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The purpose of this work is to study the effect of charging conditions on hydrogen damage. The effects of electrochemical hydrogen charging current density and time on hydrogen-induced blistering (HIB), cracking behavior, and mechanical properties of Q690 steel are studied by electrochemical hydrogen charging, microstructure observation, and slow strain rate tensile (SSRT) tests. The results show that HIB and internal cracks occur when the Q690 steel is charged at different current densities. The charging conditions have a significant effect on the HIB characteristics of the material and the morphology, number, size, and location of internal cracks. The geometrical parameters of blisters on the surface of Q690 steel are quantitatively evaluated, and deeper cracks are found at higher hydrogen concentrations. At high hydrogen charging current density (50 mA/cm2), due to the accumulation of a large number of hydrogen atoms and the precipitation of hydrogen, the active sites on Q690 steel surface increase dramatically, leading to the initiation of a large number of blisters. At this time, high current density is responsible for the initiation of blisters. The relationship between hydrogen charging current density and mechanical properties of Q690 steel is studied, and the change in the fracture morphology is observed. The Q690 steel was damaged and failed due to an internal crack caused by excessive hydrogen pressure. On the other hand, electrochemical hydrogen charging leads to the degradation of mechanical properties and the transition from ductile fracture to brittle fracture. [ABSTRACT FROM AUTHOR]

Mechanism of Hydrogen-induced defects and cracking in Ti and Ti–Mo alloy was systematically analyzed from the perspective of microstructure. Results show that more hydrogen atoms can be dissolved in Ti–Mo alloy due to the existence of β phase, and the precipitation amount of hydride is reduced under the same hydrogen charging conditions. The analysis of positron annihilation results show that the solid solution of Mo element reduces the hydrogen induced defect concentration in the alloy and inhibit the combination of hydrogen and defects. At the same time, Mo atom can affect the electronic structure of Titanium hydrides, and then reduce the bonding between titanium atoms and hydrogen atoms, thereby reducing the formation of brittle titanium hydride. The reduction of hydride and hydrogen induced defect concentration significantly decrease the hardening rate of Ti–Mo alloy, thereby effectively reducing the hydrogen embrittlement sensitivity of titanium and inhibiting its hydrogen absorption cracking tendency. • The formation of β phase in Ti–Mo alloy improves the solubility of hydrogen, thereby reducing the precipitation of hydride. • The solid solution of Mo can affect the electronic structure of titanium hydride and weaken the bonding between Ti–H. • The solid solution of Mo can reduce the concentration of hydrogen-induced defects in Ti–Mo alloy. • Reducing the concentration of hydride and hydrogen induced defect can inhibit hydrogen absorption cracking tendency. [ABSTRACT FROM AUTHOR]

• Develop a phase field method for assessing the threshold conditions to initiate hydrogen-induced cracks on dented pipelines. • Define the influences of dent depth, initial hydrogen concentration and internal pressure on crack initiation. • Determine the threshold values of dent depth and internal pressure to initiate cracks under hydrogen impact. An accurate determination of the threshold conditions to initiate cracks on aged hydrogen pipelines is paramount for ensuring energy transport safety. In this work, a finite element-based phase field method was developed to assess the crack initiation on dented pipelines while considering the hydrogen (H) impact. Theoretical and multi-physics numerical formulas were derived for prediction of the elastic-plastic fracture behavior of H-contained steel. A critical phase field parameter, ϕ =0.69, is defined for predicting crack initiation at the dent on pipelines. The presence of H within the steel decreases the threshold dent depth for initiating H-induced cracks. When the initial H concentration increases from 0 to 0.5 wppm, the maximum dent depth for crack initiation reduces from 17.5 mm to 10.7 mm. The maximum dent depth required for crack initiation reduces from 17.5 mm to 7.8 mm when an internal pressure of 8 MPa is applied on the steel pipe. The site with the maximum phase field parameter changes during indentation, implying that the location initiating cracks depends on the dent dimension. The existing criteria in ASME B31.12 standard are not applicable for predicting H-induced crack initiation on dented pipelines. This study proposes a new method to predict hydrogen-induced cracking on aged pipelines when transporting hydrogen. [Display omitted] [ABSTRACT FROM AUTHOR]

