Cr Ferritic-martensitic Steels Research Articles

The effect of deformation temperature in metastable austenite zone on microstructure evolution, mechanical and corrosion properties of high Cr ferritic-martensitic steel

The effect of deformation temperature in metastable austenite zone on microstructure evolution, mechanical and corrosion properties of high Cr ferritic-martensitic steel

Effect of Impurities on the Compatibility of Steels in Supercritical CO2 at 450°–650°C

Abstract Direct-fired supercritical CO2 (sCO2) power cycles are a pathway to low-CO2 fossil energy but contain O2 and H2O in the sCO2 from combustion. The effect of impurities on structural steels was investigated at 450°–650 °C in 30 MPa sCO2. The test matrix included 9 and 12%Cr ferritic-martensitic (FM) steels and conventional and advanced austenitic steels exposed for 1000–2000 h with and without additions of 1%O2 and 0.1%H2O to simulate the cycle after water removal. For FM steels, the mass gains and scale thicknesses were similar with and without impurities with the formation of thick, duplex Fe-rich scales in all cases including the observation that Fe2O3 only formed with 1%O2. For the austenitic steels, higher mass gains were observed at all temperatures with increased formation of Fe-rich oxides when impurities were added. Carbon ingress was assessed by bulk combustion analysis, glow discharge optical emission spectroscopy (GDOES) and measuring postexposure room temperature tensile properties. Bulk C content was strongly increased at 650 °C but not at 450° or 550 °C.

Internal carburization and scale formation on austenitic steels in supercritical carbon dioxide

ABSTRACT Direct-fired supercritical CO2 (sCO2) power cycles are being commercialised to revolutionise fossil energy as a low-emission power source. To lower the cost of this technology, less expensive steels are needed in the lower temperature segments of the cycle. However, there are concerns about internal carburisation of steels in sCO2. 1 A consistent observation is that thin, Cr-rich oxides appear to reduce C ingress compared to thick Fe-rich oxides formed on 9–12% Cr ferritic-martensitic steels. Advanced austenitic stainless steels (SS) like alloy 709 (20Cr-25Ni) are able to continue to form Cr-rich oxides at 650°C, while a conventional type 316 H SS formed a Fe-rich scale. The C diffusion profiles in SS specimens were quantified at 550°C–650°C using glow discharge optical emission spectroscopy and electron probe microanalysis. Analytical transmission electron microscopy was used to compare the thin protective Cr-rich oxide formed on alloy 709 in sCO2 at 650°C to that formed in ambient air.

Heat Treatment of High-Performance Ferritic (HiperFer) Steels.

High-performance Ferritic (HiperFer) steels are a novel class of heat-resistant, fully ferritic, Laves phase precipitation hardened materials. In comparison to conventional creep strength-enhanced 9-12 wt.% Cr ferritic-martensitic steels, HiperFer features increased mechanical strength, based on a thermodynamically stable distribution of small (Fe,Cr,Si)2(Nb,W) Laves phase precipitates, and-owing to its increased chromium content of 17 wt.%-improved resistance to steam oxidation, resulting in superior temperature capability up to 650 °C. Previous publications focused on alloying, thermomechanical processing, and basic mechanical property evaluation. The current paper concentrates on the effect of heat treatment on microstructural features, especially Laves phase population, and the resulting creep performance. At 650 °C and a creep stress of 100 MPa, an increase in rupture time of about 100% was achieved in comparison to the solely thermomechanically processed state.

A REVIEW: FERRITIC-MARTENSITIC STEELS – TREATMENT, STRUCTURE AND MECHANICAL PROPERTIES

The constantly growing consumption of electricity requires the development and implementation of more powerful and energy-intensive systems of the new generation. Fusion and fission reactors of the 4th generation (Gen-IV) will make it possible to cover the growing demand for electricity. Since Gen-IV reactors will operate at higher temperatures and radiation doses, the problem of selecting scientifically based structural materials arises, since conventional reactor materials are not suitable for use in such severe operating conditions. Among the structural materials under consideration for future generations of reactors, special attention is paid to 9…12% Cr ferritic-martensitic steels due to their higher radiation tolerance and excellent mechanical properties compared to traditionally used austenitic steels. This review presents the main ferritic-martensitic steels that will be used as structural materials, their structure, mechanical properties and various thermal and thermomechanical treatments applied to them.

