Effect of welding polarity on bead geometry, microstructure, microhardness, and residual stresses of 1020 steel

This work examines the effect of welding polarity as a measure of heat input on the bead geometry, microstructure, microhardness and residual stresses of AISI 1020 carbon steel that was processed by shielded metal arc welding (SMAW). Single weld beads were deposited on the steel plate using a constant current but with different welding polarities of AC, DC- and DC+. Optical microscopy indicates that welding by DC- provides the widest weld bead and largest heat affected zone (HAZ) due to the large heat input subjected to the plate. Nevertheless, similar microstructures in the HAZ and fusion zone (FZ) of the weld were found for all welding polarities. Vickers microhardness tests also show that the large heat input by DC- polarity provided the minimum microhardness for all microstructures of the weld. In addition, across-weld measurements of the residual stresses by neutron diffraction indicate that the three welding polarities produced similar profile. High tensile longitudinal residual stresses were found to extend horizontally from the weld center to the HAZ, which become compressive further away from the bead. The highest tensile residual stress in the HAZ occurred for DC- polarity, while it has the lowest value for AC polarity. Throughthickness measurements also indicate that the residual stresses within the HAZ are approximately constant for AC polarity. This suggests that equal distribution of heat input by AC polarity to the electrode and plate is not only important for reducing residual stresses but also on minimizing the differences in residual stresses through the weld thickness.