The cracking behaviors of VCoNi medium-entropy alloys with and without short-range order (SRO) structures under different durations of hydrogen charging were investigated. The findings demonstrate that the presence of short-range order in the VCoNi alloy effectively traps hydrogen within the grain interior, thereby mitigating hydrogen segregation at grain boundaries. Consequently, the SRO structure within the VCoNi alloy prolongs the incubation period for hydrogen-induced grain boundary cracking and results in a higher density of grain interior cracks after 12 h of hydrogen charging, indicating that SRO impedes the diffusion of hydrogen into the alloy's interior compared to the alloy lacking SRO. Grain boundary cracking is a consequence of stress concentration and hydrogen segregation at grain boundaries. The initiation of cracks within grains is primarily ascribed to the generation of nanovoids at the intersection of intersecting slip bands. • Aging treatment can promote the generation of SRO in VCoNi alloy. • SRO can directly capture hydrogen confirmed by secondary ion mass spectrometry directly. • SRO reduces hydrogen segregation and delays cracking at grain boundaries. • Hydrogen-induced grain interior crack initiation and propagation mechanism is elucidated. [ABSTRACT FROM AUTHOR]

Quenching and partitioning (Q&P) steels are extensively used in the automotive industry due to their exceptional strength and ductility. However, the impact of hydrogen embrittlement (HE) poses a huge challenge to the safe service of Q&P steels, highlighting the necessity to investigate the hydrogen-induced cracking mechanisms. This study investigates the hydrogen-induced failure mechanisms in Q&P steels through experiments and finite element simulations, focusing on microstructure, local stress heterogeneity, hydrogen distribution characteristics, and hydrogen-induced cracking. The results reveal that hydrogen cracking nucleation in Q&P steel is primarily due to the further accumulation of hydrogen in local high-stress martensitic/austenitic regions. [Display omitted] • Hydrogen-induced cracks in Q&P steel are mainly nucleated in M(M/A). • During the stretching process, the hard M(M/A) region tends to produce many local high stress regions. • In the stress-free state, hydrogen is mainly adsorbed in M(M/A). • The hydrogen will accumulate further in the high-stress regions in M(M/A), thus inducing hydrogen cracking. [ABSTRACT FROM AUTHOR]

The mechanical properties and microstructural evolution of high-strength bainitic steel before and after hydrogen charging were investigated by nanoindentation creep tests and multiscale morphology observations. The results showed that hydrogen charging led to higher local nanoindentation hardness and lower stress exponent of the steel. The elongation of the material decreased, and the susceptibility to hydrogen embrittlement increased after charging. SEM observation showed that the presence of hydrogen changes the material's fracture characteristics from ductile to a mixture of ductile and transgranular cleavage fracture. TEM and SADP analysis indicated that the energetically favorable and stress-promoted hydrogen induced the enhanced dislocation activity in the lath, and the hydrogen might accumulate at the interface between the harder M-A and the softer substrate, as well as at GB. The influence of hydrogen on material properties is caused by the interaction of hydrogen-induced local hardening and hydrogen-induced cracking. [ABSTRACT FROM AUTHOR]

• Modifying microstructure influences hydrogen diffusion and cracking of steels. • HIC susceptibility of steels is dependent on their diffusion characteristics. • Phase composition was seen to play the most significant role in influencing HIC. • Low local misorientaions in steels facilitates the elimination of HIC. This work examines the effect of stress-relief (SR) and quenching and tempering (QT) heat treatments on the hydrogen-induced cracking (HIC) susceptibility of pipeline steels. Two trials of a similar X65 pipeline steel were used in this study. They had distinct differences in microstructural characteristics, which significantly affected their cracking behaviour. The SR treatment reduced local misorientations, without significantly altering the microstructural appearance of the steels. Hence, this treatment resulted in a hydrogen permeation transient and cracking response similar to the as-received steels. However, the QT treatments significantly modified the microstructure of the steels, with some treatments resulting in equiaxed polygonal ferrite grains and lower local misorientations, depending on the quenchant used. These microstructural changes contributed to increasing hydrogen diffusion and permeation through the steels, as well as alleviating or eliminating HIC after hydrogen charging. Comparing the permeation transients and diffusion characteristics of the steels without cracks to those with significant cracking, a dependence of HIC susceptibility on the diffusion and trapping behavior within the steels was observed. [ABSTRACT FROM AUTHOR]

