High-Temperature Melt Treatment in the Production of Heat-Resistant Alloys with a High Content of Foundry Wastes

Metallurgical wastes are used increasingly in alloy production. Given the great volume and age of the recovered material, the content of undesirable elements and nonmetallic inclusions in the batch tends to increase. That sharply impairs the structure and properties of the castings obtained. In turn, that is reflected in the polytherms of the melts’ physical properties and correspondingly the necessary smelting temperature and time for heat-resistant alloys. In the present work, the temperature dependence of the electrical resistivity, the kinematic viscosity, and the surface tension are studied for heat-resistant Ni–Nb–Cr–Mo melts. For EP902 alloy, the critical temperatures leading to irreversible changes that improve the state of the melt are established. The quantity of foundry wastes in the batch affects the temperature dependence of the melt’s physicochemical properties. With increase in the quantity of foundry wastes in smelting, the critical temperatures rise. The influence of the state of the melt on the solidification and structure of the hard metal is investigated. Solidification of EP902 alloy is studied by differential thermal analysis. It is found that solidification begins with the deposition of a solid solution based on γ phase and ends with the formation of a eutectic based on the intermetallide Ni₃Nb. If the melt is heated above the critical temperatures, greater supercooling is observed but the eutectic temperature is practically unaffected. On the basis of the results for the melt’s physicochemical properties and its solidification, we propose high-temperature treatment of the melt to significantly improve the quality of castings obtained from heat-resistant alloys of EP902 type with a considerable content of foundry wastes in the batch. Experimental melts are subjected to mechanical tests with optimal high-temperature treatment of the melt. For melts containing 50% foundry waste, high-temperature treatment of the melt permits the production of castings with strength and plasticity that exceed the technical specifications and are consistent from melt to melt.