TWI Frequently asked questions
The first important point is that not all electrodes can be used with all polarities. Electrode manufacturer information and specifications such as BS EN ISO 2560:2005 and AWS A5.1-2004, define the polarity with which differentelectrodes may be used. The choice of polarity also depends on the type of the material and joint design. A welding procedure should specify the polarity for each run.
Unlike TIG/GTAW welding, more heat is generated at the cathode than the anode. This effects the choice of polarity. With DC electrode positive (reverse) polarity, more heat is generated at the workpiece. This results in deeperpenetration of the weld bead, and can reduce the incidence of lack-of-fusion defects. An example of an application may be the root run in a 25mm (1in) thick low-carbon steel fillet joint.
DC electrode negative (straight) polarity generates more heat at the electrode. This results in a higher burn off rate, and therefore a higher deposition rate at a given current. However, it also produces a less penetrating beadcompared to DC electrode positive. Therefore, it is often used for welding thin sheet materials, or joints with poor fit-up, where more control of the weld pool is needed due to the increased risk of burn through.
Thus, in choosing the polarity of the electrode, the details and nature of job need to be considered.
References
ANSI/AWS A5.1-2004 Specification for carbon steel electrodes for shielded metal arc welding
BS EN ISO 2560:2005 Welding consumables. Covered electrodes for manual metal arc welding of non alloy and fine grain steels. Classification
