In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by an axial-flow compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through an axial-flow turbine. The flow of gas turns the turbine by contacting an airfoil portion of the turbine blade, which in turn provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
The various stages of the compressor and the turbine, as well as a turbofan if present, are mounted upon and linked together by shafts and shaft segments extending along the centerline of the gas turbine engine. Some of the shafts are made of high-strength steels. These shafts must have good strength, but equally importantly they must have good low-cycle-fatigue lives in torsion because of the types of loadings imposed upon the shafts.
Traditionally, the shafts had been made of maraging steels, which contain titanium nitride precipitates. After studies showed that these precipitates limit the low-cycle torsional fatigue lives, a family of high-strength, low-titanium maraging steels was developed. These steels are strengthened by aluminum additions on the order of from about 0.5 to about 1.3 weight percent, which replace the titanium additions of the earlier generation of steels. These higher-aluminum steels are described in U.S. Pat. No. 5,393,488, whose disclosure is incorporated by reference. The steels of the '488 patent result in significantly improved fatigue lives in the shafts.
However, an opportunity for improvement remains. There is an ongoing need to further increase the fatigue lives of the steels of the '488 patent, without adversely affecting the strength, toughness, and other properties of the steels, and without requiring major alterations to the processing parameters. The present invention fulfills this need, and further provides related advantages.