Shaft steels for engines
In the cooler, front end of gas turbine engines titanium alloys dominate because of their high strength. At the back end, the high temperatures require nickel based alloys to be used. However steel is still the material of choice in the main shaft of the engine owing to the combinations of very high strength and creep resistance it can provide.
Steels for aero engine main shafts require high strength to reduce the weight and physical size of the engine. For aero engine applications creep strength over relatively short durations of only a few thousand hours is required, as during a flight the engine is only under high loads for a few minutes during take-off and landing. (This compares with high temperature steels used in power plant where the design lives are in excess of 30 years for most of which time the plant may be in operation.)
The steels used in the shafts are medium carbon martensitic grades which can give strengths of about 2,000 MPa. Examples of two such grades are shown below:
Aircraft undercarriage materials are only under load when the plane is on the ground and during take-off and landing. A commercial aircraft may make 50,000 flights during its life and for each the plane will cover several miles on the ground taxiing and during take-off and landing. So the undercarriage may travel a distance similar to that for a car during its life. In flight the undercarriage is stowed in the wings which also act as the fuel tanks. It is important to minimise the weight and size of the undercarriage to reduce fuel consumption but also to allow space for the maximum amount of fuel in the tanks.
Anyone who has ever flown will be aware landings are often not smooth. A fully laden A380 has a weight well in excess of 500 tonnes and a landing speed of 150 mph so the undercarriage material must also be able to withstand high impact loads for which it requires a good level of toughness. Ultrahigh strength martensitic steels are used, such 300M – 0.4%C, 1.6%Si, 0.8%Cr, 0.5%Mo, 1.8%Ni, 0.07%V, or other similar NiCrMoV variants. When oil quenched and tempered at 300 oC these give yield strength of about 1650 MPa or above.
At these very high strength levels toughness is very sensitive to the presence of small defects such as non-metallic inclusions. To reduce their numbers the material often undergoes a secondary refining step, such as vacuum arc remelting, during production. This results in very high fracture toughness with a K1c value in excess of 60 MPa m0.5
These ultrahigh strength steels are very susceptible to cracking from hydrogen and stress corrosion. To prevent this, undercarriage components are often cadmium plated. However there are major environmental concerns with the use of cadmium owing to its high toxicity. There are moves to ban its use which means there is a need for alternative coating solutions or the development of a stainless steel with tensile strength of about 2,000 MPa.
A significant proportion of the world’s undercarriage steels are produced by Liberty Speciality Steel in the UK and it is estimated that a plane lands on their steel about every eight seconds!
Apart from the engine main shaft and the undercarriage, steels are used in a variety of other components in modern commercial aircraft.
This is illustrated in the image for the Airbus A340. The steels tend to be high strength and frequently stainless, so are highly alloyed. The range of alloys used includes 12%Cr martensitic stainless steels, precipitation hardening stainless steels (15/5PH) and maraging steels. These are used on flap and slat tracks which are integral parts of the systems which modify the wing shape to alter the degree of lift during take-off and landing. Other applications include the use of 15/5PH for more mundane, but no less important, applications such as door hinges and controls.