This work evaluates the effect of hydrogen on the mechanical integrity of a weld in a martensitic base metal. Different regions in the heat-affected zone (HAZ) are reproduced to investigate the interaction of the local microstructure with hydrogen. The hardness strongly depends on the distance from the welding joint due to the different phases present. The HAZ contains zones of acicular ferrite, coarse martensite and tempered martensite. Additionally, the entire weld is subjected to a constant load while being simultaneously electrochemically charged with hydrogen. During this test, a crack initiates in the filler, showing the highest hydrogen solubility as demonstrated by thermal desorption spectroscopy, while propagation occurs along the microstructure of the HAZ with the highest hardness level. [ABSTRACT FROM AUTHOR]

Hydrogen diffusivity and trapping have been studied in two advanced high strength steel grades and model samples using electrochemical permeation test. Microstructures of CP1000 and DP1000 steels consist of ferrite, martensite and a small fraction of retained austenite. In addition, bainite is present in CP1000. Model phases with predominance of a particular phase have been prepared by specific heat treatment. DP1000 has shown the lowest diffusivity among all materials, while ferritic model sample has shown the highest. Differences in hydrogen diffusion coefficient values are linked to trapping microstructural characteristics and grain size. [ABSTRACT FROM AUTHOR]

In the present work, the critical crack nucleation size and the hydrogen trapping capability of inclusion were discussed with a numerical simulation considering the factors of inclusion/matrix heterojunction. The results showed that the inclusion composition had a significant effect on hydrogen capacity and its critical nucleation size of hydrogen-induced cracking (HIC). The MnS inclusions exhibited a larger critical size tolerance of HIC nucleation (approximately 2.5 μm), but for some typical oxide inclusions, it ranged between 0.1 and 0.4 μm. In addition, two sheets of steel containing different composition, morphology, and distribution of inclusions were studied by the standard-based test and thermal desorption spectroscopy (TDS) to evaluate the HIC susceptibility and hydrogen trapping behaviour. The results complementarily demonstrated that when controlling the non-metallic inclusions into the proper size and compound with MnS into the sphere, the HIC resistance of steel could be efficiently improved. • Values of critical size triggering HIC of some typical inclusions are determined. • Oxides with big shear modules exhibit the lower critical size. • Compounding effect of MnS can enlarge the critical size of oxides. • Finely dispersed inclusions can relieve the local H-pressure and optimize HIC. [ABSTRACT FROM AUTHOR]

[Display omitted] • The HE indexes of tensile properties of BM are more sensitive than that of the WJ. • The FAC coupling with HE shrinks from the outside to the inside in the FAD. • The reduction trend of safety factors with HE for crack depth differs from pressure. • The impact fracture morphology with HE for WJ presents brittle characteristics. Currently, no systematic approach exists for the safety assessment method of hydrogen transmission low-strength pipeline, it is necessary to study the effect of hydrogen embrittlement (HE) on the method of failure assessment diagram (FAD) for the defective pipe. The hydrogen charging experiments of tensile and Charpy impact specimens for the L245 pipe steel were carried out with charging time of 0, 30, 90, and 120 h, and then the tensile and Charpy impact tests were immediately conducted. The HE indexes of tensile and Charpy impact properties were presented to quantitatively analyze the effect of HE under tensile and impact deformation processes. Subsequently, the FAD method coupled with the effect of HE was investigated to analyze the difference in the safety assessment method between hydrogen and non-hydrogen environments. Finally, the fracture morphology of tensile and Charpy impact specimens was observed to analyze the responses of microscopic fracture modes between hydrogen and non-hydrogen environments. The results show that the effect of HE on the tensile properties of the base material is more sensitive than that of the weld joint. The FAC with the effect of HE for safety assessment of defective pipes shrinks by reducing the safe zone. The decrease ratio of safety factors affected by HE presents an increasing tendency as the increase of crack depth while showing a decreasing tendency as the increase of internal pressure. [ABSTRACT FROM AUTHOR]

