Abstract Creep-fatigue (CF) interaction in a tempered martensitic Fe-9%Cr-based oxide dispersion strengthened (ODS) steel was studied at 650 °C by introducing hold-time of up to 30 min at peak tensile strain of 0.7%. The symmetrical loops under pure fatigue/continuous cycling (PF/CC) became asymmetrical due to stress relaxation during hold-time. Moreover, this also resulted in a reduction of cyclic life. For the investigated hold-time durations, the increase in tensile hold (TH) period had a negligible effect on peak stresses, but led to a further reduction in lifetime. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to visualize microstructural evolution under both PF/CC and TH waveforms. In general, PF/CC resulted in: (1) rearrangement and/or annihilation of dislocations, (2) partial elimination of the original sub-grain structures, (3) grain growth, (4) M23C6 carbides coarsening and (5) Cr-W enriched Laves phase precipitation. Nevertheless, upon introducing tensile hold-time, no substantial additional microstructural changes were identified. To uncover reasons for specimens premature failure under TH waveforms, detailed investigations on their surfaces, cross-sections and fracture surfaces were carried out. These investigations led to two important conclusions. Firstly, due to comparatively longer high-temperature exposure, the extent of oxidation increased upon introducing TH which expedited damage progression. Secondly, TH induced intergranular damage in the form of creep cavities does not only provide additional crack initiation sites but also their growth/coalescence under tensile stresses act as a bridging link for accelerated crack propagation. These two findings are associated with a reduction of cyclic life due to introduction of hold-time. Hence, the effect of hold-times is primarily due to (1) oxidation-fatigue interaction and (2) creep-fatigue interaction.
#1Ulrich Führer(KIT: Karlsruhe Institute of Technology)H-Index: 2
#2Jarir Aktaa(KIT: Karlsruhe Institute of Technology)H-Index: 25
Abstract Ferritic-martensitic (F/M) steels are a common choice for high-pressure, high-temperature boilers and steam piping in power plants, but cyclic softening without saturation is a major degradation mechanism of F/M steels. This degradation mechanism is more pronounced and complex under creep-fatigue loadings. For reliable, but not overly conservative design a better understanding of this phenomenon is necessary. The intent of this work is to characterize and model the softening and lifetim...
#1Ankur Chauhan(KIT: Karlsruhe Institute of Technology)H-Index: 9
#2Jan Hoffmann(KIT: Karlsruhe Institute of Technology)H-Index: 19
Last. Jarir Aktaa(KIT: Karlsruhe Institute of Technology)H-Index: 25
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Abstract Low cycle fatigue (LCF) behavior of a tempered martensitic Fe-9%Cr-based oxide dispersion strengthened (ODS) steel at elevated temperatures is correlated to the microstructural evolution after cyclic straining. The fully reversed strain-controlled tests were conducted in air at 550 °C and 650 °C for different strain amplitude values ranging from ± 0.4% to ± 0.9%. Apart from the higher cyclic stress levels, the steel manifests complex cyclic softening which is significantly lower in comp...
Last. A. Ulbricht(HZDR: Helmholtz-Zentrum Dresden-Rossendorf)H-Index: 19
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Abstract The collaborative study is focused on the relationship between microstructure and yield stress for an ODS Fe-9%Cr-based transformable alloy and an ODS Fe-14%Cr-based ferritic alloy. The contributions to the total room temperature yield stress arising from various strengthening mechanisms are addressed on the basis of a comprehensive description of the microstructures uncovered by means of transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), small-angle neutro...
#1Ankur Chauhan(KIT: Karlsruhe Institute of Technology)H-Index: 9
#2Luis Straßberger(KIT: Karlsruhe Institute of Technology)H-Index: 4
Last. Jarir Aktaa(KIT: Karlsruhe Institute of Technology)H-Index: 25
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Abstract The present study reports on the creep-fatigue (CF) behavior of a ferritic bimodal 12Cr-ODS steel. The tests were performed in air at 550 °C and 650 °C by introducing hold-time at peak tensile, peak compressive and both peak tensile as well as compressive strains of ±0.5%. The symmetrical loops under pure fatigue/continuous cycling (CC) became asymmetrical upon introducing hold-time at either peak tensile or peak compressive strain. On the contrary, hold-time on both sides had no influe...
