Hydrogen-enhanced fatigue crack growth behaviors in a ferritic Fe-3wt%Si steel studied by fractography and dislocation structure analysis

The effect of hydrogen (H) on the fatigue behavior is of significant importance for metallic structures. In this study, the hydrogen-enhanced fatigue crack growth rate (FCGR) tests on in-situ electrochemically H-charged ferritic Fe-3wt%Si steel with coarse grain size were conducted. Results showed strong difference between the H-charged and the non-charged conditions (reference test in laboratory air) and were in good agreement with the results from literature. With H-charging, the fracture morphology changed from transgranular (TG) type to “quasi-cleavage” (“QC”), with a different fraction depending on the loading frequency. With the help of electron channeling contrast imaging (ECCI) inside a scanning electron microscope (SEM), a relatively large area in the failed bulk specimen could be easily observed with high-resolution down to dislocation level. In this work, the dislocation sub-structure immediately under the fracture surfaces were investigated by ECCI to depict the difference in the plasticity evolution during fatigue crack growth (FCG). Based on the analysis, the H-enhanced FCG mechanisms were discussed.     

Using a pulsed electric current to reduce the susceptibility of high-strength steel to the stress corrosion cracking caused by hydrogen

Under the tensile loading, the damage of metals in the corrosive medium is the most destructive and harmful. In this study, the stress corrosion cracking behavior of H-charged high-strength steel in 3.5 wt% NaCl solution after electropulsing treatment was investigated. The experimental results from elongation, yield strength, fracture morphology, and polarization curves all demonstrate the positive effect of the pulsed processing, as it reduced the susceptibility of steel to stress corrosion cracking by removing hydrogen by electropulsing. The reduction in hydrogen content of the pulsed high–strength steels was attributed to electromigration and increased system free energy, which drove the hydrogen atoms in the steel to de–trap and reduced the susceptibility to stress corrosion cracking.     

Influence of surface martensite layer on hydrogen embrittlement of Fe-Mn-C-Mo steels in wet H2S environment

This study investigated the effect of thermally induced surface martensite layer on hydrogen embrittlement of Fe-16Mn-0.4C-2Mo (wt.%) (16Mn) and Fe-25Mn-0.4C-2Mo (wt.%) (25Mn) steels through slow strain rate stress corrosion cracking testing and proof ring testing in wet H₂S environment. The 16Mn steel had a surface layer of less than 150 μm in depth containing ε-martensite, α′-martensite and austenitic twins. The martensite layer is found to reduce the hydrogen embrittlement resistance of the steel. In comparison, the 25Mn steel developed a full α′-martensite surface layer, which exhibited practically nil effect on the hydrogen embrittlement resistance of the steel. The ε-martensite provides much larger interface areas with the mechanical twins of the austenite in the 16Mn steel than the α′-martensite/austenite interfaces in the 25Mn steel. These interfaces are hydrogen trapping sites and are prone to initiate surface cracks, as observed in the scanning electron microscope. The formation of the cracks is attributed to hydrogen concentration at the ε-martensite and austenitic twin interfaces, which accelerates material fracture.     

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 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.     

Effect of low partial hydrogen in a mixture with methane on the mechanical properties of X70 pipeline steel

In this study, the effect of a low partial hydrogen in a mixture with natural gas on the tensile, notched tensile properties, and fracture toughness of pipeline steel X70 is investigated. An artificial HE aging is simulated by exposing the tested sample to the mixture gas condition for 720 h. In addition, a series of tests is conducted in ambient air and 10 MPa of 100% He and H₂. Overall, 10 MPa of 100% H₂ significantly degrades the mechanical properties of an X70 pipeline steel. However, it is observed that the 10 MPa gas mixture with 1% H₂ does not affect the mechanical properties when tested with a smooth tensile specimen. In the notched tensile test, a significant reduction in loss in the area is observed when tested with a notched specimen with a notch radius of 0.083 mm. It is also confirmed that a 10-MPa gas mixture with 1% H₂ causes a remarkable reduction in the toughness. The influence of the exposure time to 1% hydrogen in a mixture with natural gas was found to be minor.     

Effects of finish rolling deformation on hydrogen-induced cracking and hydrogen-induced ductility loss of high-vanadium TMCP X80 pipeline steel

An optimum finish rolling deformation (FRD) of thermomechanical controlled processing (TMCP) is suggested to improve the hydrogen-induced ductility loss of high-vanadium X80 pipeline steel in this study. The results demonstrate that with increasing FRD the microstructure refines, the grain size of the steel decreases and the recrystallization degree deepens. The increase of FRD leads to the reduction of low angle grain boundaries (LAGBs) and the grains oriented with plane {100} parallel to normal direction ({100}//ND) fibres, which plays a significant role in improving the resistance of crack propagation. Besides, the differences of effective hydrogen diffusion coefficient and diffusible hydrogen concentration are negligible among four experimental steels with various FRD. However, the best hydrogen-induced ductility loss resistance is obtained in the steel with 40% FRD containing the most nano-scale precipitates acting as effective hydrogen traps.     

