Microstructural based creep life assessment of 1CrMov Turbine rotor steels after long-term service
- Nyembe, Hlanganani Siphelele
- Authors: Nyembe, Hlanganani Siphelele
- Date: 2023-04
- Subjects: Materials – Creep –South Africa , Steel -- Creep
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10948/61380 , vital:70622
- Description: Creep-resistant 1CrMoV steels are used for high-pressure turbine rotors in fossil fuel power stations operating at steam temperatures up to 565 °C in the creep regime. The upper bainitic microstructure of 1CrMoV rotors changes when exposed to high temperature service over long periods. The aim of this study was to relate these microstructural changes for service exposed 1CrMoV rotor steels to the remaining creep life as determined using existing methods. Analytical electron microscopy was used to quantitatively study the microstructural evolution of service-exposed 1CrMoV steel rotors. 1CrMoV rotor steel samples were characterised in the initial state, and three long-term (272 h to 300 kh) service exposed states. Detailed microstructural investigations were conducted to analyse the cavities; bainitic laths and packets; dislocations; carbide phases and their size, population, composition. The creep life fractions were estimated using conventional life assessment methods based on creep cavitation and hardness. The creep life fraction consumed of the rotors, estimated based on the creep cavities and hardness data, ranged from ~0.5 to 1. Electron backscattered diffraction (EBSD) was used to analyse the bainitic packets, laths, misorientations, and dislocations, which showed that the bainitic lath surface area to volume (SV) correlated with service time. The precipitates were extracted from the Fe-matrix using extraction replication and were further investigated using transmission Kikuchi diffraction (TKD), scanning transmission electron microscopy (STEM), selected area electron diffraction (SAED), and energy dispersive spectroscopy (EDS). STEM-EDS and TKD identified M3C, MC, M2C, M7C3, and M23C6 (M = Fe, Cr, Mo, V, Mn) in the 1CrMoV steels. STEM-EDS measurements revealed that Fe/Cr ratios in M3C decreased from 4.5 (initial state) to 2.5 (>272 kh). The Fe/Cr and Fe/Mo ratios in overall carbides decreased from 5 and 10 (initial state) to 1 and 2 (>272 kh), respectively. The TKD analysis indicated that the relative phase proportion of M2C and M7C3 carbides, relative to the total extracted carbides, is positively correlated with service exposure. These microstructural features correlate with the creep life fraction consumed and could potentially be used as another indicator of the remnant creep life. , Thesis (PhD) -- Faculty of Science, School of Computer Science, Mathematics, Physics and Statistics. 2023
- Full Text:
- Date Issued: 2023-04
- Authors: Nyembe, Hlanganani Siphelele
- Date: 2023-04
- Subjects: Materials – Creep –South Africa , Steel -- Creep
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10948/61380 , vital:70622
- Description: Creep-resistant 1CrMoV steels are used for high-pressure turbine rotors in fossil fuel power stations operating at steam temperatures up to 565 °C in the creep regime. The upper bainitic microstructure of 1CrMoV rotors changes when exposed to high temperature service over long periods. The aim of this study was to relate these microstructural changes for service exposed 1CrMoV rotor steels to the remaining creep life as determined using existing methods. Analytical electron microscopy was used to quantitatively study the microstructural evolution of service-exposed 1CrMoV steel rotors. 1CrMoV rotor steel samples were characterised in the initial state, and three long-term (272 h to 300 kh) service exposed states. Detailed microstructural investigations were conducted to analyse the cavities; bainitic laths and packets; dislocations; carbide phases and their size, population, composition. The creep life fractions were estimated using conventional life assessment methods based on creep cavitation and hardness. The creep life fraction consumed of the rotors, estimated based on the creep cavities and hardness data, ranged from ~0.5 to 1. Electron backscattered diffraction (EBSD) was used to analyse the bainitic packets, laths, misorientations, and dislocations, which showed that the bainitic lath surface area to volume (SV) correlated with service time. The precipitates were extracted from the Fe-matrix using extraction replication and were further investigated using transmission Kikuchi diffraction (TKD), scanning transmission electron microscopy (STEM), selected