More Efficient Turbines
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The evolution of gas turbines is linked to nickel alloy development By John MilneNickel Magazine, March 2007 -- A typical jet engine today contains about 1.8 tonnes of nickel alloys and includes a long list of tailor-made nickel-base alloys to meet specific needs. The useful life of a modern commercial jet engine is about 20,000 operating hours between overhauls, compared with just 5 hours for prototype engines in the late 1940s. Sixty years ago, Germany and England began experimenting with gas turbines as a source of power for aircraft. The intake fan of the turbine compressed air and fed it into a combustion chamber where the burning of a liquid fuel caused the hot gas to expand; this expansion drove the hot section turbine and the air intake fan. Today’s gas turbines function in much the same way. The life of the early gas turbines was limited to about 5 hours because the steel alloys available then could not withstand the high temperatures (950-1,100°C) in the combustion section of the jet engine. The engine functioned, but its practical use was limited by the corrosion of the materials then available for the hot gas section. For the gas turbine to become the efficient and reliable engine it is today, improved alloys were needed. Nickel, because of its inherent strength, resistance to corrosion, and ability to alloy with other metals, was the base metal of choice for the development of better alloys for jet engines. Metallurgists in the 1940s and ’50s were aware that nickel-chromium and nickel-chromium-cobalt alloys were stronger and more resistant to corrosion and oxidation than the stainless steel alloys then in general use. These nickel-chromium and cobalt alloys could extend the life of gas turbines. Early alloys, such as N06600 (containing 72% nickel), encouraged metallurgists to develop more efficient and durable alloys for jet engines. Has the development of nickel-base alloys and subsequent processing techniques for their wrought and cast product forms reached a plateau? The efficiency of a gas turbine depends on the difference in temperature between the air intake, ambient air and the combustion chamber. The hotter the combustion zone, the greater the amount of energy that can be derived from the fuel. That’s why we needed alloys that could handle hotter combustion chamber temperatures. As the metallurgical industry evolved, new alloys were developed to meet this challenge. They were mostly nickel-base alloys with additions of chromium and other elements to enhance strength and oxidation/corrosion resistance at higher temperatures. Alloy development, as with the gas turbine itself, evolved in three stages. First, the alloys were improved by increasing nickel and chromium content in both wrought and cast alloys and by using vacuum melting techniques to reduce the formation of harmful oxides. Researchers then considered enhanced alloy compositions. Alloy additions of elements other than nickel, chromium or cobalt were the next step for higher-temperature service. In particular, the addition of tungsten, vanadium, molybdenum and niobium led to the creation of complex alloys containing as many as 12 different metallic elements. Among the alloys created was N06102 (containing 68% nickel), which had very good properties for jet engine use. Improvements in vacuum melting techniques made the creation of such complex alloys possible while maintaining alloy cleanliness and a homogenous microstructure. Next came the use of coatings, such as aluminides (CoAl or NiAl for example), which could be applied to the basic alloy parts for better resistance to corrosion or oxidation. In the third stage of development, cast alloy components, such as the hot section turbine blades, were a weak point, owing to grain boundary segregation of some elements during solidification of the molten metal. This problem was resolved by the development of new casting techniques for the hot section turbine blades. Directional solidification and single crystal castings, for example, allowed gas turbines to operate at even higher temperatures, which translated into greater fuel efficiency. Today we await further metallurgical refinements that will take gas turbines to the next level. Such improvements will rely on the continued evolution of nickel-base alloys. John Milne is a consultant to the Nickel Institute. PHOTOS: Rolls-Royce
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