Hydrogen-induced Crack Propagation Research Articles

In-depth understanding in the effect of hydrogen on microstructural evolution, mechanical properties and fracture micro-mechanisms of advanced high-strength steels welded joints

The hydrogen embrittlement (HE) mechanism of QP980 steels laser welded joints with complex microstructures is unclear. The joint with hydrogen pre-charging fractured at the inter-critical heat affected zone (ICHAZ) in slow strain rate tensile test, and the relative loss of elongation amounted to 84.8 %. Hydrogen-induced microcracks mainly initiated inside martensite (∼ 60 %). Hydrogen-induced crack propagation paths of the joint were transgranular propagation of martensite and intergranular propagation of ferrite/martensite interface. The HE mechanism of the joint was the combination of the effects in hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE).

In Situ Investigation of Hydrogen-Induced Cracking Behavior in Linepipe Steel Under Different Environments

NACE MR0175/ISO 15156 provides the severity of a sour environment defined as a function of pH and H2S partial pressure for sulfide stress cracking. Although hydrogen-induced cracking (HIC) is also a major issue of linepipes exposed to a sour environment, the severity diagram does not necessarily correspond to the HIC susceptibility. Recently, from the viewpoint of fitness-of-purpose, the severity diagrams for HIC have been proposed based on the concept of equal hydrogen concentration. From this concept, HIC susceptibility corresponds well to the steady-state hydrogen permeability obtained from the hydrogen permeation test method. In this study, in order to clarify the relationship between HIC susceptibility and hydrogen diffusion behavior, in situ crack inspection during the HIC test and finite element method analysis of hydrogen diffusion were performed. The in situ inspection technique using phased-array ultrasonic testing which can capture the time dependence of crack distribution, and HIC initiation and propagation behaviors were clearly visualized and investigated. It was found that crack initiation and propagation stabilize as hydrogen concentration reaches a steady-state value throughout the specimen and the result was considered evidence that the HIC susceptibility corresponded with the steady value of hydrogen permeability.

Comparative study on hydrogen induced cracking sensitivity of two commercial API 5L X80 steels

The cracking susceptibility of two commercial API 5 L X80 steels was compared under acidic environment and electrochemical hydrogen charging. The microstructure and internal cracks of both steels were characterized by optical microscope (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) to elucidate the main factors affecting crack resistance. In comparison, the steel with slightly higher strength and hardness showed much higher hydrogen induced cracking susceptibility. Moreover, cracks in the steel with low hydrogen susceptibility exhibited a predominantly transgranular propagation mode, while a mixed intergranular and transgranular mode in the steel with high hydrogen susceptibility. Long grain boundaries connected along the rolling direction and irregular M/A constituents distributed along the grain boundaries was the main factor causing the reduction of hydrogen-induced crack propagation resistance.

Effect of prior austenite grain morphology on hydrogen embrittlement behaviour under plastic straining in as-quenched 500 HBW steels

Prior austenite grain (PAG) structure is an important factor influencing hydrogen embrittlement (HE) susceptibility of ultrahigh-strength steels. In this study, the effect of PAG shape and size on HE behaviour is investigated using a novel tuning-fork testing method and hydrogen thermal desorption spectroscopy (TDS). Different PAG structures were acquired via re-austenitization (860°C = A860, 960°C = A960) and rapid quenching of an as-received 500 HBW direct-quenched (DQ) steel, which has an auto-tempered lath-martensitic microstructure and elongated PAG morphology. Fractography reveals different crack propagation mechanisms depending on the PAG shape. With the elongated PAG structure, hydrogen-induced crack propagation transverse to elongated PAGs was transgranular quasi-cleavage. Propagation was partially intergranular with the equiaxed PAG structures, regardless of the PAG size, leading to equally faster fracture. The TDS results show that there are no significant differences between the total hydrogen contents, but re-austenitized A860 and A960 steels contain a higher fraction of weakly trapped hydrogen. This indicates that the PAG boundaries are not the dominant hydrogen traps, and the different crack propagation mechanisms are rather linked to the geometrical shape of the grain structure.

