo steel were investigated both in the quenched and tempered state, and after a subsequent heat treatment that simulated the thermal cycling in the heat-affected zone around a weld joining thick-walled components. This simulating treatment included two alternative stress relieving procedures, at 650 and 580°C respectively. The simulating treatment caused marked coarsening of the prior austenitic grains; had no substantial effect on the static and dynamic fracture toughness, nor on the transition temperatures; but reduced the resistance against hydrogen embrittlement, and during the development of hydrogen-induced cracks produced some intercrystalline facettes. A supplementary study of surface segregation, by Auger electron spectroscopy on the free surfaces of the specimens, indicated that this incidence of intercrystalline failure may be at least partly ascribed to the pronounced segregation of phosphorus during stress relieving at both of the temperatures employed. This made an interesting comparison with 20Mn steel, where the simulating treatment profoundly reduced both the fracture and notch toughness levels but led to favourable resistance to hydrogen embrittlement. The fact that no intercrystalline failures were detected in hydrogen-charged 20Mn steel is ascribed in part to the lower observed segregation activity at the stress relieving temperatures, in part to the resultant microstructure of this non-alloyed steel.     

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 behaviors at different deformation temperatures in as-quenched low-carbon martensitic steel

The present study investigated hydrogen-related fractures at different deformation temperatures ranging from ?100 °C to 100 °C in low-carbon martensitic steel. The sensitivity to hydrogen embrittlement increased as the temperature decreased from 100 °C to 0 °C, while it decreased as the temperature decreased further below 0 °C. We characterized the fracture surface types from the morphological and crystallographic aspects and found that the fraction of hydrogen-embrittled surfaces exhibited a similar temperature dependence on the sensitivity to hydrogen embrittlement. The qualitative discussion suggested that the degree of hydrogen accumulation exhibits a peak value in the medium temperature range, which has the same tendency as the sensitivity to hydrogen embrittlement confirmed experimentally. Thus, we proposed that the effect of deformation conditions on the sensitivity to hydrogen embrittlement could be explained on the basis of the hydrogen accumulation behavior.     

Ductility and fatigue properties of low nickel content type 316L austenitic stainless steel after gaseous thermal pre-charging with hydrogen

The susceptibility of low nickel content type 316L austenitic stainless steel to hydrogen was quantified using low strain rate tensile tests and strain-controlled low-cycle fatigue life measurements. Both tests were performed under air condition after charging with high-pressure 10-MPa hydrogen gas at 300 °C for eight days. No significant influence of hydrogen was recognized in 0.2% proof stress, but the strain at fracture and reduction area was decreased significantly in both hydrogen pre-charged and in gaseous hydrogen conditions compared to companion tests conducted in air. The decrease of fatigue life in the high strain amplitude region was related to a significant decrease in the plastic component while the effect of hydrogen on the elastic component was negligible. Highly localized deformation and a pronounced martensite transformation occurred near the site of the fracture surface in the high strain amplitude regime, resulting in the early formation of abundant micro-surface cracks in this regime of the hydrogen pre-charged samples.     

Hydrogen trapping in high strength martensitic steel after austenitized at different temperatures

Hydrogen trapping behavior has been investigated by means of thermal desorption spectrometry (TDS) for a high strength steel after it is austenitized at the temperature range of 880–1250 °C, oil quenched, and tempered at 200 °C. Results show that with increasing austenitizing temperature, the pre-charged hydrogen concentration in the steel first decreases and then increases, being the lowest value at the austenitizing temperature of 1050 °C. The variation of hydrogen concentration with austenitizing temperature is related to the differences in the prior austenite grain size and solute Nb content, which may act as shallow hydrogen traps in the steel. The difference in the pre-charged hydrogen concentration can account for the previously reported result on delayed fracture resistance of the steel after austenitized at different temperatures.     

