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.     

Influence of machining parameters on surface roughness and susceptibility to hydrogen embrittlement of austenitic stainless steels

In the present work, an investigation on the susceptibility to hydrogen embrittlement of AISI 304 and 310 austenitic stainless steels was performed. The hydrogen embrittlement process leads to degradation of mechanical properties and can be accelerated by the presence of surface defects combined with elevated surface hardness. Tensile test specimens of the selected materials were machined by turning with different cutting parameters in order to create variations in surface finish conditions. The samples thus prepared were submitted to tensile tests before and after hydrogen permeation by cathodic charging. Regarding the AISI 304 steel, it was possible to notice that the presence of strain-induced martensite on the material surface led to severe hydrogen embrittlement. In the case of the AISI 310 steel, due to its higher nickel amount, no martensite formation could be detected, and this steel was found to be less susceptible to embrittlement in the tested conditions.     

Effect of hydrogen charging on the mechanical properties of advanced high strength steels

The present work investigates the influence of hydrogen on the mechanical properties of four multiphase high strength steels by means of tensile tests on notched samples. This was done by performing mechanical tests on both hydrogen charged and uncharged specimens at a cross-head displacement speed of 5 mm/min. A considerable hydrogen influence was observed, as the ductility dropped by 8–60%. In order to demonstrate the influence of diffusible hydrogen, some parameters in the experimental set-up were varied. After tensile tests, fractography was performed. It was found that hydrogen charging caused a change from ductile to transgranular cleavage failure near the notch with a transition zone to a fracture surface with ductile features near the centre.     

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.     

Origin of deformation twins and their influence on hydrogen embrittlement in cold-rolled austenitic stainless steel

The effect of cold rolling on hydrogen embrittlement in stable 18Cr–1Mn–11Ni-0.15 N austenitic stainless steels was investigated. Alloy plates were cold-rolled to 15% or 30% reduction, then pre-charged with hydrogen and subjected to tensile testing with slow strain rate. Hydrogen-induced degradation of tensile elongation became increasingly severe with the increase in the degree of cold rolling. During cold rolling, deformation twins with various orientations were actively generated, and twins with specific orientations were vulnerable to hydrogen-induced cracking. Cold rolling also increased the density of defects, and thereby facilitated penetration of hydrogen into the steels. The combination of cracks generated at the twin boundaries, and the promoted hydrogen diffusion caused severe hydrogen embrittlement in the cold-rolled steels.     

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.     

Study of the effect of hydrogen charging on the tensile properties and microstructure of four variant heat treatments of nickel alloy 718

This study examined the hydrogen embrittlement sensitivity of nickel alloy 718 given four different heat treatments to obtain various microstructural states. The four heat treatments examined are the oil and gas 718 heat treatment, the aerospace 718 heat treatment, and two variant two-step heat treatments, with a difference in aging heat treatment named 718 low band and high band. Each heat treatment leads to differences in the precipitate morphologies of γ′, γ′′ and δ phases.Material characterisation and fractography of the examined heat treatments were performed using a high resolution FEG SEM. Three specimens of each condition were pre-charged with hydrogen and tensile properties were compared with those of non-charged specimens. It was observed that hydrogen embrittlement was associated with intergranular and transgranular microcrack formation, leading to an intergranular brittle fracture. δ phase may assist the intergranular crack propagation, and this was shown to be particularly true when this phase is coarse enough to produce crack initiation, but this is not the only factor determining embrittlement. Other microstructural features play a role, as does the strength of the material. The aerospace heat treatment, which gives the highest strength and ductility in the uncharged state, shows the greatest reduction in properties with hydrogen charging.     

Effect of pre-strain on hydrogen embrittlement of metastable austenitic stainless steel under different hydrogen conditions

The effects of internal hydrogen and environmental hydrogen on the hydrogen embrittlement of 304 austenitic stainless steels (ASSs) with varying degrees of pre-strain were investigated by a tensile test under cathodic hydrogen-charged, gaseous hydrogen and hydrogen-charged and gaseous hydrogen combined conditions. The internal hydrogen embrittlement of the 304 ASSs increased with increasing pre-strain, while the hydrogen embrittlement caused by the environment hydrogen increased and then decreased with increasing pre-strain. The hydrogen embrittlement mechanisms caused by the internal hydrogen or environmental hydrogen were different. The cracks caused by internal hydrogen or environmental hydrogen are mainly initiated in grain interior or at grain boundary, respectively. Under the coupling condition of internal hydrogen and environmental hydrogen, the hydrogen embrittlement of 304 ASSs was the strongest and increased with increasing pre-strain. Environmental hydrogen was dominant for low levels of pre-deformed specimens. Internal hydrogen was dominant for high levels of pre-deformed specimen.     

