Embrittlement Conditions Research Articles

Guided analysis of fracture toughness and hydrogen-induced embrittlement crack growth rate in quenched-and-tempered steels using machine learning

This study focuses on developing a machine learning (ML) model, specifically a Bayesian-optimized deep neural network, leveraging numerical simulation data for the prediction of fracture toughness (KIC) and crack growth rate (CGR) in Quenched-and-Tempered Steel (AISI 4140 alloy) under hydrogen embrittlement conditions. The proposed model demonstrated superior accuracy with Root Mean Squared Error (RMSE) values of 0.052 for KIC and 0.084 for CGR, surpassing conventional ML models including random forest (0.094 for KIC, 0.139 for CGR), gradient boosting (0.184 for KIC, 0.196 for CGR), support vector regression (0.142 for KIC, 0.110 for CGR), and decision tree regression (0.119 for KIC, 0.133 for CGR). The findings also revealed that input parameters such as temperature, hydrogen concentration, and hydrostatic stress carry higher weight factors compared to other parameters in predicting KIC. Similarly, the prediction of high CGR values, ranging from 5.9 × 10−5 to 9.5 × 10−5 m/s, was associated with the importance of hydrostatic stress, strain rate, and initial crack length in the prediction model. This underscores the model's ability to capture the intricate dependencies of output objectives on input features. Furthermore, a comprehensive case study, informed by the ML model, highlights the potential for tuning specific processing input parameters to manage hydrogen embrittlement effectively, contributing to a deeper understanding of its dynamics.

In-situ neutron imaging of delayed crack propagation of high strength martensitic steel under hydrogen embrittlement conditions

This paper presents an in-situ observation, using neutron imaging, of delayed crack propagation in a high-strength martensitic steel specimen. Delayed cracking is believed to be caused by hydrogen embrittlement occurring due to the slow diffusion and accumulation of hydrogen ahead of a crack front, causing decreased ductility and eventual cracking under constant load. The experiment involved mechanical loading of a single-edge-notch bend specimen while submerged in an electrolyte solution (H2O + 3.5% NaCl) under cathodic polarization to facilitate hydrogen ingress. Intermittent crack propagation was observed for 12 h after the environment had been removed. The stress state at each crack configuration was extracted from a three-dimensional elastic–plastic finite element simulation, which was tailored to match the quantitative information acquired from the neutron radiographs of the fracture process. To gain insight into the evolution of hydrogen concentration with crack propagation, a modeling scheme for stress-assisted hydrogen diffusion was also employed.

Development of a Methodology for Calculating the Stress State and Resource of a Hydrogen Generator Using the Finite Element Method

An experimental stand was created to study the thermobaric and chemical influence of hydrogen on the identification of hydrocarbon production. The said stand allows to reproduce chemical-technological processes as close as possible to real formation ones. This stand makes it possible to study the kinetics of not only hydrogen, thermobaric and chemical effects, but also other thermal gas chemical processes, including hydrogen generation. The main element of the experimental stand is a hydrogen generator, the components of which work at high pressures and temperatures under conditions of hydrogen embrittlement of mechanical properties and an aggressive environment that causes corrosion of its inner surface. Based on this, the development of a methodology for calculating the thermal stress state of the generator, its strength under hydrogen embrittlement conditions, and its resource becomes relevant. Based on the finite element method, a methodology for calculating non-stationary temperature fields and the thermal stress state that occur in the hydrogen generator during thermobaric and chemical processes of varying intensity is proposed. The methodology allows to take into account the features of the geometry of the structure, the time-varying temperature and pressure distributions of the reaction products, the temperature dependence of the thermophysical and mechanical properties of the hydrogen generator material. Thanks to the application of the developed software, a study of the hydrogen generator thermal stress state during two real thermobaric and chemical processes of different intensity was carried out. Graphs of temperature and pressure changes of the reaction products of hydroreactive substances in the generator over time, which were registered during the experiment conduction, were used. The distribution of non-stationary temperature fields and stresses in the hydrogen generator elements was obtained. Areas of maximum load of generator elements are defined. It was established that during the flow of the studied thermobaric and chemical processes, pressure makes a greater contribution to the thermal stress state. The obtained results and the developed theory and software can be used in the study of generators of other designs with other thermobaric and chemical processes occurring in them.