Assessment of Outliers in Alloy Datasets Using Unsupervised Techniques

Advancements in data analytics techniques have enabled complex, disparate datasets to be leveraged for alloy design. Identifying outliers in a dataset can reduce noise, identify erroneous and/or anomalous records, prevent overfitting, and improve model assessment and optimization. In this work, two alloy datasets (9–12% Cr ferritic-martensitic steels, and austenitic stainless steels) have been assessed for outliers using unsupervised techniques and supplemented with domain knowledge. Principal component analysis and k-means clustering were applied to the data, and points were assessed as outliers based on their distance from other points in the cluster and from other points in the dataset. The outlier characteristics were investigated to determine both cluster-specific and overall trends in the properties of the outlier points. The approach demonstrated here is extensible to other alloy datasets for outlier identification and evaluation to improve the reliability of machine learning and modeling predictions for advanced alloy design.

Effect of High Temperature Thermomechanical Treatment on the Tendency of Low-Activation 12% Cr Ferritic-Martensitic Steel EK-181 to Low-Temperature Embrittlement

Effect of High Temperature Thermomechanical Treatment on the Tendency of Low-Activation 12% Cr Ferritic-Martensitic Steel EK-181 to Low-Temperature Embrittlement

Uncertainty quantification for Bayesian active learning in rupture life prediction of ferritic steels

Three probabilistic methodologies are developed for predicting the long-term creep rupture life of 9–12 wt%Cr ferritic-martensitic steels using their chemical and processing parameters. The framework developed in this research strives to simultaneously make efficient inference along with associated risk, i.e., the uncertainty of estimation. The study highlights the limitations of applying probabilistic machine learning to model creep life and provides suggestions as to how this might be alleviated to make an efficient and accurate model with the evaluation of epistemic uncertainty of each prediction. Based on extensive experimentation, Gaussian Process Regression yielded more accurate inference (Pearson;correlation;coefficent> 0.95 for the holdout test set) in addition to meaningful uncertainty estimate (i.e., coverage ranges from 94 to 98% for the test set) as compared to quantile regression and natural gradient boosting algorithm. Furthermore, the possibility of an active learning framework to iteratively explore the material space intelligently was demonstrated by simulating the experimental data collection process. This framework can be subsequently deployed to improve model performance or to explore new alloy domains with minimal experimental effort.

Characterization of thermal aging effects on the creep performance of T91 using small punch creep testing

AbstractThis paper is concerned with the development and application of a small punch creep testing method for assessment of the effects of thermal aging and associated precipitate coarsening on the high temperature creep life and deformation behavior of a 9Cr ferritic–martensitic steel. The tests are conducted on P91 steel, for both aged (up to 1 year) and unaged specimens, at temperatures of 600°C. The unaged test results are shown to be consistent with previously published creep tensile test rupture life data in terms of Chakrabarty membrane stress. Aging is shown to have a significant, detrimental effect on creep life and deformation rate. The steady‐state displacement rate, time to failure, and displacement at failure data, for both aged and unaged, were successfully correlated via the modified Monkman–Grant relationship.

A machine learning aided interpretable model for rupture strength prediction in Fe-based martensitic and austenitic alloys

The class of 9–12% Cr ferritic-martensitic alloys (FMA) and austenitic stainless steels have received considerable attention due to their numerous applications in high temperature power generation industries. To design high strength steels with prolonged service life requires a thorough understanding of the long-term properties, e.g., creep rupture strength, rupture life, etc., as a function of the chemical composition and processing parameters that govern the microstructural characteristics. In this article, the creep rupture strength of both 9–12% Cr FMA and austenitic stainless steel has been parameterized using curated experimental datasets with a gradient boosting machine. The trained model has been cross validated against unseen test data and achieved high predictive performance in terms of correlation coefficient (R^{2} > 0.98 for 9–12% Cr FMA and R^{2} > 0.95 for austenitic stainless steel) thus bypassing the need for additional comprehensive tensile test campaigns or physical theoretical calculations. Furthermore, the feature importance has been computed using the Shapley value analysis to understand the complex interplay of different features.

Data Science Techniques, Assumptions, and Challenges in Alloy Clustering and Property Prediction

Data analytics methods have been increasingly applied to understanding materials chemistry, processing due to the manufacturing approach, and uni-axial and cyclic property relationships in the highly complex space of alloy design. There are several benefits to applying data analytics to this space, including the ability to manage non-linearities in the responses of the alloy attributes and the resulting mechanical properties. However, key difficulties in applying and understanding the results of data analytics include the often lack of reported assumptions and data processing steps necessary to improve interpretation and reproducibility in derived results. In this work, the methods used to generate clustering and correlation analyses for experimental 9% Cr ferritic-martensitic steel data were investigated and the resulting implications for mechanical property predictions were assessed. This work uses principal component analysis, partitioning around medoids, t-SNE, and k-means clustering to investigate trends in composition, processing and microstructure information with creep and tensile properties, building on work done previously using a smaller version of the same dataset. The initial assumptions, preprocessing steps and methods are investigated and outlined in order to depict the fine level of detail required to convey the steps taken to process data and produce analytical results. The variations in the resulting analyses are explored due to the influence of new and more varied data.

