Study on the effects of pulsed TIG welding parameters on delta-ferrite content, shape factor and bead quality in orbital welding of AISI 316L stainless steel plate

The TIG pulse welding parameters of AISI 316L stainless steel plate of 3 mm thickness at the welding positions of 6–12 h were investigated. The weld bead profiles corresponded to DIN 8563 class BS. The studied parameters were welding speed, pulse/base currents, pulse frequency, and % on time. Pure argon and argon with nitrogen contents of 1–4 vol.% were used as shielding gas with a flow rate of 8 l/min at both the top and the back sides of welds. Preliminary welding results at the 6 h welding position showed that the appropriate parameters were: base current of 61 A, pulse frequency of 5 Hz, and 65% on time. With these constant parameters the effects of welding speeds of 2–8 mm/s and nitrogen contents of 0–4 vol.% in argon shielding gas on pulse currents were examined to attain acceptable weld bead profile corresponding to DIN 8563 class BS with complete penetration. The results showed that the lowest pulse currents were observed at the 9 h welding position. Increasing nitrogen content in argon gas decreases the pulse currents. At the welding positions of 6 and 12 h, the maximum welding speed is limited to 6 mm/s, and with a welding speed of 7 mm/s the formation of slag inclusion at the top of weld metal was observed. The maximum welding speed of 5 mm/s is found for the welding positions of 8, 9, 10 h, but the welding speed of 6 mm/s is not applicable because of incomplete filled groove. The depth/width ratios (D/W) are between 0.34 and 0.40. Increasing welding speed decreases in the weld width and increases in the D/W ratio. The delta-ferrite contents of the weld metal are about 6–10 vol.%, and are minimum at the welding position of 9 h, because of the high nitrogen content of the cover gas and lowest pulse currents compared to all other welding positions. Radiography showed acceptable weld beads free of porosity.