Effect of nanosized NbC precipitates on hydrogen-induced cracking of high-strength low-alloy steel

We investigated the effect of nanosized NbC precipitates on hydrogen-induced cracking(HIC)of high-strength low-alloy steel by conducting slow-strain-rate tensile tests(SSRT)and performing continuous hydrogen charging and fracture analysis.The results reveal that the HIC resistance of Nb-bearing steel is obviously superior to that of Nb-free steel,with the fractured Nb-bearing steel in the SSRT exhibiting a smaller ratio of elongation reduction(Iδ).However,as the hydrogen traps induced by NbC precipitates approach hydrogen saturation,the effect of the precipitates on the HIC resistance attenuate.We speculate that the highly dispersed nanosized NbC precipitates act as irreversible hydrogen traps that hinder the accumulation of hydrogen at potential crack nucleation sites.In addition,much like Nb-free steel,the Nb-bearing steel exhibits both H-solution strengthening and the resistance to HIC.

Hydrogen-induced cracking behaviors of Incoloy alloy 825

The effect of hydrogen on the fractttre behaviors of Incoloy alloy 825 was investigated by means of slow strain rate testing （SSRT） Hydrogen was introduced into the sample by electrochemical charging. The results show that surface microcracks form gradually during ag- ing at room temperature when desorption of hydrogen takes place after hydrogen charging at a current density of 5 mA/cm^2 for 24 h. SSRT shows that the increase of ductility loss is significantly obvious as the hydrogen charging current density increases. Scanning electron microscopy （SEM） images reveal ductile fracture in the pre-charged sample with low current densities, while the fracture includes small quasi-cleavage regions and tends to be brittle fracture as the hydrogen charging current density increases to 5 mA/cm^2.

Investigation on hydrogen-induced cracking susceptibility of HG980D steel with yield strength 900 MPa

In this paper, the microstructure and hardness of HG980D heat-affected zone （HAZ） at different cooling rate t8/3 were studied, the implant critical fracture stress under three diffusible hydrogen conditions were measured, and the hydrogeninduced cracking （H1C） fructograph of steel HG980D were analyzed, The experimental results show that martensite exists in HAZ of HG980D till ts/3 ≥ 150 s, the harden quenching tendency of HG980D is greater; The implant critical fracture stress is related to difJhsible hydrogen content significantly, at low hydrogen level, high restraint stress is needed to nucleate HIC, the fraetograph is mainly mierovoid coalescence, bat at high hydrogen level, only small restraint stress can cause H1C occurrence, the fractograph is mainly quasicleavage. It is very important to choose ultra-low hydrogen welding consumable to weld steel HG980D to prevent hydrogen-induced cracking.

Threshold Stress Intensity of Hydrogen-Induced Cracking and Stress Corrosion Cracking of High Strength Steel

The threshold stress intensity of stress corrosion cracking(SCC) for 40 CrMo steel in 3.5%NaCl solution decreased exponentially with the increase of yield strength.The threshold stress intensity of hydrogen-induced cracking during dynamical charging for 40 CrMo steel decreased linearly with the logarithm of the concentration of diffusible hydrogen.This equation was also applicable to SCC of high strength steel in aqueous solution.The critical hydrogen enrichment concentration necessary for SCC of high strength steel in water decreased exponentially with the increase of yield strength.Based on the results,the relationship between K_(ISCC) and σ_(ys) could be deduced.

TEM Study on Hydrogen-Induced Cracking of Fe_3Al Alloy

Hydrogen-induced cracking (HIC) of Fe3Al alloy was studied by in situ transmission electron microscope (TEM). Electron transparent specimens were mounted onto a constant displacement device. Stress was applied to the specimen by using a bolt through the device. The results showed that hydrogen enhanced the dislocation emission and motion in Fe3Al alloy. A dislocation free zone (DFZ) was formed following the dislocation emission. Microcrack initiated in the DFZ or at the main crack tip when the emission reached a critical extension. Hydrogen played an important role in the process of brittle fracture of Fe3Al alloy.

