MICROSTRUCTURAL CHARACTERIZATION OF INCOLOY 903 WELDMENTS

The fusion zone and the heat-affected zone (HAZ) microstructures obtai ned during electron beam welding of thermomechanically processed INCOL OY 903 have been characterized by analytical electron microscopy. The microsegregation observed during solidification in the fusion zone ind icates that while Fe and Co segregate in the gamma dendrites, Nb, Ti, and C are extensively rejected into the interdendritic liquid. Electro n diffraction and energy dispersive X-ray microanalyses on secondary p hases extracted from the fusion zone, from the HAZ microfissures, and from the HAZ grain boundaries on carbon replicas established the major secondary solidification constituent formed from the interdendritic l iquid to be cubic niobium-rich MC carbides. Laves phase was observed o nly in trace amounts. Continuous sheets of the MC carbides were observ ed in the HAZ grain boundary microfissures, while the HAZ grain bounda ries that resisted microfissuring were devoid of the continuous sheets of carbides and/or extensive fine carbide precipitation. The microstr ucture observed in the HAZ microfissures suggests grain boundary liqua tion and formation of low melting liquid films enriched in niobium and carbon. The liquid originates primarily from the constitutional liqua tion of primary MC carbides, MNP phosphides, and fine MC carbides alre ady present on the grain boundaries of the base metal. The solidificat ion pattern of the liquid films on the microfissures is observed to fo llow a similar pattern as that of the fusion zone. The observation of microfissuring on boundaries which show continuous and/or semicontinuo us sheets of resolidified structure suggests that HAZ microfissuring o ccurs due to the presence of low melting liquid films. The existence o f continuous and/or semicontinuous liquid films and the added presence of phosphorus in them appear to be one of the major causes for extens ive HAZ microfissuring in thermomechanically processed INCOLOY 903.