Aircraft Metals PDF

AIRCRAFT METALS DENSITY Knowledge and understanding of the uses, strengths, Density is the weight of a unit volume of a material. limitations, and other characteristics of structural In aircraft work, the specified weight of a material metals i...

AIRCRAFT METALS DENSITY Knowledge and understanding of the uses, strengths, Density is the weight of a unit volume of a material. limitations, and other characteristics of structural In aircraft work, the specified weight of a material metals is vital to properly construct and maintain per cubic inch is preferred since this figure can be any equipment, especially airframes. In aircraft used in determining the weight of a part before actual maintenance and repair, even a slight deviation manufacture. Density is an important consideration from design specification, or the substitution of when choosing a material to be used in the design inferior materials, may result in the loss of both of a part in order to maintain the proper weight and lives and equipment. The use of unsuitable materials balance of the aircraft. can readily erase the finest craftsmanship. The selection of the correct material for a specific repair MALLEABILITY job demands familiarity with the most common A metal which can be hammered, rolled, or pressed physical properties of various metals. into various shapes without cracking, breaking, or leaving some other detrimental effect, is said to be PROPERTIES OF METALS malleable. This property is necessary in sheet metal Of primary concern in aircraft maintenance are that is worked into curved shapes, such as cowling's, such general properties of metals and their alloys fairings, or wingtips. Copper is an example of a as hardness, malleability, ductility, elasticity, malleable metal. toughness, density, brittleness, fusibility, conductivity contraction and expansion, and so DUCTILITY forth. These terms are explained to establish a basis Ductility is the property of a metal which permits for further discussion of structural metals. it to be permanently drawn, bent, or twisted into various shapes without breaking. This property is HARDNESS essential for metals used in making wire and tubing. Hardness refers to the ability of a material to resist Ductile metals are greatly preferred for aircraft use abrasion, penetration, cutting action, or permanent because of their ease of forming and resistance to distortion. Hardness may be increased by cold failure under shock loads. For this reason, aluminum working the metal and, in the case of steel and certain alloys are used for cowl rings, fuselage and wing aluminum alloys, by heat treatment. Structural skin, and formed or extruded parts, such as ribs, parts are often formed from metals in their soft state spars, and bulkheads. Chrome molybdenum steel is and are then heat treated to harden them so that also easily formed into desired shapes. Ductility is the finished shape will be retained. Hardness and similar to malleability. strength are closely associated properties of metals. ELASTICITY STRENGTH Elasticity is that property that enables a metal to One of the most important properties of a material return to its original size and shape when the force is strength. Strength is the ability of a material to which causes the change of shape is removed. This resist deformation. Strength is also the ability of a property is extremely valuable because it would material to resist stress without breaking. The type be highly undesirable to have a part permanently of load or stress on the material affects the strength distorted after an applied load was removed. Each it exhibits. metal has a point known as the elastic limit, beyond which it cannot be loaded without causing permanent distortion. In aircraft construction, members and 1.2 Module 06 - Materials and Hardware parts are so designed that the maximum loads to Cooling and heating affect the design of welding which they are subjected will not stress them beyond jigs, castings, and tolerances necessary for hot rolled their elastic limits. This desirable property is present material. MATERIALS FERROUS in spring steel. FERROUS AIRCRAFT METALS TOUGHNESS Many different metals are required in the repair A material which possesses toughness will of aircraft. This is a result of the varying needs withstand tearing or shearing and may be stretched with respect to strength, weight, durability, and or otherwise deformed