| 1. |
- He, Junjing, et al.
(author)
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Application of Fundamental Models for Creep Rupture Prediction of Sanicro 25 (23Cr25NiWCoCu)
- 2019
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In: Crystals. - : MDPI. - 2073-4352. ; 9:12
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Journal article (peer-reviewed)abstract
- Creep rupture prediction is always a critical matter for materials serving at high temperatures and stresses for a long time. Empirical models are frequently used to describe creep rupture, but the parameters of the empirical models do not have any physical meanings, and the model cannot reveal the controlling mechanisms during creep rupture. Fundamental models have been proposed where no fitting parameters are involved. Both for ductile and brittle creep rupture, fundamental creep models have been used for the austenitic stainless steel Sanicro 25 (23Cr25NiWCoCu). For ductile creep rupture, the dislocation contribution, solid solution hardening, precipitation hardening, and splitting of dislocations were considered. For brittle creep rupture, creep cavitation models were used taking grain boundary sliding, formation, and growth of creep cavities into account. All parameters in the models have been well defined and no fitting is involved. MatCalc was used for the calculation of the evolution of precipitates. Some physical parameters were obtained with first-principles methods. By combining the ductile and brittle creep rupture models, the final creep rupture prediction was made for Sanicro 25. The modeling results can predict the experiments at long-term creep exposure times in a reasonable way.
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| 2. |
- He, Junjing, et al.
(author)
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Application of soft constrained machine learning algorithms for creep rupture prediction of an austenitic heat resistant steel Sanicro 25
- 2023
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In: JOURNAL OF MATERIALS RESEARCH AND TECHNOLOGY-JMR&T. - : Elsevier BV. - 2238-7854. ; 22, s. 923-937
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Journal article (peer-reviewed)abstract
- Creep rupture extrapolation is crucial for high-temperature materials served in power plants. Many analytical models can be used for creep rupture analysis, and fundamental models are also available. Machine learning is also an alternative. However, unphysical prediction curves occur readily in common machine learning algorithms, where one must manipulate the best results or ignore the less satisfactory ones. Using just high regression coefficients and low errors is not enough to obtain high accuracy of the methods. Never-theless, five soft constrained machine learning algorithms (SCMLAs), where soft con-straints, stability analysis by culling long-time or low-stress data, extrapolation from short to long times, and errors of solutions and algorithms are considered, are used for creep rupture prediction in this work. The models can generate reasonable results for fitting all data, extrapolating from short to long times, and stability analysis for Sanicro 25 after a number of tests. The errors of solutions for all the analyses are in a quite reasonable range, including extrapolation and stability analysis. The average relative standard deviation of the five SCMLAs is less than 2.5% at three times the maximum experimental creep rupture time. Creep rupture strength of the austenitic stainless steel Sanicro 25 can be predicted quantitatively by taking the average predicted stresses of the five SCMLAs. The method can also be used for other high-temperature alloys with similar creep degradation mechanisms.
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| 3. |
- He, Junjing, et al.
(author)
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Basic modelling of creep rupture in austenitic stainless steels
- 2017
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In: Theoretical and applied fracture mechanics (Print). - : Elsevier BV. - 0167-8442 .- 1872-7638. ; 89, s. 139-146
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Journal article (peer-reviewed)abstract
- Creep rupture can happen in two ways, brittle and ductile creep rupture. Brittle creep rupture of austenitic stainless steels proceeds with the nucleation, growth and coalescence of grain boundary cavities. A creep cavity nucleation model has been developed previously, which considers cavity nucleation at particles and sub-boundary corners due to grain boundary sliding. A modified constrained cavity growth model has been used to describe the cavity growth behavior with combination of the cavity nucleation models. In this paper, the brittle creep rupture has been analyzed by combining the cavity nucleation and growth models. The physically based models where no adjustable parameters were involved were used to predict the brittle creep rupture strength. On the other hand, previously developed basic models for ductile creep rupture based on exhaustion of the creep ductility have been used. The creep rupture strength of common austenitic stainless steels has been predicted quantitatively by taking both ductile and brittle rupture into account. The predicted rupture times for ductile rupture are longer than those for brittle rupture at high stresses and low temperatures with a reversed situation at low stresses and high temperatures. This reproduces the characteristic change in slope in the creep rupture curves.
