Development of shielded metal arc welding electrodes to achieve carbide-free bainitic weld microstructures

Carbide-free bainite (CFB) microstructures containing a mixture of nano-sized retained austenite laths and bainitic ferrite exhibit a good combination of strength, ductility, and toughness. In this work, an attempt was made to identify welding electrode compositions with a carbon content of about 0.35 to 0.5 wt.% to achieve carbide-free bainite microstructures in multi-pass shielded metal arc welds. Suitable alloy compositions were designed using a commercial neural network-based database, considering thermodynamic parameters such as allotropic phase boundary (X-To), Delta G(Delta) (r-a;) (driving force for transformation) and martensite start temperatures. Three different alloy compositions were identified using this approach. Shielded metal arc welding (SMAW) electrodes were fabricated with the compositions identified and the samples extracted from the weld deposits were used for dilatometer studies, metallurgical characterisation, and mechanical property evaluation. Based on the results, an optimised electrode composition and welding parameters were identified to stabilise the carbide-free bainitic microstructures in weld metal.