High Temperature Functional Alloys
Nickel based Superalloys are an exclusive group of metallic substances providing outstanding blend of elevated temperature strength, hardness and resistance to degradation in corrosive or oxidizing conditions. These materials are commonly employed in aircraft and power production turbine units, rocket engines and other industrial conditions such as nuclear power and chemical treatment plants. Exhaustive alloy and process production performances while the last decades have driven the production of alloys that can perform under average temperatures up to 1050oC with suitable expeditions to temperatures up to 1200oC that is almost 90% of melting point of a substance.
Nickel based superalloys normally make up 40 to 50% of whole weight of an aircraft engine components that operate at the high temperature while in use. Creep resistant turbine blades and vanes are commonly made by rigid investment casting processes that are important to extent quenching processes and for controlled grain structure. These parts may make up of equiaxed grain or columnar grains, discarding totally the high angle grain limits. Since the grain limits are places for damage occurrence at the elevated temperatures, the blades in initial levels of turbines are mono crystals and thereafter these are made from equiaxed alloys. Turbine disks are made through wrought processing using cast ingots or strengthened superalloy powder. Outstanding blend of strength, hardness and cracking resistance is obtainable in these materials through close control of microstructure through multiple levels of wrought treatment.
Various elements are included in nominal percentages to control the grain structure and mechanical characteristics that are manipulated by grain boundaries. Nominal inclusions of boron, carbon, hafnium and zirconium cause the production of borides or carbides, usually present at the grain boundaries. These elements are essential for controlling grain size while wrought treatment and for reduction of damage growth at the grain boundaries in operation. Carbon attains a large affinity for elements like hafnium,zirconium, tantalum, titanium, niobium, tungsten, molybdenum, vanadium and chromium and causes production of primary carbide directly from the liquid while solidification of nickel based superalloys.
Superalloy treatment starts with the production of big size ingots that are immediately employed for either of major treatment routes:
1. Remelting and immediate investment casting
2. Remelting before wrought treatment
3. Remelting to produce superalloys powder that is consolidated and subjected to wrought processing applications.
Ingots are produced through vacuum induction melting (VIM) in a refractory crucible to consolidate elemental and revert materials to produce a base alloy. However chosen alloys can significantly be melted in air or slag conditions in electric arc furnaces. VIM melting process of superalloys is widely effective in the removal of low melting point trace contaminants. Subsequent to vaporization of the contaminants, the carbon boil reaction is followed to deoxidize the melt prior the inclusion of reactive gamma’ producing elements like titanium, aluminum and hafnium. Upon receiving the alloy composition of VIM ingot, the solidified ingot is subjected to additional melting or consolidation processes that are based on the final use of the material.
Accounting to stringent needs for reducing defects in turbine engine parts, a complete understanding of structure development in any of these processing techniques is important.
The elevated temperature applications undoubtedly need cast superalloys that can offer the highest elevated temperature strength. although in some applications like combustion chambers, low strength and sheet alloys may be used. Nickel and Cobalt base alloys sheet can be used. Hastelloy X,Inconel 617, HA 188 and other alloys provide prolong service.