Hydrogen-induced Cracking of Weld Metal of Austenitic Stainless Steels

For 308L and 347L weld metals of austenitc stainless steels (ASS), hydrogen induced cracking (HIC) occurred during dynamically charging under constant load. The threshold stress intensity for HIC, Km, decreased linearly with the logarithm of the concentration of diffusible hydrogen C0 in the weld metals and the rolled plate of type 304L ASS, i.e., KIH=85.2-10.71nC0 (308L), KIH=76.1-9.31n C0(347L), and KIH=91.7-10.11nC0(304L). The fracture mode for HIC in the three type of ASS changed from ductile to brittle with the decrease in the applied stress intensity KI or/and the increase in C0. The boundary line between ductile and brittle fracture surfaces was KI-54+25exp(-C0/153)=0.

Molecular dynamics study of hydrogen-induced cracking behavior of ferrite–pearlite gas transmission pipeline steel

Hydrogen embrittlement of pipelines depends on the hydrogen-induced cracking behavior of the pipeline steel microstructure.Based on molecular dynamics analysis,the ferrite–cementite(α-Fe/Fe3C)lamellar atomic structure with the Bagaryatskii orientation relationship was established,and stepwise relaxation of the conjugate gradient energy minimization and constant-temperature and constant-pressure relaxation were performed under NPT(the isothermal–isobaric)conditions.The mechanical property curves of theα-Fe/Fe3C models were obtained under different cementite terminal plane structures,and the evolution of the atomic structure was analyzed in detail.In addition,the influence of different hydrogen concentrations,different temperatures,different strain rates,changes in voids,and different micro-degrees of freedom on the deformation and failure mechanism of the model was investigated,aiming to provide a reliable way to explore the micro-mechanism of macro-cracking behavior of pipeline steel.

Synergistic effects of Nb and Mo on hydrogen-induced cracking of pipeline steels:A combined experimental and numerical study

The synergistic effects of Nb and Mo on hydrogen-induced cracking(HIC)of pipeline steels were studied experimentally and numerically.The results showed that Mo was primarily segregated at grain-boundaries(GBs)or solid-dissolved in the matrix,while most Nb and a small amount of Mo formed dis-persed(Nb,Mo)C nano-precipitates and refined the microstructure.Compared with Nb alloying,the multi-ple additions of Nb-Mo played dual roles in affecting H diffusion:primarily,the H-traps densities such as GBs,precipitates,and solute Mo atoms increased,providing an advantage;however,Mo slightly reduced the H-trapping capacity of precipitates,playing an adverse role.Nonetheless,the beneficial effects far outweighed the adverse effects,thereby reducing H diffusivity and inhibiting crack initiation.Addition-ally,Nb and Mo hindered crack propagation synergistically as follows:(i)Mo enhanced GB cohesion by repelling H,impeding intergranular cracking and hydrogen-enhanced decohesion(HEDE);(ii)Nb reduced the proportion of3/high-angle grain boundaries,increasing cracking resistance;(iii)(Nb,Mo)C precip-itates impeded H-dislocation interactions,reducing the hydrogen-enhanced localized plasticity(HELP).

Hydrogen-enhanced dislocation emission, motion and nucleation of hydrogen-induced cracking for steel

The change in dislocation configuration ahead of a loaded crack tip before and after charging with hydrogen was in situ investigated in TEM using a special constant deflection loading device The results showed that hydrogen could facilitate dislocation emission, multiplication and motion The change in displacement field ahead of a loaded notch tip for a bulk specimen before and after charging with hydrogen was in situ measured by the laser moire interferometer technique. The results showed that hydrogen could enlarge the plastic zone and increase the plastic strain The in situ observation in TEM showed that when hydrogen-enhanced dislocation emission and motion reached a critical condition, a nanocrack of hydrogen-induced cracking ( HIC) would nucleate in the dislocation-free zone (DFZ) or at the main crack tip. The reasons for hydrogen-enhanced dislocation emission, multiplication and motion, and the mechanisms of nucleation of HIC have been discussed

Hydrogen-induced cracking by nanovoids in 310 stainless steel

Hydrogen-induced cracking was investigated by TEM in-situ tension in hydrogenated stainless steel of type 310. It was found experimentally that hydrogen-induced cracking happens via nanovoid nucleation followed by quasi-cleavage along {111} planes when C H is higher. Otherwise, in the case of lower C H, hydrogen enhances ductile fracture via hydrogen-enhanced microvoid nucleation, growth and connection. A new model was proposed based on the present experiments. Dislocations break away from defect atmospheres and move away from the DFZ, leaving vacancy and hydrogen clusters along {111} planes. Hydrogen tends to combine with vacancy clusters and initiate nanovoids along {111} planes. Dense nanovoids connect each other, resulting in brittle cracking. Scattered nanovoids grow into microvoids or even macrovoids, leading to ductile fracture.

