The weldability of Fe-36%Ni alloy (Invar alloy) was investigated to clarify the main factor influential for weld cracking in multi-pass weld metal. The tests were carried out using the Fe-36%Ni plates containing various contents of sulfur. The weld cracking susceptibility was evaluated by double bead longitudinal Varestraint test and restraint weld cracking test using gas tungsten arc welding (GTAW). The ductility of weld metal at an elevated temperature was evaluated by hot ductility test. The crack in multi-pass weld metal occurred in the grain boundary away from the fusion boundary of the subsequent weld pass. From the result of hot ductility test of weld metal, the reduction of area was lower at temperature range from 1073 to 1173K. This range was consistent within the temperature range where cracking portion in the multi-pass weld metal was heated by subsequent weld thermal cycles. Consequently, the crack in the multi-pass weld metal was specified as ductility dip cracking. The ductility dip cracking susceptibility was increased in the higher sulfur content weld metals and sulfur concentration was detected on that crack surface. Further, the oxides and sulfides, such as Mn2SiO4 and α-MnS, were very slightly observed on the grain boundary in weld metal. From these results, the ductility dip cracking in weld metal was concluded to be mainly caused by grain boundary embrittlement by segregation of sulfur.