Experimental studies of the distribution of hydrogen in metal samples after a standardized saturation procedure in a neutral solution are described. It is shown that the hydrogen concentrates in a thin surface layer which plays the role of a shield for the further propagation of hydrogen. We refer to this phenomenon as a skin effect in the hydrogen saturation since the internal regions of the metal are not saturated by hydrogen. A review of similar experimental results for other standardized methods for saturation of metals with hydrogen (cathodic polarization, saturation in gaseous hydrogen) is provided, and the universal nature of the skin effect for metals is shown. The analysis of mechanisms of the skin effect is carried out by methods of continuum mechanics based on a bi-continual model of a solid. The problem of saturating a metal sample with hydrogen without external load is posed. It is shown that the model taking into account the mutual influence of hydrogen and mechanical stresses allows one to obtain solutions describing the highly non-uniform distribution of hydrogen at standardized saturation. [ABSTRACT FROM AUTHOR]

In this work, the non‐metallic inclusions contained in a trial X70 pipeline steels are characterized by optical microscopy, scanning electron microscopy, energy‐dispersive X‐ray spectrum, and transmission electron microscopy. Statistical analysis is conducted to summarize the size and shape of the inclusions. The hydrogen trapping and the resulting hydrogen‐induced cracking (HIC) susceptibility of the steels are tested. Density functional theory is used to calculate the binding energy of hydrogen at MnS inclusions, and the impact of MnS inclusions on hydrogen trapping and the HIC susceptibility is evaluated. It is found that the majority of submicron scale inclusions are MnS, which serve as irreversible hydrogen traps. The content of the trapped hydrogen can be effectively decreased by controlling the size of MnS inclusions below submicron scale and distributing the inclusions uniformly in the steel. As a result, the susceptibility of the steel to HIC is reduced. [ABSTRACT FROM AUTHOR]

Medium-entropy alloys (MEAs) have shown exceptional hydrogen tolerance. Here we systematically investigated the hydrogen resistance evolution of a non-equiatomic Co 35 Ni 36 Cr 23 Mo 6 MEA from microstructure, trapping site, mechanical properties and cracking. The ultimate tensile strength and ductility of Co 35 Ni 36 Cr 23 Mo 6 at room temperature were about 840 MPa and 71%, respectively. In the case of hydrogen pre-charging for 12 h, the above values were almost unchanged, indicating excellent hydrogen resistance. Grain boundaries were the main initiation and propagation paths for hydrogen-induced secondary cracks, while no secondary crack was on the Co 35 Ni 36 Cr 23 Mo 6 alloy without hydrogen. We also found that the non-ductile region in the fracture morphology could be generated only when the hydrogen concentration reached a certain threshold. Moreover, hydrogen not only had negative effect on the MEA, but also promoted the formation of nanotwins during deformation process in this alloy. • The ultimate tensile strength and ductility of Co 35 Ni 36 Cr 23 Mo 6 MEA were about 840 MPa and 71%, respectively. • In the case of hydrogen pre-charging for 12 h, the above values were almost unchanged. • The non-ductile region in fracture surface could be generated only when the H concentration reached a certain threshold. • In this MEA, H promoted the formation of nanotwins during deformation, and increased the KAM value at grain boundaries. [ABSTRACT FROM AUTHOR]

In this study a thermomechanical processing was suggested to improve the hydrogen-induced cracking (HIC) resistance in X70 pipeline steel. Proposed thermomechanical treatment produced favorable crystallographic textures and significantly increased HIC resistance which is of great interest to petroleum industry. The results showed that the high angle grain boundaries and Kernel parameter values acted as hydrogen-trapping sites, leading to increased HIC susceptibility. Highest HIC resistance was obtained in sample rolled isothermally at 850 °C, due to the high proportion of grains oriented with {110} planes parallel to the normal direction and {111}//ND fibers accompanied by negligible fraction of grains oriented in {001}//ND. [ABSTRACT FROM AUTHOR]

Failure investigation was carried out on boiler water wall tubes of a thermal power plant through visual inspection, chemical analysis, and metallurgical analysis. Failure was in the form of thin/micro cracks along the length of the tubes which were located at the girth welding joint of tubes. Experimental results revealed that the cracking was from inward to outward of the tube thickness. Discontinuities/cavities were observed in the welded region which might have occurred due to lack of fusion of base metal and the weld metal. Cracks were initiated from the sharp corner/crack tip of the cavities/discontinuities present at the welded region under the action of hoop/thermal stress existed during the operation. Nature of the crack propagation indicates the case of typical hydrogen-induced cracking. Moreover, the presence of the cavities/discontinuities reduced the cross-sectional area of tubes resulting increased stress intensity. Increased stress beyond the flow stress of the material assisted by hydrogen-induced effect resulted the cracking of the tubes. In order to mitigate the problem, proper welding of tubes joints should be carried out followed by proper inspection after weld. Secondly, hydrogen dissolution during welding should be prevented and treatment for its removal after welding should be carried out. [ABSTRACT FROM AUTHOR]

