The effects of air in combination with up to 40% H2O and temperatures up to 1000 ºC on S35315 are reported. The exposure time varied from 20 h to 1000 h in a tube furnace, and up to 200 h in a thermobalance, under cyclic and isothermal conditions. The results show that the oxidation behavior of S35315 in water vapor is characterized by a protective chromium rich oxide scale and no breakaway oxidation was observed for this steel. The mechanism of oxidation in an atmosphere containing water vapor is discussed, linked to the ability for chromium to diffuse to the surface and some assumptions are proposed regarding the effects of different alloying elements such as Ce, Si, Ni and Cr.
Outokumpu 153 MA (S30415), 253 MA (S30815) and 353 MA (S35315) is a family of heat resistant austenitic stainless steels with increased contents of silicon and nitrogen, and micro alloyed (MA) with rare earth metals (REM, reactive elements, RE). S35315 has a significantly higher nickel content than the other steels. The alloying concept has resulted in characteristic family features:
? High mechanical strength at elevated temperatures, i.e. creep strength.
? Excellent isothermal oxidation resistance and, above all, excellent cyclic oxidation resistance and oxidation resistance under erosive/abrasive conditions.
Furthermore, MA steels are optimized to complement each other:
? S30415 for medium-high to high temperatures and moderately aggressive, mainly oxidizing atmospheres. This grade is extra resistant against embrittlement after service at medium-high temperatures.
? S30815 for high to very high temperatures and/or rather aggressive, mainly oxidizing atmospheres. This grade is the work-horse of the family.
? S35315 for the highest temperatures and/or toughest conditions, which often means environments that are strongly carburizing, strongly nitriding, or contain some halogens/halides. The alloy is designed primarily for service above 1000 ºC (1830 ºF), although it has been used at temperatures as low as 600 ºC (1110 ºF) in more aggressive environments.
From the MA family, only S35315 is considered in this study and compared to standard heat resistant grades, such as 310S, 309S and 800H.
It is well known that the presence of water vapor changes the oxidation behavior of metals and alloys. Chromia-forming alloys are affected by water vapor in the sense that the critical amount of chromium to form protective Cr2O3 or (Cr,Fe)3O4 scales increases. The concept that volatilization of chromium species was responsible for this effect was first proposed by Ebbinghaus, who reviewed the thermodynamic properties of volatile chromium-containing species and calculated that CrO3 was the dominant evaporating species in dry oxygen and CrO2(OH)2 in moist atmospheres.
The influence of alloying elements is observed and discussed, for example, silicon is assumed to enhance oxidation resistance in water vapor. Moreover, cyclic conditions, often reflecting service conditions, may influence the behavior of the heat resistant steels in moist atmospheres. The ability for the steel to maintain its protective oxide scale is essential in environments where an oxide scale is exposed to high stress, such as erosive/abrasive conditions and/or when large temperature variations frequently occur.
The present study shows how S35315, compared to other standard heat resistant steels, is affected by different testing conditions, such as: temperature, water vapor content in the atmosphere and testing time under isothermal and cyclic conditions. Effects of the water vapor, alloying elements, and cyclic conditions are discussed.
EXPERIMENTAL
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