Effect of H₂ Addition to Conventional Shielding Gases for GMAW of C Steel with Short-Circuit Transfer. Part 1: Weld Bead Quality and Transfer Stability

Abstract

This work has the objective to evaluate the effect of the addition of a reducing gas, in this case H₂, in the conventional mixtures Ar, Ar+10%CO₂, Ar+25%CO₂ and CO₂ in the welding of carbon steel by the GMAW process using short circuit transfer. There were evaluated weld bead appearance and transfer stability. Weld beads were produced with the conventional gases and with the addition of 2%, 4% and 6% H₂. The welds were of the type bead on plate, in flat position and mechanized process, wire ER 70S-6 of 1.2 mm, distance from the nozzle to the workpiece was 12 mm, torch angle of 90° with the face of welding, welding speed of 300 mm/min and shielding gas flow rate of 15 l/min. The wire feed speed was set to voltages within the range of 20 V and 23 V. The welds were evaluated by visual inspection, dye penetrant and macrographs. The electrical signals were captured during welding and analyzed by means of the indexes: Standard Deviation of Current Peaks (SDCP), Standard Deviation of Open Arc Period (SDOP), Standard Deviation of Short-circuit Period (SDSP) and Vilarinho index regularity of the short-circuit transfer (IVcc). The results of visual tests, dye penetrant and macrographs indicated the possibility of welding carbon steel with Ar with up to 4%H₂, Ar+10%CO₂ with up to 4%H₂, Ar+25%CO₂ with up to 2%H₂ and CO₂ with up to 6%H₂. According to the transfer stability indexes SDCP, SDOP, SDSP and IVcc, the addition of H₂ in mixtures with a higher oxidation potential produced welds with stability equal to or better than those for commercial shielding gases without the addition of H₂. Already for the shielding gases with low oxidation potential, the addition of H₂ promoted transfer instability. Mixtures Ar+25%CO₂, Ar+25%CO₂+2%H₂, CO₂ and CO₂+4%H₂ showed best transfer stability.

Key-words:
GMAW; Transfer stability; Effect of hydrogen; Oxidation potential