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

Abstract

In recent years a new group of ferritic-martensitic chromium steels for the use in fossilpower stations has been developed with chromium contents between 9 and 12%. Typicalrepresentatives of these steels are P91, E911 and Nf616, which are nowadays widely used in themore advanced power plants. In the development phase the focus was on the mechanicalproperties of these steels but when taking them to practical operation conditions it turned out thatmuch 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 ischromium, silicon, manganese and molybdenum that decide their oxidation performance andsince the contents especially of the four elements can be significantly different for the differentsteels there can be clear differences in oxidation behaviour. One of the most important issuesfrom this point of view is how the concentrations of these elements change in the metalsubsurface zone during operation/oxidation since if their level drops below a critical leveloxidation resistance of the steels will be lost. In the work to be reported the influence of alloycomposition and metal subsurface zone concentration as a function of oxidation time up to 10000hrs was investigated in dry air and air up to 10% water vapour at 650�C. The investigationscomprised several of the advanced commercial 9% Cr steels including P91, E911, Nf616 and sixlaboratory melts of Nf616 with different amounts of silicon. As the results of the investigationsshow humidity, which is omnipresent in combustion environments, can dramatically accelerateoxidation. Silicon as an alloying element reduces the detrimental effect of water vapoursignificantly while molybdenum has a negative effect. The effects of the key alloying elements inthese steels was quantified for conditions with and without water vapour in the environmentincluding the role of mechanical load and recommendations were developed on how to guaranteethe optimum oxidation resistance of these steels