| Abstract: | Influence of scale adhesion on the carbon concentration in the neighbourhood of the steel surface The oxidation of carbon-iron alloys can theoretically proceed with decarburization or carburization depending on the temperature and pertial pressure of carbon monoxide and carbon dioxide in the furnace atmosphere. In the usual conditions only decarburization is observed. Nevertheless a superficial carbon enrichment can temporarily appear despite these theoretical previsions. The latter effect is very important between 730 and 800°C. It disappears above 950°C. The enrichment exists only where the scale is adherent. Ferrous ions cross the oxide but the carbon does not. Furthermore the carbon solubility in wustite is less that 0.5 ppm. The presence of silicon, manganese or nickel includes spalling and consequently the decarburization of subscale metal. With silicon alloys spalling occurs periodically during soaking and the external scale layers consist of Fe3O4 and Fe2O3. With manganese alloys the scale is always non adherent. Separation proceeds either at high temperature or during cooling. In the firs case, over-oxidation of the scale is induced. With nickel alloys the nickel enriched zone, which lies between the compact scale and the metal, adheres very tightly. The porosity of this zone allows rapid decarburization. |
