The effects of carbon and niobium on solidification cracking susceptibility of Fe-36%Ni alloy (Invar alloy) were investigated to prove the validity of the addition of both carbon and niobium for preventing the solidification cracking as well as the ductility dip cracking. The tests were carried out using the Fe-36%Ni plates containing various contents of carbon and niobium. The solidification cracking susceptibility was evaluated by transverse Varestraint test using gas tungsten arc welding (GTAW). The microstructure of weld metal quenched by liquid tin was also observed. The solidification cracking susceptibility decreased with the increase of niobium in a range less than 1.5 mass% in weld metal containing 0.2 mass% carbon. In weld metal with 0.2% carbon, the eutectic microstructure of niobium carbide and austenite was produced in the dendrite space of primary austenite. Additionally, the residual liquid at the final stage of solidification disappeared at a higher temperature in weld metal containing 0.2% carbon and 1% niobium compared with the weld with 0.02% carbon and 1% niobium. From these results, the decrease of solidification cracking susceptibility by the addition of both carbon and niobium is considered to be due to reducing the temperature range, which both solid and liquid exist together, as a result of the decreasing niobium content in residual liquid by the formation of niobium carbide.