Effects of hydrogen pressure and prior austenite grain size on the hydrogen embrittlement characteristics of a press-hardened martensitic steel

The effects of hydrogen gas pressure and prior austenite grain size (PAGS) on the susceptibility of a 22MnB5 press-hardened martensitic steel (PHS) to hydrogen embrittlement were studied. The hydrogen test apparatus at NIST-Boulder was modified for tensile testing of plate-type and sheet-type specimens in gaseous hydrogen. This modification made it possible to evaluate the slow strain rate tensile (SSRT) properties of the PHS with three different PAGS at various hydrogen pressures (0.21 MPa–5.5 MPa). SSRT testing in gaseous hydrogen resulted in significant reductions of both the tensile strength and ductility, as compared to those measured in air. In addition, the presence of gaseous hydrogen resulted in a transition in fracture morphology from the near-45° slant fracture to a more brittle fracture along a plane perpendicular to the tensile axis. The hydrogen-affected fracture zones were connected to the sheet specimen free surfaces, signifying the effect of external hydrogen. The fracture surfaces of the hydrogen-embrittled specimens contained relatively flat, “cleavage-like” facets, the size of which depended on the PAGS or packet size. The PHS having the largest PAGS represented generally larger secondary cracks and straighter crack paths in addition to a greater area fraction of the “cleavage-like” facets, likely indicative of a lower frequency of crack deflections. Compared to the largest PAGS condition, the two PHS with smaller PAGS were more resistant to the hydrogen-induced fracture especially at relatively low hydrogen gas pressures (<0.52 MPa). In contrast, with an increase in hydrogen pressure, all PHS specimens exhibited significant decreases in tensile strength and ductility. The positive effect of refining martensitic microstructure, at the low hydrogen pressures, is likely associated with improved toughness of the smaller grain-sized specimens.     

Hydrogen gaseous embrittlement effect over mechanical properties of an experimental X-120 microalloyed steel subjected to heat treatments and different cooling rates

This research work aimed to determine the hydrogen gas pressure effect on the mechanical properties of an experimental X-120 microalloyed steel, subjected to heat treatments and quenched in different mediums. The steel in its as-received condition was reheated at 900 °C and quenched in spray water (900QSW), pressurized air (900QPA), and emulsion of water-oil medium (900QWO) which produces complex microstructures formed by martensite–bainite–acicular ferrite; otherwise, reheated at 820 °C and quenched in oil media (820QO) which produces a banded martensite-polygonal ferrite microstructure. To determine the hydrogen embrittlement susceptibility, in-situ tension tests were developed at 1, 4, and 7 MPa of hydrogen gas pressure. The results showed that as hydrogen gas pressure increases, the mechanical properties reduce in all quenched conditions, being the most susceptible condition the 820QO sample which presented the higher embrittlement index; on the contrary, the less susceptible condition was the 900QPA sample.     

Effect of PTFE coating on enhancing hydrogen embrittlement resistance of stainless steel 304 for liquefied hydrogen storage system application

The phenomenon of hydrogen embrittlement phenomenon is known to be a major obstacle to proposed to overcome this phenomenon. In the present study, polytetrafluoroethylene (PTFE), which is known to be an effective hydrogen adsorption and desorption material, was coated on the surface of stainless steel 304 to improve its hydrogen embrittlement resistance. To make a hydrogen embrittlement environment, electrochemical hydrogen pre-charging was applied to the PTFE-coated stainless steel 304. To investigate the effects of PTFE coating on the hydrogen embrittlement resistance of stainless steel 304, the Charpy V-notch impact (CVN) test was performed under three different temperatures: 25, −83, and −196 °C. Additionally, hydrogen concentration, electron back scatter diffraction (EBSD), and scanning electron microscopy (SEM) evaluations were carried out to verify the results of the CVN impact test. The PTFE coating did not have a significant effect on the quantitative reduction of hydrogen concentration; however, we confirmed its excellent performance in terms of toughness reduction due to the increase in hydrogen loading time at room temperature.     