Характеристики кривых ударного нагружения и их взаимосвязь с параметрами излома образцов стали 32Г2Р

Impact tests are mandatory for a wide range of structural steel products to evaluate the required service life and operational safety. This test is especially relevant for products used in operation in climatic areas with a cold climate, since it allows one to determine the temperature range of the ductile-brittle transition. Depending on the structural-phase state of a metal, the type of the temperature relationship of impact toughness and its displacement on the temperature axis may differ significantly. For ductile materials, the temperature range of the ductile-brittle transition is shifted towards ultra-low negative temperatures whereas for materials in the embrittled condition it lies in the region of higher temperatures. Ductile-brittle transition behavior of medium-carbon steel with 0.32 % C-1.35 % Mn-0.003 % B in two conditions (after normalization and after quenching and tempering) by instrumented impact testing and fracture analysis was studied. On the descending branch of the force-displacement curves the periods corresponding to different zones of propagation of the main crack are distinguished: period I is the ductile zone of pure shear, period II is the zone of brittle fracture, period III is the ductile zone of break fracture. Each period can be quantitatively described by a set of strength, plastic and gradient parameters. It is established that the displacement to the beginning of linear period II (SIIb) and slope of the curve on it (ΔF / ΔS)II have a better correlation with shear fracture zone percentage (B) than the other distinguished parameters of force-displacement curves.

Hydrogen embrittlement processes in microalloyed steel notched tensile samples

In this work, two microalloyed steels, one used for oil&gas pipelines and the other one for structural components in hydrotreating reactors in petrochemical industries, have been studied under hydrogen embrittlement (HE) conditions. They have been cathodically charged with hydrogen under different aggressive conditions, its hydrogen content being determined afterwards. In order to know the effect of triaxiality on the HE behaviour of this steel, two different types of axisymmetric round notched tensile specimens have been tested under continuous hydrogen charging processes, whilst varying cathodic charging conditions and loading rate. The effects on the global mechanical characterization have been analysed, as well as the correlation between the observed fracture micromechanisms with both local environmental (as hydrogen concentration) and mechanical conditions (as hydrostatic stress and plastic strain determined with a finite elements analysis). Also, subcritical cracking due to embrittlement has been analysed and compared with those obtained by fracture mechanics standardised specimens.

Chip Breaking During Automated Processing by Cutting Hard-to-Work Steels by Integrated Use of Chip Breaking Tools and High-Temperature Embrittlement

Purpose of research is to increase productivity when cutting hard-to-work steels by integrated use of chip breaking tool geometry and cutting modes, which reduce the viscous properties of the chips and the processed material. The article discusses the problems of mechanical processing of parts of oil production equipment at enterprises in Western Siberia. It is presented the main condition for ensuring chip crushing and revealed the problem of low efficiency of existing methods in modern industry, to one degree or another guiding, curling and chip-cutting method.Methods. Simulation modeling, calculation of chip direction and crushing were performed; effective methods for solving the problem of chip crushing during the machining of parts made of materials with corrosion-resistant, heat-resistant properties were identified; laboratory, persistent tests of the complex method were carried out.Results of studying the mechanism of chip formation during processing by cutting heat-resistant and corrosionresistant alloys are given here. To solve the problem, we have formulated two tasks that were solved using information taken from the theory of material resistance, tensile diagrams when testing samples of corrosionresistant, heat-resistant materials with high operational properties. A chip breaking exchangeable insert with a variable rake angle is designed and presented. The results and analysis of laboratory test data, in which a plate with chip breaking geometry of the front surface is used, are presented, the operating conditions of effective chip breaking are shown. The conclusions on the solution of the first problem are formulated, it is shown that the determining factor in chip breaking is the physical and mechanical characteristics of the processed material, which vary with different cutting conditions.Conclusion. The work shows that chip removal from the cutting zone prevents it from entering the cutting edge area, reducing impact loads on it and improving the quality of machining. It is possible to form stress concentrators in the chips, leading to chip self-destruction even when processing viscous heat-resistant alloys. The hypothesis is used that the temperature conditions for the maximum workability of heat-resistant and corrosion-resistant steels and alloys correspond to the conditions of high-temperature embrittlement, with the help of which it is possible to create conditions for effective chip crushing and conditions for maximum workability when cutting heat-resistant steels and alloys.

