Transmission Electron Microscopy in the Heat Affected Zone of an AISI 304 Austenitic Stainless Steel Welded with the Application of a Magnetic Field of Low Intensity

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A transmission electron microscopy (TEM) study was carried out of the heat affected zone (HAZ) of an AISI 304 austenitic stainless steel gas metal arc welded with a magnetic field of 14.7 mT. Thin foils for TEM observation were prepared from the as-received 304 stainless steel and the HAZ of samples welded with and without magnetic field. M₇C₃ carbides were observed in conventional bright field (BF) and high resolution (HR) confirmed their presence in the as-received stainless steel. Elemental line scans performed in the base metal showed that the austenite/M₇C₃ interface was Cr-depleted in the austenite side. The results revealed that welding with magnetic field modified the distribution of Cr within the carbides and healed the Cr-depleted zones. This evidence accounts for the enhanced corrosion behaviour previously reported by the authors and strengthens the proposed mechanism where the interaction between the external magnetic field and the magnetic field generated by the direct current of the welding process promotes diffusion of Cr in short distances, healing thus Cr depletion.