Влияние термомеханической обработки на особенности деформированной микроструктуры ферритно-мартенситной стали ЭК-181

Ferritic-martensitic steels with a chromium content of 9-12 % are currently considered as the promising structural materials for nuclear power. Interest in steels of this class is caused by their higher resistance to radiation swelling compared with austenitic steels used in the existing fission reactors. The operating temperature range of these steels is limited from below by their tendency to low-temperature embrittlement (cold fracture) under the radiation influences, and from above - by the long-term strength level (heat resistance). The authors studied the features of the microstructure of 12 % Cr ferritic-martensitic EK-181 steel near the neck of the samples deformed by tension at T=20 °С and within the range of temperatures close to the operating temperatures of a nuclear reactor (T=650 and T=720 °C). The authors carried out the comparative study of the materials processed by two methods: traditional and high-temperature treatment. The study showed that plastic deformation at T=20 °C after two treatments is similar in quality and leads to curvature and fragmentation of martensitic lamella, as well as to the formation of new low-angle boundaries. Deformation near the operating temperature range (T=650 and T=720 °C) contributes to the development of the processes of dynamic polygonization, recrystallization, increasing the density, and the size of carbide particles. After high-temperature thermomechanical treatment, these processes are less intensive compared to the state after traditional thermal treatment. After high-temperature thermomechanical treatment, EK-181 steel has an increased level of strength and has a higher resistance to plastic deformation compared to the state after traditional treatment. It is related to the high density of vanadium carbonitride nano-particles V(C, N) and the increased dislocation density after high-temperature thermomechanical treatment.

Effects of chloride on electrochemical and stress corrosion cracking behavior of 9Cr ferritic–martensitic steel

The electrochemical and stress corrosion cracking behavior of 9Cr ferritic–martensitic steel is investigated in the chloride environment by using the traditional electrochemical method, the scanning vibrating electrode technique and the slow strain rate test (SSRT). Results of the static corrosion tests and corrosion morphology show that the prior austenite grain boundaries and martensite lath boundaries are the preferred sites for pit nucleation and growth in chloride environment. Results of SSRT coupled with in situ electrochemical test show that the transition from pitting corrosion to uniform corrosion, as well as the nucleation of stress corrosion crack, is the synergistic effects of the chloride and applied load. Stress corrosion cracking of the steel in the chloride environment can be divided into three different regions as follows: fast and uniform corrosion activity, microcrack nucleation and propagation, and active crack growth regions.

Long-term stability of precipitated phases in CLAM steel during thermal aging

China Low Activation Martensitic (CLAM) steel as one of 9%Cr ferritic-martensitic steels was designed as the structural materials for fusion reactors and lead-based Reactor in China. The long-term thermal stability of precipitated phases in CLAM steel at 550 °C was systematically studied in this paper, including coarsening kinetics, number density and shape stability. Based on large data statistics, the coarsening kinetics of M23C6 carbide, MX carbonitride and Laves phase were analyzed to develop a coarsening model of each precipitated phase, respectively. The results showed that the coarsening rate of Laves phase was about 100 times than that of M23C6 carbide after aging for 20,000 h at 550 °C. The number density of precipitated phases increased firstly and then decreased with increasing of aging time due to a dynamic equilibrium process of the precipitation and dissolution of precipitated phase. The results will help to deep understand degradation mechanisms after thermal aging.

Modelling and Analysis of the Corrosion Characteristics of Ferritic-Martensitic Steels in Supercritical Water.

The dependencies of weight gain of 9-12 Cr ferritic-martensitic steels in supercritical water on each of seven principal independent variables (temperature, oxygen concentration, flow rate, exposure time, and key chemical composition and surface condition of steels) have been predicted using a supervised artificial neural network (ANN). The relative significance of each independent variable was uncovered by fuzzy curve analysis, which ranks temperature and exposure time as the most important. The optimized ANN, not only satisfactorily represents the experimentally-known non-linear relationships between the corrosion characteristics of F-M steels and the key independent variables (demonstrating the effectiveness of this technique), but also predicts and reveals that the effects of oxygen concentration on the weight gains, to a certain degree, is influenced by the flow rate and temperature. Finally, according to the ANN predicted-results, departure of oxidation kinetics from the parabolic law, and basic cause of chromium content in steel substrate influencing the corrosion rate, and the synergetic effects of dissolved oxygen concentration, flow rate, and temperature, are discussed and analyzed.

