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1.
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 (ΔKth) could not be distinctly identified. The fatigue crack was also observed to exhibit uneven propagation immediately before ΔKth. 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 ΔKth in the hydrogen gas environment. Conversely, no facet was observed immediately before ΔKth 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 ΔKth in the hydrogen gas environment.  相似文献   

2.
    
We introduced a coupled peridynamic hydrogen diffusion and fracture model to solve the hydrogen embrittlement fracture of low alloy steel AISI 4340. In this model, the influence of temperature on hydrogen diffusion coefficient is considered, and a new peridynamic constitutive analysis method is used to simulate the crack propagation of hydrogen embrittlement. We verified the model in 3D using the experimental test of the hydrogen embrittlement cracking process of AISI 4340 steel in 0.1 N H2SO4 solution from the literature. Considering different ambient temperatures, it is found that the crack propagation is highly similar to the experimental results. Based on the numerical analysis of peridynamics, the model can numerically simulate the hydrogen embrittlement fracture of AISI 4340 steel, and obtain a visual demonstration of the entire process of hydrogen atom diffusion and crack growth.  相似文献   

3.
Hydrogen embrittlement (HE) together with the hydrogen transport behavior in hydrogen-charged type 304 stainless steel was investigated by combined tension and outgassing experiments. The hydrogen release rate and HE of hydrogen-charged 304 specimens increase with the hydrogen pressure for hydrogen-charging (or hydrogen content) and almost no HE is observed below the hydrogen content of 8.5 mass ppm. Baking at 433 K for 48 h can eliminate HE of the hydrogen-charged 304 specimen, while removing the surface layer will restore HE, which indicates that hydrogen in the surface layer plays the primary role in HE. Scanning electron microscopy (SEM) and scanning tunnel microscopy (STM) observations show that particles attributed to the strain-induced α′ martensite formation break away from the matrix and the small holes form during deformation on the specimen surface. With increasing strain, the connection among small holes along {111} slip planes of austenite will cause crack initiation on the surface, and then the hydrogen induced crack propagates from the surface to interior.  相似文献   

4.
    
Alloy hardened steels offer excellent combination of mechanical properties, hardenability and corrosion resistance. 34CrMo4 is a medium carbon, low alloy steel widely used due to a good combination of high-strength, toughness and wear resistance. However, this steel experiences hydrogen embrittlement (HE), a complex phenomenon depending on the composition and microstructure. This work estimates de loss of the mechanical properties caused by hydrogen in electrochemically H-charged specimens in absence of mechanical stress but also, at low strain rate and constant load. H-charging for 2 and 6 h induce YS losses of about 40% and 71% and UTS losses of 39% and 59%, respectively. The synergistic effect of the stress and the H-charging process leads to a higher loss, 91%, and a faster brittle fracture even though hydrogen content is similar to those firstly H-charged and then tested in air.  相似文献   

5.
    
The present work aims to investigate the role of hydrogen induced blisters cracking on degradation of tensile and fatigue properties of X65 pipeline steel. Both tensile and fatigue specimens were electrochemically charged with hydrogen at 20 mA/cm2 for a period of 4 h. Hydrogen charging resulted in hydrogen induced cracking (HIC) and blister formation throughout the specimen surface. Nearly all the blisters formed during hydrogen charging showed blister wall cracking (BWC). Inclusions mixed in Al-Si-O were found to be the potential sites for HIC and BWC. Slow strain rate tensile (SSRT) test followed by fractographic analysis confirmed significant hydrogen embrittlement (HE) susceptibility of X65 steel. Short fatigue crack growth framework, on the other hand, specifically highlighted the role of BWC on accelerated crack growth in the investigated material. Coalescence of propagating short fatigue crack with BWC resulted in rapid increase in the crack length and reduced the number of cycles for crack propagation to the equivalent crack length.  相似文献   

6.
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.  相似文献   

7.
We studied the fracture strain of polycrystal ɑ-iron at three different hydrogen concentrations and for three crystal sizes with molecular dynamics simulations (MDs). As the hydrogen concentration increases, a fine crystal model's fracture resistance is prone to below a comparatively coarse grain model. This finding elucidates that the most vulnerable area can alter from coarse grain zone to fine-grain zone in the welding heat-affected zone (HAZ) with the effect of hydrogen. A simplified model is thus built to predict the strain energy increment in different crystal size systems caused by the introduction of hydrogen atoms. This strain energy increment is not equal to the fracture energy reduction induced by the same amount of hydrogen insertion, demonstrating that elastic volume expansion of grain boundary (GB) caused by hydrogen is not the determining mechanism of intergranular failure. The density of triple or multi-junctions of GBs, which is typically dependent on the volume fraction of GBs, is the crucial factor for intergranular failure caused by hydrogen embrittlement.  相似文献   

