High temperature oxidation behaviour of boiler steels with emphasis on shot-peening effects - experimental results and simulation

The goal of this work is to improve the high temperature oxidation resistance of boiler steels. These steels are categorized as low (9 wt.%), medium (12 wt.%) and high (18 wt.%) Cr steels. The isothermal oxidation studies are done with the help of thermogravimetry in lab air in the temperature range of 700-750°C. The oxide scales grown on the steels are analysed and oxidation mechanisms were understood. Cr- content and the diffusion of Cr towards the oxidation front in the alloy has a major ascendency on the protection behaviour of the steels. Main emphasis is kept on the effect of shot-peening on the oxidation behaviour of medium and high Cr steels. It is found that shot-peening has a considerable influence on the diffusion of Cr and protection behaviour of medium Cr steels. This effect is more pronounced in high Cr steel, where shot-peening has largely reduced the oxidation rate. A protective chromia layer is found to be formed on the surface of the shot-peened high Cr steel. ‘Dislocation engineering’ is applied on the shot-peened steels to improve the oxidation resistance of the steels. This is done by pre-annealing the shot-peened steels under high vacuum for different times at the same temperature as the oxidation temperature. The pre-annealed steels are subjected to oxidation at 750°C. With the help of Focussed Ion Beam (FIB) and Transmission Electron Microscope (TEM), the subsurfaces of these steels are analysed. It is found that the pre-annealing for shorter times has resulted in forming a stable network of dislocations. The dislocation engineering method has proved to be very effective in protecting the medium Cr steels against the oxidation. The diffusion of Cr has been enhanced through this network and a protective chromia layer is developed at the oxidation front of these steels. Based on the oxidation mechanisms observed from the experiments, a model for the simultaneous oxide scale growth (internal and external oxide formation) is developed. This model is incorporated in the existing simulation tool Incorr. This tool simulates the internal corrosion phenomenon only. It works basically by solving Fick’s second law using the finite difference method (diffusion kinetics) in combination with thermodynamic equilibrium calculations. The major contribution of this work is the addition of external scale growth in the simulation. The effect of shot-peening on the microstructure of the alloying system is also considered and implemented in the model. By modifying the Incorr the simultaneous oxide layer growth on the studied steels is simulated and the results are found to be in reasonable agreement with the experimental results. The modification of the mesh from a square shaped grain structure to the honeycomb shaped structure was implemented by using a more advanced Finite Element Method (FEM). Simulation of the internal oxide scale growth is performed using FEM and the results are presented in the work.Das Ziel dieser Arbeit ist es, die Hochtemperatur-Oxidationsbeständigkeit von Kesselbaustählen zu verbessern. Diese Stähle werden in niedriglegierte (9 Gew.%), mittlere (12 Gew.%) und hochlegierte (18 Gew.%) Chrom (Cr)-Stähle eingeteilt. Die isothermen Oxidationsuntersuchungen werden mit Hilfe der Thermogravimetrie in Laborluft im Temperaturbereich von 700-750 °C durchgeführt. Die Oxidschicht, welche auf den Stählen wächst, wurde analysiert und die Oxidationsmechanismen wurden aufgeklärt. Der Cr- Gehalt und die Diffusion von Cr aus der Legierung in Richtung der Oxidationsfront hat einen großen Einfluss auf den Schutz der Stähle. Der Schwerpunkt liegt auf der Wirkung einer Kugelstrahlbehandlung auf das Oxidationsschutzverhalten von mittleren- und hohlegierten Cr-Stählen. Es wurde festgestellt, dass Kugelstrahlen einen erheblichen Einfluss auf die Diffusion von Cr und das Oxidationsschutzverhalten der mittleren Cr-legierten Stähle hat. Dieser Effekt ist noch stärker bei hohlegierten Cr-Stählen ausgeprägt, bei denen Kugelstrahlen die Oxidationsgeschwindigkeit erheblich senkt, da sich auf der Oberfläche eine schützende Chromoxid-Schicht gebildet hat. Auf die kugelgestrahlten Stähle wird das „Dislocation-Engineering“ angewendet, um die Oxidationsbeständigkeit der Stähle zu verbessern. Dies wird durch das Vorglühen der kugelgestrahlten Stähle unter Hochvakuum für unterschiedliche Zeiträume bei der gleichen Temperatur wie der Oxidationstemperatur erreicht. Die vorgeglühten Stähle werden einer Oxidation bei 750°C unterzogen. Mit Hilfe von Focused Ion Beam (FIB) und Transmissionselektronenmikroskopie (TEM) wurden die Oberflächen dieser Stähle untersucht. Es wurde festgestellt, dass das Vorglühen über kurze Zeiträume zur Bildung eines stabilen Netzwerks von Versetzungen führt. Die Dislocation-Engineering-Methode hat sich als sehr effektiver Oxidationsschutz der mittleren Cr-legierten Stähle erwiesen. Die Diffusion von Cr wurde durch dieses Netzwerk verbessert und eine Schutzschicht aus Chromoxid hat sich auf der Oxidationsfront dieser Stähle entwickelt. Basierend auf den Oxidationsmechanismen, die in den Experimenten beobachtet wurden, wird ein Modell für das gleichzeitige Oxidschichtwachstum (interne und externe Oxidschichtbildung) entwickelt. Dieses Modell wird in das bestehende Simulationstool Incorr eingearbeitet. Dieses Tool simulierte bisher nur das Phänomen der inneren Korrosion. Es funktioniert grundsätzlich durch das Lösen des zweiten Fick'schen Gesetzes mit Hilfe der Finite-Differenzen-Methode (Diffusionskinetik) in Kombination mit thermodynamischen Gleichgewichtsberechnungen. Der wichtigste Beitrag dieser Arbeit ist das Einfügen des externen Oxidschichtwachstums in die Simulation. Die Wirkung vom Kugelstrahlen auf die Mikrostruktur der Legierung wird auch berücksichtigt und in das Modell implementiert. Durch die Modifizierung von Incorr kann das gleichzeitige Oxidschichtwachstum auf den untersuchten Stählen simuliert werden. Die errechneten Ergebnisse zeigen eine gute Übereinstimmung mit den experimentellen Ergebnissen. Unter Verwendung einer weiter fortgeschrittenen Finite Elemente Methode (FEM) wurde das Netz von quadratischen zu wabenförmigen Strukturen modifiziert. Die Simulation des internen Oxidschichtwachstums wird unter der Verwendung von FEM durchgeführt und die Ergebnisse sind in der vorliegenden Arbeit dargestellt

