Development of a new coated electrode with low nickel content for welding ductile iron and its response to austempering

Coated Electrodes with small amounts of nickel were developed for welding ductile iron (DI) and conversion of the same into austempered ductile iron (ADI) after austempering. Among six electrodes, Trials 3 and 4 were selected for establishing crack-free weld deposits via preheating and post-weld heat treatment. Austenitization was performed at 900℃ for 2 h and austempering at 300 or 350℃ for three different holding times to observe the results of austempering with respect to the microstructure, hardness, and austempering kinetics of the samples. The microstructures of the weld deposits showed needle-like bainitic ferrite with small amounts of retained austenite when treated at 300℃ and feathery bainitic ferrite with large amounts of retainedaustenite when treated at 350℃. The electrode labeled with Trial 3 revealed greater austenite contents than that labeled with Trial 4 when treated with a 2 h holding time regardless of the austempering temperature applied. The transformation rate of the bainitic ferrite of Trial 3 was relatively higher than that of Trial 4 and showed a lower rate constant, leading to a higher diffusion rate of carbon in austenite.