The low carbon rods obtained from commercial hot rolling are the raw material for the drawing processing of wires. The oxide scales developed during this process promote the loss of metallic performance and compromises the subsequent processes of the wires. For that reason the reduction or elimination of oxide scales is a problem of interest and considerable effort is made to solve it. The usual thermo mechanical deformation processes for manufacturing low carbon rods include an on line cooling stage. The temperature and the cooling rate of the process determine not only the thicknesses of the scales but also the oxide phases present in the scale namely Wüstite (FeO, cubic), Magnetite (Fe3O4, cubic) and Hematite (Fe2O3, trigonal)
According to the nature of the oxide scales, different methodologies are employed to remove them. As a consequence, the characterization of the scales, including the measurement of layer thicknesses and identification of the different oxides in a short time is decisive to modify processing parameters or to select the methodology for appropriate descaling. The conventional techniques to analyze the oxide scales are optical and scanning electron microscopy. Those studies have been approached with relative success because the problem is complex: differences in composition are difficult to detect: it is not easy to distinguish between Magnetite and Hematite; and the similarities in crystal structure made difficult to distinguish between Wüstite and Magnetite. However, with the combined use of techniques such as EDS (Energy Dispersive X-ray spectroscopy) and EBSD (Electron Backscatter Diffraction), it should be possible to recognize the different phases, and also characterize microstructural aspects such as grain size, morphology and grain boundaries.
The main goal of this work is to analyze the characteristics of the oxides developed in low carbon wire rods using EDS and EBSD to set up an on line characterization tool. In this work SAE 1008 wire rods of 6.4 mm in diameter were treated at two different temperatures at the Stelmor conveyor. Microestructural aspects, like thickness and composition of the constituent phases of scales and interfaces between the grain boundaries of the substrate and the oxides, are considered. The results are also correlated with a thermodynamic study of the formation of oxides contemplating the cooling conditions applied in the industry.
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