Aircraft Material-Ferrous PDF

Metal Alloy Steels Ferrous Metal Characteristics and Properties In this resource the term 'Metal characteristics' is used to identify a feature or quality of the material. When using metal in manufacturing and construction, a given metal is known to possess several characteristics which determine wh...

Metal Alloy Steels Ferrous Metal Characteristics and Properties In this resource the term 'Metal characteristics' is used to identify a feature or quality of the material. When using metal in manufacturing and construction, a given metal is known to possess several characteristics which determine where and how it can be used. These characteristics includes strength, hardness, malleability, ductility, brittleness, conductivity, expansion, elasticity, toughness, fusibility and density. In this resource the term Metal property is used to identify an attribute, quality, or characteristic of a metal. By adding (alloying) small amounts of other materials, the properties of the material characteristics are changed dramatically. As an example, in a basic form adding carbon to steel increases the hardness property of the core material. Aircraft in assembly line at Boeing 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 11 of 229 Strength One way to classify metals is according to the amount of strength they possess. A metal’s strength is determined by the percentages of the parent metal and other elements used to make an alloy. There are many different types of strength, including: Tensile strength Compressive strength Shear strength Torsional strength Bending strength Fatigue strength Impact strength (also known as toughness). 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 12 of 229 Metal Loading Forces Each type of strength listed is a measure of how a metal reacts to a specific type of loading. Tensile strength is the ability of a piece of sheet metal to withstand stress in tension. There are three definitions of tensile strength: Yield strength – the stress at which material strain changes from elastic deformation to plastic deformation, causing it to deform permanently. Ultimate strength – the maximum stress a material can withstand when subjected to tension, compression or shearing; the maximum stress on the stress-strain curve. Breaking strength – the stress coordinate on the stress-strain curve at the point of rupture. Reference numbers on the stress vs strain curve for structural steel include: Ultimate strength Yield strength (elastic limit) Rupture (or fracture) Strain-hardening region Necking region. Stress, σ Strain hardening Necking Ultimate strength Fracture Yield strength Strain, ε Image by under the Creative Commons Licence Stress vs strain curve for structural steel (tensile strength) 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 13 of 229 The remaining four fundamental loading forces (excluding tension which is described above) are listed below: Compressive strength is a metal’s ability to withstand being pressed or squeezed. Shear strength is a metal’s ability to withstand shear stress. Torsional strength is a metal’s ability to resist rotational shear. Bending strength is a metal’s bending strength. Fundamental loading forces The remaining types of strength include: Fatigue strength, or endurance strength, which is a metal’s ability to resist repeated loading. Impact strength (toughness), which is a metal’s ability to resist shock. This will be elaborated on in the following section. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 14 of 229 Fractured metal due to fatigue The following sections will describe the specific properties of metals, each of which typically relate to a type or several types of metallic strengths outlined above. Hardness A metal’s hardness is its ability to resist cutting, penetration or abrasion. The tensile strength of steel relates directly to its hardness, but for most metals this relationship is not absolute. Some metals are hardened through heat-treating or work hardening, while others are softened by a process called annealing. Metal hardness testing 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 15 of 229 Malleability A material’s ability to be bent, formed or shaped without cracking or breaking is called malleability. Hardness and malleability are generally considered opposite properties. To help increase malleability, several metals are annealed, or softened. Under this condition, complex shapes can be formed. After forming is complete, the metal is then heat-treated to increase its strength. A metal may be fully annealed when the forming is started, but hammering and shaping can harden it to such an extent that it must be re-annealed before forming is completed. A malleable metal is able to be hammered, pressed, or rolled into thin sheets without breaking. Metal pressed to form structures in aircraft panels A malleable metal is highly ductile and a non-malleable metal is brittle. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 16 of 229 Ductility The ability of metal to be drawn into wire stock, extrusions or rods is called ductility. Ductile metal is used for control cables which must be regularly inspected. Ductile metals are preferred for aircraft use because of their ease of forming and resistance to failure under shock loads. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 17 of 229 Brittleness Brittleness is a material’s tendency to break or shatter when exposed to stress, and is the opposite of ductility and malleability. A brittle metal is more apt to break or crack before it changes shape. Because structural metals are often subjected to shock loads, brittleness is not a desirable property. Cast iron and very hard steel are examples of brittle ferous metals. Ductile fracture vs brittle fracture 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 18 of 229 Elasticity Elasticity is a metal’s tendency to return to its original shape after normal stretching and bending. The flexibility of spring steel used to construct landing gear is a good example of elasticity. Another form of elasticity is demonstrated when aircraft skins expand and contract when an aircraft is pressurised. A metal’s elastic limit is the point beyond which the metal does not return to its original shape after a deforming force is removed. Some non-ferous metals have very low elastic limits, while the elastic limit of hard spring steel is very high. Elasticity is a metal’s tendency to return to its original shape after normal stretching and bending 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 19 of 229 Toughness Toughness is a material’s ability to resist tearing or breaking when it is bent or stretched. Hammer faces and wrenches are examples of metal that must be tough as well as hard to be useful. Toughness of metals 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 20 of 229 Conductivity Conductivity is the property which enables a metal to carry heat or electricity. If a metal is able to transmit heat, it is said to be thermally conductive. However, before a metal can carry heat away from its source, it must first absorb it. This ability to conduct heat away is called heat exchange. The fins on the cylinder heads of an air-cooled piston engine remove heat in this fashion. Metals that can carry heat also carry electrons, making them good electrical conductors. Electrical conductivity is the measure of a material’s ability to allow electron flow. A metal conductor can be a wire, an aircraft frame or an engine. Conductivity of metals 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 21 of 229 Thermal Expansion The ability of a metal to expand when heated and shrink when cooled is called thermal expansion. The amount of expansion or contraction is predictable at specific temperatures and is called its coefficient of expansion. All aircraft experience thermal expansion and contraction as the ambient temperature changes. Thermal expansion Fusibility The ability of metal to be joined by heating and melting is called fusibility. To fuse metal means to melt two or more compatible pieces of metal into one continuous part. The correct term is fusion joining or welding. Photo by Rob Lambert on Unsplash Welding is a form of fusing 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 22 of 229 Ferrous Metals Iron (Ferrite) Any alloy containing iron as its chief constituent is called a ferrous metal. Ferrous metals include steel, cast iron, and titanium, as well as alloys of iron with other metals (such as with stainless steel). The most common ferrous metal in aircraft structures 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 colour and quite heavy. Iron combines readily with oxygen to form iron oxide, which is more commonly known as rust. This is one reason iron is usually mixed with various forms of carbon and other alloying agents or impurities. Iron ore Iron poured from a furnace into moulds is known as cast iron and normally contains more than 2% 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 valve guides, where its porosity and wear characteristics allow it to hold a lubricant film. It is also used in piston rings. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 23 of 229 Steel Composition Steel is a material composed primarily of iron. All types of steel contain a second element: carbon. Many other alloying elements are used in most steel, but iron and carbon are the only elements found in all steel. Composition of Steel The difference between steel, cast iron and wrought iron is primarily based on the carbon content. