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Ferrous Metal (Iron)
Any alloy containing iron as its chief constituent is called ferrous metal. The most common ferrous metal in aircraft structure is steel, an alloy of iron with a controlled amount of carbon added.
Iron is a chemical element which is fairly soft malleable and ductile in its pure form. It is silvery white in color and is quite heavy. Iron combines readily with oxygen to form iron oxide, which is more commonly known as rust. Iron poured from a furnace into moulds is known as cast iron and normally contains more than two percent carbon and some silicon.
Cast iron has few aircraft applications because of its low strength to weight ratio. However, it is used in engines for items such as piston rings where its porosity and wear characteristic allow it to hold a lubricant film.
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Steel
To make steel, pig iron is re-melted in a special furnace. Pure oxygen is then forced through the molten metal where it combines with carbon and burns. The molten steel is then poured into moulds where it solidifies into ingots. The ingots are placed in a soaking pit where they are heated to a uniform temperature of about 2,200º F/ 1204.4º C. They are then taken from the soaking pit and passed through steel rollers to form plate or sheet plate.
Much of the steel used in aircraft construction is made in electric furnaces, which allow better control of alloying agents then gas-fired furnaces. An electric furnace is loaded with scrap steel, limestone and flux. The intense heat from the arcs melts the steel and the impurities mix with flux. Once the impurities are removed, controlled quantities of alloying agents are added, and the liquid metal in poured into moulds.
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Stainless Steel
Stainless steel is the classification of corrosion-resistant steels that contain large amounts of chromium and nickel. Their strength and resistance to corrosion make them well suited for high-temperature application such as firewalls and exhaust system components.
The principal alloy stainless steel is chromium. The corrosion resistant steel most often used in aircraft construction is known as 18-8 steel because of its content of 18 percent chromium and 8 percent nickel. Stainless steel may be rolled, drawn, bent or formed to any shape.
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Molybdenum
One of the most widely used alloying elements for aircraft structural steel is molybdenum. It reduces the grain size of steel and increases both its impact strength and elastic limit. Molybdenum steels are extremely wear resistant and possess a great deal of fatigue strength and it’s used in high-strength structural members and engine cylinder barrels.
Chrome-molybdenum (chrome-moly) steel is the most commonly used in aircraft. Its Society of Automotive Engineers (SAE) designation of 4130 denotes an alloy of approximately 1 per cent molybdenum and 0.30 percent carbon. It machines readily, is easily welded by either gas or electric arc, and responds well to heat treatment.
Heat- treated SAE 4130 steel has an ultimate tensile strength about four times that of SAE 1025 steel, making for landing gear structure and engine mounts.
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Heat Treatment of Steel
Iron is an allotropic metal, meaning it can exist in more than one type of lattice structure, depending on temperature.
Pure molten iron begins to solidify at 2,800 º F. Its structure at this point is known as the Delta form. If cooled to 2,554 º F, the atoms rearrange themselves into a Gamma form. Iron in this form is nonmagnetic. When nonmagnetic gamma iron in this form is cooled to 1,666 º F, another change occurs and the iron is transformed into a nonmagnetic form of Alpha structure.
There are two basic forms of steel when it comes to heat treatment. They are ferrite and austenite.
Ferrite is an alpha solid solution of iron containing some carbon and exists at temperature below the lower critical temperature. Above this lower critical temperature, the steel begins to turn into austenite, which consists of gamma iron containing carbon. As the temperature increases the transformation of ferrite into austenite until the upper critical temperature is reached.
Ferrite Austenite
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Below the alloy’s lower critical temperature, the carbon which exists in the steel in the form of iron carbides is scattered throughout the iron matrix as a physical mixture. When the steel is heated to its upper critical temperature, this carbon dissolves into matrix as a physical mixture.
Heat Treatment
Heat treatment is a series of operations involving the heating and cooling of metal in the solid state. Its purpose is to make the metal more useful, serviceable, and safe for a definite purpose. By heat treating a metal can be made harder, stronger and more resistant to impact. Heat treating can also make a metal softer and more ductile.
All heat-treating processes are similar in that they involve the heating and cooling of metals. They differ however in the temperatures to which the metal is heated and the rate at which it is cooled. A pure metal cannot be hardened by heat treatment because there is little change in its structure when heated.
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Heat –Treating Equipment
Successful heat treating requires close control over all factors affecting the heating and cooling of metals. The furnace must be of the proper size and type and must be so controlled that temperature are kept within prescribed for each operation. Even the atmosphere within the furnace affects the condition of the part being heat–treated. The quenching equipment and the quenching medium must be selected to fit the metal and the heat–treating operation.
There are many different types and sizes of furnaces used in heat treatment. Furnaces are designed to operate in certain specific temperature ranges and attempted use in other rangers frequently results in work of inferior quality. Furnaces heated by electricity the heating elements are generally in the form of wire or ribbon. Such furnaces commonly operate up to a maximum temperature of about 2000 º F.
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Heating
The object in heating is to transform parasite (a mechanical mixture of iron carbide that exists in a finely mixed condition) to austenite as the steel is hated through the critical range. Steel begins to appear dull red at about 1000 º F and as the temperature increases the colour changes gradually through various shades of red to orange, to yellow and finally to white.
