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Description
This work is focused on the oxidation behavior of an alumina-forming heat resistant steel used in steam cracking furnaces, in which a mixture of steam and ethane is heated up to 850°C in order to produce ethylene. In this application, the main problem arises from the formation of coke leading to pipe obstruction, carburization of the alloy and reduced heat exchanges. One of the possible strategies against the coke formation is to achieve a stable and protective alumina layer on the inner surface of the pipes.
The oxidation behavior of a newly developed alumina forming steel was studied at 900°C, both in air and Ar-H2O atmospheres, with a special emphasis on the combined influence of indigenous sulfur and steam. Several pipes with different sulfur contents - from 1 to 82ppm - were centrifugally cast. Samples containing very low sulfur level, formed a thin and protective α-Al2O3 layer, in both atmospheres. However, for high sulfur contents, the oxide layer becomes less protective: a significantly thicker external oxide layer and Al internal oxidation could be observed. This degradation is even faster under steam, indicating that the protectiveness of the alumina layer greatly depends on the sulfur content in the material and the oxidizing atmosphere.
To understand this phenomenon, oxide structures were analyzed at various scales using, Raman Spectroscopy, SEM, TEM and Nano-SIMS. Analyses demonstrate that steam oxidation leads to finer alumina grains as compared to air oxidation. Sulfur is detected at all metal-oxide interfaces and periodically distributed inside the alumina layer, in correlation with local Cr enrichments. The impact of such co-segregations of S and Cr in the alumina layer on the alumina grain size and the oxidation kinetic of the alloy is discussed.
| Speaker Country | France |
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