Correlation between Microstructure and Hydrogen Degradation of 690 MPa Grade Marine Engineering Steel.

Electrochemical H charging, hydrogen permeation, and hydrogen-induced cracking (HIC) behavior of 690 MPa grade steel substrate and different heat-treatment states (annealed, quenched, normalized, tempered) are investigated by cyclic voltammetry (CV), hydrogen permeation, electrochemical H charging, and slow strain rate tensile test (SSRT). The results show that hydrogen diffuses through the steel with the highest rate in base metal and the lowest rate in annealed steel. The hydrogen-induced cracks in base metal show obvious step shape with tiny cracks near the main crack. The cracks of annealed steel are mainly distributed along pearlite. The crack propagation of quenched steel is mainly transgranular, while the hydrogen-induced crack propagation of tempered steel is along the prior austenite grain boundary. HIC sensitivity of base metal is the lowest due to its fine homogeneous grain structure, small hydrogen diffusion coefficient, and small hydrogen diffusion rate. There are many hydrogen traps in annealed steel, such as the two-phase interface which provides accommodation sites for H atoms and increases the HIC susceptibility.

Crack propagation rate of hydrogen-induced cracking in high sulfur-containing pipelines

Hydrogen-Induced Cracking (HIC) is one of the main failure models of pipeline steels that are utilized to gather pipeline networks and long-distance pipeline networks within high sulfur-containing gas fields. This phenomenon is mainly caused by the combined effects of hydrogen and stress after the hydrogen atoms or ions in the transmission medium entering into the steel. This can produce a stress corrosion effect on the materials and thus lead to a decrease of fracture ductile or degradation of other properties. In this paper, a micro-fracture shear band fractal model of HIC has been proposed and the mechanism of HIC has been studied. Both the shape of the fracture process zone and the step length of hydrogen-induced crack propagation were integrated, thus building expressions that describe the fractal rate of each step of HIC. Based on this, the fractal crack propagation rates of HIC in pipelines 16 Mn and 20 steel were calculated under certain working conditions and H2S concentrations. Our study adds valuable insight into the life prediction of pipeline failure under the HIC environment.

Effects of Alloying Elements (C, Mo) on Hydrogen Assisted Cracking Behaviors of A516-65 Steels in Sour Environments.

This study examined the effects of alloying elements (C, Mo) on hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC) behaviors of A516-65 grade pressure vessel steel in sour environments. A range of experimental and analytical methods of HIC, SSC, electrochemical permeation, and immersion experiments were used. The steel with a higher C content had a larger fraction of banded pearlite, which acted as a reversible trap for hydrogen, and slower diffusion kinetics of hydrogen was obtained. In addition, a higher hardness in the mid-thickness regions of the steel, due to center segregation, resulted in easier HIC propagation. On the other hand, the steel with a higher Mo content showed more dispersed banded pearlite and a larger amount of irreversibly trapped hydrogen. Nevertheless, the addition of Mo to the steel can deteriorate the surface properties through localized pitting and the local detachment of corrosion products with uneven interfaces, increasing the vulnerability to SSC. The mechanistic reasons for the results are discussed, and a desirable alloy design for ensuring an enhanced resistance to hydrogen assisted cracking (HAC) is proposed.

Finite element modeling of HIC propagation in pipeline steel with regard to experimental observations

In this research, hydrogen induced cracking (HIC) phenomenon in pipeline steel has been investigated by finite element modeling (FEM) with the help of experimental observations. Abaqus software has been utilized to model the crack. To this, first an API 5L X70 pipeline steel was electrochemically charged by hydrogen for 8 h to create different types of HIC cracks. Then, SEM was used to observe different types of hydrogen cracks. Based on the observations, most of HIC cracks were observed at the center of cross section where center segregation of some elements occurred. The results showed that HIC cracks propagated in stepwise, sinusoidal, straight and disordered manner. Moreover, HIC crack nucleated from a point with high stress concentration factor which was between non-metallic inclusion or void and metal matrix. The initiated micro-cracks from two neighbor inclusions link together to form a long HIC crack. Based on the experimental observations and FEM modeling, it was concluded that the driving force for the HIC crack propagation is the presence of hydrogen at the crack tip after it is initiated. Crack tip usually acts as a very small void and the combination of hydrogen atoms makes a high pressure which propel the crack forward. Moreover, the HIC crack propagation path was predicted by fracture mechanics approach showing that the J-integral had its maximum amount when the HIC crack tended to propagate horizontally.