#1Ankur Chauhan(KIT: Karlsruhe Institute of Technology)H-Index: 9
#2Mario Walter(KIT: Karlsruhe Institute of Technology)H-Index: 10
Last. Jarir Aktaa(KIT: Karlsruhe Institute of Technology)H-Index: 25
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Within the frame of this work, the mechanical behaviour of a bimodal ferritic 12Cr-ODS steel as well as of a ferritic-martensitic 9Cr-ODS steel under alternating load conditions was investigated. In general, strain-controlled low-cycle fatigue tests at 550°C and 650°C revealed similar cyclic stress response. At elevated temperatures, the two steels manifest transitional stages, ie, cyclic softening and/or hardening corresponding to the small fraction of the cyclic life, which is followed by a li...
#1Ankur Chauhan(KIT: Karlsruhe Institute of Technology)H-Index: 9
#2Dimitri Litvinov(KIT: Karlsruhe Institute of Technology)H-Index: 22
Last. Jarir Aktaa(KIT: Karlsruhe Institute of Technology)H-Index: 25
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Abstract In this study, the tensile properties of a tempered martensitic 9Cr oxide dispersion strengthened (ODS) steel are investigated. The tensile tests were performed in the temperature range of 25 °C to 800 °C at a nominal strain rate of 10 −3 s −1 . At room temperature, the material exhibited high yield strength and ultimate tensile strength of 929 and 1052 MPa, respectively. A decrease in strength was observed with increase in temperature down up to 156 MPa of ultimate tensile strength at ...
Abstract Three high Cr steels prepared by powder metallurgy and strengthened by fine oxide particles were subjected to low cycle fatigue testing at room temperature and microstructural characterisation. While the strength of the three materials is roughly comparable, the strong variance in the fatigue life of studied materials was measured. The reasons for this difference are discussed in terms of microstructure and differences in the crack nucleation mechanisms and early crack growth.
Last. Tomáš Kruml(CAS: Academy of Sciences of the Czech Republic)H-Index: 22
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Abstract Low cycle fatigue properties at room temperature, 650 °C and 750 °C of three high chromium steels (9%Cr ferritic–martensitic and two 14%Cr ferritic steels) strengthened by oxide dispersion were studied and compared. Cyclic softening/hardening curves, cyclic deformation curves, S–N curves and Coffin–Manson curves are presented together with microstructural observations. Differences in cyclic response, stress level and fatigue life are attributed to differences in the matrix microstructur...
Last. André Pineau(CNRS: Centre national de la recherche scientifique)H-Index: 66
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Cyclic tests with or without tensile holding periods were conducted in air at 823 K on a modified 9Cr–1Mo martensitic steel. In addition to stress–relaxation fatigue (RF) tests with a hold time at maximum load, creep–fatigue (CF) experiments were carried out. These CF tests were strain-controlled during the cyclic part of the stress–strain hysteresis loop and then load controlled when the stress was maintained at its maximum value to produce a prescribed value of the creep strain before cyclic d...
#1Fujio Abe(National Institute for Materials Science)H-Index: 52
AbstractIt is crucial for the carbon concentration of 9% Cr steel to be reduced to a very low level, so as to promote the formation of MX nitrides rich in vanadium as very fine and thermally stable particles to enable prolonged periods of exposure at elevated temperatures and also to eliminate Cr-rich carbides M23C6. Sub-boundary hardening, which is inversely proportional to the width of laths and blocks, is shown to be the most important strengthening mechanism for creep and is enhanced by the ...
Last. Peter K. Liaw(UT: University of Tennessee)H-Index: 94
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Abstract High total strain-controlled creep-fatigue tests of 1.8% were performed for a directionally-solidified Ni-based superalloy, DZ445, at 900 °C in air. Dwell times of 0, 2, and 5 min. were introduced at the tensile peak strain for each cycle. Experimental results show a decrease in fatigue life with increasing dwell time. The significant cyclic softening is observed after the application of the dwell time. The mean compressive stress and large plastic-strain accumulation are produced due t...
