GMAW root pass of shipbuilding steel plates with different thicknesses

The root pass represents a challenge for welders. Being the first pass of the joint, it requires full penetration and is more prone to metallurgical defects. There needs to be a balance between the forces acting on the molten pool to avoid incomplete penetration or burnthrough. Additionally, the hardness in the heat affected zone (HAZ) should not exceed 350 HV, beyond which there is susceptibility to cold cracking. When different thicknesses are present in the joints, it is often thought that greater thicknesses require higher welding energy (the ratio between power and welding speed). This has also been verified in the literature. The present work aims to test if it be possible to weld the root pass of four plates of different thicknesses (7 mm, 10 mm, 12.7 mm, and 25.4 mm) considering a similar welding energy. This would make the parameterization robust, as the welder would not need to change the welding energy to perform the process under different conditions. An experimental evaluation was conducted on shipbuilding steel ASTM A131 DH36 using the GMAW process, evaluating both the geometric characteristics of the weld bead and the microstructure at different thicknesses. Cooling rates were predicted based on an in-house finite volume method (FVM) computational code. The results indicated that although all welds met the main requirement of full penetration, the metallurgical requirement of a maximum hardness of 350 HV in the HAZ was only achieved at thicknesses of 7 mm and 10 mm. This occurred because, in greater thicknesses (12.7 mm and 25.4 mm), the cooling rate was elevated due to the thickness itself and the use of a higher feed rate. Consequently, in the coarse grain heat affected zone (CGHAZ), there was a shift from the ferritic field to the bainitic field. To meet the requirements, it is advisable to adjust parameters, such as increasing weld energy or applying preheat treatment. Another alternative involves planning subsequent passes to induce a tempering effect on the root. In summary, for geometrical purposes, a constant energy can be used, whereas metallurgical objectives might necessitate greater energy input with increasing thickness.