8.
Due to its high strength and acceptable corrosion resistance, Alloy 718 is a viable material for the use in subsea applications. The alloy's susceptibility to hydrogen embrittlement is however limiting its viability. In this work the effect of microstructure on the hydrogen embrittlement susceptibility of Alloy 718 was examined by the use of slow strain rate testing on three different heat treatments. Cathodic pre-charging and polarization during testing were used to introduce hydrogen into the samples. A severe reduction in the ductility of the alloy due to the precipitation phases γ″ and γ′ were found. In addition, one of the heats had a continuous film of a Nb and C rich phase on the grain boundary giving an intergranular fracture mechanism. This intergranular fracture mode was further enhanced by the presence of hydrogen. In the solution annealed condition, the presence of hydrogen reduced the dimple size on the fracture surface.  相似文献   

9.
    
The tensile properties and crack propagation rate in a type 316 austenitic stainless steel prepared by vacuum induction melting method with different residual hydrogen contents (1.1–11.5 × 10−6) were systematically investigated in this research work. The room temperature tensile properties were measured under both regular tensile (12 mm/min) and slow tensile (0.01 mm/min) conditions, and the fracture properties of the tensile fractures with both rates were analyzed. It shows that the hydrogen induced plasticity loss of stainless steel strongly depends on the tensile rate. Under regular tensile condition, there is no plastic loss even when the hydrogen content is up to 11.5 × 10−6 while in the slow tensile condition, the plastic loss can be clearly identified rising with the increasing H contents. The fatigue crack propagation rate was tested at room temperature, and the crack growth rate formula (Paris) of the 316 stainless steels with varied H contents were obtained. The fatigue crack propagation rate test shows that the crack growth rate of the 316 stainless steel with 8.0–11.5 × 10−6 hydrogen is significantly higher than that of benchmark steel.  相似文献   

10.
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.  相似文献   

11.
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.  相似文献   

12.
The effect of the Ni/Cu ratio on hydrogen embrittlement (HE) behavior is studied in the context of the Cu–Ni binary alloy system. When classifying HE sensitivity as a function of the Ni fraction, two regimes are obtained: Regime 1 (wherein Ni fraction is less than 80 wt%) with moderate HE and Regime 2 (where Ni fraction exceeds 80 wt%) with more severe HE, although hydrogen- (H-)induced intergranular (IG) cracking is noted to be the common primary cause of H-induced degradation. Necessities of H-transportation towards grain boundaries (GBs) via H–dislocation interaction and/or dynamic H-diffusion are discussed via the slow strain rate tensile tests at −196 °C. Specifically, in Regime 1, H-transportation is necessary for the triggering of IG cracking. Conversely, in Regime 2, such H-transportation plays a minor role and the initially concentrated hydrogen along GBs can sorely cause IG cracking.  相似文献   

13.
    
This work investigated the hydrogen embrittlement mechanism of Ni fabricated by laser-based powder bed fusion (L-PBF). In the presence of hydrogen, the L-PBFed Ni failed with a brittle mode, while its fracture surface had a “transgranular-like” appearance. This unusual fracture morphology is rooted in the special grain shape induced by the laser-based manufacturing process, and the failure process is actually predominated by the intergranular decohesion. An annealing process of the as-printed sample enhanced its elongation and mitigated the hydrogen embrittlement. The special dislocation cellular pattern formed in additive manufacturing is considered to be detrimental to hydrogen embrittlement resistance.  相似文献   

14.
    
