Metallurgical Investigation of Heat Input on Robot-Welded Joints Using GMAW Pulsed Process on Structural Steel

Abstract

The effect of heat input on microstructural zones, hardness and fracture of T-joints in EN S235JR carbon steel was evaluated using the Vickers hardness, SEM, and fractography on impact fractured specimens. The gas metal arc welding (GMAW) process with pulsed metal arc transfer and different welding speeds was performed by an industrial robot using a teaching pendant. Microstructural observations revealed differences in the size and quantity of phases exhibited in the solidified weld bead and recrystallized heat affected zone (HAZ) as well as a refinement of ferrite with different morphologies when a greater heat input was generated. The hardness results indicated greater hardening with greater heat input for the weld bead than the HAZ, which was associated with fine secondary Widmastatten ferrite sideplates growing from columnar grains separated by a larger amount of fine acicular ferrite. At a speed of 35 cm/min, the greater heat input produced a greater increase in volume fraction and width of HAZ with complex microstructures formed by fine-grained ferrite and acicular ferrite. The weld bead reached a greater hardening and volume fraction compared to the HAZ linked to fine acicular ferrite. The hardening of weld bead was confirmed by fractography that showed the presence of a small microvoid morphology caused by Charpy ductile failure with reduction of energy. The engineering and scientific relevance of this work is the original knowledge regarding the fractography and HAZ microstructural evolution of robotic welded joints as a function of heat input. The main problem solved with this research is the lack of knowledge regarding the welding metallurgy of robotic GMAW process.