A study on the dynamic fracture toughness of weld heat-affected zone in ductile cast iron

The microstructure and the fracture toughness of the weld-affected zone of a ductile cast iron have been investigated with a thermal cycle simulator to simulate the actual welding process. The matrix structure of the heat-affected zone transformed to martensite, mixture of martensite and pearlite, pearlite, and pearlite including small amount of ferrite, in turn, with increasing cooling time from 800 deg C to 500 deg C. The use of a proper preheat temperature prevented the formation of martensite. The welding condition for this involved a preheat temperature of 200 deg C and a heat input of 30 KJ/cm. Also, a post-weld-heat treatment of 675 deg C for 2 hours was effective in reducing the hardness of ductile cast iron weldments by eliminating martensite. The secondary graphite formed due to martensite decomposition at that time. An instrumented Charpy impact test has been carried out on a welded and post-weld-heat-treated ductile cast irons, respectively. Results showed that the dynamic fracture toughness increased slightly with higher preheat temperature and heat input. The post-weld-heat treatment improved the dynamic fracture toughness of the weldments to three times that of the as-welded condition. (Author)