Increase in the local yield stress near surface of austenitic stainless steel due to invasion by hydrogen

In order to determine the effect of hydrogen on the local yield stress near the surface of austenitic stainless steel, an indentation test combined with inverse problem analysis was employed. For austenitic stainless steel, the indentation test is an effective method since the hydrogen is distributed near to the surface because of its high solubility and low diffusion coefficient. Although uniaxial tensile tests can also provide useful data, greater variations in the mechanical properties due to the presence of hydrogen can be detected through indentation tests. In this study, Secondary Ion Mass Spectrometry (SIMS) was used to measure hydrogen depth profiles in order to establish the relationships between the hydrogen absorption depth and the effects due to hydrogen evaluated using the indentation test. The results showed that the yield stress doubled due to hydrogen absorption and then reverted to its initial state due to hydrogen desorption at room temperature. Also, hardening due to the presence of hydrogen, which was determined using an indentation test, was found to be dependent on the relationship between the plastic deformation depth and the hydrogen absorption depth.     

Hydrogen embrittlement behavior in interstitial Mn–N austenitic stainless steel

The susceptibility to hydrogen embrittlement behavior was investigated in an interstitial Mn–N austenitic steel HR183 and stainless steel 316L. Hydrogen was introduced by cathodic hydrogen charging at 363 K. HR183 has stronger austenite stability than 316L despite its lower nickel content, the addition of manganese and nitrogen inhibited martensitic transformation during the slow strain rate tensile deformation. Due to the diffusion of hydrogen being delayed by the interstitial solution of nitrogen atoms and the uniform dislocation slips, hydrogen permeates more slowly in HR183 than 316L, contributing to an 84.79 μm thinner brittle fracture layer in HR183 steel. Hydrogen charging caused elongation losses in both 316L and HR183 steels associated with the hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) mechanism. However, the hydrogen embrittlement susceptibility of HR183 is 3.4 times lower than that of 316L according to the difference in elongation loss between the two steel after hydrogen charging. Deformation twins trapped a lot amount of hydrogen leading to brittle intergranular fracture in 316L. The multiple directions of slip in HR183 steel suppressed the strain localization inside grains and delayed the adverse effects conducted by HELP and HEDE mechanism, eventually inhibiting server hydrogen embrittlement in the HR183 steel. This study is assisting in the development of low-cost stainless steel with excellent hydrogen embrittlement resistance that can be used in harsh hydrogen-containing environments.     

Effects of stress concentration on the mechanical properties of X70 in high-pressure hydrogen-containing gas mixtures

This study aims to investigate the mechanical properties of X70 pipeline steel under the synergistic influence of hydrogen and stress concentration. Slow strain rate tensile tests and low-cycle fatigue tests were performed on the specimens with different stress concentration factors (Kt) in 10 MPa nitrogen/hydrogen mixtures. Results show that the degradation degree of the ductility and fatigue life of X70 steel induced by hydrogen increases with the increase of Kt, and as the hydrogen partial pressure in mixtures increases, the influence of Kt on hydrogen-induced degradation increases as well. In addition, finite element analysis was performed via a modified hydrogen diffusion/plasticity coupled model to study the effect of Kt on hydrogen distribution in the specimens, which can influence the mechanical properties of X70. The maximum hydrogen concentration consistently appears at the notch tip of the specimen and increases with the increase of Kt, which is proposed to be one of the reasons for the severe hydrogen embrittlement of the specimens with large Kt. As the axial tensile force on the specimen increases, the maximum hydrogen concentration at the notch tip begins to be dominated by hydrogen in the normal interstitial lattice sites and, subsequently, in the trapping sites.     

The effect of nanosized NbC precipitates on electrochemical corrosion behavior of high-strength low-alloy steel in 3.5%NaCl solution

The effect of nanosized NbC precipitates on electrochemical corrosion behavior of high-strength low-alloy (HSLA) steels in 3.5%NaCl solution has been investigated by the means of precipitate modulation, microstructure observation, electrochemical and immersion tests. The results showed that NbC precipitates markedly enhance the corrosion resistance of the H-contained steel, and the mechanism is that the plentiful and highly dispersed nanosized NbC particles acting as massive and effective hydrogen traps play a decisive role in the resistance to hydrogen activated corrosion. Moreover, it is evident that the inhibiting effect is related with the amount, size and distribution of the precipitates, and the optimized microstructures and precipitated phases improve the mechanical properties and resistance to hydrogen activated corrosion of HSLA steel.     

