Hitting the heights - steel in aeroplanes
Martin Worrall from Corus Group, Stocksbridge, UK, discusses the steels
used in modern aviation.
Media stories about new civil airliners have stressed the increasing use of lighter materials in constructions, such as in the Airbus A350 and Boeing 787 Dreamliner. However, traditional metals, including steel alloys, continue to play a major part in the efficient functioning of most aircraft and helicopters.
Aircraft and helicopter designers look to reduce weight to improve operational efficiency, and therefore attempt to minimise the use of denser and heavier materials (including steel alloys). Aluminium and titanium alloys initially replaced some of the heavier materials used in airframes and modern carbon fibre composites have begun to replace these alloys. However, the specific properties of specialised steel alloys, such as strength-to-weight ratio and strength-to-cost ratio, ensure that steels remain competitive.
Most aircraft still use many tonnes of steel alloys (up to ~20% by weight) in their construction, to take advantage of their physical properties.
Globally, the civil market for aerospace steel is at least 260,000t per annum. The major uses are l landing gear/undercarriage components l engine shafts, discs, rings, and containment and exhaust casings l control surface actuators and wing pylons l gears l bearings.
The loaded weight of the largest modern jets can exceed 300t on landing. Absorbing this impact and carrying the weight of the aircraft itself requires strong, resilient landing gear systems, with excellent fracture toughness to withstand the stresses and temperature increases. A number of ultra-high strength alloy steels with tensile strengths of ~2GPa are routinely used. These engineering grades all contain ~0.30-0.40% carbon, but more importantly, have high levels of nickel, chromium, molybdenum and silicon, plus small amounts of vanadium, to give strength in larger sections, together with a high degree of ductility, toughness and temper resistance. These steel grades are then fabricated and
heat-treated to develop their ultimate strength. These steels are shown in the table below (Ultra-high strength alloy steels).
Following suitable heat treatment, the alloys develop mechanical properties for use in undercarriage components such as:
0.2% Proof strength 1,500 MPa minimum
Ultimate tensile stress 1,700-2,000MPa
Elongation six per cent minimum
Reduction of area 25-40% minimum.
Although these grades are capable of attaining ultra-high strength, they are still prone to oxidation and (stress) corrosion due to atmospheric conditions, and, in operational use, need to be coated to prevent this. The long established coatings – chromium and/or cadmium – raise environmental concerns. Research is underway to develop an equally effective, and more environmentally friendly product for the existing alloy steels, or a corrosion resistant stainless grade of steel.
Although steel cannot be used in the highest temperature regions towards the rear of most jet engines (where nickel alloys are used), in the cooler parts of the engine steel alloys are used, as they are more cost-effective. Applications are commonly the engine shaft, discs in older engine designs, rings, and containment and exhaust casings, depending on the engine manufacture and type. Jet engines can contain ~10% by weight of stainless steel, or steel alloys.
One of the most popular steel grades for shaft manufacture is the 3 1/4%CrMoV. This offers a good balance of hot (torsional) strength and ductility and is able to operate at moderately elevated temperatures (typically up to 200oC, but reaching 450oC during take-off and landing) along the centre-line of the engine. With engine shafts, it is important that the steel alloys are clean and free from harmful inclusions that could lead to torsional fatigue failure.
Discs attached to the engine shaft operate in higher temperature environments than the shaft itself, and hold the moving blades of the fans, compressors and turbines. Martensitic stainless grades, alloyed with varying small quantities of strengthening elements such as nickel, molybdenum, vanadium, niobium, cobalt and tungsten (depending on the specific steel grade) are therefore used. Steel discs are now mainly used on older types of jet engines.
As opposed to the discs, rings are hoops of steel that hold the fixed rows of blades within the engine. Variants of alloyed martensitic stainless steel are used.
Containment casings (or rings) are protective armour surrounding the main engine compartments. These collect and contain any debris from broken discs or blades and prevent it from leaving the engine and penetrating the fuselage. Specialist high manganese and nitrogen alloyed austenitic steel is used for these applications. In service, these grades have high yield strengths, are ductile and tough, so as to contain high-energy debris without rupturing.
Exhaust casings are part of the cowlings that surround the rear-most part of the engine. A martensitic steel grade containing three per cent tungsten to improve hot strength can be used for this area.
Passengers seated in an aircraft window seat near the wings can see how, on take-off or landing, large parts of the surfaces move to transform the wing shape and area. These slats and flaps extend the wings’ areas and improve lift during take-off and landings. In flight they retract to reduce drag and slide within the main wing assemblies in channels or tracks. They are manipulated by metal rods and gears that form the actuation and control mechanisms. Steel alloys are used for these and, although small, they play an important part in ensuring smooth and consistent operation of the moving surfaces.
Flaps are the main movable surfaces on the wing. The track assemblies to support them use high strength steels – coated alloy grades such as 300M or martensitic stainless steel are often used. For some flap track assemblies, a precipitation hardening martensitic stainless grade such as AMS5659 (known as 15/5PH) is also used. Inherently oxidation resistant, unlike 300M, such grades operate without the need for coating. They are capable of developing high strength levels exceeding 1,000MPa. Both grades are used in the remelted condition to optimise their cleanliness and mechanical properties.
Slats are the movable surface on the leading edge of the wing and create extra lift on landing. Slat track assemblies often use different steels to flaps, such as an 18% nickel maraging steel. These have excellent strength and wear resistance, and develop their strength by intermetallic precipitation within a relatively soft martensitic matrix. Maraging steel slats are ideal for use in this part of aircraft as they help resist abrasive wear from the passage of air and airborne particulates.
Transferring the rotational movement of an engine shaft is accomplished via a series of shafts and gears. Steels therefore require core strength and toughness, and wear resistance for gear/shaft contact surfaces. Most alloys meet these needs with a heat-treated lower carbon alloy steel for intrinsic strength and toughness, and hardened contact faces due to carburising or nitriding.
The AMS 6308 alloy is a more specialist grade designed to operate under ‘run dry’ conditions. Its high molybdenum content allows the gearbox to continue normal operation at higher frictional temperatures (should there be a loss of lubrication/coolant) for long enough to safely land the helicopter.
Although a number of sophisticated bearing grades are used in the aerospace industry, one of the most common still in use is the one per cent CCr through-hardening bearing steel, produced super-clean by a remelting route to meet the requirement for use in aircraft and helicopters.
It is necessary to guarantee that the materials entering the aircraft and helicopter supply chains are made to the highest possible quality standards. Therefore, aircraft-grade steels must meet the relevant national and industry-based standards to help ensure the intrinsic properties required by customers, such as chemistry, steel cleanliness and mechanical properties, are consistently met.
Further information:Corus Speciality Steels