Effects of Molybdenum on HIC Susceptibility in Normalized Pressure Vessel Steels for Sour Service Applications

This paper discusses a molybdenum-added alloy design for normalized pressure vessel steels, to reduce hydrogen induced cracking (HIC) and inhibit crack propagation. The change in microstructure produced by the modified alloy composition was analyzed to determine its effect on HIC characteristics. The microstructural change was observed by optical microscopy, hardness tests, scanning electron microscopy, and electron backscattered diffraction. The crack length ratio and crack thickness ratio were evaluated using the NACE TM 0284 standard, and ultrasonic testing was used for HIC analysis. The formation of polygonal ferrite and pearlite during the processing of the alloy creates localized areas of high stress concentration at the polygonal ferrite/pearlite interface, due to the expansion/contraction of various structures during the transformation. This results in the generation of potential hydrogen-trapping sites, subsequent HIC, and crack propagation. The addition of molybdenum leads to a decrease in the volume fraction of the pearlite structure in the steel in favor of a more beneficial bainitic ferrite microstructure, which is generated during the normalizing process. This bainitic transformation creates a more favorable expansion/contraction compatibility and reduces/breaks up the ferrite/pearlite banding. The combination of these two characteristics can result in an overall lower stress-intensity state, which can minimize hydrogen-trapping and crack propagation. This study demonstrates that the resistance of normalized pressure vessel steels to HIC can be significantly improved by incorporating molybdenum in the alloy design. Key words: sour service, molybdenum, hydrogen induced crack (HIC), normalizing steel, pressure vessel steel

A Proposal for the Application of Failure Assessment Diagrams to Subcritical Hydrogen Induced Cracking Propagation Processes

In this work, an optimization proposal for a model based on the definition of regions for crack propagation by means of the micromechanical comparison by SEM images and its application to failure assessment diagrams (FADs) is presented. It consists in three approaches. (1) The definition of the crack propagation initiation in the elastic-plastic range. (2) A slight modification of the zones in which the FAD is divided for hydrogen induced cracking (HIC) conditions. (3) The introduction of a simple correction for the definition of the Kr coordinate of the FAD to take into account the fracture toughness reduction caused by an aggressive environment, instead of using a fracture parameter obtained from a test in air. For the experimental work, four medium and high strength steels exposed to a cathodic charge and cathodic protection environments were employed, studying two different loading rates in each case, and testing C(T) samples under slow rates in the environment. The study was completed with a subsequent fractographic analysis by SEM. A good degree of fulfilment was appreciated in both materials and environmental conditions, showing the validity of the predictions supplied by the FAD optimization model proposal, which constitutes an advance in the accuracy of the FAD predictive model.

Effect of electrochemical hydrogen charging on an API X70 pipeline steel with focus on characterization of inclusions

Environmental conditions of service affect the resistance of pipeline steels to failure, as a result, the applied stress which would be otherwise considered safe can be responsible for fracture due to hydrogen induced cracking (HIC). The presence of hydrogen sulfide in natural gas is one of the main sources of hydrogen in steel. HIC testing showed that cracks initiated from inclusions and propagated through the steel matrix along the crack path that was occupied by inclusions. The number of inclusions and the distance between them as well as the grain size between them was considered to be important for predicting failure. Shorter distance between inclusions and smaller grains were found to facilitate HIC propagation. The hydrided specimens were subjected to tensile stress until failure to generate stress-strain curves, ductility and toughness data. It was revealed that even though most of the cracks initiate from rectangular and spinel inclusions, the globular inclusions have also contributed to reducing the tensile strength, ductility and toughness. Synchrotron radiation 3D imaging of steel structure was used to explain the features of the HIC cracks formation during tensile tests.

