INVESTIGATION OF BOUNDARIES AND STRUCTURES IN DISSIMILAR METAL WELDS

Cracking, or disbonding, along the fusion boundary in dissimilar metal welds has been a persistent problem, particularly in applications whe re austenitic alloys ale clad on to structural steels for corrosion pr otection. Many failures in dissimilar metal welds occur as a result of cracking along a boundary that runs parallel to the fusion boundary i n the adjacent weld metal. A preliminary investigation was under taken to determine the nature and evolution of boundaries and structure in dissimilar metal welds using a simple ternary system composed of a pur e iron substrate and a 70Ni-30Cu (Monel) filler metal. Changes in base metal dilution were found to alter the evolution of boundaries and st ructures near the fusion boundary dramatically. Optical metallography and electron microanalysis reveal that the resulting weld microstructu res and boundaries are similar to those observed in engineering materi als used for cladding and corrosion resistant overlay. Transmission el ectron diffraction analysis revealed orientation relationships between adjacent base metal and weld metal grains at the fusion boundary to b e different from the cube on cube relationship normally observed in si milar metal welds. A model is proposed describing the evolution of the boundary most susceptible to cracking in dissimilar welds.