Materials used in the manufacture of the blades consist of mainly three different elements, namely iron, nickel, and cobalt with chromium which forms one of the major alloying elements, since it gives high resistance to oxidation. Other alloying elements that have been used include most of the metals of the periodic table.
Materials working under high stress and high temperature have a particular rate of creep. It means elongation under stress is not a fixed quantity as it is in case of normal temperature. Thus, elongation continues to increase with time and the blades gradually take up the original gap provided at their tips. Therefore, contact with casing results in failure. The repeated heating and cooling of the material affect the physical properties of the material.
1 Selection
The turbine blades are the most conditioned members of a gas turbine. They must withstand the following:
- Effects of high operating temperature
- Centrifugal tensile stresses due to rotational speeds of the order of 8000 to as high as 30,000 r.p.m.
- Bending stress due to equivalent impulse load of the gases acting at a certain distance from fixing of the cantilever blade.
- Hot erosive and corrosive effects due to high temperature combustion products e.g., CO2 and CO with O2.
Turbine blade material must, therefore, be selected from considerations of the working conditions.
2 Requirements of Blade Material
The material should possess the following characteristics:
- Maximum strength at high temperature
- High creep strength
- High resistance of corrosion
- Maximum erosion resistance
- Structural stability when exposed to varying temperatures
- Castability or forgability
- Weldability, if welding is used in the manufacture
- Machinability
- Absence of embrittlement in the service
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