Rotating Parts in Gas Turbines
The Rotating Parts in Gas Turbines require outstanding creep & stress rupture properties combined with good fatigue strength at elevated temperatures. Alloys selected for the manufacture of rotating parts such as discs, blades, spools and shafts, have to be strong and structurally reliable at extreme conditions.
Tight dimensional tolerance within the turbine casings allow no room for creep growth even after tens of thousands of rotations at temperatures as high as 900°C. Under such high centrifugal stress and temperature, a single micro-sized defect can rapidly propagate leading to catastrophic stress-rupture failure.
In the hot-end section, rotating parts are made of Super alloys such as Alloy 718, Alloy 80A, Alloy 90, Alloy 105, Rene 41, 188, L605 (Alloy 25) and Waspaloy®.
Cobalt and molybdenum, found in some of these Super alloys, enhances the alloys’ high temperature mechanical properties. Other highly engineered alloys used are Alloy 418 (Greek Ascoloy), Titanium 6-2-4-2. Titanium 6-2-4-6 and Titanium 6AL-4V (Ti Grade 5).
Alloy 105, Alloy 90, Rene 41, and Alloy 718 for instance, possess exceptionally high creep and stress-rupture resistance up to 950°C, 920°C, 982°C and 705°C respectively. Stainless steel Alloy 418 (Greek Ascoloy) is also a popular choice for high-stress components up to 649°C. In rotating parts, Waspaloy® is used at temperatures up to 870°C.
Titanium alloys, being 50% less dense than average nickel alloys, are preferable for use in the aircraft rotating parts as they offer high strength-to-weight ratio.
Titanium 6-2-4-2 has high tensile strength, good creep resistance, toughness and high temperature stability for long term application at temperatures up to 538°C in the annealed and aged condition.
Titanium 6-2-4-6 Properties of this alloy are influenced by its thermo-mechanical history. Enhanced strength, ductility, and low-cycle fatigue properties are contained in alpha-beta forged material. Beta-forged material contains the best combination of good low-cycle fatigue and fatigue-crack growth resistance.
Titanium 6AL-4V (Ti Grade 5) is utilised at service temperature up to 400°C but thinner sections can be heat treated to increase mechanical properties for use up to 600°C.
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