Thermodynamics of nitrogen solubility in nickel-based alloys at plasma-arc remelting

Heat-resistant nickel-based alloys are widely used in the domestic aircraft industry, rocket engineering, and instrument making. Increasing the basic mechanical and operational characteristics of the metal is achieved mainly by alloying the base with various elements such as rhenium, ruthenium, hafnium, etc. However, increasing the operational properties can also be achieved by metal nitriding, as a result of which interstitial solid solutions are formed (finely dispersed nitrides), which increase the strength of the alloy. The work is devoted to the study of nitriding process of nickel-based complex alloys. Various options of the interaction of nitrogen with the melt are considered under conditions of open melting and processing with low-temperature nitrogen-containing plasma. The use of plasma-arc remelting allows one to obtain various types of gas in form of atoms, ions and molecules in plasma. The first two forms are much more active than molecular nitrogen, which leads to super-equilibrium concentrations in the alloy. A thermodynamic analysis of nitrogen solubility in a nickel-based melt was performed during open melting and under plasma-arc remelting. The nitrogen solubility in model EP741NP alloy was calculated depending on the partial pressure of nitrogen above the melt surface and in the plasma-forming gas. It was shown that treatment of the melt with nitrogen-containing plasma makes it possible to obtain higher content of nitrogen in the alloy. In this work, temperature of the metal in the zone of contact with plasma arc was estimated using the technique according to which the evaporation of the melt components from the arc spot area occurs when the metal boiling point is reached.

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