Determination of hydrogen embrittlement stress intensity threshold by fractography

Electroplating of aerospace parts generates hydrogen embrittlement, which is controlled by 200-h sustained-load tests performed on cylindrical notched samples. Those tests are costly and offer only a pass-or-fail assessment, with limited interpretability. In this article, we propose a new fractographic method to obtain a quantitative indication of the embrittlement severity of electroplated samples that failed the sustained-load test. On the fracture surfaces of failed samples, intergranular (IG) zones were observable at fracture initiation sites, surrounded by ductile zones, with intermediate regions containing mixed (ductile+IG) features. Within the framework of the recently proposed Hydrogen-Enhanced Plasticity-Mediated DEcohesion mechanism (HELP-mediated HEDE), we argue that there is a correlation between the intergranular appearance of the fracture surfaces and the subcritical nature of the crack propagation. Using this argument, we computed the stress intensity factor K for each crack and loading condition, with the input of intergranular zone sizes and finite element analysis. Minimum and maximum K values were identified and statistics of the extremes was used to calculate the minimum expected K values for various embrittlement conditions. It is proposed that this value is an estimate of the threshold for embrittlement, Kth. Values of Kth between 9 MPa m0.5 and 30 MPa m0.5 were measured for various plating conditions. Those estimations are more conservative than the thresholds reported in the literature, which were measured using more complex experimental procedures. Moreover, the fracture toughness estimated with the maximum K values was possibly affected by high initial hydrogen concentrations as per the HELP+HEDE embrittlement mechanism.

Demonstration of Structural Integrity of Boiling Water Reactor Pressure Vessels Under Ultimate Response Guideline Operation

In recent years, the compound beyond-design-basis accident (BDBA), which combines earthquake, tsunami, or some other severe events to impact a nuclear power plant (NPP), has received more attention. After the Fukushima nuclear disaster, the licensee of NPPs in Taiwan established the ultimate response guideline (URG) that instructs operators to perform reactor depressurization, low-pressure water injection, and containment venting to prevent core meltdown and hydrogen explosion once long-term loss-of-power and water-supply events occur. In this paper, we employed the probabilistic fracture mechanics (PFM) method to evaluate the structural integrity of boiling water reactor (BWR) pressure vessels under URG operation. At first, models of the beltline shell welds for BWR vessels associated with the Pressure Vessel Research Users Facility-Exponential flaw distribution were built for the PFM Fracture Analysis of Vessels–Oak Ridge (FAVOR) code. Then, the thermal-hydraulic data of URG transients for Taiwan domestic BWRs were imposed as the loading conditions. The analysis results demonstrate that performing URG operation will not cause significant fracture probability even at extreme embrittlement conditions. If long-term station blackout occurs due to a compound BDBA, the URG procedures can prevent core damage and hydrogen explosion, while maintaining the structural integrity of the reactor pressure vessels.

Influence of Hydrogenation on the Character of Fracture of Nitinol Alloy in Tension

We establish the macro- and microfractographic features of the tensile fracture of cylindrical specimens made of nitinol (Ni–Ti alloy) at room temperature and at 0°C caused by their preliminary electrolytic hydrogenation. The revealed hydrogen embrittlement of the metal was associated with the facilitation of martensite transformation, which is more clearly pronounced at the lower testing temperature. This explains the phenomenon of reorientation of the macroscopic fracture surfaces of specimens from the tensile mode into the shear mode under the influence of the conditions of embrittlement. On the microlevel, the appearance of brittle cleavages along the boundaries of martensite crystals is regarded as a specific feature of the fracture of nitinol caused by its hydrogenation. Under more severe hydrogenation conditions, the width of the cleavage zone in the subsurface layers of the specimens increases, which is explained by the deeper penetration of hydrogen into the metal favorable for the structural and phase transformations.

