Environment Embrittlement Research Articles

Hydrogen environment embrittlement in nickel-base superalloy Alloy 617

Hydrogen environment embrittlement (HEE) in a nickel-base superalloy, INCONEL® Alloy 617, was scrutinized using a notch tensile test in a 99 MPa hydrogen atmosphere to determine the function of grain boundary carbides. The fracture surface ratio of the intergranular surface increased monotonically with increasing grain boundary coverage, which is defined as the ratio between the length of the grain boundary covered with carbide and the total length of the grain boundary. Strain distribution analysis clarified that more strain locally accumulated near the grain boundary covered with grain boundary carbide compared to the uncovered one. The suggested mechanism of HEE in the alloy is that a large amount of plastic strain near the grain boundary covered with grain boundary carbide triggers crack formation at the interface between the matrix and the grain boundary carbide, followed by crack growth along the grain boundary, ultimately leading to fracture.

µ-CT Investigation of Hydrogen-Induced Cracks and Segregation Effects in Austenitic Stainless Steel

Hydrogen can drastically degrade the mechanical properties of a variety of metallic materials. The so-called hydrogen environment embrittlement of austenitic CrNi-type steels is usually accompanied by the formation of secondary surface cracks, which can be investigated in order to assess the embrittlement process. The occurrence of hydrogen-induced cracks is often related to element segregation effects that locally impact the austenite stability. Since there is as yet a lack of investigation methods that can visualize both structures three-dimensionally, the present study investigates the imageability of hydrogen-induced cracks and element segregation structures in austenitic CrNi-steel via micro-computed tomography (CT). In order to improve the X-ray visibility of segregation structures, modified versions of the reference steel, X2CrNi18-9, that contain W and Si are designed and investigated. The investigations demonstrated that small differences in the X-ray attenuation, caused by the W or Si modifications, can be detected via CT, although segregation structures could not be imaged due to their small size scale and image noise. Hydrogen-induced cracks were characterized successfully; however, the detection of the smaller cracks is limited by the resolution capability.

Cause analysis of the cooler leakage in a stirling engine

After a stirling engine operated for 1750 h, the cooler in the engine was found leaking. The leakage led to a rapid decrease of the cycle efficiency. After dismantling, a cooling tube was found leaked by air pressure test. Through macro- and microscopic fracture morphologies, the cause of the leakage was determined as fatigue under alternating stress during operation, but the fatigue source region shows abnormal brittle fracture morphologies. By XRT and inner wall inspections, more cracks initiated from the extrusion bands on the inner wall, and F-containing lubricant was also detected there. Considering the fabrication process of the tube, the formation mechanism of the inner cracks is determined as hydrogen environment embrittlement (HEE) under the conditions of the local surface damage, deformed microstructure and pure hydrogen circumstance during bright annealing. Remedial methods are also proposed to increase the service life and service safety.

Machine learning approach for prediction of hydrogen environment embrittlement in austenitic steels

This study introduces a machine learning approach to predict the effect of alloying elements and test conditions on the hydrogen environment embrittlement (HEE) index of austenitic steels for the first time. The correlation between input features and the HEE index was analyzed with Pearson's correlation coefficient (PCC) and Maximum Information Coefficient (MIC) algorithms. The correlation analysis results identified Ni and Mo as dominant features influencing the HEE index of austenitic steels. Based on the analysis results, the performance of the four representative machine learning models as a function of the number of top-ranked features was evaluated: random forest (RF), linear regression (LR), Bayesian ridge (BR), and support vector machine (SVM). Regardless of the type and the number of top-ranking features, the RF model had the highest accuracy among various models. The machine learning-based approach is expected to be useful in designing new steels having mechanical properties required for hydrogen applications.

Ammonia mitigation and induction effects on hydrogen environment embrittlement of SCM440 low-alloy steel

The effect of ammonia (NH 3 ) contained in hydrogen (H 2 ) gas on hydrogen environment embrittlement (HEE) of SCM440 low-alloy steel was studied in association with the NH 3 concentration, loading rate, and gas pressure. NH 3 worked as both mitigator of the HEE and inducer of hydrogen embrittlement (HE) depending on the testing conditions. The mitigation of the HEE was achieved by the deactivation of the iron (Fe) surface for H 2 dissociation caused by the preferential adsorption of NH 3 on the Fe surface, which is enhanced by the increase in the NH 3 concentration and decrease in the H 2 gas pressure. NH 3 induced HE was caused due to creating hydrogen by the NH 3 decomposition. Since the NH 3 decomposition rate is low, the induction effect was observed when the loading rate was low. The effect of NH 3 was determined by the competition of the mitigation and induction effects. • NH 3 worked as both mitigator and inducer of hydrogen embrittlement. • The effects of NH 3 depended on NH 3 concentration, loading rate, and gas pressure. • NH 3 mitigated hydrogen environment embrittlement by hindering hydrogen uptake into the material. • NH 3 induced hydrogen embrittlement by creating hydrogen by its decomposition.

