Suppression of hydrogen-assisted fatigue crack growth in austenitic stainless steel by cavitation peening

In this paper we demonstrate the suppression of hydrogen-assisted fatigue crack growth in type 316L austenitic stainless steel by cavitation peening employing a cavitating jet in air. Plate bending fatigue tests on pre-cracked samples were conducted after cathodic hydrogen charging with and without cavitation peening. Without cavitation peening, the hydrogen effect on the crack growth behavior at low applied stress was clearly demonstrated compared with high applied stress in the fatigue test. The coalescence of sub-cracks and the main crack propagating from the pre-crack were observed in the hydrogen charged specimen. This phenomenon significantly accelerated the crack growth. This unexpected fracture was suppressed by introducing compressive residual stress by cavitation peening regardless of the length of processing time. In addition, lengthier treatment reduced the crack growth rate of the hydrogen charged specimen by 75% compared to an untreated one.     

Fatigue crack growth behavior of JIS SCM440 steel near fatigue threshold in 9-MPa hydrogen gas environment

In this study, stress intensity factor range (ΔK) decreasing tests were conducted and the in-situ observations were used to investigate the fatigue crack growth behavior of JIS SCM440 steel near the fatigue threshold in a 9-MPa hydrogen gas environment. The fatigue crack growth rate reflected the threshold behavior of the material, although the crack propagation knee point immediately before the threshold stress intensity factor range (ΔK_th) could not be distinctly identified. The fatigue crack was also observed to exhibit uneven propagation immediately before ΔK_th. In contrast, the knee points in a helium gas environment and air were very distinct. Fractographic analysis further revealed the existence of intergranular facets, which were observed immediately before ΔK_th in the hydrogen gas environment. Conversely, no facet was observed immediately before ΔK_th in the helium gas environment and air. The formation of the facets was considered to be one of the causes of the uneven crack propagation immediately before ΔK_th in the hydrogen gas environment.     

Effects of hydrogen on fatigue crack growth behavior of austenitic stainless steels

The effect of hydrogen on fatigue crack growth behavior of three stainless steels has been investigated from the viewpoint of microscopic fatigue mechanisms, martensitic transformation and hydrogen content. Fatigue crack growth rates in the hydrogen-charged SUS304 and SUS316 were accelerated with respect to crack growth rates in uncharged specimens. The crack growth rate in the hydrogen-charged SUS316L was only slightly higher than that in the uncharged SUS316L. Martensitic transformation on the fatigue fracture surfaces was detected using X-ray diffraction both in the hydrogen-charged and uncharged specimens of SUS304, SUS316 and SUS316L. Materials with increased tendency for martensitic transformation also showed increased acceleration in fatigue crack growth rate due to hydrogen. It was concluded that martensitic transformation in the vicinity of the fatigue crack tip increased the local diffusion of hydrogen thus increasing crack growth rate.     

Assessment of resistance to fatigue crack growth of natural gas line pipe steels carrying gas mixed with hydrogen

Hydrogen embrittlement of line pipe steels in the natural gas transmission and distribution network is investigated. The objective is to assess whether the existing network can be used to safely transport a mixture of hydrogen and natural gas. The surveyed literature indicates that the hydrogen-induced acceleration of fatigue crack growth induced by natural gas pressure fluctuations can be the most probable type of failure. We analyzed the fatigue crack growth in line pipe steels containing a long axial crack in the inner diameter (ID) surface by accounting for random cyclic loading due to random and realistic pressure fluctuations, crack closure, and accurate calculation of the stress intensity factor. Using the available experimental data for the crack growth rate vs. stress intensity factor range in the presence of hydrogen, we simulated crack growth over a period of 100 years. The results show that under typical pressure fluctuations in the natural gas network, cracks with depths less than 40% of the wall thickness will never reach depths equal to 75% of the wall thickness. This is a conservative estimate that results from i) the nature of the geometry of the initial flaw in the ID surface that we used in the analysis, ii) the fact that the existing experimental data for the effect of hydrogen on the Paris law are for pressures that are orders of magnitude larger than the partial pressures intended for the hydrogen gas in the mixture, and iii) the experimental data are for fatigue crack growth in pure hydrogen gas without impurities normally present in natural gas, such as oxygen or methane, that can inhibit hydrogen uptake.     

