The scaling behaviour of austenitic Fe-Cr-Ni alloys at high temperatures in air

Literature concerning the growth and chemical and mechanical stability of scales formed on iron-based alloys at high temperatures is reviewed. The experimental work is designed to enable a study of the effects of the minor alloying elements, silicon and yttrium, on the scaling behaviour of an austenitic Fe-Cr-Ni alloy under both isothermal and athermal conditions of exposure. The rates of scale growth are evaluated from thermogravimetric data and results obtained from metallographic examination. The mechanical stability of scales is obtained using thermogravimetric data and results from a resonant frequency technique. From the latter, changes in the effective moduli of surface scales are used to evaluate the ability of a scale to provide a physical barrier to an oxidising environment. Isothermal oxidation tests of 336 hours duration (maximum) are carried out at temperatures in the range 8500 to 10000C. At 10000C, it is shown that silicon exerts a strong influence on the ability of an alloy to form a protective scale, yttrium having little influence on growth kinetics. Thermal cycling tests are performed. The main conclusions of the work are that both minor alloying elements, silicon and yttrium, exert an influence on thermal cycling behaviour. (author)