Thermal Stability of the Microstructure and Mechanical Properties of the Ferritic-Martensitic Steel EK-181

Thermal stability of the microstructure and high-temperature mechanical properties of the low-activate 12% Cr ferritic-martensitic steel EK-181 are studied at temperatures from 700 to 800°C. It is shown that after traditional heat treatment and annealing at temperatures from 700 to 800°C, the ferritic-martensitic structure of the steel is retained and the density of coarse-dispersed M23C6 particles is increased. After annealing at T = 800°C, the initial recrystallization stages with the formation of new submicrocrystalline ferritic grains are detected. An increase in the annealing temperature from 700 to 800°C leads to a decrease in the strength properties of the steel. Wherein, the character of the temperature dependence of the strength at temperatures from 700 to 800°C is similar to that observed at lower temperatures (from 500 to 700°C).

Frequency and hold time influence on crack growth behavior of a 9–12% Cr ferritic martensitic steel at temperatures from 300 °C to 600 °C in air

Due to an increase of renewable energies proportion, e.g. wind power and photovoltaics, which cannot supply energy constantly, modern power plants must be able to be operated flexibly in order to compensate the residual load. As a consequence of increasing alternating load, fatigue damage becomes more and more important, while creep damage caused by ever shorter holding times at high operating temperature decreases. In this study a turbine bypass valve, one of the most fatigue loaded power plant components, manufactured from widespread standard 12% Cr ferritic/martensitic steel X20 was investigated. Fatigue crack growth experiments showed that the crack growth rate increases slightly with decreasing frequency (20 Hz → 5 Hz). In hold time tests (300 s → 600 s, effective frequency 3.33 × 10−3 Hz → 8.33 × 10−4), larger crack propagation rates per cycle occur than in the fatigue crack growth experiments with 5 and 20 Hz. In comparison to pure cyclic loading maximum load holding time further required significantly higher ΔK values to start crack growth.

Roles of Mn in the High-Temperature Air Oxidation of 9Cr Ferritic–Martensitic Steel After Severe Plastic Deformation

Oxidation resistance of 9Cr ferritic–martensitic steel before and after cold-swaging process in air at 923 K was compared, and the oxide scales were characterized in detail by secondary ion mass spectrometry (SIMS). Both the cold-swaged sample and post-annealed samples showed considerably greater oxidation resistance than the initial as-tempered sample. Mn played a key role in scale formation of the cold-swaged sample and post-annealed sample; although the Mn content was only 0.5 wt% in the matrix, formation of Mn oxide (MnCr2O4 and Mn2O3) was favored. Roles of Mn in the oxidation behaviors of the cold-swaged sample and post-annealed sample were discussed.

Heterogeneous Creep Deformations and Correlation to Microstructures in Fe-30Cr-3Al Alloys Strengthened by an Fe2Nb Laves Phase

A new Fe-Cr-Al (FCA) alloy system has been developed with good oxidation resistance and creep strength at high temperature. The alloy system is a candidate for use in future fossil-fueled power plants. The creep strength of these alloys at 973 K (700 °C) was found to be comparable with traditional 9 pct Cr ferritic–martensitic steels. A few FCA alloys with general composition of Fe-30Cr-3Al-.2Si-xNb (x = 0, 1, or 2) with a ferrite matrix and Fe2Nb-type Laves precipitates were prepared. The detailed microstructural characterization of samples, before and after creep rupture testing, indicated precipitation of the Laves phase within the matrix, Laves phase at the grain boundaries, and a 0.5 to 1.5 μm wide precipitate-free zone (PFZ) parallel to all the grain boundaries. In these alloys, the areal fraction of grain boundary Laves phase and the width of the PFZ controlled the cavitation nucleation and eventual grain boundary ductile failure. A phenomenological model was used to compare the creep strain rates controlled by the effects of the particles on the dislocations within the grain and at grain boundaries. (The research sponsored by US-DOE, Office of Fossil Energy, the Crosscutting Research Program).

Corrosion-Mechanical Behavior of T91 and 14-19Cr ODS Steels Exposed to Flowing Lead-Bismuth

This work describes the behavior of T91 (9Cr ferritic-martensitic steel), MA957 ODS (14%Cr, 0.3Mo, 0.9Ti, 0.25%Y2O3) and PM2000 ODS (19%Cr, 5%Al, 0.25%Y2O3) steels produced by mechanical alloying process, after long-term exposure to liquid lead-bismuth eutectic. Small bend specimens were pre-strained, then exposed to flowing lead-bismuth at 350°C for 100, 500 and 1000 hours. After exposure, the specimens were examined by using SEM equipped with EDX. The resulting changes of surface and built-up oxide layers are discussed. Liquid Metal Embrittlement, LME, crack initiation was not observed in the specimens. The absence of LME in these conditions is discussed.