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  • 1.
    He, Junjing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    High temperature performance of materials for future power plants2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    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.

  • 2.
    He, Junjing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Basic modelling of creep rupture in austenitic stainless steels2017In: Theoretical and applied fracture mechanics (Print), ISSN 0167-8442, E-ISSN 1872-7638, Vol. 89, p. 139-146Article in journal (Refereed)
    Abstract [en]

    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.

  • 3.
    He, Junjing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Brittle rupture of austenitic stainless steels due to creep cavitation2016In: 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21), Elsevier, 2016, p. 863-870Conference paper (Refereed)
    Abstract [en]

    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.

  • 4.
    He, Junjing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Creep cavity growth models for austenitic stainless steels2016In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 674, p. 328-334Article in journal (Refereed)
    Abstract [en]

    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.

  • 5.
    He, Junjing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Formation of creep cavities in austenitic stainless steels2016In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 51, no 14, p. 6674-6685Article in journal (Refereed)
    Abstract [en]

    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

  • 6.
    He, Junjing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Modelling grain boundary sliding during creep of austenitic stainless steels2016In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 51, no 6, p. 2926-2934Article in journal (Refereed)
    Abstract [en]

    Two models are presented for grain boundary sliding (GBS) displacement during creep. GBS is considered as crucial for the formation of creep cavities. In the first model, the shear sliding model, GBS is accommodated by grains freely sliding along the boundaries in a power-law creeping material. The GBS rate is proportional to the grain size. In the second model, the shear crack model, the sliding boundaries are represented by shear cracks. The GBS rate is controlled by particles in the boundaries. In both models, the GBS displacement rate is proportional to the creep strain rate. Both models are consistent with existing experimental observations for GBS during creep of austenitic stainless steels. For cavity nucleation at particles, Harris’ model (1965) for the relationship between GBS and a critical particle size has been analysed and found to be in agreement with observations.

  • 7.
    He, Junjing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. KTH, School of Industrial Engineering and Management (ITM), Centres, Brinell Centre - Inorganic Interfacial Engineering, BRIIE.
    Notargiacomo, Sandro
    Low-Cycle Fatigue Properties of a Nickel-Based Superalloy Haynes 282 for Heavy Components2017In: Journal of materials engineering and performance (Print), ISSN 1059-9495, E-ISSN 1544-1024, Vol. 26, no 5, p. 2257-2263Article in journal (Refereed)
    Abstract [en]

    Low-cycle fatigue (LCF) tests of the nickel-based superalloy Haynes 282 from a large forged ingot were conducted at 25 and 750 degrees C with total strain ranges from 0.7 to 1.7%. Compared with other tests on this alloy, it was found that the LCF properties showed similar results at room temperature, but improved number of cycles to failure at high temperatures. The number of cycles at a given total strain range showed no large differences between the core and rim positions. By comparing with two other types of low gamma' volume fraction nickel-based superalloys, Haynes 282 gave the best LCF properties at high temperatures. The reason may be due to the dominating transgranular fracture in the current work. A mixture of intergranular and transgranular fractures had been observed in the other alloys. The results demonstrate that heavy components of Haynes 282 can be produced with good LCF properties.

  • 8.
    He, Junjing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Vujic, Stojan
    Creep, low cycle fatigue and creep-fatigue properties of a modified HR3C2016In: 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21), Elsevier, 2016, p. 871-878Conference paper (Refereed)
    Abstract [en]

    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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