Microstructural stability of modified 9Cr–1Mo steel during long term exposures at elevated temperatures

This paper describes the stability of microstructure in normalized and tempered modified 9Cr–1Mo steel exposed to service temperatures in the range of 773–873 K for different time durations. A detailed microstructural and microchemical analysis of the secondary phases was carried out using optical and electron microscopy techniques. The microstructural observations, supported by hardness measurements showed that the lath morphology of the tempered martensite was retained even after 10 000 h of aging. The coarsening of M₂₃C₆ carbide was observed until 5000 h, when the Laves phase started appearing. The microstructural features observed are discussed in conjunction with the embrittlement observed in this steel on high temperature aging exceeding 5000 h.     

Creep behaviour of modified 9Cr-1Mo ferritic steel

Creep deformation and fracture behaviour of indigenously developed modified 9Cr-1Mo steel for steam generator (SG) tube application has been examined at 823, 848 and 873 K. Creep tests were performed on flat creep specimens machined from normalised and tempered SG tubes at stresses ranging from 125 to 275 MPa. The stress dependence of minimum creep rate obeyed Norton’s power law. Similarly, the rupture life dependence on stress obeyed a power law. The fracture mode remained transgranular at all test conditions examined. The analysis of creep data indicated that the steel obey Monkman-Grant and modified Monkman-Grant relationships and display high creep damage tolerance factor. The tertiary creep was examined in terms of the variations of time to onset of tertiary creep with rupture life, and a recently proposed concept of time to reach Monkman-Grant ductility, and its relationship with rupture life that depends only on damage tolerance factor. SG tube steel exhibited creep-rupture strength comparable to those reported in literature and specified in the nuclear design code RCC-MR.     

Creep–fatigue interaction damage model and its application in modified 9Cr–1Mo steel

Creep–fatigue interaction damage evolution of the nuclear engineering materials modified 9Cr–1Mo steel is studied with Continuum Damage Mechanics (CDM) theory. Based on the Norton creep damage and fatigue dissipate potential theory, an effective stress controlled creep–fatigue interaction damage model has been developed in this paper, in which the creep and fatigue damage function are both considered as nonlinear variables. The damage evolution function consists of the stress amplitude and the range of mean strain. The damage parameters in the model have clear physical meaning and can be determined easily. A stress controlled creep–fatigue interaction experiment has been performed with the P91 steel to obtain the damage model. The experimental results indicated that the damage model is applicable to describe the damage evolution for creep–fatigue interaction.     

High-temperature mechanical properties improvement on modified 9Cr-1Mo martensitic steel through thermomechanical treatments

In the framework of the development of generation IV nuclear reactors and fusion nuclear reactors, materials with an improved high temperature (≅650 °C) mechanical strength are required for specific components. The 9-12%Cr martensitic steels are candidate for these applications. Thermomechanical treatments including normalisation at elevated temperature (1150 °C), followed by warm-rolling in metastable austenitic phase and tempering, have been applied on the commercial Grade 91 martensitic steel in order to refine its microstructure and to improve its precipitation state. The temperature of the warm-rolling was set at 600 °C, and those of the tempering heat-treatment at 650 °C and 700 °C thanks to MatCalc software calculations. Microstructural observations proved that the warm-rolling and the following tempering heat-treatment lead to a finer martensitic microstructure pinned with numerous small carbide and nitride particles. The hardness values of thermomechanically treated Grade 91 steel are higher than those of the as-received Grade 91. It is also shown that the yield stress and the ductility of the thermomechanically treated Grade 91 steel are significantly improved compared to the as-received material. Preliminary creep results showed that these thermomechanical treatments improve the creep lifetime by at least a factor 14.     

Creep-fatigue evaluation of normalized and tempered modified 9Cr---1Mo

Creep-fatigue is a fatal failure mode of the high temperature structural materials of liquid metal fast breeder reactors (LMFBRs). In this report, two important issues are discussed for creep-fatigue evaluation of normalized and tempered modified 9Cr---1Mo (modified 9Cr---1Mo(NT)) steel which is a promising structural material for the steam generator of large-scale LMFBRs in Japan. Several evaluation methods based on the ductility exhaustion concept are discussed for the prediction of tension strain hold creep-fatigue damage of this material. A time-fraction type of linear damage summation concept based on a new ductility exhaustion theory is proposed from the point of view of its appropriate conservatism for time extrapolation and its simplicity.Also, a life reduction mechanism of low cycle fatigue with strain hold at the compression side is discussed, based on the data observed by a scanning type electron microscope. Creep damage or the tension mean stress caused by compression strain hold hardly reduce the low cycle fatigue life of this material. A new concept based on the location of oxidation on the test specimen surface can explain the reduction in low cycle fatigue life of modified 9Cr-1Mo(NT) steel.     