Condition Monitoring of a Power Tower Using Metallurgical Investigations of Structural Members

Abstract A spectacular damage occurred at the end of 2005 in Münsterland when a large number of power towers in 110 kV transmission lines collapsed due to increased loads of ice and snow, which raised awareness amongst experts for the evaluation of a continuous safe operation of historical towers. At the time, it was found that embrittlement by strain ageing was a contributing factor. The in-house client ordered the condition evaluation of a power tower located on his premises. The evaluation was performed by testing four so-called structural members from this tower. Mechanical-technological material tests were performed exclusively on samples from the flat, mostly not cold-formed areas of the structural members' angle irons. The results of all these tests were inconspicuous, including the notch impact tests at −20 °C. Therefore, it is safe to say that the material at hand is generally not in an embrittled condition. In this context it is also interesting that the nitrogen content of the batch the angle irons of the examined structural members were made from, did not exceed the embrittlement sensitivity threshold of 0.01 percent by weight set by the Federal Institute for Materials Research. From a material-technological perspective, there is no objection to a continued safe operation of the power tower on the client's premises. Further tests and repairs were not considered necessary.

МОДЕЛИРОВАНИЕ ПОВЕДЕНИЯ МАТЕРИАЛОВ И ЭЛЕМЕНТОВ КОНСТРУКЦИЙ, НАХОДЯЩИХСЯ ПОД ВОЗДЕЙСТВИЕМ АГРЕССИВНЫХ СРЕД (ОБЗОР)

The review of a number of scientific works in the field of modeling processes of interaction of an aggressive medium with materials and structural elements and their behavior under the action of loads is offered. The investigated phenomena are described using theoretical-experimental methods for the further prediction of short-term and long-term strength characteristics of materials and structural elements subject to the influence of an aggressive medium. In view of the complexity of the phenomena under consideration, an interdisciplinary scientific approach is used that takes into account both the physics-chemical interaction of the aggressive medium with materials and the systematization of mechanical-mathematical (phenomenological) models. The main attention is paid to the study and modeling of high-temperature corrosion damage of materials and structural elements. It is taken into account both the diffusion penetration of the aggressive medium and the chemical interaction of its elements with the material. The effect of various factors, such as liquidmetal media and radiation exposure; oxide films formed on corroding surfaces; cracking of stressed stainless steels in the conditions of hydrogen embrittlement, on the corrosion process is presented. The dependence of corrosion rate on the stressed state with elastoplastic deformations also is represented. Publications with various versions of phenomenological models of corrosion wear processes, loss of plasticity and embrittlement in aggressive media are given, as well as publications using the probabilistic-statistic approach to modeling corrosive failure. An extensive list of wellknown literature sources, that reflect scientific research taking into account all aspects of interaction of the material with an aggressive environment is given. Keywords: modeling, physics-chemical models, mechanical-mathematical models, aggressive environment, liquid metal environment, strength, corrosion, corrosion layer, corrosion failure, diffusion, chemical reaction, ionizing radiation, experiment.

Using Small Punch tests in environment under static load for fracture toughness estimation in hydrogen embrittlement

In this paper, the response of three medium and high-strength steels to hydrogen embrittlement is analyzed by means of the quasi-non-destructive test known as the Small Punch Test (SPT). SPT tests on notched specimens under static load are carried out, applying Lacaclle’s methodology to estimate the fracture toughness for crack initiation, comparing the results to KIEAC fracture toughness obtained from C(T) precracked specimens tested in the same environment; SPT showed good correlation to standard tests. A novel expression was proposed to define the parameter KIEAC-SP as the suitable one to estimate the fracture toughness for crack initiation in hydrogen embrittlement conditions by Small Punch means, obtaining good accuracy in its estimations. Finally, Slow Rate Small Punch Tests (SRSPT) are proposed as a more efficient alternative, introducing an order of magnitude for the adequate rate to be employed.