Microstructure modification of EB-pvd gadolinium zirconate thermal barrier coatings and the effect on their resistance against siliceous CMAS melts

Please click Additional Files below to see the full abstract

Effects of EB-PVD microstrural features on CMAS infiltration of Yttria-rich zirconia coatings

Please click Additional Files below to see the full abstract

Non-destructively capturing CMAS degradation of EB-PVD thermal barrier coatings through 3D confocal Raman renderings

Please click Additional Files below to see the full abstract

Synthesis of multi component rare-earth silicate systems for T/EBC application: Study of their high temperature interactions with CMAS

Please click Additional Files below to see the full abstract

Influence of Nb coating on the oxidation behavior of ZrB2

Please click Additional Files below to see the full abstract

Microstructure Modification of EB-PVD GZO TBCs and the effect on their Resistance against Siliceous CMAS melts

Gadolinium zirconate TBCs are proven to be very effective in restricting the CMAS attack by forming quick crystalline reaction products such as apatite and garnet that seal the porosity against infiltration. However, the microstructural effects on the efficacy of GZO against CMAS attack is less explored and especially the microstructures that are obtained by the Electron Beam-Physical Vapor Deposition method. Several distinct GZO microstructures are created by EB-PVD deposition method and assessed with regard to their microstructural characteristics. The response of elected microstructures to CMAS melts with different chemical compositions was studied for up to 50h at 1250°C. An optimized columnar microstructures with smaller intercolumnar gaps and long featherarms was found to significantly lower the CMAS infiltration compare to those microstructures created with standard parameters. The formation the reaction products such as Apatite, Fluorite, Garnet etc. was found to be strongly dependent on the CMAS melt composition as well as on the local microstructure. Garnet, which formed as a continuous layer on top of Apatite and Fluorite, is identified as a beneficial reaction product that improves the CMAS resistance, as it slows down the consumption of GZO, binds high amounts of CMAS and stabilizes the subjacent Apatite-Fluorite reaction layer

