LIMITS OF THE OXIDATION RESISTANCE OF SEVERAL HEAT-RESISTANT STEELS UNDER ISOTHERMAL AND CYCLIC OXIDATION AS WELL AS UNDER CREEP IN AIR AT 650-DEGREES-C

In order to guarantee the oxidation resistance of Cr-steels the Cr con tent in the alloy must be above a critical limit. Recently developed 9 Cr 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 oxidat ion time. Four ferritic heat-resistant commercial steels containing 9- 12% Cr and the austenitics AISI 304 and Alloy 800 were investigated at 650 degrees C in air to determine their oxidation behaviour and the c ourse of Cr-depletion in the metal subsurface zone for times up to 300 0 hours. In addition to isothermal tests, thermal cycling tests and cr eep tests were also performed. Surprisingly large differences in oxida tion behaviour were found between the two 9Cr steels. Furthermore, of the two steels designated as 12Cr steel, one was even worse than the 9 Cr steels while the other one was best. Thermal cycling improved the o xidation behaviour of the steels which was worse under isothermal cond itions by almost two orders of magnitude. The oxidation behaviour as a function of time very much reflected the amount of Cr in the metal su bsurface zone. The breakaway effects observed could be correlated with a drop in the Cr content in the subsurface zone below a critical valu e which had been determined by model calculations. The tendency observ ed under isothermal conditions is enhanced by superimposed creep defor mation. It is concluded from the results that growth stresses in the o xide scales combined with the actual Cr-concentration in the metal sub surface zone play a major role in oxidation resistance.