Machine Learning Approach for Prediction of Hydrogen Environment Embrittlement in Austenitic Steels

Machine Learning Approach for Prediction of Hydrogen Environment Embrittlement in Austenitic Steels

Effect of Gas Pressure on Hydrogen Environment Embrittlement of Carbon Steel A106 in Carbon Monoxide Mixed Hydrogen Gas

Effect of Gas Pressure on Hydrogen Environment Embrittlement of Carbon Steel A106 in Carbon Monoxide Mixed Hydrogen Gas

Stacking Fault Energy in Relation to Hydrogen Environment Embrittlement of Metastable Austenitic Stainless CrNi-Steels

Metastable austenitic steels react to plastic deformation with a thermally and/or mechanically induced martensitic phase transformation. The martensitic transformation to α’-martensite can take place directly or indirectly via the intermediate stage of ε-martensite from the single-phase austenite. This effect is influenced by the stacking fault energy (SFE) of austenitic steels. An SFE < 20 mJ/m2 is known to promote indirect conversion, while an SFE > 20 mJ/m2 promotes the direct conversion of austenite into α’-martensite. This relationship has thus far not been considered in relation to the hydrogen environment embrittlement (HEE) of metastable austenitic CrNi steels. To gain new insights into HEE under consideration of the SFE and martensite formation of metastable CrNi steels, tensile tests were carried out in this study at room temperature in an air environment and in a hydrogen gas atmosphere with a pressure of p = 10 MPa. These tests were conducted on a conventionally produced alloy AISI 304L and a laboratory-scale modification of this alloy. In terms of metal physics, the steels under consideration differed in the value of the experimentally determined SFE. The SFE of the AISI 304L was 22.7 ± 0.8 mJ/m2 and the SFE of the 304 mod alloy was 18.7 ± 0.4 mJ/m2. The tensile specimens tested in air revealed a direct γ → α’ conversion for AISI 304L and an indirect γ → ε → α’ conversion for 304 mod. From the results it could be deduced that the indirect phase transformation is responsible for a significant increase in the content of deformation-induced α’-martensite due to a reduction of the SFE value below 20 mJ/m2 in hydrogen gas atmosphere.

Evaluation of hydrogen related degradation of API X42 pipeline under hydrogen/natural gas mixture conditions using small punch test

This study investigates the effect of a hydrogen/natural gas mixture on the mechanical properties of API X42 pipeline steel using small punch (SP) test. The hydrogen-assisted fracture behavior demonstrated a transition from ductile to brittle failure, similar to the temperature-assisted brittle fracture mode. The preferential sites for hydrogen-assisted crack initiation in the X42 pipeline steel were located at the ferrite/pearlite interface, where hydrogen diffusion is enhanced by localized stress/strain conditions. The feasibility of the hydrogen environment embrittlement susceptibility obtained in the SP test and the correlation to the conventional test method is also discussed.

Effects of external hydrogen on hydrogen-assisted crack initiation in type 304 stainless steel

PurposeThe purpose of this paper is to present a crack initiation mechanism of the external hydrogen effect on type 304 stainless steel, as well as on fatigue crack propagation in the presence of hydrogen gas.Design/methodology/approachThe effects of external hydrogen on hydrogen-assisted crack initiation in type 304 stainless steel were discussed by performing fatigue crack growth rate and fatigue life tests in 5 MPa argon and hydrogen.FindingsHydrogen can reduce the incubation period of fatigue crack initiation of smooth fatigue specimens and greatly promote the fatigue crack growth rate during the subsequent fatigue cycle. During the fatigue cycle, hydrogen invades into matrix through the intrusion and extrusion and segregates at the boundaries of α′ martensite and austenite. As the fatigue cycle increased, hydrogen-induced cracks would initiate along the slip bands. The crack initiation progress would greatly accelerate in the presence of hydrogen.Originality/valueTo the best of the authors’ knowledge, this paper is an original work carried out by the authors on the hydrogen environment embrittlement of type 304 stainless steel. The effects of external hydrogen and argon were compared to provide understanding on the hydrogen-assisted crack initiation behaviors during cycle loading.

Hydrogen effects in X30MnCrN16‐14 austenitic steel

AbstractChrome‐manganese‐nitrogen austenitic steels show a technically relevant combination of proprties, i. e. high strength, high ductility, non magnetic and good corrosion resistance at costs being much lower compared to conventional chrome‐nickel austenitic stainless steels which are widely used for hydrogen applications. Hydrogen environment embrittlement of steel X30MnCrN16‐14 is investigated by slow displacement rate tensile testing in hydrogen atmosphere at 10 MPa and room temperature. Compared to the values in air, the elongation at fracture as well as the reduction of area are severely reduced in the presence of hydrogen. The microstructure is characterized in detail and the deformation modes are previously reported. It is assumed that the inherent planar deformation modes are facilitated by hydrogen resulting in premature failure.