The effect of continuous hydrogen charging on tensile properties of EUROFER 97 and ODS-EUROFER steels was studied at room and elevated temperatures of 100 °C and 300 °C. The hydrogen effect decreases with increase of the temperature for ODS-EUROFER steel, while susceptibility to hydrogen of EUROFER 97 steel remains approximately the same at all testing temperatures. Continuous hydrogen charging results in a reduction of the grain boundary cohesion of the EUROFER 97 and ODS-EUROFER steels tested at RT. With increase of the testing temperature up to 300 °C EUROFER 97 steel exhibits relatively high amount of micro-cracks which agglomerate in sub-micrometer size cracks, while the hydrogen-induced intergranular crack nucleation in ODS-EUROFER steel is effectively suppressed. Possible mechanism of the hydrogen-induced crack nucleation and propagation under applied external stress is discussed. [ABSTRACT FROM AUTHOR]

Precipitation strengthened austenitic stainless steels are widely used in hydrogen related applications. However, their applications may face hydrogen damage resulting in hydrogen-induced delayed failure. Results show that the weld is more sensitive to fracture and hydrogen-induced failure than the matrix. High density curved dislocations, abundant of large size precipitates and considerable γ′ precipitates coarsening are found in the weld. Large size precipitates are found to be major hydrogen traps and preferential microcrack nucleation sites. The γ′ precipitates coarsening make the weld more ductile than the matrix. With the decrease of the applied stress, hydrogen-induced cracking mechanism in the weld changes from brittle transgranular fracture to brittle intergranular fracture. [ABSTRACT FROM AUTHOR]

Addition of trace lanthanum salt in acid solution was found to be enhanced inhibitory effect on hydrogen permeation to X70 pipeline steel. The hydrogen permeation curves were performed to show that the addition of La3+ inhibits hydrogen permeation by accelerating the recombination of the adsorbed hydrogen atoms and the desorption of hydrogen molecule, which decrease the subsurface hydrogen concentration. The inhibitory effect of La3+ involves the film formation process. Moreover, electrochemical impedance spectroscopy analysis (EIS) depict that the inhibitive effect is caused by the catalysis of the recombination-desorption-evolution of adsorbed hydrogen atoms by La(OH) 3 /La 2 O 3. This provides a novel solution for the prevention of hydrogen-induced cracking of high-strength steel, which often causes catastrophic accidents. [ABSTRACT FROM AUTHOR]

ABSTRACT Hydrogen is known to have a deleterious effect on most engineering alloys. It has been shown repeatedly that the strength of steels is inversely related to the ductility of the material in hydrogen gas. However, the fatigue properties with respect to strength are not as well documented or understood. Here, we present the results of tests of the fatigue crack growth rate (FCGR) on API X70 from two sources. The two materials were tested in air, 5.5 and 34 MPa pressurized hydrogen gas, and at both 1 and 0.1 Hz. At these hydrogen pressures, the FCGR increases above that of air for all values of the stress intensity factor range (Δ K) greater than ~7 MPa · m1/2. The effect of hydrogen is particularly sensitive at values of Δ K below ~15 MPa · m1/2. That is, for values of Δ K between 7 and 15 MPa · m1/2, the FCGR rapidly increases from approximately that found in air to as much as two orders of magnitude above that in air. Above 15 MPa · m1/2, the FCGR remains approximately one to two orders of magnitude higher than that of air. [ABSTRACT FROM AUTHOR]

The factors affecting hydrogen trapping at the non-metallic inclusions (NMIs) are identified by a combined experimental and theoretical study. The calculated results based on the proposed models show that the hydrogen trapping capacities at the NMIs decrease in the order of E b TiC > E b TiN > E b Al 2 O 3 > E b FeO > E b MnS . The interfacial sinking of hydrogen leads to the formation of a silver particle-denuded zone around the NMIs. The elastic property of NMIs is more important for hydrogen trapping than the misfit strain. This work unveils the hydrogen trapping-dependent mechanism on the composition, the misfit strain, the elastic properties, the shape and the size of NMIs. • Theoretical models are built to calculate hydrogen trapping at the inclusions. • Hydrogen distribution around the inclusions is experimentally observed. • The hydrogen trapping capacities of the inclusions are theoretically calculated. • The experimental results are explained using the proposed theoretical models. • The key factors that affect hydrogen trapping at inclusions are determined. [ABSTRACT FROM AUTHOR]

