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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

In order to guarantee the oxidation resistance of Cr-steels the Cr content in the alloy must be above a critical limit. Recently developed 9Cr steels are close to that limit and as ongoing oxidation leads to Cr subsurface zone depletion the question arises as to how the oxidation behaviour is affected by the decrease in Cr concentration with oxidation time. Four ferritic heat-resistant commercial steels containing 9-12% Cr and the austenitics AISI 304 and Alloy 800 were investigated at 650°C in air t? determine their oxidation behaviour and the course of Cr-depletion in the metal subsurface zone for t1mes up to 3000 hours. In addition to isothermal tests, thermal cycling tests and creep tests were also performed. Surprisingly large differences in oxidation behaviour were found between the two 9Cr steels. Furthermore, of the two steels designated as 12Cr steel, one was even worse than the 9Cr steels while the other one was best. Thermal cycling improved the oxidation behaviour of the steels which was worse under isothermal conditions by almost two orders of magnitude. The oxidation behaviour as a function of time very much reflected the amount of Cr in the metal subsurface zone. The breakaway effects observed could be correlated with a drop in the Cr content in the subsurface zone below a critical value which had been determined by model calculations. The tendency observed under isothermal conditions is enhanced by superimposed creep deformation. It is concluded from the results that growth stresses in the oxide scales combined with the actual Cr-concentration in the metal subsurface zone play a major role in oxidation resistance.

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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