Effect of sodium environment on the low cycle fatigue properties of modified 9Cr-1Mo ferritic martensitic steel

Modified 9Cr-1Mo ferritic steel is the material of current interest for the steam generator components of liquid metal cooled fast breeder reactors (LMFBRs). The steam generator has been designed to operate for 30-40 years. It is important to accurately determine the life of the components in the actual environment in order to consider the extension of life beyond the design life. With this objective in view, a programme has been initiated at our laboratory to evaluate the effects of flowing sodium on the LCF behaviour of modified 9Cr-1Mo steel. LCF tests conducted in flowing sodium environment at 823 K and 873 K exhibited cyclic softening behaviour both in air and sodium environments. The fatigue lives are significantly improved in sodium environment when compared to the data obtained in air environment under identical testing conditions. The lack of oxidation in sodium environment is considered to be responsible for the delayed crack initiation and consequent increase in fatigue life. Comparison of experimental lifetimes with RCC-MR design code predictions indicated that the design curve based on air tests is too conservative.     

Effects of an intermediate heat treatment during a cold rolling on the tensile strength of a 9Cr–2W steel

The effects of an intermediate heat treatment during a cold rolling on the tensile strength of a 9Cr–2W steel were evaluated. Before a cold rolling, the steel was normalized at 1050 °C and tempered at 550 °C in order to avoid the formation of M₂₃C₆ and V-rich MX precipitates in the martensitic structure. A 75% cold rolling and a heat treatment at 750 °C for 30 min induced the formation of large M₂₃C₆ carbides in a fully recrystallized structure. However, three cold rollings with an intermediate heat treatment at 750 °C for 10 min after each cold rolling led to the formation of fine and uniform M₂₃C₆ carbides in a partially recrystallized structure, providing an enhanced tensile strength at 650 °C. It is thus concluded that an intermediate heat treatment during a cold rolling could be an effective procedure for fabricating a high strength 9Cr–2W steel at high temperatures.     

Fatigue-creep life prediction of Cr-1Mo steel under combined tension-torsion at 600°C

The results of fatigue-creep life prediction for Cr-1Mo steel under multiaxial stress conditions of combined tension-compression and cyclic torsion are summarized. This work was performed as the second task A-II of the cooperative project of phase 3 by the Subcommittee on Inelastic Analysis and Life Prediction of High Temperature Materials, JSMS. The ratios of axial to torsional strain range, Δε/Δ(γ/3^1/2), were set to be non-unity in these phase 3 tests for both in-phase and out-of-phase (with 90° phase difference) strain-controlled wave patterns, including pure torsion, while the ratios were unity in phase 2. By comparing the evaluated failure lives with the corresponding data of phase 2, some discussion on the effect of the wave patterns is presented and the validity of the life prediction methods is evaluated.     

Corrosion characteristics of reduced activation ferritic steel, JLF-1 (8.92Cr–2W) in molten salts Flibe and Flinak

Static corrosion tests were performed in molten salts, LiF–BeF₂ (Flibe) and LiF–NaF–KF (Flinak), at 500 °C and 600 °C for 1000 h. The purpose is to investigate the corrosion characteristics of reduced activation ferritic steels, JLF-1 (8.92Cr–2W) in the fluids. The concentration of hydrogen fluoride (HF) in the fluids was measured by slurry pH titration method before and after the exposure. The HF concentration determined the fluoridation potential. The corrosion was mainly caused by dissolution of Fe and Cr into the fluids due to fluoridation and/or electrochemical corrosion. Carbon on the surface might be dissolved into the fluids due to the corrosion, and this resulted to the decrease of carbide on the surface. The corrosion depth of the JLF-1 specimen, which was obtained from the weight losses, was 0.637 μm in Flibe at 600 °C and 6.73 μm in Flinak at 600 °C.     

Influence of dynamic sodium environment on the creep-fatigue behaviour of Modified 9Cr-1Mo ferritic-martensitic steel

The use of liquid sodium as a heat transfer medium for sodium-cooled fast reactors (SFRs) necessitates a clear understanding of the effects of dynamic sodium on low cycle fatigue (LCF), creep and creep-fatigue interaction (CFI) behaviour of reactor structural materials. Mod. 9Cr-1Mo ferritic steel is the material of current interest for the steam generator components of sodium cooled fast reactors. The steam generator has a design life of 30-40 years. The effects of dynamic sodium on the LCF and CFI behaviour of Mod. 9Cr-1Mo steel have been investigated at 823 and 873 K. The CFI life of the steel showed marginal increase under flowing sodium environment when compared to air environment. Hence, the design rules for creep-fatigue interaction based on air tests can be safely applied for components operating in sodium environment. This paper attempts to explain the observed LCF and CFI results based on the detailed metallography and fractography conducted on the failed samples.     