This study reported the significant influence of Ta on hydrogen-induced cracking of pipeline steels. It was found that with decreasing TaC size, the hydrogen diffusivity of steels was decreased, as the hydrogen trapping of TaC is surface-dominant. Ta inhibited crack initiation through deep H-traps provided by small semi-coherent/incoherent TaC (<35 nm), whilst trigger cracks directly at incoherent TaC/α-Fe interfaces when TaC > ~223 nm. Moreover, Ta suppressed crack propagation as it increased effective H-trap densities, optimized crystallographic characteristics (changing the proportions of Σ3, Σ11, Σ17 and high-angle grain boundaries), and inhibited H-dislocation interaction, which was more pronounced when TaC was smaller. [Display omitted] • Ta markedly changed the HIC resistance and its influence depended on TaC size. • Ta has different functions in crack initiation and propagation of HIC. • The critical TaC size for crack initiation at the TaC/matrix interface was ~223 nm. • Ta impeded crack growth via changing crystallographic features, H traps and HELP. • The hydrogen trapping of semi/in-coherent TaC precipitates is interface-dominant. [ABSTRACT FROM AUTHOR]

The effect of various experimental conditions (i.e., hydrogen charging current density, charging time, solution concentration, and temperature) on the embrittlement and cracking susceptibility of 2205 duplex stainless steel was studied by electrochemical hydrogen charging and slow strain rate tests. The results showed that the choice of the experimental conditions had obvious effect on the hydrogen concentration in the specimens. A relationship between the embrittlement and hydrogen charging conditions was established by the investigation of the fracture morphology. Under the free-charging condition, the fracture surfaces were characteristic of dimples, while on the condition of the dynamic hydrogen charging, the hydrogen-induced fracture showed the appearance of cleavage. Further examination of fracture cracks confirmed that the ferrite phase acts as a preferential path for crack propagation. [ABSTRACT FROM AUTHOR]

Abstract: The microstructure, lattice parameters, mechanical properties, and electrical conductivity mechanisms for Cu doping on B-site of (La0.8Ca0.2)(Cr0.9Co0.1)O3−δ have been systematically investigated. In this study, the concept of defect chemistry is used to explain the relationship between the concentration of hole with the electrical conductivity. The information of charge compensation mechanisms and defect formation may be valuable for a better understanding of the interconnect of (La0.8Ca0.2)CrO3−δ -based ceramics used for solid oxide fuel cells (SOFCs). The electrical conductivity increases with the increase in temperature in air, whereas the electrical conductivity has a maximum value at a certain temperature in 5% H2–95% Ar. The concentration of hole at high oxygen activity is larger than that at low oxygen activity. Because (La0.8Ca0.2)CrO3−δ -based ceramics are p-type conductors, the electrical conductivity is dominated by the concentration of hole. Obviously, the electrical conductivities of Cu doping on B-site of (La0.8Ca0.2)(Cr0.9Co0.1)O3−δ in air are larger than those in 5% H2–95%. At higher Cu-doping levels, higher temperatures, and high oxygen activity, the compensation mechanism is significantly dominated by the formation of oxygen vacancy, i.e. ionic compensation. On the contrary, the compensation mechanism is significantly dominated by the formation of the formation of Cr4+, i.e. electrical compensation at lower Cu-doping levels, higher temperatures, and lower oxygen activity. Because mechanical properties of SOFCs interconnect materials are very important in reducing atmosphere, the effect of atmospheres on fracture toughness and microhardness for specimens is investigated, which results revealed that when specimens were exposed to 5% H2–95% Ar forming gas, all specimens appeared to have hydrogen-induced cracking (HIC) except for (La0.8Ca0.2)(Cr0.87Co0.1Cu0.03)O3−δ . Although the exact mechanism of HIC is still not clear, it is known from this study that Cu doped in the B-site of (La0.8Ca0.2)(Cr0.9Co0.1)O3−δ enhanced HIC damage and with the increase of Cu-doping level, the degree in HIC is increased. [Copyright &y& Elsevier]