Stacking Fault Energy in Relation to Hydrogen Environment Embrittlement of Metastable Austenitic Stainless CrNi-Steels

Stacking Fault Energy in Relation to Hydrogen Environment Embrittlement of Metastable Austenitic Stainless CrNi-Steels

Stacking Fault Energy in Relation to Hydrogen Environment Embrittlement of Metastable Austenitic Stainless CrNi-Steels

Stacking Fault Energy in Relation to Hydrogen Environment Embrittlement of Metastable Austenitic Stainless CrNi-Steels

On the role of nitrogen on hydrogen environment embrittlement of high-interstitial austenitic CrMnC(N) steels

This work investigates the susceptibility of high-interstitial CrMn austenitic stainless steel CN0.96 to hydrogen environment embrittlement. In this context, an N-free model alloy of CN0.96 steel was designed, produced, and characterized. Both steels were subjected to tensile tests in air and in a high-pressure hydrogen gas atmosphere.Both steels undergo severe hydrogen embrittlement. The CN0.96 steel shows trans- and intergranular failure in hydrogen, whereas the N-free model alloy shows exclusively intergranular failure. The different failure modes could be related to different deformation modes that are induced by the presence or absence of N, respectively. In the CN0.96 steel, N promotes planar dislocation slip. Due to the absence of N in the model alloy, localized slip is less pronounced and mechanical twinning is a more preferred deformation mechanism. The embrittlement of the model alloy could therefore be related to mechanisms that are known from hydrogen embrittlement of twinning-induced plasticity steels.

Role of surface oxide layers in the hydrogen embrittlement of austenitic stainless steels: A TOF-SIMS study

Hydrogen environment embrittlement (HEE) of low-nickel austenitic stainless steels (AISI 300 series) with different chemical compositions was studied focusing on the impact of the steels surface oxides, grain sizes and dislocation arrangements. The susceptibility of the steels to HEE is judged with respect to the relative reduction of area (RRA), where the HEE susceptibility is lower for larger RRA values.For many AISI 300 steels a linear trend is observed correlating RRA and the probability of strain induced martensite formation in tensile tests. Some steels, however, depart from the general trend, revealing greater HEE resistances.A careful examination of possible factors influencing HEE of the investigated steels reveals that high RRA values are linked to a specific type of oxide layer, namely the “high constant level oxide”, as categorized by TOF-SIMS evaluation. Thus, this type of oxide layer may be able to lower the steels HEE susceptibility. Other types of surface oxides, grain sizes and dislocation arrangements in the matrix of the particular AISI 300 steels appear to be of secondary importance.

Inhibitory effect of oxygen on hydrogen‐induced fracture of A333 pipe steel

AbstractThe effect of oxygen contained in hydrogen gas environment as an impurity on hydrogen environment embrittlement (HEE) of A333 pipe steel was studied through the fracture toughness tests in hydrogen gases. The oxygen contents in the hydrogen gases were 100, 10, and 0.1 vppm. A significant reduction in the J‐Δa curve was observed in the hydrogen with 0.1‐vppm oxygen. Under given loading conditions, the embrittling effect of hydrogen was completely inhibited by 100 vppm of oxygen. In the case of the hydrogen with 10‐vppm oxygen, initially the embrittling effect of hydrogen was fully inhibited, and then subsequently appeared. It was confirmed that 1‐vppm oxygen reduced the embrittling effect of hydrogen. The results can be explained by the predictive model of HEE proposed by Somerday et al.

Investigation of the Local Austenite Stability Related to Hydrogen Environment Embrittlement of Austenitic Stainless Steels

Hydrogen is increasingly considered as fuel for future mobility or for stationary applications. However, the safe distribution and storage of pure hydrogen is only possible with suitable materials. Interstitially dissolved hydrogen atoms in the lattice of numerous metals are responsible for hydrogen embrittlement (HE). If hydrogen is introduced by an external source, it is called hydrogen environment embrittlement (HEE). Commonly, steels like AISI 316L with a high resistance to HEE include a large number of alloying elements and in high amount. High alloying levels result in a decrease of cost-efficiency. Therefore, the systematic investigation of lean-alloyed austenitic stainless steels is necessary in order to understand the mechanism of HEE. For that purpose, the steel grades AISI 304L and AISI 316L are selected in this work. Tensile tests in air and 400 bar hydrogen gas atmospheres are performed. After tensile testing in H, AISI 304L revealed secondary cracks at the specimen surface, which are related to the local austenite stability, which in turn is affected by the level of micro-segregation. The microstructural investigations of the crack environment directly contribute to the understanding of the micro-mechanisms of HEE. Property-maps generated from experimentally measured distributions of alloying elements allow to correlate the impact of micro-segregations on the local austenite stability. It is shown, that local segregation-bands affect the initiation and propagation of secondary cracks. In this context, the local austenite stability which is significantly affected by the Ni distribution will be discussed in detail by comparison of the metastable austenitic steel grades AISI 304L and AISI 316L.

