In recent years a new group of ferritic-martensitic chromium steels for the use in fossil power stations has been developed with chromium contents between 9 and 12%. Typical representatives of these steels are P91, E911 and Nf616, which are nowadays widely used in the more advanced power plants. In the development phase the focus was on the mechanical properties of these steels but when taking them to practical operation conditions it turned out that much of the life-time of the materials and components is determined by their oxidation properties. Oxidation resistance is first of all a function of alloy composition. For the steels of this group it is chromium, silicon, manganese and molybdenum that decide their oxidation performance and since the contents especially of the four elements can be significantly different for the different steels there can be clear differences in oxidation behaviour. One of the most important issues from this point of view is how the concentrations of these elements change in the metal subsurface zone during operation/oxidation since if their level drops below a critical level oxidation resistance of the steels will be lost. In the work to be reported the influence of alloy composition and metal subsurface zone concentration as a function of oxidation time up to 10000 h was investigated in dry air and air up to 10% water vapour at 650 °C. The investigations comprised several of the advanced commercial 9% Cr steels including P91, E911, Nf616 and six laboratory melts of Nf616 with different amounts of silicon. As the results of the investigations show humidity, which is omnipresent in combustion environments, can dramatically accelerate oxidation. Silicon as an alloying element reduces the detrimental effect of water vapour significantly while molybdenum has a negative effect. The effects of the key alloying elements in these steels was quantified for conditions with and without water vapour in the environment including the role of mechanical load and recommendations were developed on how to guarantee the optimum oxidation resistance of these steels
The lifetime of thermal barrier coating (TBC) systems on gamma titanium aluminides was determined in the temperature range between 850°C and 950°C under cyclic oxidation conditions in air. Coupons of the alloy Ti-45Al-8Nb (at.%) were coated by pack aluminizing. A subset of samples was subsequently annealed at 910°C for 312 h in argon. During this heat treatment, the two-phase (Nb,Ti)Al 3 plus TiAl 2 microstructure of the coating transformed into single phase c-TiAl. On pre-oxidised aluminized, annealed and bare samples, TBCs of yttria partially stabilized zirconia were deposited using electron-beam physical vapour deposition (EB-PVD). No spallation of the TBCs was observed in cyclic oxidation tests at 850°C for up to 3,000 cycles of 1 h dwell time at high temperature. The two-phase aluminide coating provided effective oxidation protection due to the formation of a continuous alumina scale. The lifetime of this TBC system exceeded 1,400 cycles at 950°C, whereas an aluminized and annealed sample failed after approximately 500 cycles. The TBC on bare substrate failed when thermally cycled at 900°C. In contrast, no spallation occurred with an aluminized and annealed specimen at this temperature during the maximum exposure length of 1,000 cycles, probably related to an increased aluminium concentration in the subsurface region.EB-PVD zirconia top coats were well adherent to the alumina scale and the thermally grown mixed oxides. Failure of the TBC systems observed with bare and annealed samples was associated with spalled oxide scales formed on c-TiAl.
The time and temperature dependent evolution of the microstructure of thermal barrier coating systems under isothermal conditions between 950 and 1100°C up to 5000 h are investigated for two APS -TBC systems and two EB-PVD systems. Kinetics for the thermally grown oxide thickness values, the b phase depletion underneath the oxide scale and the physical defects in and around the thermally grown oxide are determined by extensive SEM and subsequent interactive image analysis. In the case of physical defects, the size of pores, interacting pore populations and maximum crack lengths are measured. Additionally, the latter are classi ed with respect to their local orientation in the thermally grown oxide or its vicinity. Finally, the results are discussed with regard to their signi cance in lifetime modelling of gas turbine components.MST/5908
Minor alloying elements can significantly influence the oxidation behaviour of ferritic martenstic steels, and above all the time to the beginning of breakaway oxidation. This has been found to be particularly true when oxidation occurs in the presence of water vapour. The oxidation of 9%Cr steels has been investigated for times of up to y10 000 h in dry air and in air with both 4% and 10% water vapour. The steels were commercial and quasicommercial versions of P91, E911 and Nf616. The content of the alloying elements, primarily Si, but also W, Mn, and Mo, was varied for the purpose of investigating their effect on breakaway oxidation. This study not only involved weight gain measurements but also characterising the Cr depletion in the substrates. Additionally, the role that intrinsic growth stresses play with regard to the incubation time to breakaway was investigated by high temperature in situ X-ray diffraction and in situ acoustic emission analysis. The results show that the occurrence of breakaway is the result of cracking processes in the oxide scale in combination with a severe chromium depletion of the subsurface zone.CEST/2115Keywords: breakaway oxidation, 9%Cr steels, alloy composition, Cr-subsurface depletion, water vapour M. Schütze (schuetze@dechema.de), D. Renusch and M. Schorr are in the Karl-Winnacker-Institut der DECHEMA e.V., D-60061 Frankfurt am Main, Germany. Manuscript
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