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| 4. |
- He, Junjing, et al.
(author)
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Brittle rupture of austenitic stainless steels due to creep cavitation
- 2016
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In: 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21). - : Elsevier. ; , s. 863-870
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Conference paper (peer-reviewed)abstract
- Basic creep cavitation models have been used to predict brittle rupture of austenitic stainless steels. It involves the grain boundary sliding models, which is the basis of the creep cavitation models, the recently developed cavity formation models and the constrained cavity growth models. The individual creep cavitation models are verified with experimental observations. Brittle rupture due to creep cavitation that appears as intergranular failure is found to be dominant at high temperatures and long creep exposure times.
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| 5. |
- He, Junjing, et al.
(author)
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Creep cavity growth models for austenitic stainless steels
- 2016
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In: Materials Science & Engineering. - : Elsevier. - 0921-5093 .- 1873-4936. ; 674, s. 328-334
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Journal article (peer-reviewed)abstract
- Diffusion controlled cavity growth models tend to exaggerate the growth rate. For this reason it is essential to take into account the restrictions caused by creep rate of the surrounding material, so called constrained growth. This has the consequence that the stress that the cavities are exposed to is reduced in comparison to the applied creep stress. Previous constrained growth models have been based on linear viscoplasticity. To avoid this limitation a new model for constrained growth has been formulated. Part of the work is based on a FEM study of expanding cavities in a creeping material. Compared with the previous constrained cavity growth models, the modified one gives lower reduced stresses and thereby lower cavity growth rates. By using recently developed cavity nucleation models, the modified creep cavity growth model can predict the cavity growth behaviour quantitatively for different types of austenitic stainless steels, such as 18Cr10Ni, 17Cr12NiNb and 17Cr12NiTi.
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| 6. |
- He, Junjing, et al.
(author)
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Creep, low cycle fatigue and creep-fatigue properties of a modified HR3C
- 2016
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In: 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21). - : Elsevier. ; , s. 871-878
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Conference paper (peer-reviewed)abstract
- Creep, low cycle fatigue (LCF) and creep fatigue tests have been conducted for modified HR3C (25Cr20NiNbN) at high temperatures ranging of 650-750 degrees C. Both LCF and creep fatigue test results could be described with the Coffin-Manson relationship. The number of cycles to failure in the creep fatigue tests was more than one order of magnitude lower compared with LCF. The effect of the total hold time in tension (the total creep time) was compared to creep rupture data. The creep fatigue results were in reasonable agreement with the creep tests. The short creep fatigue lives may be due to the low creep ductility which was found in the creep tests. Fractography showed that the rupture mode was intergranular. Cavities were observed at grain boundaries due to the fracture of the primary Z phase particles in both LCF and creep fatigue tests. In comparison to Sanicro 25, the modified HR3C showed better LCF properties.
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| 7. |
- He, Junjing, et al.
(author)
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Evaluating creep rupture life in austenitic and martensitic steels with soft-constrained machine learning
- 2023
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In: Journal of Materials Research and Technology. - : Elsevier BV. - 2238-7854. ; 27, s. 5165-5176
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Journal article (peer-reviewed)abstract
- Machine learning is extensively utilized for predicting creep rupture of high-temperature steels. Recently, five soft-constrained machine learning algorithms (SCMLAs) have been developed to enhance the extrapolation capabilities of machine learning. These SCMLAs were applied to the austenitic steel Sanicro 25, showing their potential. To improve SCMLAs, this study has introduced new guidelines that address temperature culling within the input range and temperature extrapolation beyond the input range. Leveraging these guidelines, the SCMLAs were extended to various austenitic and martensitic stainless steels. The predicted results of TP316H, the data of which is representative of austenitic stainless steels, were validated through error estimates. Furthermore, notable agreement has been reached for temperature culling and temperature extrapolation, as demonstrated for TP91 and TP92 martensitic steels. The effects of single casts and the temperature dependence of the predictions have been analyzed for the studied materials. Consistent results can be readily achieved through systematic evaluations of SCMLAs for extrapolating up to 300,000 h or three times the maximum experimental rupture time for the studied materials. It is demonstrated that SCMLAs can provide reliable creep rupture life prediction across various high-temperature materials.
