EVALUATION OF FATIGUE DAMAGE BY X-RAY DIFFRACTION TECHNIQUE OF STEAM TURBINE ROTOR STEELS AT ELEVATED TEMPERATURES

ABSTRACT In many industrial applications materials are subjected to degradation of mechanical properties as a result of real service conditions. The assessment of the remaining lifetime of components and structures is commonly based on correlated procedures including numerous destructive, nondestructive and mathematical techniques that should guarantee reasonable precise assessment of the current damage extent of materials in question and the remnant lifetime assessment. The assessment of the remaining lifetime by X-ray diffraction technique is based on the fact that mechanical properties of the most materials depend strongly on crystallite size and orientation of ingredients, which are the crucial parameters for the determination of the ageing state and the prediction of residual lifetime of the components

Study of influence of aluminum nitride nanoparticles on the structure, phase composition and mechanical properties of AZ91 alloy

In this work, magnesium-based composites were obtained by shock-wave compaction of a powder mixture of Mg-5 wt.% AlN at a shock-wave pressure of 2 GPa. Their microstructure was investigated and the phase composition was determined, from which it follows that the nanoparticles retain their phase composition and are uniformly distributed in the magnesium matrix. The materials obtained by shock-wave compaction were used as master alloys for the production of magnesium alloys by die casting. The amount of aluminum nitride nanoparticles in the AZ91 magnesium alloy was 0.5 wt.%. Studies of the microstructure of the magnesium alloys showed a decrease in the average grain size of the magnesium matrix from 610 to 420 m. Studies of mechanical properties have shown that the introduction of aluminum nitride nanoparticles increases the yield strength from 55 to 119 MPa, the tensile strength from 122 to 171 MPa and the plasticity from 4 to 6.5%, respectively. The effect of nanoparticles on the fracture behavior of the magnesium alloy under tension was determine

Einsatz der Röntgenbeugung zur in-Situ-Untersuchung der Hochtemperaturkorrosion und ihrer Kinetik

Der Einsatz der zeit- und temperaturaufgelösten Röntgenbeugung zur Untersuchung der Hochtemperaturkorrosion ermöglicht eine Identifizierung der Korrosionsprodukte in situ und die Bestimmung der kinetischen Parameter sowohl für jedes Korrosionsprodukt separat als auch für die gesamte Schicht. Die Ausbildung von Texturen kann während der Oxidation kontinuierlich beobachtet werden. Die thermische Ausdehnung von Substrat und Korrosionsprodukt wird simultan verfolgt, wobei aus der Betrachtung der einzelnen Gitterparameter Informationen über Anisotropien im Ausdehnungsverhalten zu entnehmen sind. Röntgenbeugung bei streifendem Einfall ermöglicht die Untersuchung sehr dünner Schichten und der Oxidation im Anfangsstadium. Es werden Serien von Röntgenbeugungsdiagrammen in Abhängigkeit von der Zeit oder Temperatur aufgezeichnet. Daraus werden über ein numerisches Verfahren Kurven gewonnen, die das Wachstum einer Oxidschicht enthalten. Die Absorption des Röntgenstrahls in der wachsenden Schicht w ird in die mathematische Beschreibung dieser Kurven einbezogen und das zu erwartende Zeitgesetz eingesetzt. Mit Hilfe der Methode der kleinsten Quadrate wird an die gemessene Kurve die berechnete Kurve angepaßt, woraus sich die kinetischen Parameter ergeben. Die kinetische Auswertung läßt sich sowohl bei isothermen, als auch bei nichtisothermen Versuchen anwenden. Bei letzteren kann die Temperaturabhängigkeit der Zunderkonstante aus einem Aufheizversuch gewonnen werden, vorrausgesetzt, daß sich im untersuchten Temperaturbereich die Aktivierungsenergie nicht ändert und keine Phasenumwandlungen auftreten. Die Untersuchungsmethode wurde an dem l-Schichtsystem Ni/Ni0, dem 2 Schichtensystem Cu/Cusub2O/CuO und an dem System Fesub3Osub4/Fesub2Osub3 bei der Eisenoxidation demonstriert. Bei letzterem wurde die Röntgenbeugung bei streifendem Einfall angewandt. Die Untersuchungen zeigen die Vorgehensweise der beschriebenen Methode und liefern Ergebnisse, die in die aus der Literatur bekannten Sa

Influence of the slurry thickness and heat treatment parameters on the formation of aluminium diffusion coating

