VALERIU GABRIEL GHICA, MIHAI BUZATU, GHEORGHE IACOB, MIRCEA IONUT PETRESCU ELECTRON MICROSCOPY RESEARCHES ON THE MICROSTRUCTURE OF INCONEL 718 ALLOY
Identification and characterization of basic austenitic matrix, carbides, and hardening precipitates present in superalloys are crucial for establishing and controlling the heat treatments to which the material must be subject in order to achieve the required technological parameters: high mechanical properties, resistance to creep, fatique and corrosion. The microstructure of Inconel 718 nickel-iron based superalloy, used for execution of anchoring heads in Cernavoda nuclear power station units, is characterized and analyzed in this paper. Forged Inconel 718 superalloy was subject to the following heat treatment: solution annealing treatment at 1000°C, air cooling, and two steps ageing treatment (heating at 771°C for 8 h, cooling in furnace up to 660°C, 10 h maintained at that temperature, air cooling). Several calibrations of the heat treatment furnace were carried out to meet the required heat treatment regime (very restrictive ± 1°C). The microstructure was investigated by optical and electron microscopy using JEOL 100 CX / ASID 2D equipment and by energy-dispersive X-ray spectroscopy (EDX). Two-steps replicas were used for the identification and characterization of the phases present in the austenitic matrix. The ratio of the alloy phases is affected by small variations in the applied heat treatment. The purpose of the study was to investigate the characteristic phases present in the Inconel 718 superalloy, especially identification and analysis of the g’’(Ni3Nb) hardening precipitates morphology.