Improved resistance to hydrogen environment embrittlement of warm-deformed 304 austenitic stainless steel in high-pressure hydrogen atmosphere

The resistance of warm-deformed 304 austenitic stainless steel to hydrogen environment embrittlement (HEE) was investigated by adjusting the deforming temperature and deforming strain in hydrogen atmosphere. Higher deforming temperatures (100∼300 °C) can improve the resistance to HEE, while higher deforming strains weakened the resistance to HEE. Higher deforming temperatures caused a more uniform distribution of the high-density dislocations and less deformation twins, which weakened hydrogen segregation to prevent hydrogen-induced cracking. As the deforming strain increased, more primary deformation twins formed, which promoted the formation of strain-induced α′ martensite and then accelerated the growth of hydrogen-induced cracks.

Impact of chemical inhomogeneities on local material properties and hydrogen environment embrittlement in AISI 304L steels

This study investigated the influence of segregations on hydrogen environment embrittlement (HEE) of AISI 304L type austenitic stainless steels. The microstructure of tensile specimens, that were fabricated from commercially available AISI 304L steels and tested by means of small strain-rate tensile tests in air as well as hydrogen gas at room temperature, was investigated by means of combined EDS and EBSD measurements. It was shown that two different austenitic stainless steels having the same nominal alloy composition can exhibit different susceptibilities to HEE due to segregation effects resulting from different production routes (continuous casting/electroslag remelting). Local segregation-related variations of the austenite stability were evaluated by thermodynamic and empirical calculations. The alloying element Ni exhibits pronounced segregation bands parallel to the rolling direction of the material, which strongly influences the local austenite stability. The latter was revealed by generating and evaluating two-dimensional distribution maps for the austenite stability. The formation of deformation-induced martensite was shown to be restricted to segregation bands with a low Ni content. Furthermore, it was shown that the formation of hydrogen induced surface cracks is strongly coupled with the existence of surface regions of low Ni content and accordingly low austenite stability. In addition, the growth behavior of hydrogen-induced cracks was linked to the segregation-related local austenite stability.

Fatigue crack propagation of aerospace aluminum alloy 7075-T651 in high altitude environments

Fracture mechanics fatigue testing of 7075-T651 in environmental conditions pertinent to high altitude airframe operation established the fatigue crack growth behavior over a wide span of stress intensity range (ΔK). Two distinct methods were used to control the environmental severity parameter of water vapor pressure over frequency (PH2O/f): (1) at 23°C the PH2O value was controlled by an ultra-high vacuum system into which purified water was leaked, and (2) the testing temperature was decreased to temperatures as low as −65°C allowing an equilibrium PH2O over ice at a given temperature to be maintained. Decreasing PH2O at 23°C (at a constant f) results in a drastic reduction in the fatigue behavior, strongly suggesting that the reduction in moisture is a primary mechanism controlling the retarded fatigue crack growth behavior observed at low termperatures. Additionally, studies conducted at a corresponding PH2O/f with various testing temperatures down to −15°C show that fatigue crack growth rate (FGCR) behavior is consistent with data generated at 23°C. However, at temperatures below −30°C, low T tests exhibit crack growth rates below the 23°C results despite having the same PH2O/f value. This suggests that the strong role of environmental moisture reduction is augmented by a separate temperature dependent effect on either the inherent dislocation behavior and/or a separate aspect of the Hydrogen Environment Embrittlement (HEE) process. Mechanisms based purely on temperature dependent dislocation behavior fail to fully describe the experimental observations. Temperature dependent impacts on the HEE process (apart from the reduction of the PH2O) via influencing the surface reaction process to generate atomic H at the crack tip or the diffusion of the H within the crack tip process zone do not adequately describe the observed experimental data. Furthermore, a dip in the crack growth rate at −30°C suggests that molecular flow of H2O from the bulk to the crack tip is in part responsible for the temperature dependent behavior. Such behavior is hypothesized to be caused by the preferential onset of a rough crack wake morphology (likely slip band cracking) at low temperatures, that impedes molecular flow from the bulk to the crack tip. This behavior strongly suggests an important temperature dependence of the dislocation motion or H-dislocation interactions on the observed FCGR behavior.