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.     

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.     

Compressive behaviour of thin catalyst layers. Part I - Experimental study

In this study, the effect of compression is investigated experimentally on deformation and porosity of catalyst layers (CLs). Compression tests are performed on five CL samples with various microstructures using a thermomechanical analyzer and a custom-made machine Tuc-Ruc (Thickness under compression-Resistivity under compression). The results indicate that CLs have a linear behaviour with no plastic deformation at pressures less than 2 MPa even after 12 cycles. However, CLs showed plastic deformation, work hardening, and elastic shakedown under cyclic compression up to 5 MPa. In this pressure range, the material becomes stiffer and Young's modulus has increased by 50–113% after 8 loading cycles. Moreover, the material “settles down” after 6 cycles showing no further significant plastic deformation at higher pressures (up to 5 MPa). This behaviour suggests that CLs enter elastic shakedown region since after several cycles, plastic strain diminished, and they behave elastically afterwards. The compression tests on five samples yield Young's modulus of 30–45 MPa for pressures up to 2 MPa and Young's modulus of 37–70 MPa for pressures up to 5 MPa. The reason for slight change in Young's modulus is that the microstructure of CL changed, and the porosity decreased at higher pressures.     

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.     

Analysis of martensitic transformation in 304 type stainless steels tensile tested in high pressure hydrogen atmosphere by means of XRD and magnetic induction

Tensile specimen of several 304 type stainless steels tested under pressurized hydrogen and helium atmospheres were investigated with the focus on the γ → α′ transformation as a function of Ni content. Martensite contents on the fracture surfaces increased with decreasing Ni content and were independent of the test atmosphere (He or H₂) despite different macroscopic plastic deformations. This was attributed to similar plastic deformations at the crack tip which governs the γ → α′ transformation at the fracture surface. The severity of hydrogen environment embrittlement was quantified by RA measurements which is a measure of the maximum macroscopic plastic deformation. RA values in H₂ decrease with decreasing Ni content and RA is almost exactly inverse proportional to the martensite content measured by Feritscope in the uniform elongation area. This implies that the influence of hydrogen of the steels investigated here is dominated by surface effects.     

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.     

Strain variation on the reaction tank of high hydrogen content during hydrogen absorption-desorption cycles

The strain generated in a reaction tank of hydrogen storage alloys were measured in order to analyze strain variation in different locations during hydrogen absorption-desorption cycles. Strain gauges were set on the various locations of the tank. The change of strain was continuously monitored using the strain analyzer. The results indicate that plastic deformation had occurred during activation period, and it did not become larger in the later hydriding-dehydriding cycles though the tank still had the ability for elastic deformation. The stress induced by alloy swell on the tank was over 8 MPa. Tangential strain was greater than longitudinal strain in each place. There wasn't much more difference in Longitudinal at respective points while tangential strain in the middle part was much larger than that in two sides' parts of the tank. The nonuniform packing density lead the deformation tended to occur more serious in tail part than in head part.     

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.     

Analyses of hydrogen distribution around fatigue crack on type 304 stainless steel using secondary ion mass spectrometry

Secondary Ion Mass Spectrometry (SIMS) analyses were carried out on type 304 austenitic stainless steel. On annealed specimen exposed to hydrogen (10 MPa, 358  K), Element Depth Profiles SIMS mode was able to describe quantitatively the hydrogen profile content computed by the Fick’s law. Based on SIMS analyses on the wake of a fatigue crack (propagation in hydrogen gas at 0.6 MPa and RT), it was possible to compute an apparent diffusivity and solubility in the crack tip region. The apparent solubility and diffusivity in the deformed regions were two times and five orders of magnitude higher than the ones on annealed material, respectively. High hydrogen content was found around the crack tip, where the plastic deformation was well developed (pronounced slip activity). The high apparent diffusivity is presumed to result from enhanced hydrogen transport induced by cyclic plastic activity at the crack tip.     

