Processing and Characterization of Inconel 625 Nickel Base Superalloy

Abstract

Nickel-based superalloy Inconel 625 is widely used in aeronautical, aerospace, chemical, petrochemical and marine applications due to its good mechanical properties, weldability and resistance to high temperature corrosion on prolonged exposure to aggressive environments. It is a solid solution strengthened medium strength superalloy, which contains chromium, molybdenum and niobium as alloying additions. Considering the chemistry and specification requirements of the alloy, it was processed through vacuum induction melting (VIM) process followed by electro slag remelting (ESR) route to obtain alloy with controlled gas and inclusion contents. Homogenisation cycle was selected and was carried out at 1170°C temperature to obtain uniformity in chemistry and microstructure. Chemical homogeneity was confirmed through analysis of samples from top, middle and bottom of the secondary ESR ingot. Hot working range was decided considering the flowability of superalloy and the same was carried out under close monitoring of temperature and with specified amount of reduction per stroke. Intermediate reheating and reduction during forging was noted to be an important aspect so to avoid cracking during forging. Processing parameters were established to obtain forgings of different thicknesses with sound ultrasonic quality. Microstructure analysis revealed single phase austenitic grain structure with ASTM grain size no. 4-7, confirming that material has undergone sufficient amount of mechanical working. Mechanical testing was carried out and the mechanical properties were found to be meeting the requirement. Present paper provides details of melting process selection, thermomechanical processing and characterization of the superalloy to achieve the targeted mechanical properties.