Effect of high temperature deformation on the microstructure,mechanical properties and hydrogen embrittlement of 2.25Cr–1Mo-0.25 V steel

In this paper, the effects of high temperature deformation on the microstructure, mechanical properties and hydrogen embrittlement (HE) of the 2.25Cr–1Mo-0.25 V steel was investigated by a scanning electron microscope (SEM), a transmission electron microscope (TEM) and tensile tests. The SEM and TEM images demonstrated that high temperature plastic deformation (HTPD) led to the coarsening of carbides and the dislocation density increase. The tensile tests displayed that the HTPD resulted in the cracking susceptibility increase, as indicated by the increased numbers and sizes of cracks at the fractured surface. This was attributed to the coarsening of carbides during high temperature deformation. In contrast, the HTPD highly decreased the loss of ductility compared to the un-deformed specimens, although the amount of ductility losses (elongation or reduction of area) did not change significantly as the HTPD increased. The correlations among carbides, hydrogen and cracks were discussed.     

Strengthening mechanism and hydrogen-induced crack resistance of AISI 316L stainless steel subjected to laser peening at different power densities

Microstructural response of AISI 316L stainless steel to laser peening (LP) was examined by means of optical microscopy (OM) and transmission electron microscopy (TEM) in order to analyze the effects of LP on hydrogen-induced cracking (HIC) resistance. Depth profiles of near-surface microhardness and surface compressive residual stress (CRS) of LP treated specimens were presented respectively. Slow strain rate tensile tests were performed on the hydrogen-charged samples and their corresponding stress-strain curves as well as fracture morphologies were finally investigated in detail. The results demonstrated that LP induced a grain refinement effect on the treated surface while a maximum refining rate of 56.18% was achieved at the laser power density of 10 GW/cm². The near-surface microhardness also exhibited an attenuation trend with the increasing depth. The surface CRS positively correlated with power density before it reached a threshold value. A special U-shaped dislocation tangle band was observed in the LP treated specimen which served as hydrogen trapping sites. The LP treated samples exhibited better toughness after hydrogen charging from both macro mechanical properties and micro fracture morphologies. LP-induced grain refinement and CRS are believed to be the main contributing factors towards inhibiting the diffusion of hydrogen atoms which ultimately leads to the reduction of the hydrogen embrittlement sensitivity of AISI 316L stainless steel.     

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.     

Anisotropy of cold-worked Type-304 austenitic stainless steel: Focus on the hydrogen diffusivity

Anisotropic nature of effective hydrogen diffusivity was investigated on a cold-worked (CW) Type-304 stainless steel. The material was characterized by using disk-shaped specimens sampled from two directions of steel plates with various rolling ratio. The thickness direction of the disks was parallel to the rolling direction for SL specimens and perpendicular for LT ones. Electromagnetic induction (EMI) and electron backscatter diffraction (EBSD) clarified the content and distribution of strain-induced martensite (SIM). The effective diffusivities and solubilities were jointly determined by desorption method and thermal desorption analysis (TDA) in H-charged specimens with high-pressure gas. The increase of SIM with CW ratio and the differences of SIM distribution observed between LT and SL specimens could justify the anisotropic effective diffusivities. Finite element method (FEM) was used to simulate permeation tests based on multiple EBSD maps. Simulations supported the experimental findings: at the CW ratio of 60%, the CW process increased the diffusivity by twenty and the diffusivity was five time greater in the SL specimen than the LT one. The inhomogeneous SIM distribution justified the modifications of diffusion properties by CW in both specimens.     

The potential for using high chromium ferritic alloys for hydroprocessing reactors

This paper outlines the development of hydroprocessing reactors and the parallel development of applicable steels for their high temperature and high pressure process environments. Trends in the development of newer processes for severe hydroprocessing applications have been increasing in operating hydrogen partial pressures and operating temperatures that require the development of new alloys to meet these more severe process environments.The paper outlines the properties of conventional hydroprocessing reactor materials and discusses the advantages of the advanced high chromium ferritic steel alloy Grade 91 (9Cr–1Mo–V) for high temperature hydroprocessing applications. Additionally, the alloys permitted for ASME Section I and Section VIII Division I construction, Grade 92 (Code Case 2179), and what will probably be called Grade 122 (Code Case 2180) are briefly introduced as possible future choices for hydroprocessing reactor construction. These three alloys contain 9–12% Cr and have time independent allowable stress values above 566 °C.These high, time independent, strength values provide materials that will in some cases permit extending hydroprocessing temperature limits by 112 °C. The paper provides room temperature and elevated temperature mechanical and toughness properties for the low chrome and Grade 91 materials and discusses the effects of hydrogen attack, and hydrogen and isothermal embrittlement. Fabrication aspects, including forming and welding are addressed.The paper discusses the environmental resistance of these alloys and investigates the possibility of utilizing excess wall metal thickness in these materials in less severe applications in lieu of the deposition of a higher chromium alloy weld overlay to overcome the corrosive effects of the process environment.     