Effect of the loading mode and temperature on hydrogen embrittlement behavior of 15Cr for steam turbine last stage blade steel

The hydrogen embrittlement of 15Cr martensitic stainless steel, for steam turbine last stage blades, was systematically studied by using slow strain rate tensile (SSRT) test and constant loading tensile (CLT) test at room temperature and 80 °C to simulate the service conditions. It was shown that, despite the lower hydrogen concentration absorbed during SSRT, the hydrogen-induced fracture strength of 15Cr steel for SSRT was lower than the threshold fracture strength for CLT. This was due to the remarkable enhancement in local hydrogen concentration due to the transportation of hydrogen by mobile dislocation during SSRT. In addition, although the higher hydrogen concentration was absorbed during SSRT at 80 °C, the hydrogen embrittlement susceptibility of 15Cr steel for SSRT at 80 °C was lower than that at room temperature, because the degree of local hydrogen accumulation decreased at a higher temperature.     

Low-temperature tensile and impact properties of hydrogen-charged high-manganese steel

Low-temperature mechanical properties of a high-manganese austenitic steel were evaluated with and without hydrogen pre-charging to examine the applicability of the alloy as a material for hydrogen infrastructure. The high-manganese steel, along with the conventional 304 and 316 L austenitic steels, was examined for hydrogen-related properties including hydrogen content after gas-phase pre-charging, tensile properties, and Charpy impact toughness at different temperatures ranging from room temperature to −80 and −196 °C, respectively, and the resultant fracture surfaces. Under hydrogen-charged conditions, the high-manganese steel showed low-temperature mechanical properties comparable to those of conventional austenitic steels, suggesting the potential of the alloy for structural applications in hydrogen environment.     

Crack propagation analysis of hydrogen embrittlement based on peridynamics

We introduced a coupled peridynamic hydrogen diffusion and fracture model to solve the hydrogen embrittlement fracture of low alloy steel AISI 4340. In this model, the influence of temperature on hydrogen diffusion coefficient is considered, and a new peridynamic constitutive analysis method is used to simulate the crack propagation of hydrogen embrittlement. We verified the model in 3D using the experimental test of the hydrogen embrittlement cracking process of AISI 4340 steel in 0.1 N H₂SO₄ solution from the literature. Considering different ambient temperatures, it is found that the crack propagation is highly similar to the experimental results. Based on the numerical analysis of peridynamics, the model can numerically simulate the hydrogen embrittlement fracture of AISI 4340 steel, and obtain a visual demonstration of the entire process of hydrogen atom diffusion and crack growth.     

Comparative study of embrittlement of quenched and tempered steels in hydrogen environments

The study of steels which guarantee safety and reliability throughout their service life in hydrogen-rich environments has increased considerably in recent years. Their mechanical behavior in terms of hydrogen embrittlement is of utmost importance. This work aims to assess the effects of hydrogen on the tensile properties of quenched and tempered 42CrMo4 steels. Tensile tests were performed on smooth and notched specimens under different conditions: pre-charged in high pressure hydrogen gas, electrochemically pre-charged, and in-situ hydrogen charged in an acid aqueous medium. The influence of the charging methodology on the corresponding embrittlement indexes was assessed. The role of other test variables, such as the applied current density, the electrolyte composition, and the displacement rate was also studied. An important reduction of the strength was detected when notched specimens were subjected to in-situ charging. When the same tests were performed on smooth tensile specimens, the deformation results were reduced. This behavior is related to significant changes in the operative failure micromechanisms, from ductile (microvoids coalescence) in absence of hydrogen or under low hydrogen contents, to brittle (decohesion of martensite lath interfaces) under the most stringent conditions.     