Hydrogen-induced cracking and its anisotropy of a PZT ferroelectric ceramics

Threshold stress intensity factor of hydrogen-induced cracking (HIC), K IH, of a lead zirconate titanate ferroelectric ceramics (PZT-5) has been measured during dynamic charging with various current densities at constant load using notched tensile specimens with poling direction parallel or perpendicular to the crack plane. The results show that K IH reveals anisotropy, and the threshold stress intensity factor for the specimen with poling direction parallel to the crack plane, K IH a , is greater than that perpendicular to the crack plane, K IH b , similar to the anisotropy of fracture toughness, K IC. The normalized threshold stress intensity factor of HIC, however, does not reveal anisotropy, and decreass linearly with logarithm of hydrogen concentration, C o, i.e. K IH a /K IC a =K IH b /K IC b =0.4?0.15 In C o. Therefore, the anisotropy of HIC is the same as that of the fracture toughness, and is due to the anisotropy of the stress-induced 90° domain switching.

Influence of Microstructures on Polarization Resistance and Hydrogen-Induced Cracking of a Micro-Alloyed Steel Submitted to Different Cooling Rates

The effect of different microstructures on the polarization resistance (Rp) and the hydrogen-induced cracking (HIC) of a micro-alloyed steel austenitized and submitted to different cooling rates was studied.Samples 19.1 x 6 x 2 mm,containing the whole thickness of the plate were extracted from a 20 mm plate and heat treated on a quenching dilatometer,were submitted to Rp and HIC corrosion tests.Both Rp and HIC tests followed as close as possible ASTM G59 and NACE standard TM0284-2003,in this case,modified only with regard to the size of the samples.Steel samples transformed from austenite by a slow cooling (cooling rate of 0.5℃.s-1) showed higher susceptibility to hydrogen-induced cracking,with large cracks in the middle of the sample propagating along segregation bands,corresponding to the centerline of the plate thickness.For cooling rates of 10℃.s-1,only small cracks were found in the matrix and micro cracks nucleated at non-metallic inclusions.For higher cooling rates (40℃.s-1) very few small cracks were detected,linked to non-metallic inclusions.This result suggests that structures formed by polygonal structures and segregation bands (were eutectoid microconstituents predominate) have higher susceptibility to HIC.Structures predominantly formed by acicular ferrite make it difficult to propagate the cracks among non-oriented and interlaced acicular ferrite crystals.Smaller segregation bands containing eutectoid products also help inhibit cracking and crack propagation;segregation bands can function as pipelines for hydrogen diffusion and offer a path of stress concentration for the propagation of cracks,frequently associated to non-metallic inclusions.Polarization resistance essays performed on the steel in theas received condition,prior to any heat treatment,showed larger differences between the regions of the plate,with a considerably lower Rp in the centerline.The austenitization heat treatments followed by cooling rates of 0.5 e 10℃.s-1 made more uniform the corrosion resistance along the thickness of the plate.The effects of heat treatments on the corrosion resistance are probably related to the microconstituent formed,allied to the chemical homogenization of the impurities concentrated on the centerline of the plate.

Recent research progress in the mechanism and suppression of fusion welding-induced liquation cracking of nickel based superalloys

Nickel-based superalloys are extensively used in the crucial hot-section components of industrial gas turbines,aeronautics,and astronautics because of their excellent mechanical properties and corrosion resistance at high temperatures.Fusion welding serves as an effective means for joining and repairing these alloys;however,fusion welding-induced liquation cracking has been a challenging issue.This paper comprehensively reviewed recent liquation cracking,discussing the formation mechanisms,cracking criteria,and remedies.In recent investigations,regulating material composition,changing the preweld heat treatment of the base metal,optimizing the welding process parameters,and applying auxiliary control methods are effective strategies for mitigating cracks.To promote the application of nickel-based superalloys,further research on the combination impact of multiple elements on cracking prevention and specific quantitative criteria for liquation cracking is necessary.