Further improvement of the Prometey model and Unified Curve method part 1. Improvement of the Prometey model

The local approach to brittle fracture is able in principle to transfer fracture data from one type of specimens to another one for different test temperatures and different material conditions. However, in practice the capacity of existing models to perform transferability need to be improved. For this reason the probabilistic model of brittle fracture known as the Prometey model has been modified. The modified model (referred to as the Prometey-M model) has been used for analysis of the transferability of the experimental results on brittle fracture for smooth and notched cylindrical tensile specimens and cracked specimens from Reactor Pressure Vessel (RPV) steel in the initial and embrittled conditions. The Prometey-M model allows the calculation of the brittle fracture probability with the same model parameters for specimens of different types tested at different temperatures. Recommendations have been given for the parameter values of the Prometey-M model that may be taken the same for RPV steels in the initial and embrittled conditions.

Duplex stainless steels “475°C embrittlement”: influence of the chemical composition on the fatigue crack propagation

Abstract: Duplex stainless steels (DSSs) are prone to age hardening and embrittle over a wide temperature range depending on their chemical composition. This is mainly due to precipitation phenomena that may occur inside ferrite grains and at ferrite-austenite grain boundaries. The aim of this work is the analysis of chemical composition influence on fatigue crack propagation resistance of “475°C embrittled” duplex stainless steels. Fatigue crack propagation resistance of 21 Cr 1 Ni, 22 Cr 5 Ni and 25 Cr 7 Ni duplex stainless steels was investigated considering both as received and 475°C embrittled conditions (1000h). Microstructural analyses were performed using a transmission electron microscope (TEM). Concentrations of the main elements, but carbon, were evaluated using a standardless analysis program.

Evaluation of strength and fracture toughness of ferritic high strength steels under hydrogen environments

The susceptibility of high strength ferritic steels to hydrogen-assisted fracture in hydrogen gas is usually evaluated by mechanical testing in high-pressure hydrogen gas or testing in air after pre-charging the specimens with hydrogen. We have used this second methodology, conventionally known as internal hydrogen. Samples were pre-charged in an autoclave under 195 bar of pure hydrogen at 450ºC for 21 hours.Different chromium-molybdenum steels submitted to diverse quenching and tempering heat treatments were employed. Diverse specimens were also used: small cylindrical samples to measure hydrogen contents and the kinetics of hydrogen egression at room temperature, tensile specimens, notched tensile specimens with a sharp notch, and also compact fracture toughness specimens. Fractographic examination in SEM was finally performed in order to know the way hydrogen modify fracture micromechanisms.The presence of hydrogen barely affects the conventional tensile properties of the steels, but it clearly alters their notched tensile strength and fracture toughness. This is due to the strong effect that stress triaxiality (dependent also on the steel yield strength) has on the accumulation of hydrogen on the notch/crack front region, being the displacement rate used in the test another important variable to be controlled, due to its influence on hydrogen diffusion to the embrittled process zone. Moreover, the modification of fracture micromechanisms was finally determined, being ductile (initiation, growth and coalescence of microvoids) in the absence of hydrogen and brittle and intergranular under the material conditions of maximum embrittlement.

Mechanism of corrosion fatigue cracking of automotive coil spring steel

The AISI 300M ultra-high strength steel was applied for the automotive suspension coil spring. Recently, some premature failures were reported, which caused by synergistic effect of cyclic mechanical stress and corrosion, namely corrosion fatigue cracking. In this study, the accurate mechanism of corrosion fatigue cracking for coil spring steel was studied for the proper prevention method against the catastrophic failure. Fatigue life was evaluated in 5 wt% NaCl solution under the anodic dissolution and hydrogen embrittlement conditions, which is simulated by applying constant potentials. Scanning electron microscopy and energy dispersive X-ray spectroscopy analysis indicated that the corrosion fatigue cracking was initiated at the MnS inclusion of the pit initiation site. The calculation of hydrogen production corresponding to each corrosion fatigue test condition revealed the two operating mechanisms of the cracking process. The corrosion fatigue cracking failure of coil spring steel was mainly caused by the anodic dissolution combined with hydrogen embrittlement.

