Plasma Gas Flow Rate Research Articles

Plasma arc welding of pipelines: A study to optimise welding conditions for horizontal fixed joints of mild steel pipes

A plasma arc process was applied to the welding of mild steel pipes (thickness: 6·4 mm, outside diameter: 406·4 mm) and the effect of different welding parameters on the shape of welds and consistency of defects was studied in the flat, vertical and overhead positions. In conventional plasma arc welding conditions, full penetration and continuous bead formation were obtained in a range of conditions which tended towards lower plasma gas flow rates and higher travelling speeds as the welding current was increased. In the overhead position, the penetration bead presented deep continuous concaves. In the vertical-up position, it was not possible to obtain a range of welding conditions assuring full penetration and continuous bead formation. The experiment relating to the effect of the pulsating welding current on the shape and defects of welds disclosed that the use of pulse frequencies lower than 4 Hz makes the penetration bead less uniform than the use of higher pulse frequencies. It was also found that the use of high pulse frequencies results in a remarkable effect in the vertical-up position. Welding using argon gas back shielding made the penetration bead more uniform in height and width. By decreasing the atmospheric pressure inside the pipe, concaves in the penetration bead could be reduced, particularly in the overhead position. From the results described above, it is considered that pulsating welding current and the backing method are effective in obtaining a good bead over the entire circumference of mild steel by plasma keyhole welding. The experiment also revealed that when plasma arc welding is applied, mild steel pipes can be welded at four times the productivity that can be expected of the conventional GMA welding. Therefore plasma arc welding equipment and appropriate jigs will be made shortly for practical application.