Abstract: In this paper, the analysis of a brittle fracture explosion accident of a thick-walled 20 steel pipe, which connects the outlet of an ammonia separator and the inlet of a cold exchanger in an ammonia synthesis unit of a chemical fertilizer plant, is presented in detail. The causes and mechanism of the material degradation are investigated by analyzing chemical compositions (especially the elements of nitrogen, hydrogen), testing mechanical properties, observing microstructures and defects, and strain aging sensitivity, and analyzing annealing behavior of the pipe. Furthermore, the mechanism of crack initiation and propagation are analyzed carefully through macro- and micro-crack observations by means of Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The research results not only support the conclusions given by the accident survey group, but also clarify the uncertainty in the report of the survey at that time. The further research makes it clear that the decrease of material toughness due to strain aging embrittlement (SAB), and the crack initiation and propagation is caused by SAB and its lowering effect on the threshold value of hydrogen-induced cracking (HIC). The research work described in the paper has a significant meaning to recognize or eliminate SAB occurred in quite ductile plain carbon steels very commonly used in industry, such as 20 steel in China. [Copyright &y& Elsevier]

High strength low alloy (HSLA) steels are developed to provide better strength, toughness, and improved weldability. These steels are often used as better alternatives for quenched and tempered steels due to the excellent combination of these properties. The beneficial properties in these steels are achieved by careful control of composition and by adopting suitably controlled thermomechanical processing (TMCP). Improved weldability is achieved by maintaining carbon at lower level. To compensate the effect of carbon, small amounts of alloying elements, such as niobium, vanadium, titanium, aluminium, and nitrogen are added. Though these steels have good weldability, they suffer from hydrogen induced cracking especially at high strength levels. In the present study, hydrogen induced cracking susceptibility of a newly developed indigenous vanadium-titanium microalloyed high strength steel was evaluated by means of implant cracking test. Welding was carried out using the GMAW process at different heat input levels. It was found that the coarse-grained heat affected zone (CGHAZ) of the steel is prone for hydrogen induced cracking at lower heat inputs. The implant test results indicated that the critical stress level increases with increasing heat inputs. The results were correlated with the microstructural modifications that occur in the CGHAZ at different welding conditions. The microhardness variations in that region were in good agreement with the observed results. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the electrochemical hydrogen charging for X70 pipeline steel in H2SO4 solution was carried out and the influence of non-metallic inclusions and microstructure on the hydrogen-induced cracking (HIC) and healing behavior of X70 pipeline steel was investigated by SEM and TEM technique. Based on the results of hydrogen permeation tests, the diffusivity of hydrogen at room temperature and the density of hydrogen trapping sites in X70 steel were estimated. Heat treatment was used to heal the hydrogen-induced cracks in X70. It was found that non-metallic inclusions, such as nitrides and oxides, played different roles on HIC of X70 steel. Oxides are more sensitive than nitrides to HIC in X70 steel. Cracks initiated and grew preferentially along the grain boundary of polygonal ferrite instead of the acicular ferrite when the sample was charged of enough hydrogen. It also demonstrated that proper heat treatment can heal such cracks. The healing process was divided into two steps: firstly, many cracks dissociated into several detached segments; secondly, theses segments regressed into spherical voids. The mechanism of crack healing was also discussed. [Copyright &y& Elsevier]

Hydrogen-induced cracking (HIC) has been studied phenomenologically. The effect of microstructure on HIC is discussed for steels having two different levels of nonmetallic inclusions. Steels with different microstructures were produced by a thermomechanically controlled process (TMCP) from two different heats, which had different levels of nonmetallic inclusions. Ferrite/pearlite (F/P), ferrite/acicular ferrite (F/AF), and ferrite/bainite (F/B) were three representative microstructures of the tested steels. For the steels with higher inclusion levels, the minimum permissible inclusion level for HIC not to develop varied according to the steel microstructure. On the other hand, HIC occurred in the martensite/austenite (M/A) constituents regardless of steel microstructure above a certain concentration. It was shown that M/A constituents were easily embrittIed by hydrogen atoms. Steels having a F/AF were resistant to HIC in the tested service condition; they exhibited a wide range of diffusible hydrogen contents without developing HIC. [ABSTRACT FROM AUTHOR]

