Characterization of the Gas Tungsten Arc Welding (GTAW) joint of Armco iron nanostructured by Equal-Channel Angular Pressing (ECAP)

Abstract

• Ultrafine-grained Armco iron was obtained by severe plastic deformation and submitted to welding. • Gas Tungsten arc welding generates a gradient microstructure as a function of the heat input. • Hardness decreases after the welding process but still superior to the annealed material. • High heat inputs generate the formation of Widmanstätten ferrite. After Severe Plastic Deformation (SPD), the strength of the Armco iron sharply increases with the number of Equal-Channel Angular Pressing (ECAP) passes. So, the ultimate tensile stress (UTS) reached up to 800 MPa after eight passes, reflecting an increment of around four times than the as-received material strength. The original grain size of the annealed iron of about 70 μ m also experimented with drastic changes after ECAP, reaching grain sizes approaching to 300 nm. Samples were then welded by the Gas Tungsten Arc Welding (GTAW) process varying the voltage and current conditions. Metallographic analysis by Light Optical Microscopy (LOM) and Electron Backscatter Diffraction (EBSD), was carried out to analyze the weld penetration, Fusion Zone (FZ), and the size and microstructure morphology across the Heat Affected Zone (HAZ). Special attention was given to microstructure changes of the nanostructured base metal. A microstructural gradient was observed throughout the welded joint with hardness reduction concerning the nanostructured material but still superior to the material without ECAP. The temperature heterogeneity gave rise to different grain sizes and morphologies like Widmanstätten ferrite around the FZ, especially at high heat inputs.