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| 8. |
- He, Junjing, et al.
(author)
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Formation of creep cavities in austenitic stainless steels
- 2016
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In: Journal of Materials Science. - : Springer Science+Business Media B.V.. - 0022-2461 .- 1573-4803. ; 51:14, s. 6674-6685
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Journal article (peer-reviewed)abstract
- The possibility of creep cavity formation at subboundaries in austenitic stainless steels is analysed. It is demonstrated that such nucleation is thermodynamically feasible. A minimum stress must be exceeded in order to create cavities. The nucleation is assumed to take place where subboundaries on one side of a sliding grain boundary meet subgrain corners on the other side (double ledge models). Alternative cavitation positions can be found where particles meet subboundaries. The nucleation model can quantitatively predict the observed nucleation rate. The model gives a nucleation rate that is proportional to the creep rate in agreement with many experiments
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| 9. |
- He, Junjing, et al.
(author)
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Growth of creep cavities in austenitic stainless steels
- 2015
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In: 8th European Stainless Steel and Duplex Stainless Steel Conference 2015. - : Austrian Society for Metallurgy and Materials (ASMET). ; , s. 529-538
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Conference paper (peer-reviewed)abstract
- Creep rupture of austenitic stainless steels proceeds with the nucleation and growth of grain boundary cavities. A creep cavity nucleation model has been developed, which considers cavity nucleation at particles and subboundaries due to grain boundary sliding. A constrained cavity growth model has been used to describe the cavity growth behaviour. The models can reproduce available observations in an acceptable way both for cavity nucleation and growth.No adjustable parameters are involved. By combining the cavity nucleation and growth model, creep brittle rupture can be analysed.Taking the contributions from dislocation creep and cavitation into account, creep rupture strength can be predicted. The modelling results show good agreement with published creep strength data for 17Cr-12Ni-Mo (316H).
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| 10. |
- He, Junjing
(author)
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High temperature performance of materials for future power plants
- 2016
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Doctoral thesis (other academic/artistic)abstract
- Increasing energy demand leads to two crucial problems for the whole society. One is the economic cost and the other is the pollution of the environment, especially CO2 emissions. Despite efforts to adopt renewable energy sources, fossil fuels will continue to dominate. The temperature and stress are planned to be raised to 700 °C and 35 MPa respectively in the advanced ultra-supercritical (AUSC) power plants to improve the operating efficiency. However, the life of the components is limited by the properties of the materials. The aim of this thesis is to investigate the high temperature properties of materials used for future power plants.This thesis contains two parts. The first part is about developing creep rupture models for austenitic stainless steels. Grain boundary sliding (GBS) models have been proposed that can predict experimental results. Creep cavities are assumed to be generated at intersection of subboundaries with subboundary corners or particles on a sliding grain boundary, the so called double ledge model. For the first time a quantitative prediction of cavity nucleation for different types of commercial austenitic stainless steels has been made. For growth of creep cavities a new model for the interaction between the shape change of cavities and creep deformation has been proposed. In this constrained growth model, the affected zone around the cavities has been calculated with the help of FEM simulation. The new growth model can reproduce experimental cavity growth behavior quantitatively for different kinds of austenitic stainless steels. Based on the cavity nucleation models and the new growth models, the brittle creep rupture of austenitic stainless steels has been determined. By combing the brittle creep rupture with the ductile creep rupture models, the creep rupture strength of austenitic stainless steels has been predicted quantitatively. The accuracy of the creep rupture prediction can be improved significantly with combination of the two models.The second part of the thesis is on the fatigue properties of austenitic stainless steels and nickel based superalloys. Firstly, creep, low cycle fatigue (LCF) and creep-fatigue tests have been conducted for a modified HR3C (25Cr20NiNbN) austenitic stainless steel. The modified HR3C shows good LCF properties, but lower creep and creep-fatigue properties which may due to the low ductility of the material. Secondly, LCF properties of a nickel based superalloy Haynes 282 have been studied. Tests have been performed for a large ingot. The LCF properties of the core and rim positions did not show evident differences. Better LCF properties were observed when compared with two other low γ’ volume fraction nickel based superalloys. Metallography study results demonstrated that the failure mode of the material was transgranular. Both the initiation and growth of the fatigue cracks were transgranular.
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