8th International Symposium on Superalloy 718 and Derivatives Edited by: Eric Ott, Anthony Banik, Xingbo Liu, Ian Dempster, Karl Heck, Joel Andersson, Jon Groh, Tim Gabb, Randy Helmink, and Agnieszka Wusatowska-Sarnek TMS (The Minerals, Metals & Materials Society), 2014 CONTROLLING PRECIPITATION OF ALLOY 718 WITH DIFFERENT Nb CONTENT AND RELEVANT EFFECT ON MECHANICAL BEHAVIOUR 1 1 1 2 2 Oriana Tassa , Laura Alleva , Arianna Gotti , Jutta Kloewer , Ali Aghajani 1Centro Sviluppo Materiali S.p.A., Via di Castel Romano 100, 00128, Rome, Italy 2ThyssenKrupp VDM GmbH, KleffstraBe 23, 58762 Altena, Deutschland Keywords: IN718, Heat treatment, Mechanical properties, Delta phase, Precipitation modeling 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. Introduction Age-hardenable alloys are used as corrosion resistant materials in the oil and gas industry because of their combination of high strength, corrosion resistance, and ability to be heat treated to various strength levels. Nickel-base alloys 718, 725, and 925 are commonly used in oil and natural gas production [1]. Components used in these severe environments must resist SCC. The potential for SCC becomes greater with increasing temperature and concentrations of H2S and the presence of chloride ions and elemental sulfur. Alloy strength is another factor. As strength increases, environmental cracking susceptibility also increases. A proper material design is required to achieve the optimum level of strength, ductility, toughness and cracking resistance. These alloys contain high Cr/Mo levels for preventing aqueous corrosion, and also considerable levels of hardeners like Ti/Nb/Al to form gamma prime and gamma double prime precipitates for improving strength. Being heavily alloyed multi-component systems, these materials require special solutions for processing and heat treatments. Several new alloys have been developed for 849