The experiment study to assess the impact of hydrogen blended natural gas on the tensile properties and damage mechanism of X80 pipeline steel

Environmental hydrogen embrittlement has become a non-negligible problem in the hydrogen blended natural gas transportation. To qualitatively study the degradation mechanism of X80 steel used in the natural gas pipelines, the slow strain tensile experiments are carried out in this work. The nitrogen and hydrogen are adopted to simulate the hydrogen blended natural gas to explore the tensile properties of X80 steel. According to the volume proportion of hydrogen, the test atmospheres are divided into the reference atmosphere and the hydrogen-contained atmospheres of 1%, 2.2% and 5%. The tensile experiments of the smooth and notched specimens are conducted in the above gas atmospheres. Mechanical properties and fracture morphologies after stretching are further analyzed. The results show that the hydrogen blended natural gas has little effect on the tensile and yield strengths. Distinguished from the hydrogen volume proportion of 1% and 2.2%, with the increase of hydrogen proportion, the effect of hydrogen on mechanical properties of specimens increases significantly. Moreover, the deteriorated mechanical properties of notched specimens are more seriously than those of smooth specimens. This work provides the basis for safe hydrogen proportion for X80 pipeline steel when transporting hydrogen blended natural gas.     

Metallurgical and mechanical properties of hydrogen charged carbide-free bainitic weld metals

Hydrogen embrittlement is a major concern during the welding of high-strength steels. The susceptibility of the welds to hydrogen embrittlement increases with increase in weld strength. The ever-increasing demand to increase the strength of steels necessitates the development of novel welding procedures and fillers to produce welds of high strength and with resistance to hydrogen embrittlement. In this current work, the susceptibility of carbide-free bainitic weld metals to hydrogen embrittlement is studied with varying volume fractions of constituent phases. Using three different weld metal compositions, six different weld metal microstructures of carbide-free bainite were generated. The hydrogen saturation behaviour of the various weld metals was studied by cathodic electrolytic charging and subsequent diffusible hydrogen measurements by the hot extraction method. Tensile tests were conducted on various weld metals with and without hydrogen charging to evaluate their susceptibility to hydrogen embrittlement. The results show that the carbide-free bainite weld metals are highly resistant to hydrogen embrittlement despite their very high strength.     

A systematic study on the influence of electrochemical charging conditions on the hydrogen embrittlement behaviour of a pipeline steel

Although hydrogen embrittlement (HE) has been the subject of extensive research over the past century, a systematic study on the HE susceptibility of steels under different electrochemical charging conditions has been lacking. This study specifically targets this knowledge gap by evaluating the HE behaviour of a typical pipeline steel X65 after hydrogen-charging in acidic, neutral, and alkaline electrolytes that simulate various industrial environments. Results from a series of experiments show that the HE susceptibility of X65 steel varied significantly with hydrogen-charging electrolytes and, to a smaller extent, with electrochemical charging variables. The highest and lowest HE susceptibilities were found from specimens charged in acidic and alkaline electrolytes, respectively. An increase in yield strength was observed for almost all hydrogen-charged specimens, regardless of the charging conditions. Under severe electrochemical charging conditions, blistering was detected and mechanical properties were substantially decreased. Discussion has been made in comprehending these relationships.     

Hydrogen embrittlement of super duplex stainless steel – Towards understanding the effects of microstructure and strain

The effects of austenite spacing, hydrogen charging, and applied tensile strain on the local Volta potential evolution and micro-deformation behaviour of grade 2507 (UNS S32750) super duplex stainless steel were studied. A novel in-situ methodological approach using Digital Image Correlation (DIC) and Scanning Kelvin Probe Force Microscopy (SKPFM) was employed. The microstructure with small austenite spacing showed load partitioning of tensile micro-strains to the austenite during elastic loading, with the ferrite then taking up most tensile strain at large plastic deformation. The opposite trend was seen when the microstructure was pre-charged with hydrogen, with more intense strain localisation formed due to local hydrogen hardening. The hydrogen-charged microstructure with large austenite spacing showed a contrasting micro-mechanical response, resulting in heterogeneous strain localisation with high strain intensities in both phases in the elastic regime. The austenite was hydrogen-hardened, whereas the ferrite became more strain-hardened. SKPFM measured Volta potentials revealed the development of local cathodic sites in the ferrite associated with hydrogen damage (blister), with anodic sites related to trapped hydrogen and/or micro voids in the microstructure with small austenite spacing. Discrete cathodic sites with large Volta potential variations across the ferrite were seen in the coarse-grained microstructure, indicating enhanced susceptibility to micro-galvanic activity. Microstructures with large austenite spacing were more susceptible to hydrogen embrittlement, related to the development of tensile strains in the ferrite.     