In Situ Observation of Hydrogen-Induced Cracking Propagation Behavior

Hydrogen-induced cracking (HIC) is one of the major issues of line pipe exposed to sour environments. There are some guidelines on materials requirements for carbon and low alloy steels for H2S-containing environments in oil and gas production. Generally, HIC susceptibility is evaluated after the test duration, typically 96 h, in accordance with NACE Standard TM0284-2016. However, HIC propagation behavior during HIC test has not been fully understood. In this study, a new in situ HIC measurement method has been developed in order to make the connection between HIC propagation behavior and microstructure. This technique is based on the combination of an automatic ultrasonic wave inspection system and a scanning electron microscopy (SEM) observation. HIC propagation rate and HIC propagation behavior of carbon steels with different textures were investigated, using this in situ technique. Texture components of tested steels were changed by controlled rolling process in the alpha-gamma dual phase region. The {100} intensity parallel to the rolling plane was developed with increasing controlled rolling reduction in the alpha-gamma dual phase region. HIC propagation rate increased and crack length of HIC grew in a staircase pattern with time when the {100} texture was highly developed. In addition, HIC propagation behavior could be overlapped with fracture surface, just like a projection mapping. The overlapping could make the connection between HIC propagation behavior and HIC fracture surface. Microstructure and texture just under the HIC fracture surface was also characterized by SEM and an electron backscatter diffraction pattern method. The results obtained in this study showed that HIC propagation behavior was affected by a texture. In addition, the new in situ HIC observation technique, which can make the direct connection between HIC propagation behavior and microstructure, revealed a detailed HIC propagation behavior and an effect of microstructure.

베이나이트계 X70강 수소가압분위기 인장시험의 수소취성균열 전파

베이나이트계 X70강 수소가압분위기 인장시험의 수소취성균열 전파

On Modeling Hydrogen-Induced Crack Propagation Under Sustained Load

The failure of hydrogen containment components is generally associated with subcritical cracking. Understanding subcritical crack growth behavior and its dependence on material and environmental variables can lead to methods for designing structural components in a hydrogen environment and will be beneficial in developing materials resistant to hydrogen embrittlement. In order to identify the issues underlying crack propagation and arrest, we present a model for hydrogen-induced stress-controlled crack propagation under sustained loading. The model is based on the assumptions that (I) hydrogen reduces the material fracture strength and (II) crack propagation takes place when the opening stress over the characteristic distance ahead of a crack tip is greater than the local fracture strength. The model is used in a finite-element simulation of crack propagation coupled with simultaneous hydrogen diffusion in a model material through nodal release. The numerical simulations show that the same physics, i.e., diffusion-controlled crack propagation, can explain the existence of both stages I and II in the velocity versus stress intensity factor (V–K) curve.

Role of Crystallographic Texture in Hydrogen-Induced Cracking of Low Carbon Steels for Sour Service Piping

This work presents the results of ongoing investigations aimed at determining the influence of crystallographic texture on hydrogen-induced cracking (HIC) in low carbon steels for sour service piping. Electron backscatter diffraction (EBSD) and X-ray texture measurements have been performed on HIC samples of API 5L X46 and ASTM A106 steels. The results obtained in this study show that the resistance to HIC of low carbon steels for sour service piping could be improved through crystallographic texture control and grain boundary engineering. Controlled rolling schedules can be proposed in order to induce a crystallographic texture dominated by the {112}//ND, {111}//ND, and {011}//ND fibers, where ND is the sample normal direction. Such a texture is expected to decrease significantly the steel susceptibility to HIC by (1) reducing the number of available transgranular and intergranular low resistance cleavage paths provided by the {001}//ND oriented grains, (2) reducing the probability of crack coalescence and stepwise HIC propagation through large HIC-induced plastic strain, and (3) increasing the number of high resistance intergranular crack paths provided by coincidence site lattice (CSL) boundaries and low-angle boundaries between grains with orientation within the {111}//ND texture fiber.