without breaking. Toughness resistance to deterioration of specific structures or is a desirable property in aircraft metals. parts. In addition, the particular shape or form of the material plays an important role. In selecting BRITTLENESS materials for aircraft repair, these factors plus many Brittleness is the property of a metal which allows others are considered in relation to the mechanical little bending or deformation without shattering. and physical properties. A brittle metal is apt to break or crack without change of shape. Because structural metals are often Among the common materials used are ferrous subjected to shock loads, brittleness is not a very metals. The term “ferrous” applies to the group of desirable property. Cast iron, cast aluminum, and metals having iron as their principal constituent. very hard steel are examples of brittle metals. IRON FUSIBILITY If carbon is added to iron, in percentages ranging Fusibility is the ability of a metal to become liquid by up to approximately 1 percent, the product is vastly the application of heat. Metals are fused in welding. superior to iron alone and is classified as carbon Steels fuse around 2 600 °F and aluminum alloys at steel. Carbon steel forms the base of those alloy approximately 1 100 °F. steels produced by combining carbon steel with other elements known to improve the properties CONDUCTIVITY of steel. A base metal (such as iron) to which small Conductivity is the property which enables a metal quantities of other metals have been added is called to carry heat or electricity. The heat conductivity of an alloy. The addition of other metals changes or a metal is especially important in welding because improves the chemical or physical properties of the it governs the amount of heat that will be required base metal for a particular use. for proper fusion. Conductivity of the metal, to a certain extent, determines the type of jig to be used STEEL AND STEEL ALLOYS to control expansion and contraction. In aircraft, To facilitate the discussion of steels, some familiarity electrical conductivity must also be considered with their nomenclature is desirable. A numerical in conjunction with bonding, to eliminate radio index, sponsored by the Society of Automotive interference. Engineers (SAE) and the American Iron and Steel Institute (AISI), is used to identify the chemical THERMAL EXPANSION compositions of the structural steels. In this system, Thermal expansion refers to contraction and a four-numeral series is used to designate the plain expansion that are reactions produced in metals as carbon and alloy steels; five numerals are used to the result of heating or cooling. Heat applied to designate certain types of alloy steels. The first two a metal will cause it to expand or become larger. digits indicate the type of steel, the second digit also Module 06 - Materials and Hardware 1.3 generally (but not always) gives the approximate time to accommodate steels of proven merit and amount of the major alloying element, and the last to provide for changes in the metallurgical and two (or three) digits are intended to indicate the engineering requirements of industry. (Figure 1-1) approximate middle of the carbon range. However, a deviation from the rule of indicating the carbon Metal stock is manufactured in several forms range is sometimes necessary. and shapes, including sheets, bars, rods, tubing, extrusions, forgings, and castings. Sheet metal Small quantities of certain elements are present in is made in a number of sizes and thicknesses. alloy steels that are not specified as required. These Specifications designate thicknesses in thousandths elements are considered as incidental and may be of an inch. Bars and rods are supplied in a variety present to the maximum amounts as follows: copper, of shapes, such as round, square, rectangular, 0.35 percent; nickel, 0.25 percent; chromium, 0.20 hexagonal, and octagonal. Tubing can be obtained percent; molybdenum, 0.06 percent. in round, oval, rectangular, or streamlined shapes. The size of tubing is generally specified by outside The list of standard steels is altered from time to diameter and wall thickness. SERIES DESIGNATION TYPES 100xx Nonsulphurized carbon steels 11xx Resulphurised carbon steels (free machining) 12xx Rephosphorized and resulphurised carbon steels (free machining) 13xx Manganese 1.75% *23xx Nickel 3.50% *25xx Nickel 5.00% 31xx Nickel 1.25%, chromium 