Microstructure modification of EB-pvd gadolinium zirconate thermal barrier coatings and the effect on their resistance against siliceous CMAS melts

Please click Additional Files below to see the full abstract

Investigation of erosion behavior of EB-PVD-TBCs and sacrificial coatings after CMAS infiltration

Aero-engines operating in sand laden environments often encounter severe problems with thermal barrier coatings (TBCs) due to erosion damage. Since the turbine entry temperatures are raising, the life-time of TBC coatings as well as its thermal conductivity are additionally influenced by molten sand (calcium-magnesium-alumino-silicate/ CMAS). Few attempts have been made in understanding the combined impact of both erosion and CMAS effects [1,2]. Wellman and Nicholls [1] have found that a fully CMAS infiltrated electron-beam physical vapor deposited (EB-PVD) TBC behaves like a continuum during erosion and slightly improves its erosion behavior under room temperature compared to pure TBC. Development of CMAS resistant coatings has been a hot topic for the last two decades and one of the proposed method is the application of sacrificial oxide layers such as Al2O3, MgO, Sc2O3 et al. [3], on top of the TBCs. These sacrificial layers chemically react with the CMAS and modify the melting temperature or the viscosity of CMAS and thus the infiltration of CMAS into the TBC is inhibited. Since both damage mechanisms (erosion and corrosion) occur parallel and competitively in a turbine, this study focuses on deeper understanding of the erosion behavior of CMAS-infiltrated 7wt.-% yttria stabilized zirconia (7YSZ) TBCs. 400 µm thick 7YSZ coatings with two different microstructures were produced by EB-PVD. Additionally, sacrificial Al2O3 coatings were also applied on the top of 7YSZ by means of suspension plasma spraying (SPS) and suspension high velocity oxy-fuel spraying (SHVOF) using water-based suspensions. CMAS infiltration experiments were carried out at 1250 °C using different CMAS compositions and different infiltration times. Erosion tests were realized at room temperature in an in-house erosion test rig and evaluated partly by confocal microscopy. Microstructural examinations as well as crack identification before and after testing were carried out using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Infiltrated TBCs behaved as a continuum material during erosion exposure which lead mainly to surface spallation. Furthermore, the CMAS infiltration in the TBCs and partly the sintering effect at 1250 °C lead to a network of vertical cracks. These vertical cracks are weak areas where severe erosion occurs. The different TBC microstructures, infiltration times and CMAS compositions strongly influence the erosion behavior of the TBC. In case of alumina top coats the microstructure and especially the presence of porosity in the coating has strongly influenced the CMAS infiltration depth, the erosion behavior, and the stability of the entire coating system. [1] R.G. Wellman, J.R. Nicholls, Erosion, corrosion and erosion–corrosion of EB PVD thermal barrier coatings, Tribology International. 41 (2008) 657–662. doi:10.1016/j.triboint.2007.10.004. [2] S. Rezanka, D.E. Mack, G. Mauer, D. Sebold, O. Guillon, R. Vaßen, Investigation of the resistance of open-column-structured PS-PVD TBCs to erosive and high-temperature corrosive attack, Surface and Coatings Technology. 324 (2017) 222–235. doi:10.1016/j.surfcoat.2017.05.003. [3] A.K. Rai, R.S. Bhattacharya, D.E. Wolfe, T.J. Eden, CMAS-Resistant Thermal Barrier Coatings (TBC), International Journal of Applied Ceramic Technology. 7 (2010) 662–674. doi:10.1111/j.1744-7402.2009.02373.x