Inelastic stress-strain response for notched specimen of Cr-1Mo steel at 600°C

Following a series of cooperative studies A-I and A-II (phase III) concerning the inelastic behaviour of high temperature materials under uniform state of stress, finite element analyses were carried out on circumferential notched cylinders subjected to plasticity-creep interaction conditions. Using an electric capacitance type extensometer “Strain-Pecker”, which is capable of measuring a local strain response with a gauge length of 0.5 mm under high temperature conditions, stress-strain responses for both global and local regions near the notch root were evaluated. Ten kinds of inelastic constitutive model were introduced into a finite element code, and the responses for four kinds of loading pattern were examined for two types of notch shape.     

Elevated-temperature tensile properties of irradiated 214 Cr-1 Mo steel

The effect of neutron irradiation on the tensile properties of normalized-and-tempered $\text{2}\text{1}\text{4}$ Cr-1 Mo steel was determined for specimens irradiated in Experimental Breeder Reactor II (EBR-II) at 390 to 550°C. Two types of unirradiated control specimens were tested: as-heat-treated specimens and as-heat-treated specimens aged for 5000 h at the irradiation temperatures. Irradiation to approximately 9 dpa at 390° C increased the strength and decreased the ductility compared to the control specimens. Softening occurred in samples irradiated and tested at temperatures of 450, 500, and 550 °C; the amount of softening increased with increasing temperature. The tensile results were explained in terms of the displacement damage caused by the irradiation and changes in carbide precipitates that occur during elevated-temperature exposure.     

Tertiary creep behavior of annealed 214 Cr-1 Mo steel

Under various creep conditions for annealed $\text{2}\text{1}\text{4}$ Cr-1 Mo steel, nonclassical creep curves that contain two steady-state stages were observed. The transition from the first to second steady-state stage involves a quasi-tertiary (increasing creep rate) stage, thus complicating the definition of tertiary creep. Tertiary creep is important because it is often associated with the formation of gross structural instability (i.e., the formation of cracks, voids, or a neck). The present studies indicated a consistent correlation between the onset of tertiary creep and rupture life was obtained when the end of the second steady-state stage was used as the onset of tertiary creep for the nonclassical curves. The creep strains to the end of the second steady-state stage were similar to those to the end of the secondary stage of the classical curves. These results along with previous work indicate that the creep rate during the second steady-state stage of the nonclassical curves is controlled by the same process that controls creep during the secondary stage of a classical curve.     

Statistical model for the random cyclic strain–life relations of 1Cr18Ni9Ti pipe-weld metal under temperature of 240°C

Modeling of random cyclic strain–life (CSL) relations of engineering material should be a basis of strain-based fatigue reliability analysis. A statistical model for the relations of a nuclear engineering material, 1Cr18Ni9Ti stainless steel pipe-weld metal under temperature of 240°C, is presented. In the model, a verified distribution, i.e. lognormal distribution, is used as an appropriate assumed distribution of the material fatigue life data. Based on the Coffin–Manson law, the relations are modeled by mean value- and standard deviation-cyclic curves of the logarithm of fatigue life. Then, fatigue analysis at an arbitrarily given probability can be made conveniently according to the normal distribution function. An approach for estimating the curves and their confidence bounds is developed by a linear regression technique. Different from the existent reliability analysis methods that considered the material constants in the law as independently random variables, present work treats them as dependently random variables from the fit of test data. Availability of the model has been indicated by an analysis of the material test data.     

Numerical and experimental investigations into life assessment of blade–disc connections of gas turbines

The positively engaged connection between blade and disc of a gas turbine is highly stressed by fatigue and creep fatigue loadings. For this purpose, a new calculating method based on inelastic finite element analyses considering the main influences on damage was developed at MPA Stuttgart. Low cycle fatigue (LCF) tests with component-like specimens have been conducted for verification. Experimental data and life assessment results based on the Smith, Watson and Topper parameters were compared well.     