0.65% 33xx Nickel 3.50%, chromium 1.55% 40xx Molybdenum 0.20 or 0.25% 41xx Chromium 0.50% or 0.95%, molybdenum 0.12 or 0.20% 43xx Nickel 1.80%, chromium 0.5 or 0.80%, molybdenum 0.25% 44xx Molybdenum 0.40% 45xx Molybdenum 0.52% 46xx Nickel 1.80%, molybdenum 0.25% 47xx Nickel 1.05% chromium 0.45%, molybdenum 0.20 or 0.35% 48xx Nickel 3.50%, molybdenum 0.25% 50xx Chromium 0.25, or 0.40 or 0.50% 50xxx Carbon 1.00%, chromium 0.50% 51xx Chromium 0.80, 0.90, 0.95 or 1.00% 51xxx Carbon 1.00%, chromium 1.05% 52xxx Carbon 1.00%, chromium 1.45% 61xx Chromium 0.60, 0.80, 0.95%, vanadium 0.12%, 0.10% min., or 0.15% min. 81xx Nickel 0.30%, chromium 0.40%, molybdenum 0.12% 86xx Nickel 0.55%, chromium 0.50%, molybdenum 0.20% 87xx Nickel 0.55%, chromium 0.05%, molybdenum 0.25% 88xx Nickel 0.55%, chromium 0.05%, molybdenum 0.35% 92xx Manganese 0.85%, silicon 2.00%, chromium 0 or 0.35% 93xx Nickel 3.25%, chromium 1.20%, molybdenum 0.12% 94xx Nickel 0.45%, chromium 0.40%, molybdenum 0.12% 98xx Nickel 1.00%, chromium 0.80%, molybdenum 0.25% Figure 1-1. SAE numerical index. 1.4 Module 06 - Materials and Hardware The sheet metal is usually formed cold in such Steel containing carbon in percentages ranging from machines as presses, bending brakes, drawbenches, 0.30 to 0.50 percent is classed as medium carbon or rolls. Forgings are shaped or formed by pressing steel. This steel is especially adaptable for machining MATERIALS FERROUS or hammering heated metal in dies. Castings are or forging, and where surface hardness is desirable. produced by pouring molten metal into molds. The Certain rod ends and light forgings are made from casting is finished by machining. SAE 1035 steel. Spark testing is a common means of identifying Steel containing carbon in percentages ranging from various ferrous metals. In this test the piece of iron 0.50 to 1.05 percent is classed as high carbon steel. or steel is held against a revolving grinding stone The addition of other elements in varying quantities and the metal is identified by the sparks thrown adds to the hardness of this steel. In the fully heat- off. Each ferrous metal has its own peculiar spark treated condition it is very hard, will withstand high characteristics. The spark streams vary from a few shear and wear, and will have little deformation. It tiny shafts to a shower of sparks several feet in has limited use in aircraft. SAE 1095 in sheet form length. (Few nonferrous metals give off sparks when is used for making flat springs and in wire form for touched to a grinding stone. Therefore, these metals making coil springs. cannot be successfully identified by the spark test.) The various nickel steels are produced by combining Identification by spark testing is often inexact nickel with carbon steel. Steels containing from 3 unless performed by an experienced person, or the to 3.75 percent nickel are commonly used. Nickel test pieces differ greatly in their carbon content and increases the hardness, tensile strength, and elastic alloying constituents. Wrought iron produces long limit of steel without appreciably decreasing the shafts that are straw colored as they leave the stone ductility. It also intensifies the hardening effect of and white at the end. Cast iron sparks are red as heat treatment. SAE 2330 steel is used extensively they leave the stone and turn to a straw color. Low for aircraft parts, such as bolts, terminals, keys, carbon steels give off long, straight shafts having a clevises, and pins. few white sprigs. As the carbon content of the steel increases, the number of sprigs along each shaft Chromium steel is high in hardness, strength, and increases and the stream becomes whiter in color. corrosion resistant properties, and is particularly Nickel steel causes the spark stream to contain small adaptable for heat-treated forgings which require white blocks of light within the main burst. greater toughness and strength than may be obtained in plain carbon steel. It can be used for such articles TYPES, CHARACTERISTICS, AND as the balls and rollers of anti-friction bearings. USES OF ALLOYED STEELS Steel containing carbon in percentages ranging from Chrome-nickel or stainless steels are the corrosion 0.10 to 0.30 percent is classed as low carbon steel. resistant metals. The anti-corrosive degree of this The equivalent SAE numbers range from 1010 to steel is determined by the surface condition of the 1030. Steels of this grade are used for