Improvement of hardness and resistance to oxidation by electric arc alloying on ferritic steels
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In these tests, an investigation was made of the microstructure, hardness, and resistance to wear of the surface alloys obtained by the electric arc technique. The oxidation tests were carried out in a furnace at 850 8C or 950 8C, with no control of the air composition (the tests were made using the ambient air inside the furnace). Before and after oxidation, the samples were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and microprobe analysis by wave dispersion spectroscopy (WDS). It was then possible to visualize and analyze the surface alloys structure and the oxidation products formed. The surface alloys and the substrate hardness were measured by Vickers indentation under a 5 N load. The pin-on-disk wear tests were carried out in a dry environment at ambient temperature (without chip evacuation) after the surface alloys and the substrates were polished using 600 grade paper finish. The test conditions used were (a) alumina sphere diameter of 6 mm, (b) load of 10 N, (c) sphere track radius of 6 mm, (d) sphere dislocation speed on the disk of 0.1 m/s, and (e) test duration of 104 laps. The micrographic observations made from cross-sectional cuts of the treated samples revealed that the morphology and the structure of the surface alloys obtained with Fe30Cr-4C electrodes are not affected by the substrate used. Therefore, the coatings on VCO and VH13 steels are identical with an approximate thickness of 1 mm where the electrode structure is maintained after the coating solidification. In the interface with the substrate, an intergranular penetration of coating material in the substrate to about 250 mm in depth (Figure 1) can be seen. The SEM metallographics have shown that the surface alloy is formed by two distinct metallurgical structures. (1) An intergranular eutectic structure in the substrate interface (Figure 1) consisting of iron solid solution and (Fe, Cr)7C3 carbides. The small difference in the chemical composition of VCO and VH13 steel does not affect this region structure. (2) A hypereutetic structure in the upper region consisting of (Fe, Cr)7C3 primary precipitates completely surrounded by the eutectic, iron solid solution and (Fe, Cr)7C3 carbides. The average wear ratio measured on the surface alloys and substrates shows that the electric arc deposition of Fe30Cr-4C leads to a significant increase in the hardness and wear resistance in relation to the substrates. The surface average hardness of the steels increases from about 400 (VCO steel) or 450 Hv (VH13 steel), before the treatment, to about 850 Hv (after surface alloying), and the wear ratio decreases to a factor close to 2. These results are directly correlated with the microstructure. On the other hand, the increase in the hardness and wear resistance is essentially due to (Fe, Cr)7C3 carbide precipitation. Weight gains (Dm/s) vs time (t) obtained at 950 8C in isothermal oxidation are presented in Figure 2. The chromium containing surface alloy exhibit an excellent behavior compared to the untreated steels. It
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