The effects of alloyed elements on ductility dip cracking susceptibility in multi-pass weld metal of Fe-36%Ni alloy (Invar alloy) was investigated to obtain the proper chemical compositions for preventing the ductility dip cracking and to clarify its mechanism. The tests were carried out using the Fe-36%Ni plates containing various contents of manganese, titanium, calcium, carbon and niobium. The ductility dip cracking susceptibility was evaluated by double bead longitudinal Varestraint test and restraint weld cracking test using gas tungsten arc welding (GTAW). The ductility dip cracking susceptibility decreased with the addition of both carbon and niobium. By the addition of these elements, the micro structure of weld metal was changed from the cellular structure with straight grain boundary to the cellular dendritic structure with grain boundary like saw teeth. Additionally, in the weld metal containing both niobium and carbon, the eutectic microstructure, with the niobium carbide, was produced. From these results, the prevention of ductility dip cracking in weld metal, with the addition of both niobium and carbon, is thought to be due to: (1) the decrease of sulfur concentration in the grain boundary by formation of eutectic structure with niobium carbide; and (2) the locking of the intergranular slip by the anchoring effect of the cellular dendritic structure with grain boundary like saw teeth.
