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.2.6 .lloy. A286 is an austenitic iron base alloy that has been used for years in aircraft engine applications. Its use for industrial gas turbinesstarted about1965， when technological advances made the production of sound ingots sufficient in size to produce these wheels possible.
As knowledge of the capabilities of M-152 increased， production of the wheels was switched from A286 to M-152. A286 is currently being intro-duced in turbines as part of a composite aft shaft.
Compressor Blades
Compressor blading is variously made by forging， extrusion， or machining.All productionblades， untilrecently， have been made from Type 403 or 403Cb (both 12 Cr) stainless steels. During the1980s， a new compressor bladematerial， GTD-450， a precipitationhardened， martensitic stainlesssteel， wasintroduced into production for advanced and uprated machines， as shown in Table 2. This material provides increased tensile strength without sacrificing stress corrosion resistance. Substantial increases in the high-cycle fatigue andcorrosion fatigue strength are also achieved with this material， compared to Type 403. Superior corrosion resistance is also achieved due to high concentrations of chromium and molybdenum. Compressor corrosion are usually caused by moisture and salt ingested by the turbine. Coating of compressor blades is also highly recommended.
Forgings and Nondestructive Testing
Most other rotor parts in gas turbines are individually forged. This includescompressor wheels， spacers， distance pieces， and stub shafts. All are made from quenched and tempered low-alloy steels (Cr-Mo-V or Ni-Cr-Mo-V) with the material and heat treatment optimized for the specific part. The intent is toachieve the best balance of strength， toughness with ductility， processing andnondestructive evaluation capability， particularly when it is recognized that some of these parts may be exposed to operating temperatures as low as -60 oF (-51 oC).
It is recommended that parts are sonic and magnetic particle tested. Many last-stage compressor wheels are spun in a manner analogous to turbine wheels as a means of proof testing and imparting bore residual stresses. This last-stage compressor wheel is probably the next most critical rotor com-ponent after the turbine wheels， especially in the new very high pressure ratio compressors.
New nondestructive techniques to inspect turbine forgings to greater levels of sensitivity than ever before possible have been developed. These new ultrasonic inspection techniques are being applied to all the turbine forgings to ensure an even greater level of confidence in these high strength forgings.
Additional development efforts continue to improve the current pro-cessing of other forgings by working with our suppliers on the furtheroptimization of properties and forging quality. In-process， nondestructive evaluation of all rotor components continues to be emphasized as a critical aspect to produce quality forgings.
Ceramics
The day when turbines will operate at 2500-3000 oF (1371-1649 oC)，yielding double the present horsepower at half the present enginesize， may not be far off. This dream may turn into reality because of ceramics andunique cooling systems. Ceramicswere， untilrecently， dismissed as being toobrittle， hard to fabricate，and not suited to flight engines. However， the addition of aluminum to ceramics forms a compound that is more ductile.
Temperature limits of flight engine alloys have been steadily increasing about 20 oF (11 oC) per year since 1945. Transpiration and internally cooled metal blades have resulted in higher temperatures and more efficient opera-tion. But the direct correlation between efficiency and fabrication cost has resulted in a situation of diminishing returns for the superalloys. As moreand more cooling air is needed for the superalloy components， the efficiency of the engine drops to a point where turbine inlet temperatures around 2300 oF (1260 oC) are the optimumand， at thatpoint， they are uneconomic for automotive use.
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燃气涡轮工程手册 Gas Turbine Engineering Handbook 2(61)