Some material toughness properties of a series of turbine casing bolts removed from service after over 100 000 hours

The present study was aimed at examining the effects of temperature on both the Charpy and slow strain rate fracture toughness properties of a series of large steam turbine casing bolts which had been subjected to temperatures of around 500°C for some 122 000 h. Also, the various toughness properties were assessed in terms of the existing fracture toughness–Charpy correlations.The present CrMoV steel bolts had suffered varying degrees of Reverse Temper Embrittlement (RTE) during service and exhibited strong fracture toughness–temperature trends. Although no single fracture toughness–Charpy correlation effectively described the full toughness–temperature transition curves, it was evident that, for embrittled and partially embrittled bolts, the Wallin correlation best decribed the lower temperature portion of the curves. At the upper temperature region all embrittlement conditions exhibited reasonable agreement with the upper bound correlation of Barsom and Wolfe.Finally, it was shown that the fracture toughness transition temperatures, T_K, and the Charpy Fracture Appearance Transition Temperatures (FATT) exhibited good agreement with the fracture toughness transition temperature being equal to the temperature at which the fracture toughness value was 3000 N/MM^3/2.     

Modeling fatigue life and hydrogen embrittlement of bcc steel with unified mechanics theory

The fatigue life estimation of metals operating in hydrogen-rich environments such as hydrogen pipelines, hydrogen-burning internal combustion engines, etc. is important. Studies in the past 40 years have shown that the diffusion of hydrogen into steel and other metals causes various chemical reactions, hydrogen-material interactions, and microstructural changes. That leads to hydrogen embrittlement (HE) and other types of hydrogen damage mechanisms including hydrogen environmentally assisted cracking (HEAC). Hydrogen embrittlement mechanisms, such as hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) can have synergetic effects in steel depending on the hydrogen concentration level. At concentrations above and below the critical hydrogen concentration, HEDE and HELP dominate the embrittlement process, respectively. Different HE mechanisms result in distinctly different fracture modes, both ductile and fully brittle. The ultrasonic vibration fatigue life of bcc steel with a ferrite-pearlite microstructure pre-charged with hydrogen at different concentrations is studied. Modeling is based on the unified mechanics theory (UMT), which does not need any empirical dissipation/degradation potential function or an empirical void evolution function. However, the UMT does require analytical derivation of the thermodynamic fundamental equation of the material, which is used to calculate the thermodynamic state index (TSI) of the material. The UMT is ab-initio unification of the second law of thermodynamics and the universal laws of motion of Newton [1]. Dissipation/degradation evolution is governed by Boltzmann's second law of thermodynamics entropy formulation. The original contribution of this paper is the derivation of the thermodynamic fundamental equation of pre-hydrogen embrittled bcc steel subjected to ultrasonic very high cycle fatigue and the numerical simulations of fatigue life estimation using the proposed novel model. The synergetic interaction of hydrogen embrittlement mechanisms in steel and other metallic materials, i.e., HELP and HEDE at different hydrogen concentrations (HELP + HEDE model) is also studied, reviewed, and applied. The synergetic effects between ultrasonic vibration fatigue life and synergistically active hydrogen embrittlement mechanisms in low carbon bcc steel (S355J2+N, equivalent to ASTM A656), according to the HELP + HEDE model for HE, is modeled for the first time using UMT and also thoroughly discussed.     

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 of super duplex stainless steel in acid solution

Super duplex stainless steel (SDSS) is a good choice of material when resistance to harsh environments is needed. Despite the material’s excellent corrosion resistance and high strength, a number of in-service failures have been recorded. The root cause of these failures was environmentally induced cracking initiated at manufacturing and in-service metallurgical defects. In this study the hydrogen embrittlement of pre-strained super duplex stainless steel specimens was investigated after 48 h cathodic charging in 0.1 M H₂SO₄. The metallurgical changes that resulted from four levels of cold work (4, 8, 12, and 16% plastic strain) were considered and their effect on the embrittlement of the SDSS alloy was investigated. After hydrogen charging, the specimens were pulled immediately to failure and the mechanical properties evaluated. The obtaining fracture morphology was investigated using low and high magnification microscopy. Experimental results indicated that charging the super duplex stainless steel alloy with hydrogen caused varying degrees of embrittlement depending on cold work level. Increasing cold work resulted in a reduction of the elongation to failure. Microscopic investigation confirmed the significant effect of cold work on the hydrogen embrittlement susceptibility of the super duplex stainless steel alloy investigated.