Limits of the oxidation resistance of several heat-resistant steels under isothermal and cyclic oxidation as well as under creep in air at 650°C

Abstract

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 to determine their oxidation behaviour and the course of Cr-depletion in the metal subsurface zone for times 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.