Interaction Study of Carbide Precipitation and Impurity Segregation under Temper Embrittlement Conditions in a Coarse-Grained Heat-Affected Zone in Q690 Steel

This work studied the interaction between carbide precipitation and impurity segregation under temper embrittlement (TE) conditions in a coarse-grained heat-affected zone (CGHAZ) in Q690 steel, a low-alloy high-strength structural steel used in the hydraulic support in the fully-mechanized mining face. From the perspective of carbide precipitation, through thermodynamics calculation and analysis, it was found that the existence of cementite at the grain boundaries was preceded by impurities segregation (primarily phosphorus). The precedent phosphorus segregation thus enhances the nucleation rate of cementite at the grain boundaries by lowering the ferrite/cementite interfacial energy. Both carbide precipitation and impurity segregation at the grain boundaries reached a maximum as a result of their mutual role in the temperature range of TE.

Stability of vacancy-hydrogen clusters in nickel from first-principles calculations

The interactions of hydrogen (H) atoms with vacancies are investigated by means of ab initio calculations. The lowest segregation energies are −0.27 and −0.41eV at single vacancies and divacancies, respectively. These values are in excellent agreement with those corresponding to the two characteristic peaks of the thermal desorption spectra. The microscopic interpretation of the experimental data is therefore clarified. An energetic model is built from the ab initio data and used to study the influence of H bulk concentration and temperature on the concentration of vacancy-H clusters. Analytical expressions, validated by Monte Carlo simulations, are given. The mean vacancy occupation and the H-induced vacancy enrichment are calculated at two temperatures representative of H embrittlement experiments and stress corrosion cracking at high temperatures. The stability domain of VH6 clusters is found to significantly overlap with the experimental conditions for embrittlement. Therefore, vacancy clustering at high concentrations can be qualitatively discussed based on VH6–VH6 interactions that are found weakly repulsive. Consequences on H damage in Ni are discussed. The effect of metal vibrations on segregation and local hydride stability is qualitatively evaluated by off-lattice Monte Carlo simulations using a semi-empirical Ni–H potential. They are shown to shift local hydride stability towards higher H concentrations.

Fatigue crack initiation behavior in embrittled austenitic–ferritic stainless steel

Fatigue crack initiation behavior in duplex stainless steel was studied in the embrittled condition (aging at 475°C for 100h) by interrupting fatigue tests conducted at different Δσ/2 values, identifying the crack initiation sites and relating them to the crystallographic parameters obtained from electron back scattered diffraction scans. From the fatigue crack initiation studies conducted at Δσ/2=400 and 500MPa, it was concluded that the crack initiation sites are the slip markings corresponding to {111} plane in austenite at 400MPa and more cracks were observed to initiate at twin boundaries in austenite at 500MPa in the aged condition.

Embrittlement of a zinc coated high manganese TWIP steel

The liquid metal embrittlement (LME) of a high manganese austenitic TWIP steel was investigated by hot tensile testing of zinc coated specimens using Gleeble® thermo-mechanical simulator. The influence of different parameters such as temperature, strain rate and pre-exposure time on the occurrence of embrittlement was studied. The results show that the steel can be embrittled by liquid zinc within a limited temperature range depending on strain rate. A critical stress for cracking has been defined for each embrittlement condition. The effect of time of contact between the steel and the liquid zinc has been investigated by performing holding at temperature before tensile testing. It has been shown that a holding of few seconds is sufficient to regain the whole original ductility. Physical mechanisms involved in LME phenomenon are discussed.