Nickel is a tough metal that has very strong resistance to corrosion and oxidation. It is widely used as an alloying element with metals such as copper, molybdenum, and iron. Nickel is most commonly alloyed with chromium to produce stainless steel.
Nickel is extracted from nickel-rich ores and then refined into pure nickel, ferronickel (an alloy of iron and nickel), and nickel oxide. These nickel products are then melted and combined with other metallic elements and alloys to produce nickel alloys. In a molten state, nickel alloys are cast into ingots. To achieve high corrosion resistance, ingots of nickel are typically cast in special vacuum arc furnaces.
The resistance of nickel to oxidation at high temperatures makes the metal very suitable for jet engine parts and other applications involving hot, corrosive environments. Specialized nickel alloys strengthened to resist corrosion and high stresses at extreme operating temperatures (1200°F-2000°F, 650°C-1100°C) are known as superalloys.
Applications of Nickel Alloys
Nickel-based superalloys are commonly used in the manufacture of aircraft components. At temperatures above 1300°F (700°C), most ordinary steel oxidizes significantly in a few hours. Even stainless steel cannot survive for long at these temperatures. But nickel can withstand such severe conditions. Among the applications for nickel-based superalloys are gas turbine rotor blades in jet engines, where exhaust gases flow at temperatures exceeding 1800°F (980°C). The blades must resist corrosion to prevent excessive wear and any change to their shape. A typical turbine blade is designed to operate for well over 1000 hours of run time.
To achieve the properties necessary for operation, most turbine blades today are cast and then ground and polished to the final dimension. Nickel-based superalloys are very hard, and they have little ductility at room temperature. The use of cold-working processes to harden these alloys is impossible. Turbine blades are usually cast in a special fashion so that all of the grains in the metal are oriented in the long, stressed direction. Cooling methods are controlled to form long, rod-shaped grains in the metal. The final grain structure is very strong and allows the blades to resist elongation at high temperatures.