Soaking
The temperature of the furnace must be held constant during the soaking period, since it is during this period that rearrangement of the internal structure of the steel takes place. The length of the soaking period depends upon the type of steel and the size of the part. As a general rule, a soaking period of 30 minutes to 1 hour is sufficient for the average heat-treating operation.
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Cooling
Various rates of cooling are used to produce the desired results, still air is a slow cooling medium, but is much faster than furnace cooling. Liquids are the fastest cooling media and therefore used in hardening steels. There are three commonly used quenching liquids brine, water and oil. Brine is the most severe medium, water is next and oil is the least severe. Generally an oil quench is used for alloy steels and brine or water for carbon steels.
Portable Quench Tank Quenching Media
Quenching solutions act only through their ability to cool the steel. Most requirements for quenching media are met satisfactorily by water. The rate of cooling is relatively rapid during quenching in brine, somewhat less rapid in water and slow in oil. Brine usually is made of a 5 to 10 percent solution of salt (sodium chloride) in water. In addition to its greater cooling speed, brine has the ability to “throw” the scale from steel during quenching.
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STEEL APPLICATIONS General
The base material iron is a chemical element which, in its pure form, is a very soft, malleable and ductile metal which is easy to form and shape. It readily combines with oxygen to form iron oxide (rust), and so is alloyed, primarily with carbon, but also with other elements. When molten iron is alloyed with more than 2% Carbon and poured into a mould, cast iron is formed. Cast iron has limited uses in the aviation industry due to low strength to weight ratio and brittleness.
Iron is extracted from iron ore by mixing it with coke and limestone and heating it in a furnace. The process extracts the oxygen from the ore, and allows the iron to sink to the bottom of the furnace. The limestone reacts with any impurities
in the molten iron and floats to the surface to form a slag.
To make steel, the pure iron is remelted in a special furnace where carbon is introduced along with other alloying elements to achieve the desired characteristics.
Description
Steel is an excellent engineering material with many applications. For aircraft use, however, it does have some significant problems. The main restrictions are its high density (approximately 3 times the density of aluminium) and its susceptability to corrosion. The corrosion of steel can be reduced by the addition of certain alloying elements, but this can have significant effects on properties and costs.
Between 9 and 16% (Airbus A320: 9% , Boeing B777: 11%) of an aircraft’s structure is alloy steel and stainless steel. The high strength and high modulus of elasticity are the primary advantages of the high-strength steels. This is useful for designs with space limitations such as with some landing gear components.
Alloy selection considerations include service temperature, strength, stiffness fatigue properties and fabricability.
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Steel Application
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TESTING OF MATERIALS
The mechanical properties of a material must be known before that material can be incorporated into any design. Mechanical property data is compiled from extensive material testing. Various tests are used to determine the actual values of material properties under different loading applications and test conditions.
Tensile Testing
Tensile testing is the most widely-used mechanical test. It involves applying a steadily increasing load to a test specimen, causing it to stretch until it eventually fractures. Accurate measurements are taken of the load and extension, and the results are used to determine the strength of the material. To ensure uniformity of test results, the test specimens used must conform to standard dimensions and finish as laid down by the appropriate Standards Authority (BSI, DIN, ISO etc).
The cross-section of the specimen may be round or rectangular, but the relationship between the cross-sectional area and a specified "gauge length", of each specimen, is constant. The gauge length, is that portion of the parallel part of the specimen, which is to be used for measuring the subsequent extension during and/or after the test.
Tensile Strength
Tensile strength in a material is obtained by measuring the maximum load, which the test piece is able to sustain, and dividing that figure by the original cross-sectional area (c.s.a.) of the specimen. The value derived from this simple calculation is called STRESS.
Note: The units of Stress may be quoted in the old British Imperial (and American) units of lbf/in2, tonf/in2 (also psi and tsi), or the European and SI units such as kN/m2, MN/m2 and kPa or MPa.
Example 1
A steel rod, with a diameter of 5 mm, is loaded in tension with a force of 400 N. Calculate the tensile stress.
Exercise 1
Calculate the tensile stress in a steel rod, with a cross-section of 10 mm x 4 mm, when it is subjected to a load of 100 N.
Example 2
A structural member, with a cross-sectional area of 05m2, is subjected to a load of 10 MN. Calculate the stress in the member in; (a) MN/m2 and (b) N/mm2 (a)
(b)
As the load in the tensile test is increased from zero to a maximum value, the material extends in length. The amount of extension, produced by a given load, allows the amount of induced STRAIN to be calculated. Strain is calculated by measuring the extension and dividing by the original length of the material.
Note: Both measurements must be in the same units, though, since Strain is a ratio of two lengths, it has no units.
Exercise 2
Calculate the cross-sectional area of a tie rod which, when subjected to a load of 2,100N, has a stress of 60 N/mm2.
Note: When calculating stress in large structural members, it may be more convenient to measure load in Mega-Newtons (MN, or N6) and the area in square metres (m2). When using such units, the numerical value is identical to that if the calculation had been made using Newtons and mm2.
i.e. A Stress of 1 N/mm2 = l MN/m2