Análise das fases cristalinas na soldagem GTAW de aço inoxidável AISI 316L revestido por Níquel-Watts utilizando Thermo-Calc

Abstract

This study investigates the microstructure and mechanical properties of the austenitic stainless steel AISI 316L after performing GTAW (TIG) welding using commercially pure argon gas (with 2% oxygen) at the ends of sheets that were previously subjected to nickel electrodeposition, containing a nickel-Watts solution enriched with niobium particulate (with a particle size of less than 150 µm) at room temperature. To evaluate the mechanical properties and microstructural changes, tensile tests, potentiodynamic polarization, Vickers microhardness, ferritoscopy, and microstructural analyses were conducted. The welding resulted in the formation of austenitic and delta ferrite (δ) phases, with a pronounced concentration in the molten zone (MZ), attributed to the rapid cooling characteristic of the welding process. Additionally, the emergence of metallic carbides and sigma (σ) phase was observed at grain boundaries, particularly in the heat-affected zone (HAZ). Ferritoscopy revealed a significant increase in the volumetric fraction of ferromagnetic phases in the welded regions, reaching 6.7% Fe in the MZ, 3.3% Fe in the HAZ, and 0.5% Fe in the base metal (BM). The microhardness results demonstrated differences among the analyzed regions, with the ZF exhibiting the highest microhardness value (231 HV ± 7 HV), followed by the ZTA with an intermediate value (188 HV ± 6 HV), and the base metal (BM) showing the lowest value (174 HV ± 1 HV). In the potentiodynamic polarization tests, the results indicated minimal variations in corrosion and pitting potentials between the base metal and the welded samples. For the welded sample, the average corrosion potential was measured at 0.02 V ± 0.01 V, with an average pitting potential of 0.24 V ± 0.01 V. Conversely, the base metal exhibited an average corrosion potential of 0.05 V ± 0.01 V and an average pitting potential of 0.29 V ± 0.01 V. Microfractographic analyses characterized the fracture of the welded samples as ductile, with the presence of dimples indicating a good capacity for deformation prior to rupture. Despite the addition of niobium to the nickel-Watts solution, the formation of niobium precipitates in the material was not identified, which may be attributed to the low thickness of the coating compared to the thickness of the weld bead, resulting in insignificant improvements in mechanical properties and corrosion resistance.