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by Evgeny KABLOV, RAS Corresponding Member, Director General of the Chief Scientific Center of the Russian Federation, the State "VIAM" enterprise,
and Alexander PETRAKOV, Dr. Sc. (Tech.), Chief Researcher of the same Institute
The maintenance of modern aerospace technology is predetermined by rather tough requirements to the materials of which they are manufactured. This includes both the necessity to achieve high durability of the hardware and its minimal specific gravity, size, fuel consumption as well as sufficient reliability and a long working resource versus the effect of variable and considerable strains, alternation of higher (up to 450 0 C) and lower (down to - 253 0 C) temperatures, aggressive corrosion environments, different types of radiation, etc.
The competitiveness of Hying devices largely depends on the material's quality. Such characteristics as the night range, speed, mobility, accuracy, all-weather navigation as well as quality home- made raw materials - all this is important for military aircraft. The same is true of civil aviation as well - reliability, fire-proof conditions, comfort, ecology are likewise important. All that should be achieved by minimizing construction and maintenance expenses.
It is clear therefore that the materials used in aircraft building should have high specific durability (it is also called weight effectiveness), rigidity, corrosion resistance, fatigue resistance* as well as crack endurance, among other features. No doubt, one particular material cannot meet all the requirements; that is why, while manufacturing different parts of flying devices, one makes use of the most suitable of the existing materials, or else develops new composites.
Aluminum alloys have found the widest use. Upper and lower wing and fuselage surfaces (durability in the range of 450-550 Mpa is required here), parts of the so-called power set, that is different rigidity ribs and frames, which connect them, fittings, beams with the durability limit of 500-600 Mpa, etc., are manufactured of these alloys. The share of such materials in modem aircraft is as high as 50-70 percent.
Titanium alloys (used for manufacturing undercarriage components, beams and so on) and, in particular, polymer composites are rather widespread. Such composites are used in the manufacture of the wing panel, horizontal and vertical tail, doors of the landing gear hatches and power plants. With the durability of 1,700-2,500 Mpa, their specific mass does not exceed 2 gr/cm . Their portion in aircraft reaches 8-15 percent and approximately 50 percent in helicopters.
* Fatigue resistance is characterized by the endurance limit, that is the greatest tension that a material can endure without being destroyed at the assigned number of cyclic effects. - Ed.
At first notice it seems that the role of steels in such an honored set of materials should diminish. Not at all. They account for as much as 8-10 percent in passenger liners, 25-50 percent - in military aircraft, and it looks this ratio won't deteriorate in the near future. Mostly loaded parts of flying apparatuses such as landing gear parts, hydrocylinder hulls, pipelines of high-pressure hydrosystems, bolts to fasten the wing to the fuselage, gearings of engine reducers, pinions of the main reduction gear of the helicopter power plant - all that is steel-fabricated. And although this material has been known for ages, it has a number of advantages over its "successors". It is distinguished for higher rigidity and durability (that is particularly apparent in smaller parts), resistance to cyclic loads, corrosion resistance, good process- ability, that is the possibility of obtaining components and parts by various methods-high and cold deformation, mechanical processing, welding, soldering, etc. Besides, steel is nonexpensive. Therefore, since our Institute's foundation, one of the priorities has been to create new types of steel.
The constant improvement in the flying apparatuses' construction required a continuous increase in the durability and the specific durability (i.e. the correlation of durability to the material's density) while preserving all the advantages of steel. In aviation before 1941 the first of these parameters was fluctuating within the limits of 800-1,000 Mpa, whereas now it has reached 1,300-2,000 Mpa. However, the complexity of the problem does not necessarily lie in achieving such indices, but in guaranteeing the efficiency of aircraft manufactured from corresponding materials.
The point is that the increase in the durability of the steel leads to the reduction of its plasticity, viscidity, crack-proof qualities, etc. Therefore the makers of new types of steel are searching for a compromise between raising the durability and ensuring the reliability Today three groups of highly durable steels are being widely applied in aviation hardware. These are structural medium doped steel alloys, corrosion-proof alloys used to produce parts operating in hard high-friction conditions and parts that undergo chemical and thermal treatment.
In any case the appearance of such materials has made scientists reconsider their approaches to the construction and producing technologies of the parts, which were adopted earlier, because all the above steels possess a number of specific qualities and differ considerably from those created earlier that had medium durability (up to 1,400 Mpa). In particular, it turned out that the breach in their technological cycle could lead to a pre-term break of parts regardless of the high quality of the metal. Thus, the destruction points can be surface and undersurface defects occurring at different stages of manufacturing the semi-product, the part and the construction as a whole. Therefore it was very important to work out clear organizational and technical measures including instructions on the heat and mechanical processing of the parts, anticorrosion protection, welding, etc., that we managed to achieve in the early 60s of the 20th century. Besides, the approach to the products from highly durable steels has considerably changed: the major requirements to them have become a minimal strain concentration and a high surface cleanliness.
So, new steels have taken their place in aircraft building; moreover, depending on the durability limit, different parts are manufactured from them. Say, if this parameter falls within the limits of 1,600-1,800 Mpa, such a metal is suitable to produce the glider's power set (spars, various beams, frames, axles, etc.). As far as VKS-8 (1,800-2,000 Mpa) and VKS-9 (1,950-2,100 Mpa) steels are concerned, they are irreplaceable in
manufacturing big welding parts (electron and radial as well as argon-circular types of welding are possible) of the glider and the landing gear in the aircraft of the Sukhoy, Antonov, Mikoyan, and Kamov Design Offices. But that is not enough. The steels with the durability limit exceeding 1,950 Mpa successfully replace titan alloys and make it possible with their equal specific durability to reduce considerably their production costs.
