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 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.     

Hydrogen embrittlement and associated surface crack growth in fine-grained equiatomic CoCrFeMnNi high-entropy alloys with different annealing temperatures evaluated by tensile testing under in situ hydrogen charging

The effect of the annealing temperature after cold rolling on hydrogen embrittlement resistance was investigated with a face-centered cubic (FCC) equiatomic CoCrFeMnNi high-entropy alloy using tensile testing under electrochemical hydrogen charging. Decreasing annealing temperature from 800 °C to 750 °C decreased grain sizes from 3.2 to 2.1 μm, and resulted in the σ phase formation. Interestingly, the specimen annealed at 800 °C, which had coarser grains, showed a lower hydrogen embrittlement susceptibility than the specimen annealed at 750 °C, although hydrogen-assisted intergranular fracture was observed in both annealing conditions. Because the interface between the FCC matrix and σ was more susceptible to hydrogen than the grain boundary, the presence of the matrix/σ interface significantly assisted hydrogen-induced mechanical degradation. In terms of intergranular cracking, crack growth occurred via small crack initiation near a larger crack tip and subsequent crack coalescence, which has been observed in various steels and FCC alloys that contained hydrogen.     

Investigation on hydrogen induced cracking behaviors of Ni-base alloy

Hydrogen embrittlement of a Ni-base alloy at room temperature was investigated by slow strain rate tensile test (SSRT) under precharging or dynamic charging conditions. It was found that hydrogen embrittlement susceptibility of this alloy increased with increasing charging current density in both charging conditions. In-situ observation of hydrogen induced cracking revealed that surface crack initiation at both grain boundaries and slip bands, which should be attributed to decomposition of hydride phase during aging at room temperature. SSRT result exhibited that hydrogen diffusion in the alloy could be facilitated by deformation and as a result induced transgranular fracture of the sample. Both hydrogen induced cracking and the interaction between hydrogen and deformation played combined roles on hydrogen embrittlement of this alloy.     

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.     

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 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.     

Effects of internal hydrogen and surface-absorbed hydrogen on the hydrogen embrittlement of X80 pipeline steel

Effects of internal hydrogen and surface-absorbed hydrogen on hydrogen embrittlement (HE) of X80 pipeline steel were investigated by using different strain rate tensile test, annealing and hydrogen permeation tests. HE of X80 pipeline steel is affected by internal hydrogen and surface-absorbed hydrogen, and the latter plays a major role due to its higher effective hydrogen concentration. The HE susceptibility decreases with increasing the strain rate because it is more difficult for hydrogen to be captured by dislocations at the high strain rate. Annealing at 200 °C can weakened HE caused by internal hydrogen, while it has little effect on HE caused by surface-absorbed hydrogen. HE of X80 pipeline steel is mainly determined by the behavior of dislocation trapping hydrogen, which can be attributed to the interaction between hydrogen and dislocation.     

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.     

Eliminating reversible hydrogen embrittlement in high-strength martensitic steel by an electric current pulse

Hydrogen embrittlement has been a great issue in ultrahigh-strength automotive steel applications. Few studies on solving hydrogen embrittlement have been conducted in terms of removing hydrogen, and most studies focus on improving the resistance to hydrogen embrittlement through microalloying. In this study, an electric pulse was used to solve hydrogen embrittlement by removing hydrogen at 120 Hz (frequency), 180 μs (duration), and 200 A (current intensity). The temperature of the samples reached 105 °C under these parameters. Compared with the samples heat treated at the same temperature, the precharged samples processed by electric pulses had a lower hydrogen content and less ductility loss. Moreover, electric pulse processing can also reduce the approximate equilibrium concentration of hydrogen compared with heat treatment. This is due to the introduction of electrical free energy, which reduces the barrier to hydrogen diffusion, and pulsed electric current also accelerates the hydrogen diffusion.     

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 permeation in a Cr–Mo–V medium-carbon steel: Effect of the quenching medium and tempering temperature

Hydrogen permeation tests are carried out to evaluate the effect of the quenching medium and tempering temperature on the permeation parameters and density of hydrogen traps, of a Cr–Mo–V low-alloy medium-carbon steel. Three types of steel membranes are tested: 1) in the as-quenched condition, 2) tempered at 235 °C and 3) tempered at 530 °C; each one quenched in two different media: oil or brine. From the as-quenched condition, the apparent concentration of hydrogen and hydrogen flux, tend to decrease as the tempering temperature increases. The membranes in the as-quenched condition and tempered at 530 °C, show lower hydrogen diffusivity and higher density of hydrogen traps than membranes tempered at 235 °C. It is concluded that tempering at 235 °C, promotes hydrogen induced cracking, which is contrary to what has been previously determined. The cracking is related to a higher hydrogen diffusivity and lower density of hydrogen traps.     

Differences between internal and external hydrogen effects on slow strain rate tensile test of iron-based superalloy A286

To investigate the evaluation method of hydrogen compatibility of A286 superalloy in high pressure hydrogen gas, SSRT tests of hydrogen-charged specimens were conducted at ambient temperature at various strain rates. The relative reduction in area (RRA), one of the ductility parameters, was determined. The hydrogen content in the hydrogen-charged specimen was the same as the equilibrium hydrogen content on the specimen surface at 150 °C in 70 MPa hydrogen gas. The strain rate dependence of RRA was smaller than that of RRA obtained in 70 MPa hydrogen gas at 150 °C. All the hydrogen-charged specimens showed slip-plane fractures in the grains in their cores. However, the specimens in 70 MPa hydrogen gas at 150 °C showed fracture surfaces morphology ranging from dimples to quasi-cleavages and intergranular fractures with decreasing strain rate. These dissimilarities are expected to arise from differences in the hydrogen concentration behaviors of the specimens during the deformation process.     