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.     

Characterization of fatigue crack of hydrogen-charged austenitic stainless steel by electromagnetic and ultrasonic techniques

In this study, the feasibility of the fusion sensing of eddy current testing (ECT) and ultrasonic testing (UT) as effective tools to clarify the hydrogen-embrittlement mechanism of austenitic stainless steels was investigated. Fatigue testing was conducted on hydrogen-charged and uncharged AISI 304 specimens. The effect of hydrogen exposure on the martensitic transformation, and crack closure and crack face morphology were investigated by ECT and UT. The results suggest that a comparison of ECT and UT results can evaluate martensitic transformation and crack closure and crack face morphology, which are important in understanding the hydrogen embrittlement of austenitic stainless steels.     

Influence of hydrogen pressure on fatigue properties of X80 pipeline steel

The low-cycle fatigue and fatigue crack growth (FCG) properties of X80 pipeline steel in hydrogen atmosphere were determined to investigate the variation of hydrogen pressure and its influence on fatigue life. The test environment was switched to a hydrogen atmosphere after 1000, 3000, or 5000 cycles of pre-fatigue testing in a nitrogen atmosphere. Notch tensile tests were conducted in nitrogen and hydrogen atmospheres after the specimens were pre-fatigued for 3000 or 5000 cycles. The results showed that the cycles to failure of X80 decreased exponentially with increasing hydrogen pressure. When the displacement amplitude (DA) values remained steady (below 3000 cycles), the X80 steels showed no noticeable deterioration in the fatigue properties with or without hydrogen. When the DA values increased (above 5000 cycles), cracks propagated slowly and fatigue properties were strongly reduced in the hydrogen atmosphere, but not in nitrogen. Hydrogen-accelerated crack growth dominates the reduction of fatigue life below 0.6 MPa of hydrogen pressure. Hydrogen-accelerated crack initiation plays a more important role than FCG in the reduction of fatigue life with increasing hydrogen pressure.     

Hydrogen transport in solution-treated and pre-strained austenitic stainless steels and its role in hydrogen-enhanced fatigue crack growth

Hydrogen solubility and diffusion in Type 304, 316L and 310S austenitic stainless steels exposed to high-pressure hydrogen gas has been investigated. The effects of absorbed hydrogen and strain-induced martensite on fatigue crack growth behaviour of the former two steels have also been measured. In the pressure range 10–84 MPa, the hydrogen permeation of the stainless steels could be successfully quantified using Sieverts' law modified by using hydrogen fugacity and Fick's law. For the austenitic stainless steels, hydrogen diffusivity was enhanced with an increase in strain-induced martensite. The introduction of dislocation and other lattice defects by pre-straining increased the hydrogen concentration of the austenite, without affecting diffusivity. It has been shown that the coupled effect of strain-induced martensite and exposure to hydrogen increased the growth rate of fatigue cracks.     

Fatigue limit of carbon and CrMo steels as a small fatigue crack threshold in high-pressure hydrogen gas

The fatigue limit properties of a carbon steel and a low-alloy CrMo steel were investigated via fully-reversed tension-compression tests, using smooth specimens in air and in 115-MPa hydrogen gas. With respect to the CrMo steel, specimens with sharp notches were also tested in order to investigate the threshold behavior of small cracks. The obtained SN data inferred that the fatigue limit was not negatively affected by hydrogen in either of the steels. Observation of fatigue cracks in the unbroken specimens revealed that non-propagating cracks can exist even in 115-MPa hydrogen gas, and that the crack growth threshold is not degraded by hydrogen. The experimental results provide justification for the fatigue limit design of components that are to be exposed to high-pressure hydrogen gas.     