Comprehensive study on hydrogen induced cracking of electrical resistance welded API X52 pipeline steel

Different heat treatment cycles were designed in order to investigate the effect of microstructural changes on hydrogen induced cracking resistance (HIC) and mechanical properties of the electric resistance welded steel. The heat treating of the as-welded specimen improved the ductility and impact toughness. After heat treatment, the uniform hardness profile was obtained for the welded specimens. The removal of local hard zones reduced the risk of HIC. The chemical composition and clustering of inclusions have a deleterious effect on cracking resistance in the H₂S environment. Aluminosilicate compounds and MnS inclusions were favorite sites for HIC. The most promising post weld heat treatment for improving mechanical properties and the resistance to HIC was the application of two-cycle quenching followed by tempering.     

Highly sensitive secondary ion mass spectrometric analysis of time variation of hydrogen spatial distribution in austenitic stainless steel at room temperature in vacuum

Hydrogen contained in austenitic stainless steel is classified as diffusible or nondiffusible. The hydrogen distribution in austenitic stainless steel changes with time owing to hydrogen diffusion at room temperature, and such changes in hydrogen distribution cause the mechanical properties of the steel to change as well. It is therefore important to analyze the time variation of the hydrogen distribution in austenitic stainless steel at room temperature to elucidate the effects of hydrogen on the steel's mechanical properties. In this study, we used secondary ion mass spectrometry (SIMS), a highly sensitive detection method, to analyze the time variation of the distribution of hydrogen charged into 316L austenitic stainless steel. SIMS depth profiles of hydrogen that were acquired at the three measurement times were analyzed, and the results were compared among the measurement times. ¹H⁻ intensities and distribution of the intensities changed with time due to diffusion of hydrogen in the hydrogen-charged 316L steel sample at room temperature. Moreover, the time variation of the hydrogen concentration distribution of the hydrogen-charged 316L sample was calculated using a one-dimensional model based on Fick's second law. The time variations of the measured hydrogen intensities and of the calculated values are compared.     

An alternative and comprehensive approach to estimate trapped hydrogen in steels using electrochemical permeation tests

The present study critically discusses the apparent diffusivity of H during build-up and decay permeation transients in four microalloyed line-pipe steels using Devanathan-Stauchursky cell in galvanostatic-potentiostatic condition. Among the existing models, the Carvalho's solution has been found to give the best fit with the experimental build-up transient. The numerical solution of trapping and detrapping events of H during build-up transients is investigated by solving the McNabb-Foster model. The residual hydrogen was measured in the steels using LECO-DH603 hydrogen determinator after saturation H-charging. Further the Carvalho's exponent of permeation transient has been found to have good correlation with the H trapping-detrapping rates during the permeation test and also with the residual H content in the steel. Finally, empirical relationships have been proposed that can be used to estimate the residual H in the studied steels along with the trapping-detrapping parameters using the permeation test data.     

Stresses controllable analysis and optimal design of unique high pressure vessel applied in hydrogen charge station

High pressure hydrogen storage vessels are the key equipment in hydrogen charge stations. Hydrogen environment embrittlement (HEE) is always the associated problem that is inescapable and difficult to be solved completely. In order to decrease the harmfulness of HEE, a unique flat steel ribbon wound pressure vessel (FSRWPV) is designed, whose inner shell material is austenitic stainless steels 0Cr18Ni9 (304) and the steel ribbon material is 16MnR (SA516Gr70). The residual stresses in FSRWPV are analyzed and a stress controlling model is put forward. Through this model, the stress distribution in the FSRWPV wall can be controlled by adjusting the pretension in flat steel ribbons. After optimal designing, the stresses in flat steel ribbon layers are uniform, and that in the inner shell is low or negative. This kind of stress distribution can effectively prevent HEE and stress corrosion cracking (SCC), therefore the FSRWPV has good properties of bearing HEE and SCC. Furthermore, as flat steel ribbon layers are a discrete structure, the online monitoring of FSRWPVs can be conveniently realized, so they have performance of “leak only no bursting”. At the end, an applied example of high pressure hydrogen FSRWPV is given.