In recent decades a new class of highly durable, the so-called marten-site-senescent steels has been developed; these have the durability of 1,450-2,500 Mpa and possess unique physical and mechanical as well as technological properties. For instance, thanks to the low carbon and nitrogen concentration they have a high plasticity, ductility, resistance to continuous static charge and corrosion cracks. And that is not all. The said material is very practical, i.e. billets fabricated of it after chilling can undergo different types of cold processing by pressure (shell unreeling, thread knurling, etc.), can be easily processed by the cutting tool; their durability can be improved twofold by a simple ageing heat treatment technique (heating and cooling in the air) at relatively low temperatures. The above advantages of marten-site- senescent steels are fully manifested in producing parts of complex form with small tolerances (including sensitive ones) that undergo chemical and heat treatment.* The metal of this type has found application in highly loaded units of MiG-31 and MiG-29 fighter planes, in parts of the turning unit and landing gear of the "Buran" orbital spaceship of multiple use, etc.
Further development of aircraft engineering advanced new requirements to the materials. First of all, this concerns fighters, whose speed exceeds the sound by 2,5-3 times, because for these purposes they have to overcome the heat limit, i.e. temperatures of 280-300 0 C, when aluminum alloys are unacceptable. We have managed to solve this problem. High-durable corrosion-proof steels offered by us possess all the necessary qualities: high durability, plasticity, ductility, high-tech qualities as they are easy to punch and weld. This quality makes it possible to do without further heat treatment and as a result, complicated openwork constructions, say, carrying torsion boxes can be created with no sealing or riveting, which used to be widely adopted.
The VNS-2 corrosion-proof steel with the durability limit of 1,250- 1,400 Mpa has become the basic material in all-welded aircraft cells. It is applied in the form of foil and band for the coating and inner set as well as for producing power parts (rods, forgings, etc.).
However, while exploiting the flying apparatuses where the VNS-2 steel had been used, it was found that it did not fit well with the humid climate (for instance, that of the Mediterranean). Further search enabled us to obtain new steels EPS 17 (rod) and VNS-41 (foil). According to their mechanical characteristics and adapt-
* See V. Pushin, "New Functional Materials", Science in Russia, No.6, 2000. - Ed.
ability to modem technologies, they correspond to the already tested VNS-2 steel; and due to a new system of doping and optimizing the regime of durable ageing, they considerably exceed it according to the corrosion resistance; thus this concerns both major pants and welded junctures.
The VNS-5 steel with the durability limit of 1,380-1,600 Mpa has found widest use. Power parts of MiG and Su glider aircraft as well as the landing gear of the Beriev Design Bureau's hydroplane are manufactured from it. It is also applied in civil aviation (large- fuselage IL-86 aircraft and IL-96 airbus), and for making highly loaded bolts for fastening the engine to the fuselage.
Another representative of this class of metals is the SN-2A steel with the durability limit of 1,100-1,300 Mpa. It has well recommended itself as a material for power parts including industrial holders and for air and oxygen balloons, with which all types of aircraft are equipped, including naval aviation. The most important distinction of such balloons is that when hit by bullets, they do not split into fragments.
Today aviation and missilery have seen a wider application of a new type of fuel-hydrogen and its oxidizer-liquid oxygen that have the temperature of -253 0 C. For working in such conditions our institute has developed special high-durability corrosion proof steels (VNS-25, VNS-49, VNS-59) with the durability limit of 1,000- 1,400 Mpa at the room temperature and 1,700-2,100 at 20 К (- 253 0 C). This metal is successfully applied in different liquid-propellant engines, in particular, in the world's most powerful PD-170 designed by the "Energomash" Office. Parts manufactured from this material, that is frames of pumps and regulators of fuel supply, make up 50-60 percent of its mass.
Nowadays medium doped and corrosion-proof steels have been widely applied as structural materials to make reducers and units that undergo chemical and heat treatment. It is explained by the fact that as a result of long studies it has become possible to propose a technology which guarantees the combination of the required surface level characteristics of the item (high rigidity, wear resistance, fatigue resistance, etc.) and its core (plasticity, ductility, adaptability to technology, etc.). So, the VKS-7 steel with the carbonitride reinforcement has been developed for the reducers' highly loaded large module pinions. This steel provides, after due chemical and heat treatment, a depth of the reinforcing level of up to 2.5 mm and rigidity exceeding 60 HRC, which guarantees a high contact durability at temperatures of up to 250 0 C (there have been no such analogs so far).
Another case with helicopters. Our Institute has created a highly durable (up to 1,300 Mpa), wear-resistant, heat-proof VKS-10 steel for them. Unlike serial domestic and foreign analogs, working at a temperature of up to 250 0 C, this one can endure a temperature of 450 0C. Its application guarantees transmission of high torque, in which a local temperature rise occurs in the zone of cogs' contact, and even when the lub-oil supply is disrupted, the work of the reducer can continue for two hours with no break.
All this proves that in aircraft building steel has retained its role as a major material, though, like any other man-made product, it needs further improvement.
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