| 1. |
- Liu, Fang, 1975, et al.
(författare)
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Microstructure of Z-Phase Strengthened Martensitic Steels: Meeting the 650°C Challenge
- 2017
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Ingår i: Materials Science Forum. - 1662-9752 .- 0255-5476. ; 879, s. 1147-1152
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Konferensbidrag (refereegranskat)abstract
- We studied three series of Z-phase strengthened steels using scanning electron microscopy, transmission electron microscopy, and atom probe tomography to reveal the detailed microstructure of these steels. In particular, the phase transformation from M(C,N) to Z-phase (CrMN) was studied. Carbon content in the steels is the governing factor in this transformation. The impact toughness of some test alloys was rather low. This is attributed to the formation of a continuous W-rich film along prior austenite grain boundaries. Cu and C addition to the test alloys changed Laves phase morphology to discrete precipitates and improved toughness dramatically. BN particles were found in some steels. Formation of BN is directly linked to the B concentration in the steels.
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| 2. |
- Rashidi, Masoud, 1987, et al.
(författare)
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Atom probe study of MX to Z-phase transformation in Z-phase strengthened 12%Cr steels
- 2017
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Ingår i: Microscopy and Microanalysis. - 1435-8115 .- 1431-9276. ; 23, s. 360-365
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Tidskriftsartikel (refereegranskat)abstract
- In creep resistant Z-phase strengthened 12% Cr steels, MX to Z-phase transformation plays an important role in achieving a fine distribution of Z-phase precipitates for creep strengthening. Atom probe tomography was employed to investigate the phase transformation in a Nb-based Z-phase strengthened trial steel. Using iso-concentration surfaces with different concentration values, and subtracting the matrix contribution enabled us to reveal the core-shell structure of the transient precipitates between MX and Z-phase. It was shown that Z-phase forms by diffusion of Cr into NbN upon ageing, and Z-phase has a composition corresponding to Cr1+xNb1-xN with x=0.08.
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| 3. |
- Rashidi, Masoud, 1987, et al.
(författare)
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Core-Shell Structure of Intermediate Precipitates in a Nb-Based Z-Phase Strengthened 12% Cr Steel
- 2017
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Ingår i: Microscopy and Microanalysis. - 1435-8115 .- 1431-9276. ; 23:2, s. 360-365
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Tidskriftsartikel (refereegranskat)abstract
- In creep resistant Z-phase strengthened 12% Cr steels, MX (M=Nb, Ta, or V, and X=C and/or N) to Z-phase (CrMN, M=Ta, Nb, or V) transformation plays an important role in achieving a fine distribution of Z-phase precipitates for creep strengthening. Atom probe tomography was employed to investigate the phase transformation in a Nb-based Z-phase strengthened trial steel. Using iso-concentration surfaces with different concentration values, and subtracting the matrix contribution enabled us to reveal the core-shell structure of the transient precipitates between MX and Z-phase. It was shown that Z-phase forms by diffusion of Cr into NbN upon ageing, and Z-phase has a composition corresponding to Cr1+xNb1-xN with x=0.08.
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| 4. |
- Rashidi, Masoud, 1987
(författare)
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Development of a new generation of creep resistant 12% chromium steels: Microstructure of Z-phase strengthened steels
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fossil-fuel fired steam power plants provide more than 60% of the electricity generated worldwide, and account for about one third of the global CO2 emissions. Increasing the steam temperature and pressure leads to a higher thermal efficiency of the power plants and thus lower emissions. The efficiency is limited by the long-term corrosion and creep resistance of economically viable materials used in the critical components of such a power plant, 9–12% Cr steels. Increasing the Cr content from 9% to 11–12% in the best commercially available steels provides sufficient corrosion resistance for an increase from the current maximum steam temperature of 620°C to 650°C for future power plants. However, after a few years of service, formation of coarse Z-phase (Cr(Nb, V)N) precipitates at the expense of a fine distribution of VN precipitates degrades the precipitation hardening and accordingly creep resistance of the steels.In this work a new family of 12% chromium steels is studied, where Ta or Nb is used instead of V to strengthen the steel by forming a dense distribution of Z-phase rather than VN. Z-phase does not nucleate directly as Z-phase, instead it forms through a gradual transformation of existing MX and M2N precipitates. The former leads to the formation of Z-phase with a blade-like morphology and the latter promotes large bulky Z-phase precipitates. As a result of the MX to Z-phase transformation, creep strength comparable to commercially available 9% Cr steels can be achieved. Investigation on the Z-phase precipitates based on Ta or Nb showed that Ta-based Z-phase benefits from a denser distribution and a slower coarsening rate, and thus is recommended for alloy design.Carbon is found to play the most critical role in the precipitation processes of Z-phase strengthened steels. An ultra-low C content and an optimal balance between Ta and N in a model alloy lead to the formation of a fine distribution of TaN in the as-tempered condition, which are transformed to blade-like Z-phase after short-term ageing. Such a low C content leads to very little formation of M23C6 at grain boundaries, which allows for the formation of a continuous film of Laves-phase there and a low impact toughness. Although, the addition of C results in precipitation of Ta(C, N), and hence a slower phase transformation to Z-phase, a low but not ultra-low carbon content is preferred in the new Z-phase strengthened steels.
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| 5. |
- Rashidi, Masoud, 1987, et al.
(författare)
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Microstructure and mechanical properties of two Z-phase strengthened 12%Cr martensitic steels: the effects of Cu and C
- 2017
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Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093. ; 694, s. 57-65
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Tidskriftsartikel (refereegranskat)abstract
- Z-phase strengthened 12% Cr steels are designed to combine good corrosion and creep resistance for applications in fossil fuel power plants with steam temperatures up to 650 °C. Two trial Z-phase strengthened steels were investigated, Z-steel with ultra-low C content, and ZCuC-steel with relatively high C content and Cu addition. The Z-steel has better creep strength; however, the alloy has low impact toughness due to the formation of continuous Laves-phase films at grain boundaries. Atom probe tomography, transmission electron microscopy, and scanning electron microscopy were employed to study the effects of C and Cu on the microstructure of the two steels in the as-tempered condition, and after ageing for different times. The Z-steel shows a fast transformation from TaN to Z-phase. The relatively high C content in the ZCuC-steel resulted in the formation of two categories of MX: Ta(C, N) and TaN. The phase transformation from Ta(C, N) to Z-phase is slower compared to that from TaN to Z-phase. In addition, precipitation of M23C6 and Cu particles in the ZCuC-steel led to easier nucleation of Laves-phase, and hence a much improved toughness.
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