Thermodynamic Study on Denitrification and TiN Precipitation in IN718 Nickel Base Superalloy Melt

Abstract

Low nitrogen content can control the formation of TiN inclusions， thereby improving the metallurgical quality and comprehensive performance of nickel based high-temperature alloys. This article first compared the differences between the standard Gibbs free energy and the interaction coefficient of alloy elements with N， Ti activity in nickel based and iron based metal liquids， it pointed out that two different sets of thermodynamic data from nickel based and iron based metal liquids cannot be mixed.The thermodynamic model for nitrogen removal of IN718 alloy was established based on the existing thermodynamic parameters of nickel based alloys. It was found that reducing temperature and increasing vacuum degree to reduce nitrogen partial pressure while meeting other smelting conditions are the direct means to reduce nitrogen content in the alloy liquid. The addition of different alloying elements has different effects on nitrogen removal. At the same time， the thermodynamic model and segregation model for TiN precipitation were established， and the calculation results showed that under the production temperature of IN718 at 1 450 ℃， the ［N］ content in the alloy liquid was controlled below 43.69×10-6， the solute elements N and Ti are enriched and precipitated in the liquid phase during solidification. The calculated precipitation temperature is 1 468 K， and the solid fraction  is fs 0.829. If we want to control the solid phase fraction  fs to be above 90% or 95% respectively for precipitating TiN inclusions， ［N］ needs to be controlled below 25.00×10-6 and 15.00×10-6 respectively.

Keywords

• nickel base superalloy； denitrification； tin precipitation； thermodynamics