Effects of hydrogen on tensile properties and fracture surface morphologies of Type 316L stainless steel

The effects of hydrogen on the tensile properties and fracture surface morphologies of Type 316L stainless steel were investigated using virgin and prestrained specimens. Hydrogen gas exposure at 10 MPa and 250 °C for 192 h resulted in its uniform distribution in the specimens. Such internal hydrogen degraded the tensile ductility of the specimens. Cup–cone fracture occurred in the non-, Ar-, and H-exposed specimens. The fracture surfaces were covered with large and small dimples. The H-exposed specimens exhibited larger small-dimple areas than the non- and Ar-exposed ones. The diameter of the large dimples decreased with increasing small-dimple area. Three-dimensional analysis of the dimples showed that the small-dimple regions were void sheets produced by local shear strain. Hydrogen accelerated nucleation of voids and formation of the void sheets by enhancing localization of shear deformation, thereby reducing the average size of the dimples.     

The effects of double notches on the mechanical properties of a high-strength pipeline steel under hydrogen atmosphere

Tensile tests and fatigue life tests are performed on double-notched specimens in hydrogen and nitrogen atmospheres to investigate the effects of double notches on the mechanical properties of a high strength pipeline steel. The results show that the fracture occurs at the notch with a lower stress concentration factor (Kt), which is governed by the combination of the stress concentration and the strain hardening caused by plastic deformation in the tensile process. Hydrogen gas accelerates the crack initiation and growth, but it doesn't affect the competitive mechanism of stress concentration and strain hardening.     

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.     

Investigation of deformation mechanics and forming limit of thin-walled metallic bipolar plates

The present study is conducted on forming of the metallic bipolar plates made of 316 stainless steel sheet with a parallel serpentine flow field. The plastic deformation of straight and curved microchannels, forming limit criteria, and deformation mechanics during the process are investigated partially to present a reliable model for estimating fracture initiation. For this purpose, experimental stamping tests are employed to fabricate metallic bipolar plates and the process is simulated by finite element software. The validity of simulation results is examined by comparing thickness distribution and force-displacement curves reflecting 4.76% and 3.85% error rates, respectively. According to experimental observations, fracture starts at a channel depth of 0.610 mm. Hence, for determining the forming limit and predicting the fracture during the process, the deformation mechanic is studied at different points of the microchannels. Results of stress states analysis indicate that the stress state of plane-strain tension up to biaxial tension governs this process. Despite the presence of different loading paths during the process, the critical element in each channel is deformed under plane-strain tension. Therefore, a fracture model is developed based on thinning percentage and equivalent strain to predict the instability of metallic bipolar plates. According to the results, both the equivalent strain and thinning percentage criteria with critical limits of 0.56 and 33.45%, respectively, are considered as an allowable range of plastic deformation during the conventional stamping process of bipolar plates. Results indicate that maximum thinning in all directions is lower than 33.45% by using the modified stamping process.     

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.     

Hydrogen-assisted fracture resistance of pipeline welds in gaseous hydrogen

Fracture resistance of pipeline welds from a range of strength grades and welding techniques was measured in air and 21 MPa hydrogen gas, including electric resistance weld of X52, friction stir weld of X100 and gas metal arc welds (GMAW) of X52, X65 and X100. Welds exhibited a decrease in fracture resistance in hydrogen compared to complementary tests in air. A general trend was observed that fracture resistance in 21 MPa hydrogen gas decreased with increasing yield strength. To accommodate material constraints, two different fracture coupon geometries were used in this study, which were shown to yield similar fracture resistance values in air and 21 MPa hydrogen gas; values using different coupons resulted in less than 15% difference. In addition, fracture coupons were removed from controlled locations in select welds to examine the potential influence of orientation and residual stress. The two orientations examined in the X100 GMAW exhibited negligible differences in fracture resistance in air and, similarly, negligible differences in hydrogen. Residual stress exhibited a modest influence on fracture resistance; however, a consistent trend was not observed between tests in air and hydrogen, suggesting further studies are necessary to better understand the influence of residual stress. A comparison of welds and base metals tested in hydrogen gas showed similar susceptibility to hydrogen-assisted fracture. The overall dominant factor in determining the susceptibility to fracture resistance in hydrogen is the yield strength.     