Effect of icosahedral phase formation on the stress corrosion cracking(SCC)behaviors of the as-cast Mg-8%Li(in wt.%)based alloys

Through exploring the stress corrosion cracking(SCC)behaviors of the as-cast Mg-8%Li and Mg-8%Li-6%Zn-1.2%Y alloys in a 0.1 M NaCl solution,it revealed that the SCC susceptibility index(I_(SCC))of the Mg-8%Li alloy was 47%,whilst the I_(SCC)of the Mg-8%Li-6%Zn-1.2%Y alloy was 68%.Surface,cross-sectional and fractography observations indicated that for the Mg-8%Li alloy,theα-Mg/β-Li interfaces acted as the preferential crack initiation sites and propagation paths during the SCC process.With regard to the Mg-8%Li-6%Zn-1.2%Y alloy,the crack initiation sites included the I-phase and the interfaces of I-phase/β-Li andα-Mg/β-Li,and the preferential propagation paths were the I-phase/β-Li andα-Mg/β-Li interfaces.Moreover,the SCC of the two alloys was concerned with hydrogen embrittlement(HE)mechanism.

Hydrogen-induced amorphization of Zr-Cu-Ni-Al alloy

Arc melting was utilized in this study to produce Zr_(55)Cu_(30)Ni_5Al_(10) alloys under mixed atmospheres with various ratios of high-purity hydrogen to argon. The influences of hydrogen addition on the solidification structure and glass-forming ability of Zr_(55)Cu_(30)Ni_5Al_(10) alloy were determined by examining microstructures in different parts of the cast ingots. The results showed that different degrees of crystallization structures were obtained in the ascast button ingots after arc melting in high-purity Ar, and the cross-sectional solidification morphology of arcmelted ingots was found to consist of crystals with varying from the bottom up. By contrast, there were completely amorphous structures in the middle and upper areas of the as-cast button ingots fabricated by adding 10% H_2 to the high-purity Ar atmosphere. A clear solidification interface was found between the crystal and glass in the ascast button ingots, which indicates that hydrogen addition can enhance the Zr_(55)Cu_(30)Ni_5Al_(10) alloy's glass-forming ability. The precise mechanism responsible for this was also investigated.

Impact Analysis of Microscopic Defect Types on the Macroscopic Crack Propagation in Sintered Silver Nanoparticles

Sintered silver nanoparticles(AgNPs)arewidely used in high-power electronics due to their exceptional properties.However,the material reliability is significantly affected by various microscopic defects.In this work,the three primary micro-defect types at potential stress concentrations in sintered AgNPs are identified,categorized,and quantified.Molecular dynamics(MD)simulations are employed to observe the failure evolution of different microscopic defects.The dominant mechanisms responsible for this evolution are dislocation nucleation and dislocation motion.At the same time,this paper clarifies the quantitative relationship between the tensile strain amount and the failure mechanism transitions of the three defect types by defining key strain points.The impact of defect types on the failure process is also discussed.Furthermore,traction-separation curves extracted from microscopic defect evolutions serve as a bridge to connect the macro-scale model.The validity of the crack propagation model is confirmed through tensile tests.Finally,we thoroughly analyze how micro-defect types influence macro-crack propagation and attempt to find supporting evidence from the MD model.Our findings provide a multi-perspective reference for the reliability analysis of sintered AgNPs.

Characterization of alpine meadow surface crack and its correlation with root-soil properties

