Controlling Precipitation of Alloy 718 with Different Nb Content and Relevant Effect on Mechanical Behaviour

Abstract

In oil and gas fields high levels of hydrogen sulphide, carbon dioxide, chloride and sulphur at high pressures and high temperatures require material fine tuning in terms of chemical composition, microstructure and mechanical properties to assure required stress corrosion cracking resistance. 718 alloy is applied in various components operating in severe environments. Pilot scale production has been realised, with a narrow control of Nb content (4.8-5.2 mass%) to investigate chemical composition effect on material properties, for different heat treatments. SEM/FEG and TEM investigations have been carried out. The results give correlations between Nb content and hardness, tensile strength and ductility. Heat-treatment strongly affects grain boundaries delta-phase formation, volume fraction and mean size of fine precipitates inside grains.To better understand experimental results thermodynamic and kinetic calculations have been performed: results by MatCalc and JMatPro commercial software are compared and discussed in order to have information on reliability and applicability of these models.