Hot deformation behavior and microstructure evaluation of hydrogenated Ti-6Al-4V matrix composite

Ti-6Al-4V matrix composite reinforced with TiB and TiC particulates was prepared and hydrogenated. Isothermal compression tests were carried out at the deformation temperatures ranging from 750 °C to 900 °C and strain rate ranging from 0.01 s^?1 to 1 s^?1. The effects of hydrogen concentration, deformation temperature and strain rate on flow stress-strain curves and microstructure evaluation were studied. Hydrogen decreases the deformation temperature at least 100 °C or enables the composite to deform at a higher strain rate at the same flow stress level. Hydrogen improves dynamic recrystallization of α phase and accommodation deformation between reinforcements and matrix. Optimum hydrogen concentrations at different deformation temperatures were determined. The strain rate sensitivity index and apparent activation energy of the composite with 0 wt.% H and 0.35 wt.% H were calculated and discussed.     

In situ measurement of plasticity accompanying hydrogen induced cracking in a polycrystalline AlZnMg alloy

Hydrogen induced single crack propagation is studied in an embrittled aluminum alloy. Hydrogen is introduced into the system by electrochemical reactions in an acidic aqueous medium. After hydrogen charging, tensile tests are performed in air, on notched samples, with a microtensile machine under an optical microscope. A high magnification of × 2000 is used to follow the single crack initiation and propagation. Digital Image Correlation gives the displacement field on the surface with a spatial resolution of approximately 1 μm. It enables the determination of the position of the crack tip and the local velocity at a sub-grain scale. The von Mises strain is calculated and provides a precise measure of the local plastic field that accompanies crack propagation. In addition to the primary plasticity which is emitted from the crack tip or its immediate neighborhood in the form of two intense slip bands, a secondary plastic zone that spreads over several microns ahead of the tip is sytematically found. The characteristics of the plastic zone are measured, together with the velocity and the applied stress intensity factor. In addition, different fracture mechanisms are found on the fracture surface. In particular there are transitions in the fracture mode from intergranular smooth to transgranular parallel to the grain boundary plane. The local fracture mechanisms, in the vicinity of the surface, are linked to the local velocities and plastic deformations. Surprisingly no strong velocity/plasticity correlations are found while the velocities are scattered over a wide range, which is interpreted as a strong polycrystalline effect.     

Activation of TiFe for hydrogen storage by plastic deformation using groove rolling and high-pressure torsion: Similarities and differences

Intermetallics of TiFe were processed using three different routes: annealing, plastic deformation using groove rolling and severe plastic deformation using high-pressure torsion (HPT). Hydrogen absorption was less than 0.2 wt.% in the coarse-grained annealed sample because of difficult activation. The groove-rolled sample, with subgrain structure and high density of dislocations and cracks, absorbed 0.3, 1.0, 1.4 and 1.7 wt.% of hydrogen in the first, second, third and fourth hydrogenation cycles, respectively. The HPT-processed sample, containing nanograins, absorbed 1.7–2 wt.% of hydrogen in any hydrogenation cycles. Both samples activated by groove rolling and HPT were not deactivated by long time exposure to the air. No surface segregation was detected after groove rolling, while the HPT-processed sample exhibited surface segregation. The current study confirmed the significance of plastic deformation and formation of grain boundaries and cracks on activation for hydrogen storage.     

Finite element analysis of hydrogen diffusion/plasticity coupled behaviors of low-alloy ferritic steel at large strain

Hydrogen embrittlement is commonly considered as a typical failure mechanism for low-alloy ferritic steel under high pressure hydrogen environment. Currently, the hydrogen enhanced localized plasticity theory has been largely recognized for studying the hydrogen embrittlement mechanism by introducing the localized plastic flow and the hydrogen induced strain concept. However, the hydrogen induced strain and the plastic strain are often solved respectively in this theory, which may weaken the effect of hydrogen on the plastic deformation. The purpose of this paper is to propose a modified theoretical model from the microstructural level by emphasizing the coupling mechanism between the hydrogen diffusion and the plastic deformation at large strain, where the hydrogen induced strain is superimposed on the equivalent plastic strain instead of on the strain components. Fully implicit backward Euler algorithm by finite element analysis (FEA) under the corotational configuration is used to implement the proposed model, where the hydrogen induced strain is involved in the stress return process within each iteration, indicating a more direct interaction between them than existing works. FEA by using finite element software ABAQUS-UMAT subroutine is performed for the smooth tensile specimen and the notch specimen respectively under slow tensile strain rate loading and different hydrogen pressure. Developed direct coupling model is expected to further gain insight into the hydrogen embrittlement effect on the plastic deformation, especially at the trapping sites.     

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.