Overview of hydrogen embrittlement in high-Mn steels

Hydrogen and fuels derived from it will serve as the energy carriers of the future. The associated rapidly growing demand for hydrogen energy-related infrastructure materials has stimulated multiple engineering and scientific studies on the hydrogen embrittlement resistance of various groups of high performance alloys. Among these, high-Mn steels have received special attention owing to their excellent strength – ductility – cost relationship. However, hydrogen-induced delayed fracture has been reported to occur in deep-drawn cup specimens of some of these alloys. Driven by this challenge we present here an overview of the hydrogen embrittlement research carried out on high-Mn steels. The hydrogen embrittlement susceptibility of high-Mn steels is particularly sensitive to their chemical composition since the various alloying elements simultaneously affect the material's stacking fault energy, phase stability, hydrogen uptake behavior, surface oxide scales and interstitial diffusivity, all of which affect the hydrogen embrittlement susceptibility. Here, we discuss the contribution of each of these factors to the hydrogen embrittlement susceptibility of these steels and discuss pathways how certain embrittlement mechanisms can be hampered or even inhibited. Examples of positive effects of hydrogen on the tensile ductility are also introduced.     

Effects of cryogenic and tempered treatment on the hydrogen embrittlement susceptibility of TRIP-780 steels

Cryogenic and Tempered (CT) treatments were performed on commercial TRIP 780 steels in order to reduce the hydrogen embrittlement (HE) susceptibility. The HE behavior was assessed immediately after cathodically hydrogen charging on both CT treated and untreated samples. Slow strain-rate tensile (SSRT) tests were conducted to evaluate their HE performance. It is shown that samples with CT treatments behave higher resistance to HE comparing with their untreated counterparts. Meanwhile, microstructure characterization and magnetization measurements were adopted to reveal the evolution of retained austenite (A_r) and its stabilization due to CT treatment. Moreover, hydrogen-induced cracking (HIC) accompanied with martensite phase transformation in TRIP steel was studied by electron backscattering diffraction (EBSD) technique and it was proved that cracks initiated from the fresh untempered martensite inherited from phase transformation of unstable A_r upon straining. Finally, results in this study demonstrate the relationship between A_r stability and HE susceptibility, and provide a possible solution to reduce HE susceptibility in TRIP steels.     

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.     

Development of a stable high-aluminum austenitic stainless steel for hydrogen applications

A novel high-aluminum austenitic stainless steel has been produced in the laboratory with the aim of developing a lean-alloyed material with a high resistance to hydrogen environment embrittlement. The susceptibility to hydrogen environment embrittlement was evaluated by means of tensile tests at a slow strain rate in pure hydrogen gas at a pressure of 40 MPa and a temperature of −50 °C. Under these conditions, the yield strength, tensile strength and elongation to rupture are not affected by hydrogen in comparison to companion tests carried out in air. Moreover, a very high ductility in hydrogen is evidenced by a reduction of area of 70% in the high-pressure and low-temperature hydrogen environment. The lean degree of alloying is reflected in the molybdenum-free character of the material and a nickel content of 8.0 wt.%. With regard to the alloy concept, a combination of high-carbon, high-manganese, and high-aluminum contents confer an extremely high stability against the formation of strain-induced martensite. This aspect was investigated by means of in-situ magnetic measurements and ex-situ X-ray diffraction. The overall performance of the novel alloy was compared with two reference materials, 304L and 316L austenitic stainless steels, both industrially produced. Its capability of maintaining a fully austenitic structure during tensile testing has been identified as a key aspect to avoid hydrogen environment embrittlement.     

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.     

Effects of grain size and dislocation density on the susceptibility to high-pressure hydrogen environment embrittlement of high-strength low-alloy steels

The tensile properties of several high-strength low-alloy steels in a 45 MPa hydrogen atmosphere at ambient temperature were examined with respect to the effects of grain size and dislocation density on hydrogen environment embrittlement. Grain size was measured using an optical microscope and dislocation density was determined by X-ray diffractometry. Both grain refinement and a reduction in dislocation density are effective in reducing the susceptibility to embrittlement. The steel that has high dislocation density or large grain size inclines to show a smooth intergranular fracture surface. Given only the grain size and dislocation density, a simple approximation of the embrittlement property of high-strength steel could be obtained. This method could be useful in selecting candidate materials in advance of the mechanical tests in high-pressure hydrogen gas.     

Detection of voids in hydrogen embrittled iron using transmission X-ray microscopy

Hydrogen embrittlement remains a barrier to widespread adoption of hydrogen as a carbon-neutral energy source. Here, hydrogen embrittlement mechanisms are investigated across length scales in iron using transmission X-ray microscopy (TXM), digital image correlation (DIC), and notched tensile testing during in-situ electrochemical hydrogen charging. TXM reveals void size and spatial distribution ahead of a propagating crack. We find hydrogen charging leads to voids within ～10 μm of the crack tip and suppression of voids beyond this distance. Near the crack tip, voids are elongated in the direction of the crack and are smaller than voids in an uncharged sample. In the presence of hydrogen, these voids lead to quasi-cleavage fracture and a sharper crack tip. DIC shows localization and reduction of plastic strain with hydrogen charging, and tensile testing reveals a reduction in fracture energy and elongation at failure. These results are discussed in the context of hydrogen embrittlement mechanisms.