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High temperature oxidation of low carbon steels containing residual copper Kumar, Dilip

Abstract

Oxidation studies were carried out on low carbon steels containing 0.2%, 0.39% and 0.78% (wt) Cu at temperatures 1000°, 1100°, 1200° and 1275°C for times 5 minutes to 2 hrs in air. Oxide growth curves were plotted from the weight gain data. The scale surfaces were examined under optical microscope as well as SEM. Analyses by EDX, XRD as well as XPS were also performed on a few of them. Cross-sectional examination of samples was carried out under SEM in order to study the Cu enrichment at the substrate surface. The growth curves displayed parabolic behavior in general, even during the early stages of oxidation. However, the parabolic rate constants dropped to lower values after a certain time at 1200°C as well as 1275°C. Usually, the scale was adherent in the beginning. However, it became detached from the substrate with passage of time. Decreasing Cu content and decreasing temperature enhanced the scale detachment. Oxide surfaces in the steels oxidized for times 1 hour or less exhibited growth of whiskers of CuFe2O4 spinels over the scale comprising mainly of FeO. Increasing temperature increased the diameters of whiskers marginally and decreased their lengths. Increasing Cu reduced the growth of whisker. The whiskers disappeared from scale surface when the steel was oxidized for 2 hours and what remained afterwards on the surface consisted only of Fe2O3 crystals. Enrichment of Cu was observed at the oxide-metal interface and the grain boundaries close to the steel surface even in the early stages of oxidation at all temperatures. However, the enriched phase at the surface was discontinuous and separated from the substrate at 1000°C. Grain boundary enrichment of Cu was very marginal at 1000°C. At 1100°C, the size of Cu rich phase increased for lower Cu levels (e.g. 0.22% and 0.39%). However, this phase remained discontinuous and separated from substrate for these steels. On the other hand, a continuous layer of Cu was observed at the surface at most of the locations for 0.78% Cu steel. Additionally, there were locations near the interface devoid of any surface enrichment but accompanied by penetration of grain boundaries by the enriched layer. Increasing the temperature above 1100°C caused enhanced internal oxidation of Si, Mn and Fe. Increasing the time of oxidation accentuated this process. Increasing the time also increased the Cu content of the enriched layer at both 1000°C as well as 1100°C. The thickness of the enriched phase also increased with time. Increasing temperature too, had a similar effect. However, increasing the time to 2 hours at 1200°C and 1275°C caused the thickness of highly enriched layer to decrease. No penetration of grain boundaries by the enriched phase was observed at 1200°C and 1275°C.

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