Abstract: We consider delamination crack growth controlled by gas diffusion into the crack. If the gas is accumulated inside the delamination, after some incubation period, the crack starts growing under the pressure of the accumulated gas. An important example is given by hydrogen-induced delamination. Hydrogen absorbed by a metal is typically dissolved in the proton form within the lattice. Some of the protons reach the surface of pre-existing or freshly created cracks or delaminations where they recombine with electrons and form molecular hydrogen in the crack cavity. Since the molecular form of hydrogen is thermodynamically more stable, this process leads to accumulation of hydrogen gas inside the delamination crack. Under the excessive hydrogen pressure fracture often takes place even in the absence of any additional external loading. It is especially dangerous if the metal is covered with coating, where because of the dissimilarity of materials, microscopic voids appear more frequently. This leads first to hydrogen precipitation within these coating–metal interface voids and then to their development. This results in the delaminating of the coating. As it is well known, in the absence of aggressive media the cracking resistance of a material is characterized by a unique constant: so that if the stress intensity factor is smaller than this constant, the crack is fixed, and if it is larger, the crack moves in a dynamic regime. But in the presence of aggressive media, particularly hydrogen, the crack development is characterized by a smooth kinetic function v(K I), which is dependence of the crack velocity on the stress intensity factor, with the lower threshold value of resistance Kscc smaller than the static critical stress intensity factor K I c . As numerically shown in the author''s earlier work for the internal crack growth, the crack velocity first increases in accordance with the kinetic function, until reaching some value of vs , and remains at the same level afterwards. Since, as has been obtained by the author in the previous paper, kinetic equations for internal and delamination cracks are essentially identical, the same conclusion can be derived for the latter. In this paper the stability and asymptotic approach to the constant velocity of delamination growth is proved analytically. If vs is known, the corresponding value of the stress intensity factor, Ks , is obtained by substituting vs into the kinetic function v(K I). [Copyright &y& Elsevier]

The spatial distribution of plastic deformation and grain orientation surrounding hydrogen-induced cracks (HIC) is investigated in samples of API-5L-X46 pipeline steel using scanning electron microscopy and electron backscattering diffraction (EBSD). This work shows direct experimental evidence of the influence of microstructure, microtexture and mesotexture on HIC crack path. [Copyright &y& Elsevier]

• The ratio of crack depth/length increases with hydrogen concentration. • Variance of internal pressure responsible for the different crack structures. • Crack geometry parameter defined as a useful indicator of internal pressure. Quantitative prediction of the hydrogen-induced crack initiation remains challenging, partially due to the difficulties in directly measuring the hydrogen gas pressure in the crack cavity. Through detailed cross-sectional observations, this study shows that the structure of the internal cracks beneath the surface blisters, i.e., the ratio of crack depth/length, can be significantly influenced by the hydrogen charging conditions. This is attributed to the change of hydrogen gas pressure in the crack cavity at different hydrogen contents, supported by the formation of stepwise cracks near the specimen surface as well as the hydrogen concentration measurements. The post-failure crack geometry is considered a useful indicator of the level of internal gas pressure to quantitatively assess the hydrogen-induced cracking sensitivity. The mechanisms of crack initiation and propagation at different hydrogen concentrations are discussed. [ABSTRACT FROM AUTHOR]

The present work provides an overview of the work on the interaction between hydrogen (H) and the steel’s microstructure. Different techniques are used to evaluate the H-induced damage phenomena. The impact of H charging on multiphase high-strength steels, i.e., high-strength low-alloy (HSLA), transformation-induced plasticity (TRIP) and dual phase (DP) is first studied. The highest hydrogen embrittlement resistance is obtained for HSLA steel due to the presence of Ti- and Nb-based precipitates. Generic Fe-C lab-cast alloys consisting of a single phase, i.e., ferrite, bainite, pearlite or martensite, and with carbon contents of approximately 0, 0.2 and 0.4 wt %, are further considered to simplify the microstructure. Finally, the addition of carbides is investigated in lab-cast Fe-C-X alloys by adding a ternary carbide forming element to the Fe-C alloys. To understand the H/material interaction, a comparison of the available H trapping sites, the H pick-up level and the H diffusivity with the H-induced mechanical degradation or H-induced cracking is correlated with a thorough microstructural analysis. [ABSTRACT FROM AUTHOR]