The inhibitory effect of carbon monoxide contained in hydrogen gas environment on hydrogen-accelerated fatigue crack growth and its loading frequency dependency

The effect of carbon monoxide (CO) contained in H₂ gas as an impurity on the hydrogen-accelerated fatigue crack growth of A333 pipe steel was studied in association with loading frequency dependency. The addition of CO to H₂ gas inhibited the accelerated fatigue crack growth due to the hydrogen. The inhibitory effect was affected by the CO content in the H₂ gas, loading frequency, and crack growth rate. Based on these results, it was revealed that the inhibitory effect of CO was governed by both competition between the rate of fresh surface creation by the crack growth and the rate of coverage of the surface by CO and time for hydrogen diffusion in the material to the crack tip with reduced hydrogen entry by CO.     

Research and demonstration on hydrogen compatibility of pipelines: a review of current status and challenges

Hydrogen transportation by pipelines gradually becomes a critical engineering route in the worldwide adaptation of hydrogen as a form of clean energy. However, due to the hydrogen embrittlement effect, the compatibility of linepipe steels and associated welds with hydrogen is a major concern when designing hydrogen-carrying pipelines. When hydrogen enters the steels, their ductility, fracture resistance, and fatigue properties can be adversely altered. This paper reviews the status of several demonstration projects for natural gas-hydrogen blending and pure hydrogen transportation, the pipeline materials used and their operating parameters. This paper also compares the current standards of materials specifications for hydrogen pipeline systems from different parts of the world. The hydrogen compatibility and tolerance of varying grades of linepipe steels and the relevant testing methods for assessing the compatibility are then discussed, and the conservatism or the inadequacies of the test conditions of the current standards are pointed out for future improvement.     

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

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

Oxidation and creep interactions during high temperature high-R fatigue crack growth threshold determination

While knowledge relating to the determination and practical application of fatigue crack growth threshold stress intensity factors (ΔK_th) for defect assessment is relatively well established for many circumstances, this is not the case for materials and conditions which are sensitive to time dependent mechanisms. In particular, current testing standards do not address determination of the property for very high R (K_min/K_max) ratios, and those conditions when the environment can be influential and ΔK_th is sensitive to the adopted da/dN(ΔK) criterion. The incidence of creep crack growth in association with very slowly growing high-R high cycle fatigue cracks is not always discernable by the direct examination of near-to-threshold da/dN(ΔK) test records, but can be predicted by means of a time dependent failure assessment diagram analysis. In addition to a general state of knowledge review, particular attention is paid to circumstances relating to high-R ΔK_th in power plant steels at high temperatures for which oxide induced closure and creep cracking can be influential. Evidence for a low alloy 1%Cr steel, a martensitic 9%Cr steel and an austenitic 18%Cr steel is considered, and the practical implications are examined.     

A study on fatigue crack growth in the high cycle domain assuming sinusoidal thermal loading

The assessment of fatigue crack growth due to turbulent mixing of hot and cold coolants presents significant challenges, in particular to determine the thermal loading spectrum and the associated crack growth. The sinusoidal method is a simplified approach for addressing this problem, in which the entire spectrum is replaced by a sine-wave variation of the temperature at the inner pipe surface. The loading frequency is taken as that which gives the shortest crack initiation and growth life. Such estimates are intended to be conservative but not un-realistic. Several practical issues which arise with this approach have been studied using newly-developed analytical solutions for the temperature and stress fields in hollow cylinders, in particular the assumptions made concerning the crack orientation, dimensions and aspect ratio. The application of the proposed method is illustrated for the pipe geometry and loadings conditions reported for the Civaux 1 case where through wall thermal fatigue cracks developed in a short time, but the problem is relevant also for fast reactor components.     

疲劳裂纹预测的灰方法

借助灰色理论，建立了预测疲劳裂纹扩展的灰色模型，并应用此模型预测了某不锈钢构件腐蚀疲劳裂纹的扩展，得到了较高精度的预测结果，为疲劳裂纹预测提供了一种简易而可靠的新途径。     

Effect of inside diameter on design fatigue life of stationary hydrogen storage vessel based on fracture mechanics