Last. Jan Hoffmann(KIT: Karlsruhe Institute of Technology)H-Index: 19
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Abstract A major part in the EUROfusion materials research program is dedicated to characterize and quantify nuclear fusion specific neutron damage in structural materials. While the majority of irradiation data gives a relatively clear view on the displacement damage, the effect of transmutation – i.e. especially hydrogen and helium production in steels – is not yet explored very well. However, few available results indicate that EUROFER-type steels will reach their operating limit as soon as t...
Abstract A numerical process based on crystal plasticity finite element (CPFE) was implemented to predict creep-fatigue crack initiation life. CPFE-based model can describe the macroscopic cyclic deformation and reveal grain-level damage mechanism. A new life prediction approach was then constructed by introducing fatigue and creep indicator parameters. Furthermore, a series of strain-controlled creep-fatigue tests in GH4169 superalloy at 650℃ were used to validate predicted accuracy of this mod...
Abstract Dual hardening steels reach their well-balanced mechanical properties in terms of strength and toughness through the combination of secondary hardening carbide and intermetallic particle precipitation. This characteristic profile makes them well suited for hot-work applications. In this study, out-of-phase thermomechanical fatigue tests, recreating operating conditions present during hot-work applications, were performed on a dual hardening steel and a 5% Cr martensitic hot-work tool st...
#1Kaiju Lu(KIT: Karlsruhe Institute of Technology)H-Index: 2
#2Ankur Chauhan(KIT: Karlsruhe Institute of Technology)H-Index: 9
Last. Jarir Aktaa(KIT: Karlsruhe Institute of Technology)H-Index: 25
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Abstract In the present work, low cycle fatigue (LCF) behavior of an equiatomic CoCrFeMnNi high entropy alloy (HEA) is correlated to the microstructural evolution at 550 °C. The fully reversed strain-controlled fatigue tests were conducted in air under strain amplitudes ranging from 0.2 to 0.8%. The measured cyclic stress response showed three distinct stages which include initial cyclic hardening followed by a quasi-stable cyclic response until failure. The rate and amount of cyclic hardening i...
Last. Zhengxin Tang(TJU: Tianjin University)H-Index: 6
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Abstract Cyclic deformation mechanism of Sanicro 25 alloy was investigated under different strain rates ( e ˙ ) at 700 °C. When e ˙ increased, the maximum stress amplitude σ a decreased and the fatigue life N f increased. The saturated hysteresis energy Δ W p first decreased and then increased with increasing e ˙ . It is attributed to the variation of stress amplitude, which is related to the interaction of precipitates and dislocations. HAADF STEM analysis shows that the Cr23C6 precipitates wer...
Abstract Predicting the remnant creep fracture life precisely is crucial for ensuring safety of high temperature components. This study presents a microstructural damage mechanics-based model for creep fracture of 9%Cr steel under prior fatigue loading. Microstructure observation reveals that the decrease of dislocation density and the growth of martensite lath width occurred during prior fatigue process contribute to the degradation of creep strength. Particularly, coarsening of martensite lath...
Abstract The non-Masing behavior of Sanicro 25 alloy was studied during high-temperature low-cycle fatigue (700 °C). The results showed that Sanicro 25 alloy exhibits the non-Masing behavior when total strain amplitude ( e a ) increased. The analysis of stable hysteresis loops showed that the stress amplitude ( σ a ) increased with increasing e a . At the stable cyclic stage, the rate of increase in σ a with number of cycles increased when e a increased. Based on the EBSD observation, the Geomet...
Abstract The degradation of tensile and creep properties is inevitable during high temperature service operation. Hence this work aims to evaluate the effect of prior creep-fatigue interaction damages on remnant tensile and creep properties of 9%Cr steel. Prior creep-fatigue tests interrupted at different lifetime fractions and different tensile hold times are performed at 650 °C. Afterwards, subsequent tensile and creep tests are conducted at the same temperature. Results reveal that high lifet...