High temperature oxidation of Fe-9Cr-1Mo steel in stagnant liquid lead-bismuth at several temperatures and for different lead contents in the liquid alloy

This research project deals with the feasibility studies concerning the construction of an hybrid reactor for the transmutation of long-lived radioactive wastes. In this context, the liquid lead-bismuth eutectic (LBE) is considered to be a good candidate for the spallation target material needed for the neutrons production necessary to the transmutation. In this hybrid reactor, the LBE, which is enclosed in a T91 (Fe-9%Cr) steel container, can induce corrosion concerns. If the oxygen content dissolved in Pb-Bi is higher than the needed content for magnetite formation, corrosion proceeds by oxidation of the steel. Previously, specific results were reported, obtained in stagnant liquid LBE at 470 °C. An analytical model taking into account the oxide layer structure has been carried out. It involves iron, oxygen and chromium bulk diffusion and diffusion via preferential paths such as liquid lead-bismuth nano-channels incorporated in the oxide layer structure and grain boundaries. In this paper, experimental results on corrosion kinetics, obtained at different temperatures with different percentages of lead in the lead-bismuth alloy, are presented. The model, adapted to the different experimental conditions, is compared to these kinetics and to experimental points coming from the literature at different temperatures in LBE, in pure lead and in pure bismuth.     

A knowledge based system for creep–fatigue assessment

A knowledge based system was developed in the BRITE-EURAM C-FAT project to store the material property information necessary to perform complex creep–fatigue assessments and to thereby improve the effectiveness of data retrieval for such purposes. The C-FAT KBS incorporates a multi-level database which is structured to contain not only ‘reduced’ deformation and fracture test data, but also to enable ready access to the derived parameter constants for the constitutive and model equations used in a range of assessment procedures. The data management scheme is reviewed. The C-FAT KBS also has a dynamic worked example module which allows the sensitivity of predicted lifetimes to material property input data to be evaluated by a number of procedures. Complex cycle creep–fatigue endurance predictions are particularly sensitive to the creep property data used in assessment, and this is demonstrated with reference to the results of a number of large single edge notched bend specimen feature tests performed on a 1CrMoV turbine casting steel at 550°C.     

Elevated-temperature tensile properties of irradiated 9 Cr-1 MoVNb steel

Normalized-and-tempered 9 Cr-1 MoVNb steel tensile specimens were irradiated in the Experimental Breeder Reactor-11 (EBR-11) at 390, 450, 500, and 550°C to ～2.1 and 2.5 × 10²⁶ neutrons/m² (> 0.1 MeV), which produced displacement damage levels of ～10 and 12 dpa, respectively. Tensile tests were conducted at the irradiation temperature and at room temperature. In addition to the irradiated specimens, as-heat-treated specimens and as-heat-treated specimens thermally aged at the irradiation for 5000 h were also tested.Thermal aging had no effect on the unirradiated tensile properties. Irradiation at 390°C increased the 0.2% yield stress and the ultimate tensile strength above those of the unirradiated control specimens. The ductility decreased slightly. After irradiation at 450, 500, and 550°C, the tensile properties were essentially the same as the unirradiated values. The hardening at 390°C was attributed to the dislocation and precipitate structure formed during the irradiation. The lack of hardening at 450°C and higher correlates with an absence of an irradiation-induced damage structure.     

Heat treatment effects on the tensile properties of annealed 2.25 Cr-1 Mo steel

The effect of heat treatment on the tensile properties of annealed 2.25 Cr-1 Mo steel was investigated. Detailed tensile properties were determined from 25 to 593°C and strain rates between 2.67 × 10^?6 and 6.67 × 10^?3/s on steel plates (from a single heat) given three different heat treatments. For all heat treatments, dynamic strain-aging peaks were observed between 200 and 400°C. The peak height for the annealed material that was cooled fastest was largest and occurred at the highest temperature. The dynamic strain aging was concluded to be the result of interaction solid solution hardening in the proeutectoid ferrite and involves interactions between molybdenum and carbon atoms or atom clusters with dislocations. The difference in the dynamic strain-aging effects for the different heat treatments was explained in terms of the precipitation reactions that occur during the heat treatment.     

Dispersion hardening effects of Nb-V precipitates in Mod.9Cr-1Mo steels

The mechanism of high creep strength of high nitrogen Mod.9Cr-1Mo steels was metallurgically investigated by using an analytical high resolution TEM. The threshold stress in the constituent equation,

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, is strongly dependent on dispersion strengthening due to peculiar-shaped niobium-and-vanadium-precipitates, i.e. wing-like vanadium-nitrides “V-wings” adhering to spherical niobium-carbonitrides. Key factors of the strength are size and shape of the precipitates. Increase of nitrogen addition is effective to growth of V-wings leading to large threshold stress. Calculated stresses based on a dispersion strengthening showed a good fit with experimentally-measured stresses.