Microstructural characterization of INCOLOY 903 weldments

Abstract

The fusion zone and the heat-affected zone (HAZ) microstructures obtained during electron beam welding of thermomechanically processed INCOLOY 903 have been characterized by analytical electron microscopy. The microsegregation observed during solidification in the fusion zone indicates that while Fe and Co segregate in the gamma dendrites, Nb, Ti, and C are extensively rejected into the interdendritic liquid. Electron diffraction and energy dispersive X-ray microanalyses on secondary phases 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 liquid to be cubic niobium-rich MC carbides. Laves phase was observed only in trace amounts. Continuous sheets of the MC carbides were observed in the HAZ grain boundary microfissures, while the HAZ grain boundaries that resisted microfissuring were devoid of the continuous sheets of carbides and/or extensive fine carbide precipitation. The microstructure observed in the HAZ microfissures suggests grain boundary liquation and formation of low melting liquid films enriched in niobium and carbon. The liquid originates primarily from the constitutional liquation of primary MC carbides, MNP phosphides, and fine MC carbides already present on the grain boundaries of the base metal. The solidification pattern of the liquid films on the microfissures is observed to follow a similar pattern as that of the fusion zone. The observation of microfissuring on boundaries which show continuous and/or semicontinuous sheets of resolidified structure suggests that HAZ microfissuring occurs due to the presence of low melting liquid films. The existence of continuous and/or semicontinuous liquid films and the added presence of phosphorus in them appear to be one of the major causes for extensive HAZ microfissuring in thermomechanically processed INCOLOY 903.