Influence of deformation on the hydrogen behavior in iron and nickel base alloys: a review of experimental data

Most of the hydrogen-induced crack propagation mechanisms occuring in metals and alloys involve various interactions of hydrogen with deformation processes in materials. Modeling the kinetics of hydrogen entry with respect to its local concentration to assess a crack propagation rate first requires a detailed characterization of these interactions. The purpose of this paper is to review some data illustrating the influence of deformation on the hydrogen behavior in iron and nickel base alloys. Experimental data on hydrogen, deuterium and tritium diffusion and trapping show: (a) a strain dependence of the hydrogen ingress in b.c.c. alloys through natural or passive films; (b) a strain induced hydrogen trapping; (c) an accelerated transport of hydrogen in pre-strained samples or during dynamic straining; (d) a hydrogen enhanced localized plasticity; and (e) a deformation enhanced embrittlement. These effects are shown to depend on various parameters including the hydrogen activity, the material chemistry and microstructure, the strain rate and the temperature.

A THEORY OF HYDROGEN-INDUCED CRACK PROPAGATION IN AN ELASTIC CONTINUOUS MEDIUM

Induced by the crack stress field, hydrogen atoms tend to gather together to form hydro-gen atmosphere in an elastic continuous medium. The average hydrogen concentration of this hydrogen atmosphere is calculated in this paper. By regarding this hydrogen atmosphere at the crack tip as an elastic inclusion, the eigen-strain of the inclusion is calculated and the influence of the stress field pij to crack dislocation density is studied. Thus the total stress intensity factor of the crack tip is obtained. We infer that the mechanism of the hydrogen embritt-lement is the enlargement of the crack stress intensity factor by this hydrogen atmosphere. The physical process of the hydrogeninduced crack propagation is also discussed. The calculated propagation rate of the crack agrees quite well with the experimental result.

A method to evaluate the critical hydrogen concentration for hydrogen-induced crack propagation

During the investigation of hydrogen embrittlement of the low alloy high strength steel 90 MnV 8, a method was developed to evaluate the threshold hydrogen concentration for crack propagation. A fatigue precracked CT-specimen, stressed with constant load, was cathodically charged with hydrogen from the rear. Based on the numerical solution of the diffusion problem in a stress field, the critical hydrogen concentration was calculated from the incubation time of crack growth. Permeation experiments yielded data about the apparent diffusion coefficient and solubility of hydrogen. The computed critical concentrations are independent of the stress intensity factor and are, for various martensitic microstructures, in the range of 0.1–10 g atom/m 3.

Resistance of Line Pipe Steels to Wet Sour Gas

Abstract Line pipe steels with excellent resistance to wet sour gas are urgently needed because of increased use of sour gas reserves that have a higher hydrogen sulfide concentration, and because of higher operating pressures. The authors conducted extensive analyses to explore the effects of metallurgical factors on the resistance of line pipe steels to hydrogen induced cracking (HIC) and sulfide stress cracking (SSC). Both extensive desulfurization and complete inclusion shape control proved essential in eliminating potential HIC formation sites. To prevent HIC propagation, it was necessary to reduce hardness in the segregation zone, which was achieved by optimizing the chemical compositions of steels. Microstructure homogenization and complete elimination of inclusions improved SCC resistance in line pipe steels. These findings have dramatically helped improve the manufacturing conditions of line pipe steels for sour gas service.

Hydrogen-Induced Crack Propagation in a QT-Steel Weldment

A hydrogen-induced fracture process in the HAZ of a QT-steel weldment is studied numerically. Two-dimensional models are used to estimate the transient distributions of stress and hydrogen during the cooling and during the subsequent crack propagation. The distributions of hydrogen in front of a blunted crack are studied for different void volumes in the fracture zone corresponding to brittle and ductile fracture mechanisms. It is tentatively concluded that the incubation time at slow crack growth is not controlled by the hydrostatic stress. A ductile fracture mechanism leading to a relative void volume of 0.02-0.05 in the fracture zone is found to imply long incubation times resulting in a low average crack propagation speed.