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The base metal microstructure was characterized by equiaxed grains exhibiting many twins and a uniform distribution of primary W-rich MC-type carbides several microns in size throughout the grain interiors and along grain boundaries.
Examination of weld samples revealed evidence of significant amounts of eutectic melting of the W-rich MC carbides adjacent to the weld fusion boundary. In addition, cracks were observed along liquated grain boundaries in the weld HAZ (PMZ), especially in regions near the “nailhead” of keyhole welds. Changes in weld parameters were not successful in eliminating eutectic melting. However, cracking was eliminated by limiting travel speeds to 15 mm/s for keyhole-type welds or by changing focus and power to produce deep conduction mode welds.
Cracking requires the simultaneous presence of a susceptible microstructure and a strain level in excess of that which can be borne by the microstructure. Cracking can be eliminated by removal of one of these factors. In the present case, the susceptible liquated microstructure persisted despite changes in weld parameters. The lower travel speeds of the crack free welds either lowered the strain on the liquated grain boundaries or allowed backfilling of cracks. Microstructures and data from hot ductility studies will also be presented and discussed to further elucidate the eutectic melting phenomenon.