Design fatigue life of stationary hydrogen storage vessel constructed of the practical materials of low alloy steels was analyzed based on fracture mechanics in hydrogen and air of 45, 85 and 105 MPa using cylindrical model with inside diameter (Di) of 150, 250 and 350 mm. Design fatigue life of five typical model materials was also analyzed to discuss the effect of Di on the design fatigue life by hydrogen-induced crack growth of the vessel. K_IC of all the practical materials qualified the leak before burst. Design fatigue life generally increased slightly with increasing Di in air, while design fatigue life by K_IH was much shorter than that in air. Hydrogen influence on design fatigue life increased with increasing Di due to that K_I at initial crack increased with increasing Di. The design fatigue life data of the model materials under the conditions of Di, pressure, ultimate tensile strength, K_IH, fatigue crack growth rate and regulations in both hydrogen and air were proposed quantitatively for materials selection and development for stationary hydrogen storage vessel.     

The role of induced α′-martensite on the hydrogen-assisted fatigue crack growth of austenitic stainless steels

The effects of rolling on the hydrogen-assisted fatigue crack growth characteristics of AISI 301, 304L and 310S stainless steels (SSs) were investigated. In hydrogen, cold rolled specimens with a 20% thickness reduction were found to increase the fatigue crack growth rates (FCGRs) in the 301 and 304L SSs, and to a much lesser extent in the 310S SS. However, enhanced slip was observed for the 310S specimen in hydrogen. Hydrogen-accelerated FCGRs of the 301 and 304L SSs were related with the crack growth through the strain-induced martensite formed in the plastic zone ahead of the crack tip.     

The dependence of fatigue crack growth on hydrogen in warm-rolled 316 austenitic stainless steel

The fatigue crack growth rate of warm-rolled AISI 316 austenitic stainless steel was investigated by controlling rolling strain and temperature in argon and hydrogen gas atmospheres. The fatigue crack growth rates of warm-rolled 316 specimens tested in hydrogen decreased with increasing rolling temperature, especially 400 °C. By controlling the deformation temperature and strain, the influences of microstructure (including dislocation structure, deformation twins and α′ martensite) and its evolution on hydrogen-induced degradation of mechanical properties were separately discussed. Deformation twins deceased and dislocations became more uniform with the increase in rolling temperature, inhibiting the formation of dynamic α′ martensite during the crack propagation. In the cold-rolled 316 specimens, deformation twins accelerated hydrogen-induced crack growth due to the α′ martensitic transformation at the crack tip. In the warm-rolled specimens, the formation of α′ martensite around the crack tip was completely inhibited, which greatly reduced the fatigue crack growth rate in hydrogen atmosphere.     

Predictions for fatigue crack growth life of cracked pipes and pipe welds using RMS SIF approach and experimental validation

The objective of the present study is to understand the fatigue crack growth behavior in austenitic stainless steel pipes and pipe welds by carrying out analysis/predictions and experiments. The Paris law has been used for the prediction of fatigue crack growth life. To carry out the analysis, Paris constants have been determined for pipe (base) and pipe weld materials by using Compact Tension (CT) specimens machined from the actual pipe/pipe weld. Analyses have been carried out to predict the fatigue crack growth life of the austenitic stainless steel pipes/pipes welds having part through cracks on the outer surface. In the analyses, Stress Intensity Factors (K) have been evaluated through two different schemes. The first scheme considers the ‘K’ evaluations at two points of the crack front i.e. maximum crack depth and crack tip at the outer surface. The second scheme accounts for the area averaged root mean square stress intensity factor (K_RMS) at deepest and surface points. Crack growth and the crack shape with loading cycles have been evaluated. In order to validate the analytical procedure/results, experiments have been carried out on full scale pipe and pipe welds with part through circumferential crack. Fatigue crack growth life evaluated using both schemes have been compared with experimental results. Use of stress intensity factor (K_RMS) evaluated using second scheme gives better fatigue crack growth life prediction compared to that of first scheme. Fatigue crack growth in pipe weld (Gas Tungsten Arc Welding) can be predicted well using Paris constants of base material but prediction is non-conservative for pipe weld (Shielded Metal Arc Welding). Further, predictions using fatigue crack growth rate curve of ASME produces conservative results for pipe and GTAW pipe welds and comparable results for SMAW pipe welds.     

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.