Quantifying surface cracks in alpine meadows is a prerequisite and a key aspect in the study of grassland crack development.Crack characterization indices are crucial for the quantitative characterization of complex cracks,serving as vital factors in assessing the degree of cracking and the development morphology.So far,research on evaluating the degree of grassland degradation through crack characterization indices is rare,especially the quantitative analysis of the development of surface cracks in alpine meadows is relatively scarce.Therefore,based on the phenomenon of surface cracking during the degradation of alpine meadows in some regions of the Qinghai-Tibet Plateau,we selected the alpine meadow in the Huangcheng Mongolian Township,Menyuan Hui Autonomous County,Qinghai Province,China as the study area,used unmanned aerial vehicle(UAV)sensing technology to acquire low-altitude images of alpine meadow surface cracks at different degrees of degradation(light,medium,and heavy degradation),and analyzed the representative metrics characterizing the degree of crack development by interpreting the crack length,length density,branch angle,and burrow(rat hole)distribution density and combining them with in situ crack width and depth measurements.Finally,the correlations between the crack characterization indices and the soil and root parameters of sample plots at different degrees of degradation in the study area were analyzed using the grey relation analysis.The results revealed that with the increase of degradation,the physical and chemical properties of soil and the mechanical properties of root-soil composite changed significantly,the vegetation coverage reduced,and the root system aggregated in the surface layer of alpine meadow.As the degree of degradation increased,the fracture morphology developed from"linear"to"dendritic",and eventually to a complex and irregular"polygonal"pattern.The crack length,width,depth,and length density were identified as the crack characterization indices via analysis of variance.The results of grey relation analysis also revealed that the crack length,width,depth,and length density were all highly correlated with root length density,and as the degradation of alpine meadows intensified,the underground biomass increased dramatically,forming a dense layer of grass felt,which has a significant impact on the formation and expansion of cracks.

Closure Effect ofⅠ+ⅡMixed-mode Crack for EA4T Axle Steel

The crack-closure effect is a crucial factor that affects the crack growth rate and should be considered in simulation analysis and testing.A mixed-mode I+II loading fatigue crack growth test was performed using EA4T axle steel specimens.The variation of the plastic-induced crack closure(PICC)effect and the roughness-induced crack closure(RICC)effect during crack deflection in the mixed-mode is examined in this study.The results show that the load perpendicular to the crack propagation direction hinders the slip effect caused by the load parallel to the crack propagation direction under mixed-mode loading,and the crack deflection is an intuitive manifestation of the interaction between the PICC and RICC.The proportion of the RA value change on the crack side caused by contact friction was reduced by the interaction between PICC and RICC.The roughness of the crack surface before and after the crack deflection is different,and the spatial torsion crack surface is formed during the crack propagation process.With the increase of the crack length,the roughness of the fracture surface increases.During the crack deflection process,the PICC value fluctuates around 0.2,and the RICC value is increased to 0.15.

Effect of carbon material and surfactant on ink property and resulting surface cracks of fuel-cell microporous layers

Ensuring the consistency of electrode structure in proton-exchange-membrane fuel cells is highly desired yet challenging because of wide-existing and unguided cracks in the microporous layer(MPL). The first thing is to evaluate the homogeneity of MPL with cracks quantitatively. This paper proposes the homogeneity index of a full-scale MPL with an area of 50 cm~2, which is yet to be reported in the literature to our knowledge. Besides, the effects of the carbon material and surfactant on the ink and resulting MPL structure have been studied. The ink with a high network development degree produces an MPL with low crack density, but the ink with high PDI produces an MPL with low crack homogeneity. The polarity of the surfactant and the non-polarity of polytetrafluoroethylene(PTFE) are not mutually soluble,resulting in the heterogeneous PTFE distribution. The findings of this study provide guidelines for MPL fabrication.

Particle Discontinuous Deformation Analysis of Static and Dynamic Crack Propagation in Brittle Material

Crack propagation in brittle material is not only crucial for structural safety evaluation,but also has a wideranging impact on material design,damage assessment,resource extraction,and scientific research.A thorough investigation into the behavior of crack propagation contributes to a better understanding and control of the properties of brittle materials,thereby enhancing the reliability and safety of both materials and structures.As an implicit discrete elementmethod,the Discontinuous Deformation Analysis(DDA)has gained significant attention for its developments and applications in recent years.Among these developments,the particle DDA equipped with the bonded particle model is a powerful tool for predicting the whole process of material from continuity to failure.The primary objective of this research is to develop and utilize the particle DDAtomodel and understand the complex behavior of cracks in brittle materials under both static and dynamic loadings.The particle DDA is applied to several classical crack propagation problems,including the crack branching,compact tensile test,Kalthoff impact experiment,and tensile test of a rectangular plate with a hole.The evolutions of cracks under various stress or geometrical conditions are carefully investigated.The simulated results are compared with the experiments and other numerical results.It is found that the crack propagation patterns,including crack branching and the formation of secondary cracks,can be well reproduced.The results show that the particle DDA is a qualified method for crack propagation problems,providing valuable insights into the fracture mechanism of brittle materials.