With a validated fluid-structure-fracture coupling approach, this paper studied the dynamic response and crack propagation of pre-flawed square tube under internal hydrogen-oxygen detonation. Fracture of tube was judged by a bivariate failure criterion derived from the underlying failure mechanism at high strain rate conditions. A programed burn approach based on the CJ theory was applied to simulate gaseous detonation. The coupling between detonation wave and tube was realized by penalty contact algorithm with an improved contact stiffness calculation formula. It was demonstrated that the peak pressure at tube edge is 29% higher than that at the middle of tube face. The dominant crack driving force comes from the specific vibration and deformation modes of square tube, where the deformed round section of tube corresponds to the maximum stress wave that travels behind the flexural waves on the tube. Above mechanism makes the backward cracks branch or turn before the forward cracks and the speeds of front and back branch cracks comparable to each other, which is opposite or different from the cases of round tubes. The crack behaviors with different initial flaw locations and detonation pressures were summarized and identified in detail. The forward crack speed can be up to 900 m/s, while the backward crack speeds are generally 65%–85% of above and the branch cracks run at about 100 m/s. In addition, the crack speed has a certain increase immediately after crack branching or turning. Among the three initial flaw location cases, the tube with initial flaw at the middle of face is most resistant to crack propagation under internal detonations.  相似文献   

15.
It is well known that hydrogen weakens strengths of metals, and this phenomenon is called hydrogen embrittlement. Despite the extensive investigation concerning hydrogen related fractures, the mechanism has not been enough clarified yet. In this study, we applied the molecular dynamics method to the mode I crack growth in α-Fe single crystals with and without hydrogen, and analyzed the hydrogen effects from atomistic viewpoints. We estimated the hydrogen trap energy in the vicinity of an edge dislocation in order to clarify the distribution of hydrogen atoms, using the molecular statics method. We also evaluated the energy barrier for dislocation motion under a low hydrogen concentration. Based on these results, we propose a mechanism for hydrogen embrittlement of α-Fe under monotonic loading.  相似文献   

16.
    
In this work, a practical numerical model with few parameters was proposed for the prediction of environmental hydrogen embrittlement. The proposed method adopts hydrogen enhanced plasticity-based mechanism in a fracture strain model to describe hydrogen embrittlement. Fracture toughness degradation of three commercial steels SA372J70, AISI4130 and X80 in high pressure hydrogen environment were investigated. Firstly, governing equations for hydrogen distribution and material damage evolution was established. Hydrogen enhanced localized flow softening effect was coupled within fracture strain dependency on stress triaxiality. Then, the numerical implementation and identification process of model parameters was described. Model parameters of the investigated steels were determined based on experiment results from literatures. Finally, with the calibrated model, fracture toughness reduction of the steels was predicted in a wide range of hydrogen pressure. The prediction results were compared with experimental results. Reasonable accuracy was reached. The proposed method is an attempt to reach balance between physical accurate prediction and engineering practicality. It is promising to provide a simplified numerical tool for the design and fit for service evaluation of hydrogen storage vessels.  相似文献   

17.
    
The present study quantitatively evaluated mechanical response of hydrogen-related fracture in the as-quenched martensitic steel and correlated it to crack propagation behavior analyzed by microstructure observations. The crack-growth resistance curves revealed that the hydrogen-related intergranular cracks propagated in a stable manner even when the diffusible hydrogen content was large. Fracture initiation toughness was decreased significantly by small amounts of diffusible hydrogen. With further increasing diffusible hydrogen content, however, the fracture initiation toughness did not change and remained almost constant. On the other hand, tearing modulus, corresponding to crack-growth resistance, decreased rather gradually with increasing diffusible hydrogen content. The microstructure observations confirmed that the hydrogen-related crack propagated discontinuously in a stepwise manner on a microscopic scale. Accordingly, it was proposed that the microscopic discontinuous crack propagation could be the possible reason for the stable crack propagation.  相似文献   

18.
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.  相似文献   

19.
Seventeen metastable austenitic stainless steels (type 304 and 316 alloys) were tested in tension both with internal hydrogen and in external hydrogen. Hydrogen-assisted fracture in both environments is a competition between hydrogen-affected ductile overload and hydrogen-assisted crack propagation. In general, hydrogen localizes the fracture process, which results in crack propagation of particularly susceptible materials at an apparent engineering stress that is less than the tensile strength of the material. Hydrogen-assisted crack propagation in this class of alloys becomes more prevalent at lower nickel content and lower temperature. In addition, for the tests in this study, external hydrogen reduces tensile ductility more than internal hydrogen. External hydrogen promotes crack initiation and propagation at the surface, while with internal hydrogen surface cracking is largely absent, thus preempting hydrogen-assisted crack propagation from the surface. This is not a general result, however, because the reduction of ductility with internal and external hydrogen depends on the specifics of the testing conditions that are compared (e.g., hydrogen gas pressure); in addition, internal hydrogen can promote the formation of internal cracks, which can propagate similar to surface cracks.  相似文献   

20.
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.  相似文献   

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