Response of Austempering Heat Treatment on Microstructure and Mechanical Property in Different Zones of As-Welded Ductile Iron (DI)

Sound ductile iron (DI) welded joints were performed using developed coated electrode and optimized welding parameters including post weld heat treatment (PWHT). Weldments consisting of weld metal, partially melted zone (PMZ), heat affected zone (HAZ) and base metal were austenitized at 900 °C for 2 hour and austempered at 300 °C and 350 °C for three different holding time (1.5 hour, 2 hour and 2.5 hour). In as-weld condition, microstructures of weld metal and PMZ show ledeburitic carbide and alloyed pearlite, but differ with their amount. Whereas microstructure of HAZ shows pearlite with some ledeburitic carbide and base metal shows only ferrite. However, in spite of the significant variation in microstructures at different zones of weldment in as-weld condition, all the zones show similar microstructure of base metal such as bainitic ferrite along with some retained austenite after austempering heat treatment, indicating the response of heat treatment from different zones like base metal. However, the microstructure of each zone of weldment varies in shape, size and amount with changing the austempering temperature and holding time. In general, microstructure at 300 °C reveals needle shaped bainitic ferrite with lower amount of retained austenite; whereas at 350 °C microstructure shows feathery shaped bainitic ferrite with higher amount of retained austenite. After austempering weld metal shows lowest hardness followed by PMZ, HAZ and base metal, which is just opposite to as-welded condition, irrespective of austempering temperature and holding time. All the transverse tensile test weld samples austempered at 300 °C and 350 °C for 2 hour holding time, failed from the base metal indicating 100% joint efficiency.