| 1. |
- Farooq, Muhammad, et al.
(författare)
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Influence of long time ageing on ductility and toughness in the stainless steel 310 in the presence of banded microstructure
- 2011
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Ingår i: 7th European Stainless Steel Conference. - : Associazione Italiana di Metallurgia. - 9788885298842
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Konferensbidrag (refereegranskat)abstract
- During service at elevated temperatures extensive formation of particles can take place that can have a dramatic influence on mechanical properties. Precipitation of σ-phase and M23C6-carbides have been studied both experimentally and with thermodynamic modelling for 25Cr20Ni austenitic stainless steels (AISI 310) at 800 °C for up to 5000 h. Previous work has demonstrated that the modelling could describe the nucleation and growth satisfactory. After long term ageing the particles form bands in the microstructure. In the present paper the influence of these bands on ductility and toughness at room temperature is analysed. For this purpose previously developed models for ductility and toughness are utilised. Model values for banded and non-banded microstructures have been generated for casts of 310 in fine and coarse grained conditions with 0.04 and 0.12%N. The model values show that in the coarse grained condition, no reduction in ductility and toughness can be expected in the banded microstructure. In the fine grained condition a modest reduction is predicted.
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| 2. |
- Farooq, Muhammad, et al.
(författare)
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Influence of long time ageing on ductility and toughness in the stainless steel 310 in the presence of banded microstructure
- 2012
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Ingår i: La Metallurgia Italiana. - 0026-0843. ; 104:11-12, s. 33-38
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Tidskriftsartikel (refereegranskat)abstract
- During service at elevated temperatures extensive formation of particles can take place that can have a dramatic influence on mechanical properties. Precipitation of sigma-phase and M23C6-carbides have been studied both experimentally and with thermodynamic modelling for 25Cr20Ni austenitic stainless steels (AISI 310) at 800 degrees C for up to 5000 h. Previous work has demonstrated that the modelling could describe the nucleation and growth satisfactory. After long term ageing the particles form bands in the microstructure. In the present paper the influence of these bands on ductility and toughness at room temperature is analysed. For this purpose previously developed models for ductility and toughness are utilised. Model values for banded and non-banded microstructures have been generated for casts of 310 in fine and coarse grained conditions with 0.04 and 0.12%N The model values show that in the coarse grained condition, no reduction in ductility and toughness can be expected in the banded microstructure. In the fine grained condition a modest reduction is predicted.
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| 3. |
- Farooq, Muhammad, et al.
(författare)
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Precipitation during long time ageing in the austenitic stainless steel 310
- 2012
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Ingår i: Materials at High Temperature. - : Informa UK Limited. - 0960-3409 .- 1878-6413. ; 29:1, s. 8-16
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Tidskriftsartikel (refereegranskat)abstract
- Extensive precipitation can occur in high temperature materials during service. This is known to significantly affect the mechanical properties. To be able to study this phenomenon, particle formation in the austenitic stainless steel 310 has been quantified. In particular, sigma-phase and M23C6-carbides have been analysed. Specimens have been aged up to 5000 h at 800 degrees C. The precipitation of the two phases has been modelled. The basis of the model is diffusion-controlled growth. sigma-phase shows a spherical growth and an M23C6 planar growth. The soft impingement of the diffusion zones with respect to both chromium and carbon is taken into account. The influence on mechanical properties is analysed in a parallel paper.
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| 4. |
- Farooq, Muhammad, 1976-
(författare)
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Strengthening and degradation mechanisms in austenitic stainless steels at elevated temperature
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With rapid economic developments and rising living standards, the demand for electricity all over the world is greatly increased. Due to high fuel costs, the steam boilers with higher steam temperature and pressure are needed to decrease the cost of power generation throughout the world extensively. In recent years, human awareness of the gradual strengthening of environmental protection increases, therefore to reduce the CO2 emissions the power generation efficiency needs to be improved. The development of high temperature materials with improved creep rupture strength and oxidation resistance is critically needed. Materials for these demanding conditions are austenitic stainless steels such as 310, 310NbN and Sanicro 25.Fundamental models have been developed for the precipitation of coarse particles during long time ageing of austenitic stainless steels and the influence of the particles on the mechanical properties. The models have been verified by ageing experiments. The austenitic stainless steel 310 was aged for up to 5000 h at 800 ºC. The precipitation models could satisfactorily describe the influence of ageing time on the radii and the volume fractions of particles. Models for the influence of the coarse precipitates on the tensile properties and the toughness were developed and reproduce the measured mechanical properties without the use of any fitting parameters. These developed models were utilised to investigate the influence of bands on ductility and toughness at room temperature. Up to 10 % σ-phase was observed to precipitate, which has a pronounced influence of the mechanical properties. Thermodynamic analysis demonstrated that the amount of precipitates due to ageing can significantly be reduced if the nitrogen or the carbon content is increased.Microstructure investigations of austenitic stainless steel 310NbN and Sanicro 25 were carried out by light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The austenitic stainless steel 310NbN was aged for up to 10000 h at 650 and 750 ºC. The austenitic stainless steel Sanicro 25 was also aged for up to 10000 h at 650 and 700 ºC. Phase fractions and mean radii evolution of precipitates were calculated and compared to the experimental results. Size distributions of the precipitates in these steels were determined. Models for the different contributions to the creep strength have been applied: i) a recovery creep model for the dislocation hardening; ii) a climb controlled model for the precipitation hardening; iii) solid solution hardening from Cottrell clouds of solutes around the dislocations, and iv) A modified Dobes model for the effective stress. The total contributions can describe the experimental creep strength satisfactorily without the use of adjustable parameters.
