Characterization of Fillet Welded Armor Steel Performed by Robotic Gas Metal Arc Welding: Effect of Heat Input on Microstructure and Microhardness

In this research, fillet welding was conducted on 8-mm thick Miilux OY Protection 600 (MIL-A-46100) armor steel using AWS A5.9 GeKa ER307 austenitic filler wire. The welding process involved robotic MIG/MAG with five different heat inputs ranging from 0.3 to 1.2 kJ/mm. The study focused on examining the influence of heat input on the microstructure, elemental changes, microhardness, and dimensions of the weld metal and the heat-affected zone (HAZ). These investigations were conducted to determine the welding parameters that they satisfy the quality requirements of the MIL-STD-1185 standard for this steel grade and weld consumable. Through analysis of macrostructure, microstructure, and microhardness, it was observed that increasing the heat input led to a decrease in hardness for both the weld metal and the HAZ, while expanding the HAZ width. The weld metal exhibited a homogenous hardness distribution at lower and higher heat inputs, but hardness increased from the root to the face for both heat inputs of 0.5 and 0.7 kJ/mm welds. Notably, a significant decrease in hardness occurred in the transition of partial transformation region (intercritical HAZ) and tempering region (subcritical HAZ) for heat inputs above 0.7 kJ/mm, indicating softening. Moreover, the width of the subcritical heat-affected zone substantially increased. Evaluation of the distance required to reach base metal hardness from the welding toe revealed that a heat input of 1.2 kJ/mm exceeded the maximum requirement of 15.9 mm according to the MIL-STD-1185 standard. However, the requirements of the military standard were satisfied for other heat input values. These findings were associated with microstructural changes in grain size, martensite, bainite, martensite/austenite morphology and their fractions, as well as delta ferrite morphology. The results successfully demonstrated that robotic GMAW welding can be applied using lower strength (undermatched) filler metal to satisfy the requirements of the respective standard of MIL-STD-1185.