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 24 of 229 Identification of Steels Society of Automotive Engineers Steel Numbering In general, the Society of Automotive Engineers (SAE) uses a four-digit numerical index system to represent chemical composition standards for steel specifications: The first digit identifies the principal alloying element. The second digit indicates the percentage of the principal alloying element. The last two digits indicate the average carbon content in hundredths of a percent. Steel Numbering System 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 25 of 229 SAE Steel Numbering System SAE designations for major classifications of steel: 1xxx – Carbon steels 2xxx – Nickel steels 3xxx – Nickel-chromium steels 4xxx – Molybdenum steels 5xxx – Chromium steels 6xxx – Chromium-vanadium steels 7xxx – Tungsten steels 8xxx – Nickel-chromium-molybdenum steels 9xxx – Silicon-manganese steels. Aviation Australia SAE Numbering System 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 26 of 229 Alloying Agents in Steel Purpose of Ferrous Metal Alloys Iron has few practical uses in its pure state. However, adding small amounts of other materials to molten iron dramatically changes its properties. Some of the more common alloying agents include carbon, sulphur, silicon, phosphorous, nickel and chromium. Carbon Carbon is the most common alloying element found in steel. When carbon is mixed with iron, compounds of iron carbides called cementite form. The carbon in steel allows the steel to be heattreated to obtain varying degrees of hardness, strength and toughness. The greater the carbon content, the more receptive steel is to heat treatment and therefore the higher its tensile strength and hardness. However, higher carbon content decreases the malleability and weldability of steel. A metal’s hardness, or temper, is indicated by a letter designation that is separated from the alloy designation by a dash. Ferrous materials are generally classified according to their carbon content. Low-carbon or mild steel: Is primarily used in non-structural areas, but has (in the past) been used in steel-tube construction. Easily welded. Machines readily. Does not accept heat treatment. Medium-carbon steels: Will accept heat treatment. Is especially adaptable for machining or forging. Used where surface hardness is desirable. High-carbon steels: Very hard. Primarily used in springs, files and some cutting tools. The following graph shows the relationship between the percentage of carbon in steel and its properties including tensile strength, ductility and hardness. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 27 of 229 Carbon steel characteristics Other Alloying Agents Silicon When silicon is alloyed with steel, it acts as a hardener. Used in small quantities, it also improves ductility. Manganese Mangalloy (manganese steel) is a unique non-magnetic steel with extreme anti-wear properties. The material is very resistant to abrasion and will achieve up to three times its surface hardness during conditions of impact, without any increase in brittleness which is usually associated with hardness. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 28 of 229 Sulphur Sulphur is usually an undesirable impurity in steel rather than an alloying element. Sulphur causes steel to be brittle when rolled or forged, and therefore it must be removed in the refining process. If all the sulphur cannot be removed, its effects can be countered by adding manganese. The manganese combines with the sulphur to form manganese sulphide, which does not harm the finished steel. In addition to eliminating sulphur and other oxides from steel, manganese improves a metal’s forging characteristics by making it less brittle at rolling and forging temperatures. Alloying additions of sulphur in small amounts will tend to improve the machinability of a steel. Machinability is the ease with which a metal can be cut (machined). Phosphorous Phosphorous raises the yield strength of steel and improves low-carbon steel’s resistance to atmospheric corrosion. Nickel Nickel adds strength and hardness to steel and increases its yield strength. It also slows the rate of hardening when steel is heat-treated, which increases the depth of hardening and produces a finer grain structure. The finer grain structure reduces steel’s tendency to warp and scale when heattreated. Chromium Chromium is alloyed with steel to increase strength and hardness as well as improve its wear and corrosion resistance. Because of its characteristics, chromium steel is used in balls and rollers of antifriction bearings. In addition to its use as an alloying element in steel, chromium is electrolytically deposited on cylinder walls and bearing journals to provide a hard, wear-resistant surface. Nickel-Chromium Steel Nickel toughens steel, and chromium hardens it. Therefore, when both elements are alloyed, they give steel desirable characteristics for use in high-strength structural applications. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 29 of 229 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. Chrome-molybdenum (chrome-moly) steel machines readily, is easily welded by either gas or electric arc, and responds well to heat treatment. Chrome-moly steel is an ideal choice for landing gear structures and engine mounts. Furthermore, chrome-moly’s toughness and wear resistance make it a good material for engine cylinders and other highly stressed engine parts. Vanadium When combined with chromium, vanadium produces a strong, tough, ductile steel alloy. Ball bearings are made of chrome-vanadium steel. Tungsten Tungsten has an extremely high melting point and adds this characteristic to steel it is alloyed with. Because tungsten steels have high density (high mass) and retain their hardness at elevated operating temperatures, they are typically used for control surface balance weights and breaker contacts in magnetos. Tungsten balance weights 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 30 of 229 Titanium Titanium steel alloys have very high tensile strength and toughness (and at higher temperatures). Titanium steel is also lightweight and has high corrosion resistance whilst the ability to withstand extreme temperatures. Stainless Steel Stainless steel is a 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 applications such as firewalls and exhaust system components. Stainless steel engine shroud segments 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 31 of 229 Heat Treatment of Steel Purpose of Steel Heat Treatment As mentioned before, pure iron is not suitable for use as a structural material. It is weak, soft, very ductile and unresponsive to heat treatment. Steel, which is typically iron alloyed with carbon and a few percent to a few tens of percent of other alloying elements, can be heat-treated to a wide range of strengths, toughnesses and ductilities. Carbon is the most important of these alloying elements in terms of the mechanical properties of steel, and most heat treatments of steel are based primarily on controlling the distribution of carbon. Heat treatment of steel is the process of heating and cooling carbon steel to change its physical and mechanical properties without changing its original shape and size. Heat treatment is often associated with increasing the strength of the steel, but it can also be used to alter certain manufacturability objectives, such as to improve machinability or formability, restore ductility, etc. Thus, heat treatment is a useful process to bolster other manufacturing processes and to improve product performance by increasing strength or other desirable characteristics. Highcarbon steels are particularly suitable for heat treatment since carbon steel responds well to the treatment and the commercial use of steels exceeds that of any other material. Steel being heat treated There are many different types of heat treatment processes, and each process provides different desirable characteristics to the product. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 32 of 229 Annealing Annealing softens steel and relieves internal stress. Annealing steel involves heating it, soaking it for a specified time and then cooling it. The steel can be cooled by leaving it in the furnace and allowing both the furnace and steel to cool together or by packing the steel in hot sand or ash so the heat is conducted away slowly. Annealing oven 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 33 of 229 Normalising The processes of forging, welding and machining usually leave stresses within steel that could lead to failure. These stresses are relieved in ferrous metals by a process known as normalising. This process involves heating the steel and maintaining the temperature until the metal is uniformly heat-soaked. The steel is then removed from the oven and allowed to cool in still air. Although this process does allow particles of carbon to precipitate out, the particles are not as large as those formed when steel is annealed. One of the most important uses of normalising in aircraft work is on welded parts. When a part is welded, internal stresses and strains set up in the adjacent material. Allowing the heated steel to cool in still air 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 34 of 229 Hardening Carbon steel can be hardened readily. The maximum hardness attained by carbon steel depends almost entirely on its carbon content. Hardening steel involves heating it so carbon can disperse uniformly. Once this occurs, the alloy is cooled rapidly by quenching it in water, oil or brine (water with a high salt content). The speed of the quench is determined by the quenching medium. Oil provides the slowest quench, and brine is the most rapid. Quenching steel 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 35 of 229 Tempering Tempering reduces the undesirable qualities of the steel's brittleness. Tempering an alloy is done by heating it to a certain temperature and holding it there until it becomes heat-soaked. It is then allowed to cool to room temperature in still air. Tempering not only reduces hardness and brittleness, but also relieves stress and improves steel’s ductility and toughness. Tempering also involves allowing the metal to cool in room temperature air 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 36 of 229 Case Hardening Aircraft components like bearings and gears that require metal with hard, durable surfaces and core material that remains tough. This is accomplished through a process called case-hardening. Lowcarbon and low-alloy steels are best suited for case-hardening. If high-carbon steel is case-hardened, the hardness penetrates the core and causes brittleness. Aviation Australia Case hardening 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 37 of 229