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| 5. |
- Sandström, Rolf, et al.
(författare)
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Basic creep models for 25Cr20NiNbN austenitic stainless steels
- 2013
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Ingår i: Materials research innovations (Print). - 1432-8917 .- 1433-075X. ; 17:5, s. 355-359
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Tidskriftsartikel (refereegranskat)abstract
- Basic models for solid solution and precipitation hardening during creep are presented for the austenitic stainless steels 25Cr20NiNbN (TP310HNbN, HR3C, DMV310N). The solid solution hardening is a result of the formation of Cottrell clouds of solutes around the dislocations. In addition to slowing down the creep, the solutes increase the activation energy for creep. The increase in activation energy corresponds to the maximum binding energy between the solutes and the dislocations. The formation of fine niobium nitrides during service enhances the creep strength. It is found that the nitrides have an exponential size distribution. In the modelling the critical event is the time it takes for a dislocation to climb over a particle. The creep models can accurately describe the observed time and temperature dependence of the creep rupture strength.
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| 6. |
- Sandström, Rolf, et al.
(författare)
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Influence of particle formation during long time ageing on mechanical properties in the austenitic stainless steel 310
- 2012
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Ingår i: Materials at High Temperature. - 0960-3409 .- 1878-6413. ; 29:1, s. 1-7
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Tidskriftsartikel (refereegranskat)abstract
- Intense precipitation may occur in high-temperature materials during service. This precipitation significantly affects the mechanical properties. Coarse precipitates can reduce ductility and toughness dramatically. To investigate the phenomenon, precipitation in the austenitic stainless steel AISI 310 is quantified in a parallel paper. The samples were aged up to 5000 h at 800 degrees C. Especially, sigma-phase and M23C6-carbides were analysed. A model for the influence of coarse particles on ductility has been developed and successfully applied to the measured values. A model due to Chen and Mott has been used to compute the effect of particles on toughness. A satisfactory agreement with the observations was obtained.
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| 7. |
- Stojan, V., et al.
(författare)
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Modelling and optimizing precipitation in creep resistant austenitic steel 25Cr-20Ni-Nb-N
- 2014
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Ingår i: Advances in Materials Technology for Fossil Power Plants - Proceedings from the 7th International Conference. ; , s. 1163-1172
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Konferensbidrag (refereegranskat)abstract
- 25Cr-20Ni-Nb-N (Tp310HCbN) steel is a promising austenitic steel for applications in superheater tubes in coal fired thermal power plants due to the high creep strength and oxidation resistance. In this work, the microstructural evolution of this material during heat treatment and thermal ageing has been investigated. The investigations were carried out by Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS). Besides, equilibrium and Scheil calculations were carried out using the thermodynamic software MatCalc to analyse the stable phases and the solidification process, respectively. Precipitation calculations during solution annealing and subsequent ageing at 650 and 750°C were performed to predict the phase fraction and precipitates radius up to 10.000h ageing time. SEM and TEM investigations of aged specimens revealed the presence of six different precipitates: M23C 6, Cr2N, sigma, Z-phase, eta-phase (Cr3Ni 2Si(C,N)) and Nb(C,N). These precipitates were predicted and confirmed by MatCalc simulations. The calculated phase fraction and mean radius show good agreement with experimental data. Finally, simulations of different Cr-, C- and N-content in Tp310HCbN were performed.
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| 8. |
- Vujic, Stojan, et al.
(författare)
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Numerical modelling and validation of precipitation kinetics in advanced creep resistant austenitic steel
- 2012
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Ingår i: Computer Methods in Materials Science. - 1641-8581. ; 12, s. 175-182
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Tidskriftsartikel (refereegranskat)abstract
- The austenitic steel Sanicro 25 is one of the most promising austenitic steels for the application in superheater tubes in coal fired thermal power plants. In this work, the microstructural evolution of this material during heat treatment and thermal ageing has been investigated. The investigations were carried out by light microscopy (LIMI), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). Scheil calculations were carried out by thermo-kinetic software MatCalc to analyse the solidification process which indicates Nb(C,N), Cr2N and Laves phase in the melt. Long term precipitation calculations predict the formation of five precipitate types in Sanicro 25: M23C6, Z-phase, Nb(C,N), Laves and Cr2N. Phase fractions and mean radii evolution of precipitates were calculated and compared to the experimental results. Calculated precipitate evolution shows good compliance with experimental data.
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