Machinability variations in Alloy 718 —With focus on machining of turbine components

University dissertation from Lund University

Abstract: Popular Abstract in English You may have never heard of gas turbine engines, but they are used in all kinds of un¬expected places. For example, many of the helicopters you see, a lot of s¬maller power plants and even the military vehicle use gas turbines. A gas turbine consists of a compressor, a combustion chamber and a turbine. Air is taken into the compressor and mechanically compressed. It is then mixed with fuel in the combustion chamber and ignited. The exhaust gases expand and are either used to produce thrust (in aircraft engines) or to produce mechanical power (in industrial gas turbines). In the first case, the gas stream is used to propel an aircraft, that is, the purpose of the turbine is only to drive the compressor. In the latter case, the desired output is mechanical power which means that more energy is taken out of the exhaust gases due to more turbine stages. The mechanical power is provided as shaft power and can for instance be used to drive a generator to produce electrical power. The efficiency of such an industrial gas turbine can be further increased by using the exhaust gases to produce steam in a heat exchanger. The steam can then be used for heating purposes or to produce more electricity in the steam turbine. To increase the efficiency of gas turbines, one is aiming at high temperatures at the turbine entrance, which has led to a massive development of new materials (for example, superalloys and thermal barrier coatings) and also material processing techniques and technology (directionally solidified and single crystal blades, for instance) for turbine blades, discs, and combustion chambers. This is the fundamental reason for the development and application of the superalloys, such as Alloy 718, and their continued improvement. The production of turbine was studied through-out this work, which includes three different turbine discs made of Alloy 718. This material is suitable for using in this section of turbine which operates at working temperature of about 800 ˚C. Alloy 718 fully retains its strength up to 700º C, yet it maintains high enough strength, for this turbine operation, up to 800 ˚C. That is main reason for using of Alloy 718 for turbine discs. The mechanical properties of this material are very suitable for high efficiency but at the same time this material is very difficult-to-machine because of these properties. The turbine discs are amongst the most critical components in the turbine. The primary function of the turbine discs is to provide a fixture for the turbine blades located in the gas stream, from which mechanical energy is extracted. The complete assembly of discs and blades is then capable of transmitting power to the fan and compressor section, via the shafts which run along almost the complete length of the turbine/engine.