The role of alloy composition, environment and stresses for the oxidation resistance of modern 9% Cr steels for fossil power stations

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 mechani