Formation of the FeAl3 or Fe2Al5 phase during the aluminization of iron-based alloys causes detrimental behaviour of the material due to the brittleness of these phases and the different coefficient of thermal expansion between the base material and the resulting diffusion coating. In order to control the microstructure of the produced diffusion zone and its evolution, two different slurry thicknesses (30–50 and 100–150 µm) using high-purity aluminium spherical particles and three different heat treatments times (5, 10 and 20 h) were tested over ferritic–martensitic P92 steel. After the heat treatment, iron–aluminide phases rich in aluminium content were formed. After 1350 hours of exposure in air at 650 °C, it was found that for the coatings in the range of 30–50 µm neither FeAl3 nor Fe2Al5 phase remained in the diffusion coating, while Fe2Al5 did remain in those coatings in the range of 100–150 µm

Oxidation of La-Sr-Mn-coated interconnector alloys for steam electrolysis under pressure in pure oxygen and in pure steam

Pressurized steam electrolysis enables an efficient conversion of electric power from renewable energy sources into hydrogen for power-to-liquids processes. The interconnector material Crofer 22 APU, uncoated and coated by La1−xSrxMnO3 (LSM), deposited by thermal spray and by roll coating was studied in pure water vapor and pure oxygen at 850 °C and 30 bar. The uncoated Crofer 22 APU forms in both atmospheres a homogeneous oxide scale from an inner Cr2O3 and an outer MnCr2O4 layer. The chromia is locally undergrown by pits of MnCr2O4. With the LSM coating, the oxide scale is notably thinner in water vapor and the formation of pits is significantly reduced. In oxygen, this effect of the LSM coating is less pronounced. Chromium from volatile species was detected in the LSM coating, more in oxygen than in water vapor. After 3000 h in pure oxygen, Crofer 22 APU with thermally sprayed LSM shows breakaway oxidation

Lifetime Modelling for MCrAlY Coatings in Industrial Gas Turbine Blades

A novel theoretical and experimental approach for lifetime modelling of MCrAlY coatings for stationary gas turbines has been undertaken using the Inverse Problem Solution (IPS) technique. With this technique feasible experimental data acquired after a defined experimental time τ e are used as input values for the model parameters estimation. In the first stage of the approach a model, based on the oxidation and diffusion processes (Fick's first and second law) was assumed, which considers the Al concentration profile across the coating. The measured average Al concentration profiles in the two-phase γ+β and γ -regions of coating as well as base metal were used as input values for the model parameters estimation and calculational prediction of the long term diffusion and oxidation behavior of the coating was performed. The time, when the β-NiAl phase is completely consumed was assumed as the coating lifetime end. Exposure experiments were carried out with a NiCoCrAlY coating (200 micron thickness) with 8% Al in air at 900 °C and 950 °C, currently up to 10000 h. The oxide scale is growing continuously and no other oxides were observed. The average and β-NiAl phase concentration profiles of Al across the coating thickness were determined by electron microprobe and image analysis systems in the initial state after 700 and 10000 h of oxidation. The concentration profile measured after 700 h was used as input values for the model parameters estimation in order to calculate the Al and β-NiAl phase concentration profiles after 10000 h. The computational forecast for 10000 h at 950 °C and 900 °C are in good agreement with the measured data. The approach was applied for NiCoCrAlY (200 micron thickness) coating lifetime modelling at 950 °C and 900 °C as well as for different coating thicknesses at 950 °C

Lifetime modelling for MCrAlY coatings in industrial gas turbine blades

A novel theoretical and experimental approach for lifetime modelling of MCrAlY coatings for stationary gas turbines has been undertaken using the Inverse Problem Solution (IPS) technique. With this technique feasible experimental data acquired after a defined experimental time t e are used as input values for the model parameters estimation. In the first stage of the approach a model, based on the oxidation and diffusion processes (Fick's first and second law) was assumed, which considers the Al concentration profile across the coating. The measured average Al concentration profiles in the two-phase g+b and g - regions of coating as well as base metal were used as input values for the model parameters estimation and calculational prediction of the long term diffusion and oxidation behavior of the coating was performed. The time, when the b-NiAl phase is completely consumed was assumed as the coating lifetime end. Exposure experiments were carried out with a NiCoCrAlY coating (200 micron thickness) with 8% Al in air at 900 °C and 950 °C, currently up to 10000 h. The oxide scale is growing continuously and no other oxides were observed. The average and b-NiAl phase concentration profiles of Al across the coating thickness were determined by electron microprobe and image analysis systems in the initial state after 700 and 10000 h of oxidation. The concentration profile measured after 700 h was used as input values for the model parameters estimation in order to calculate the Al and b-NiAl phase concentration profiles after 10000 h. The computational forecast for 10000 h at 950 °C and 900 °C are in good agreement with the measured data. The approach was applied for NiCoCrAlY (200 micron thickness) coating lifetime modelling at 950 °C and 900 °C as well as for different coating thicknesses at 950 °C