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.     

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.     

Simulation of the impact of internal pressure on the integrity of a hydrogen-charged Type-316L stainless steel during slow strain rate tensile test

A hydrogen-charged Type-316L austenitic stainless steel represents a slight loss of tensile ductility and cup-and-cone fracture accompanied by small-sized dimple. The reduction in the dimple size is interpreted to be attributed to void sheets caused by localized slip deformations by hydrogen. This paper aims to clarify the contribution of an internal pressure to the characteristic void growth of a hydrogen-charged Type-316L stainless steel during slow strain rate tensile (SSRT) test in air at room temperature. The internal pressure of pre-existing voids in the specimen charged by 100 MPa hydrogen gas at 270 °C for 200 h was simulated by diffusion-desorption analysis of hydrogen with the finite differential method (FDM). The subsequent impact of the internal pressure on the void growth was simulated by fracture-mechanics approach with the finite element method (FEM). The simulations performed under various void morphologies and fracture toughness suggested that the internal pressure in the voids was significantly low, hardly affecting the void growth.     

The dependence of hydrogen embrittlement on hydrogen transport in selective laser melted 304L stainless steel

The mechanical property and hydrogen transport characteristics of selective laser melting (SLM) 304L stainless steel were investigated by tensile tests and thermal desorption spectroscopy (TDS). The heat treatment affected the hydrogen embrittlement (HE) susceptibility and the treatment at 950 °C showed the larger HE effects. Cellular structures and melt-pool boundaries were dissolved at 850 and 950 °C, respectively. TDS results indicate that the hydrogen diffusivity of the as-received SLM 304L was lower than that of wrought 304L and the hydrogen diffusion activation energy increased with the recrystallisation degree, which was related to the dislocation density. Dislocations, rather than strain-induced martensite, were the main cause of HE owing to the high austenite stability of the samples. The pre-existing dislocations in the SLM 304L sample heat-treated at 950 °C for 4 h affected the hydrogen transport behaviour during sample stretching and led to severe HE.     

Intercritical annealing temperature dependence of hydrogen embrittlement behavior of cold-rolled Al-containing medium-Mn steel

The present investigation attempts to evaluate the influence of intercritical annealing temperature (T_IA) on the hydrogen embrittlement (HE) of a cold-rolled Al-containing medium-Mn steel (Fe-0.2C-4.88Mn-3.11Al-0.62Si) by using electrochemical hydrogen-charging, slow strain rate tensile test and scanning electron microscope. The results show that an excellent combination of strength and ductility (the product of ultimate tensile strength and total elongation) up to ∼53 GPa·% was obtained for the specimen intercritically annealed at an intermediate temperature of 730 °C, whereas the HE index increases significantly with an increase in T_IA up to 850 °C. Being different from the typical dimple ductile fracture for the uncharged specimen, the hydrogen-charged specimen exhibits a mixed brittle interface decohesion and ductile intragranular fracture mode in the crack initiation region and the brittle fracture fraction increases with increasing T_IA. Both the stability and amount of austenite play a critical role in governing the HE behavior of TRIP-assisted medium-Mn steel. Thus, it is suggested that suitable T_IA should be explored to guarantee the safety service of automotive parts made of this type of steel in addition to acquiring excellent mechanical properties.     

Influence of copper as an alloying element on hydrogen environment embrittlement of austenitic stainless steel

A Cu alloyed (18Cr–10Ni–3Cu) and a Cu free (18Cr–12.7Ni) austenitic stainless steel were tensile tested in gaseous hydrogen atmosphere at 20 °C and −50 °C. Depending on the test temperature, the Cu alloyed steel was extremely embrittled whereas the Cu free steel was only slightly embrittled. Austenite stability and inherent deformation mode are two main criteria for the resistance of austenitic stainless steels against hydrogen environment embrittlement. Based on the well known austenite stability criteria, the austenite stability of both steels should be very similar. Interrupted tensile tests show that martensite formation upon plastic deformation was much more severe in the Cu alloyed steel proving that the influence of Cu on austenite stability is overestimated in the empirical stability equations. When tested in high pressure H₂, replacing Ni by Cu resulted in a fundamental change in fracture mode atmosphere, i.e. Ni cannot be replaced by Cu to reduce the costs of SS without compromising the resistance to hydrogen environment embrittlement.     

Hydrogen trapping sites and hydrogen-induced cracking in high strength quenching & partitioning (Q&P) treated steel

The effect of hydrogen on the tensile properties and fracture characteristics was investigated in the quenching & partitioning (Q&P) treated high strength steel with a considerable amount of retained austenite. Slow strain-rate tensile (SSRT) tests and fractographic analysis on cathodically charged specimens were performed to evaluate the hydrogen embrittlement (HE) susceptibility. Total elongation was dramatically deteriorated from 19.5% to 2.5% by introducing 1.5 ppmw hydrogen. Meanwhile, hydrogen caused a transition from ductile microvoid coalescence to a mixed morphology of dimples, “quasi-cleavage” regions and intergranular facets. Moreover, hydrogen trapping sites were directly observed by means of three-dimensional atom probe tomography (3DAPT). Results have shown that hydrogen in austenite (33.9 ppmw) is 3 times more soluble than that in martensite (10.7 ppmw). By using DENT specimen, hydrogen-induced cracking (HIC) cracks were found to initiate at martensite/austenite interfaces and then propagate through retained austenite and martensite. No crack was observed to be initiating from ferrite phase.