A methodology for multipass gas metal arc welding of shipbuilding steel plates with different thicknesses

Welding of shipbuilding steels is considered a challenge. First, because of the requirement to keep the metallurgical continuity that gives the material mechanical strength and toughness. Second, due to the great thickness that is involved, the selection of welding parameters usually occurs empirically, without understanding the thickness effects. Therefore, this work aims to develop and validate a practical methodology for welding A131 DH36 naval steel plates with different thicknesses (7 mm, 10 mm, 12.7 mm, 19 mm, and 25.4 mm) by applying the multipass gas metal arc welding (GMAW) process. For this purpose, five plates were welded and qualified based on the AWS D1.1/D1.1 M:2020 standard. After being processed in the vertical up position, the effect of plate thickness on the number of passes and electrical parameters was studied, as well as the resulting microstructure. It was noticed, differently from expected, that the root and filling passes did not show large variations of electrical parameters as a function of thickness, while the finish passes required different energy levels for each one in order to meet the standard requirements. The cooling rates were estimated through the classical equations from Rosenthal and Rikalin, which enabled the prediction of the initial microstructure (without the effects of multipass), using the CCT diagram. The influence of thickness and welding energy on the cooling rate was studied. Then, microhardness and the resultant multipass microstructure were analyzed. The predicted fuzion zone (FZ) hardness varied from 195 to 339 HV depending on the thickness and type of pass. On the other hand, the measured hardness varied from 192 to 267 HV. The differences between the predicted and measured results can be explained based on the divergences caused by the assumptions used in the analytical equations and the peak temperature of the CCT diagram available, besides the divergences caused by the subsequent passes. The joints did not present defects and were within the required specifications in all tests performed: hardness, tensile, Charpy, and bending.