area electron diffraction (SAED), and energy dispersive spectroscopy (EDS). STEM-EDS and TKD identified M3C, MC, M2C, M7C3, and M23C6 (M = Fe, Cr, Mo, V, Mn) in the 1CrMoV steels. STEM-EDS measurements revealed that Fe/Cr ratios in M3C decreased from 4.5 (initial state) to 2.5 (>272 kh). The Fe/Cr and Fe/Mo ratios in overall carbides decreased from 5 and 10 (initial state) to 1 and 2 (>272 kh), respectively. The TKD analysis indicated that the relative phase proportion of M2C and M7C3 carbides, relative to the total extracted carbides, is positively correlated with service exposure. These microstructural features correlate with the creep life fraction consumed and could potentially be used as another indicator of the remnant creep life. , Thesis (PhD) -- Faculty of Science, School of Computer Science, Mathematics, Physics and Statistics. 2023
- Full Text:
- Date Issued: 2023-04
Corrosion resistance of nanocrystalline titanium zirconium nitride layers deposited on ZIRLO
- Nyembe, Hlanganani Siphelele
- Authors: Nyembe, Hlanganani Siphelele
- Date: 2018
- Subjects: Zirconium alloys , Titanium alloys Nanocrystals Nanotechnology
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10948/34744 , vital:33435
- Description: Zirconium (Zr) alloys are used as nuclear fuel cladding in water-cooled nuclear reactors. Being in contact with steam, the Zr-alloy is oxidised and it absorbs hydrogen. The hydrogen pickup in Zr-alloys induces embrittlement of the tubes due to hydride formation. In this study, effectiveness of corrosion resistant nanocrystalline titanium zirconium nitride (nc-TiZrN) coatings on the surface of a Zr-alloy for the reduction of corrosion and hydrogen pickup is investigated. The nc-TiZrN layers were deposited onto the surface of ZIRLO® using the cathodic arc vapour deposition (CAVD) technique. The influence of nc-TiZrN coatings on the corrosion performance of ZIRLO® was investigated under the following conditions: (a) In an oxygen atmosphere at various temperatures from 400 to 900 ℃ for 30 minutes, (b) for various exposure times from 6 – 24 hours with the temperature kept constant at 500 ℃ in a tube furnace, and (c) in pure steam for 30 days at 360 ℃ and 18.7 MPa in an autoclave. The corrosion properties of the nc-TiZrN coated ZIRLO were compared to uncoated ZIRLO for all experiments. Following corrosion testing, the TiZrN coatings and oxide layers were investigated by scanning and transmission electron microscopy and energy dispersive X-ray spectroscopy. The TiZrN coating had a thickness of ~3.8 μm with a small number of Ti-rich and Nb-rich inclusions. The coating exhibited a single phase solid solution of Ti0.42Zr0.58N with a NaCl-type crystal structure and columnar nanocrystals. These columnar nanocrystals showed a strong {111} preferred orientation and the {111} planes of the TiZrN crystals are oriented parallel to the (001) planes of α-ZIRLO grains with <001> direction perpendicular to the coating/ZIRLO interface. The corrosion testing at various temperatures from 400 to 900 ℃ for 30 minutes showed that the TiZrN coating oxidises slowly and retained its microstructure, composition and crystallinity up to 600 ℃. At higher temperatures above 600 ℃, the TiZrN layer completely oxidised to a non-protective TiZr-oxide layer and resulted in rapid corrosion of the underlying ZIRLO substrate. The rapid corrosion was not observed in the uncoated ZIRLO sample for all temperatures. The results of samples subjected to annealing treatment at 500 ℃ in an oxygen atmosphere at various times up to 24 hours revealed that the TiZrN layer oxidises faster than ZIRLO. This was confirmed by sub-cubic and sub-parabolic oxide growth kinetics observed in the uncoated ZIRLO which are slower than the parabolic kinetics of TiZrN coated samples. In all coated samples, the spallation was observed near the edges of the samples. The spallation was caused by rapid corrosion of the underlying ZIRLO substrate which resulted from the penetration of oxygen ions between the TiZrN coating and ZIRLO substrate along the edges of the samples. In pure steam for 30 days at 360 ℃, a complete loss of the TiZrN layer occurred but the results show that the coating might have delayed the corrosion of ZIRLO. In summary, corrosion resistance of nc-TiZrN coated ZIRLO was lower than that of uncoated ZIRLO. The study shows that the failure of TiZrN coatings leads to rapid corrosion of the underlying ZIRLO substrate which was not observed on the uncoated ZIRLO. It is not yet understood why the presence TiZrN coatings reduced the oxidation resistance of ZIRLO.
