|Fig. 22-2 Precision forging.|
15. The engine drive shafts, compressor discs, turbine discs and gear trains are forged to as near optimum shape as is practicable commensurate with non-destructive testing i.e., ultrasonic, magnetic particle and penetrant inspection. With turbine and compressor blades, the accurately produced thin airfoil sections with varying degrees of camber and twist, in a variety of alloys, entails a high standard of precision forging, ret. fig. 22-2. Nevertheless precision forging of these blades is a recognised practice and enables one to be produced from a shaped die with the minimum of further work.
16. The high operating temperatures at which the turbine discs must operate necessitates the use of nickel base alloys. The compressor discs at the rear end of the compressor are produced from creep- resisting steels, or even nickel base alloys, because of the
high temperatures to which they are subjected. The compressor discs at the front end of the compressor are produced from titanium. The higher strength of titanium at the moderate operating tem- peratures at the front end of the compressor, together with its lower weight provides a consider- able advantage over steel.
17. Forging calls for a very close control of the temperature during the various operations. An exceptionally high standard of furnace control equipment, careful maintenance and cleanliness of the forging hammers, presses and dies, is essential.
18. Annular combustion rings can be cold forged to exacting tolerances and surfaces which alleviates the need for further machining before welding together to produce the combustion casing.
19. H.P. compressor casings of the gas turbine engine are forged as rings or half rings which, when assembled together, form the rigid structure of the engine. They are produced in various materials, i.e., stainless steel, titanium and nickel alloys.