Optimal concentration of nanostructured powder in protective gas

In the present work, a method is developed for determining the optimal concentration of nanostructured powder in the protective gas in welding by a floating electrode in argon. The theoretical analysis is confirmed by experiments with molybdenum nanopowder, which is introduced in the welding bath through a special device. The apparatus used for surfacing of the sample includes a GSP-2 welding head combined with the specially developed device and a VS-300B power source. In surfacing 12Kh18N10T steel samples, 12Kh18N19T steel welding wire (diameter 1.2 mm) is employed. To ensure a satisfactory weld joint, the dendrite dimensions must be minimized. Stable welding is ensured by transfer of a droplet of electrode metal from the end of the welding wire to the welding bath. Hence, the droplet volume must also be minimized. Before optimizing the concentration of nanostructured powder in the protective gas, the influence of the welding parameters on the microstructure of the surfaced metal is established. The results show that the grain size is smallest with a current of 240–260 A and an arc voltage of 28–30 V. In those conditions, the optimal concentration of nanostructured powder in the protective gas is determined. It is found that the optimal concentration is 20 mg per 1 m of weld seam. The use of different concentrations of nanostructured powder in the protective gas results in different microstructure of the applied metal. When the concentration of nanostructured powder in the protective gas is 20 mg per 1 m of weld seam, the branching of the dendrites is least and the dendrite size corresponds to equilibrium structure. On adding nanostructured powder to the liquid bath, the mechanical properties of the weld joints are increased by 7.5% at +20°C and by 6.5% at +500°C.