- Full Text:
- Date Issued: 2018
- Authors: Nyembe, Hlanganani Siphelele
- Date: 2018
- Subjects: Zirconium alloys , Titanium alloys Nanocrystals Nanotechnology
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10948/34744 , vital:33435
- Description: Zirconium (Zr) alloys are used as nuclear fuel cladding in water-cooled nuclear reactors. Being in contact with steam, the Zr-alloy is oxidised and it absorbs hydrogen. The hydrogen pickup in Zr-alloys induces embrittlement of the tubes due to hydride formation. In this study, effectiveness of corrosion resistant nanocrystalline titanium zirconium nitride (nc-TiZrN) coatings on the surface of a Zr-alloy for the reduction of corrosion and hydrogen pickup is investigated. The nc-TiZrN layers were deposited onto the surface of ZIRLO® using the cathodic arc vapour deposition (CAVD) technique. The influence of nc-TiZrN coatings on the corrosion performance of ZIRLO® was investigated under the following conditions: (a) In an oxygen atmosphere at various temperatures from 400 to 900 ℃ for 30 minutes, (b) for various exposure times from 6 – 24 hours with the temperature kept constant at 500 ℃ in a tube furnace, and (c) in pure steam for 30 days at 360 ℃ and 18.7 MPa in an autoclave. The corrosion properties of the nc-TiZrN coated ZIRLO were compared to uncoated ZIRLO for all experiments. Following corrosion testing, the TiZrN coatings and oxide layers were investigated by scanning and transmission electron microscopy and energy dispersive X-ray spectroscopy. The TiZrN coating had a thickness of ~3.8 μm with a small number of Ti-rich and Nb-rich inclusions. The coating exhibited a single phase solid solution of Ti0.42Zr0.58N with a NaCl-type crystal structure and columnar nanocrystals. These columnar nanocrystals showed a strong {111} preferred orientation and the {111} planes of the TiZrN crystals are oriented parallel to the (001) planes of α-ZIRLO grains with <001> direction perpendicular to the coating/ZIRLO interface. The corrosion testing at various temperatures from 400 to 900 ℃ for 30 minutes showed that the TiZrN coating oxidises slowly and retained its microstructure, composition and crystallinity up to 600 ℃. At higher temperatures above 600 ℃, the TiZrN layer completely oxidised to a non-protective TiZr-oxide layer and resulted in rapid corrosion of the underlying ZIRLO substrate. The rapid corrosion was not observed in the uncoated ZIRLO sample for all temperatures. The results of samples subjected to annealing treatment at 500 ℃ in an oxygen atmosphere at various times up to 24 hours revealed that the TiZrN layer oxidises faster than ZIRLO. This was confirmed by sub-cubic and sub-parabolic oxide growth kinetics observed in the uncoated ZIRLO which are slower than the parabolic kinetics of TiZrN coated samples. In all coated samples, the spallation was observed near the edges of the samples. The spallation was caused by rapid corrosion of the underlying ZIRLO substrate which resulted from the penetration of oxygen ions between the TiZrN coating and ZIRLO substrate along the edges of the samples. In pure steam for 30 days at 360 ℃, a complete loss of the TiZrN layer occurred but the results show that the coating might have delayed the corrosion of ZIRLO. In summary, corrosion resistance of nc-TiZrN coated ZIRLO was lower than that of uncoated ZIRLO. The study shows that the failure of TiZrN coatings leads to rapid corrosion of the underlying ZIRLO substrate which was not observed on the uncoated ZIRLO. It is not yet understood why the presence TiZrN coatings reduced the oxidation resistance of ZIRLO.
- Full Text:
- Date Issued: 2018
- «
- ‹
- 1
- ›
- »