High strength Inconel alloy grades for aerospace and nuclear plant applications
The microstructure of Inconel alloys is featured by multi-component morphology, essential for their specific characteristics. These alloys mainly comprise nickel, chromium, aluminum, titanium, cobalt, boron, zirconium etc in strict concentration limits. The effect of chemical composition on the different properties of alloys as Inconel is discussed already several times. The relation between the microstructure of a nickel superalloy and their ultimate properties are well known in the wide range of temperatures from ambient level to 1100oC.
The components of aircraft engines have usually complicated structure, hence their development needs advanced metal forming processes. The task is not easy due to the specific technological properties of Inconel alloy such as deformation resistance, high intensity of strengthening and elevated temperature of recrystallization.
For the challenging application conditions of aerospace industry use of Inconel 718 wire is recommended. The alloy is produced in different forms such as sheet, wire, rod, pipe etc, through hot or cold rolling. Besides to traditional methods of forming, to produce these type of products flow and share forming methods are used.
Helium cooled reactors
Several high temperature helium cooled reactors are produced and operated for long durations. Helium is used as coolant in the primary circuit, although it is found to have contaminants nominally after constant operation that can cause environmental corrosion of high temperature service alloys in internals and heat exchangers. Based on the contaminant level, temperature, the materials can be susceptible to oxidation, carburization and decarburization. Magnitude of water and carbon monoxide is considered due to their control on the oxygen partial pressure and carbon activity. Suitable coolant composition for prolong stability of service materials is slightly oxidizing and thus causes development of an adherent Cr2O3 layer.
The essential component for the advanced nuclear plant is heat exchanger. Here use of Inconel 617 is recommended for this application as the alloy offers excellent resistance to creeping. Inconel 617 is used at different ranges in these nuclear plants.
Stability of microstructure and characteristics of Inconel 617 after prolong exposure to the high temperature anticipated in the nuclear plant heat exchanger are taken into consideration. Furnace aging of Inconel 617 in air results in development of an adherent oxide layer and a carbide loss zone under the layer. Some grain growth with passing time at high temperature and distribution of grain sizes becomes wider. Moderate changes occur in the mechanical properties in the aging conditions.
The main material in the core of nuclear plants is graphite and the key material for metallic internal components is Inconel 617. The cooling gas used for high temperature gas reactors is helium. However helium is pure, there are contaminants in ppm levels in this gas while serving in high temperature reactors. Engineers must review the performance of materials for control rods, core internals, and heat exchangers in the reactor helium condition. The need for high temperature service resulted in the development of high performance materials such as nickel based alloys. Incoloy 800H is used in control rods and Inconel 617 for structural components.