making such metal as well as by the composition, temperature, items as safety wire, certain nuts, cable bushings, or and concentration of the corrosive agent. The threaded rod ends. This steel in sheet form is used principal alloy of stainless steel is chromium. The for secondary structural parts and clamps, and in corrosion resistant steel most often used in aircraft tubular form for moderately stressed structural construction is known as 18-8 steel because of its parts. content of 18 percent chromium and 8 percent Module 06 - Materials and Hardware 1.5 nickel. One of the distinctive features of 18-8 steel is 0.25 to 0.55 percent carbon, 0.15 to 0.25 percent that its strength may be increased by cold working. molybdenum, and 0.50 to 1.10 percent chromium. These steels, when suitably heat treated, are deep Stainless steel may be rolled, drawn, bent, or formed hardening, easily machined, readily welded by either to any shape. Because these steels expand about 50 gas or electric methods, and are especially adapted percent more than mild steel and conduct heat only to high temperature service. about 40 percent as rapidly, they are more difficult to weld. Stainless steel can be used for almost any part Inconel is a nickel-chromium-iron alloy closely of an aircraft. Some of its common applications are resembling stainless steel (corrosion resistant steel, in the fabrication of exhaust collectors, stacks and CRES) in appearance. Aircraft exhaust systems manifolds, structural and machined parts, springs, use both alloys interchangeably. Because the two castings, tie rods, control cables and firewalls. alloys look very much alike, a distinguishing test is often necessary. One method of identification is The chrome-vanadium steels are made of to use an electrochemical technique, as described in approximately 18 percent vanadium and about 1 the following paragraph, to identify the nickel (Ni) percent chromium. When heat treated, they have content of the alloy. Inconel has a nickel content strength, toughness, and resistance to wear and greater than 50 percent, and the electrochemical test fatigue. A special grade of this steel in sheet form detects nickel. can be cold formed into intricate shapes. It can be folded and flattened without signs of breaking The tensile strength of Inconel is 100,000 psi or failure. SAE 6150 is used for making springs; annealed, and 125,000 psi when hard rolled. It is chrome-vanadium with high carbon content, SAE highly resistant to salt water and is able to withstand 6195, is used for ball and roller bearings. temperatures as high as 1,600 °F. Inconel welds readily and has working qualities quite similar to Molybdenum in small percentages is used in those of corrosion resistant steels. combination with chromium to form chrome- molybdenum steel, which has various uses in ELECTROCHEMICAL TEST aircraft. Molybdenum is a strong alloying element. Prepare a wiring assembly as shown in Figure 1-2, It raises the ultimate strength of steel without and prepare the two reagents (ammonium fluoride affecting ductility or workability. Molybdenum and dimethylglyoxime solutions) placing them in steels are tough and wear resistant, and they harden separate dedicated dropper solution bottles. Before throughout when heat treated. They are especially testing, you must thoroughly clean the metal in adaptable for welding and, for this reason, are order for the electrolytic deposit to take place. You used principally for welded structural parts and may use nonmetallic hand scrubbing pads or 320 assemblies. This type steel has practically replaced to 600 grit “crocus cloth” to remove deposits and carbon steel in the fabrication of fuselage tubing, corrosion products (thermal oxide). engine mounts, landing gears, and other structural parts. For example, a heat-treated SAE X4130 tube Connect the alligator clip of the wiring assembly to is approximately four times as strong as an SAE the bare metal being tested. Place one drop of a 0.05 1025 tube of the same weight and size. percent reagent grade ammonium fluoride solution in deionized water on the center of a 1 inch × 1 inch A series of chrome-molybdenum steel most used sheet of filter paper. Lay the moistened filter paper in aircraft construction is that series containing over the bare metal alloy being tested. Firmly press 1.6 Module 06 - Materials and Hardware the end of the aluminum rod over the center of the moist paper. Maintain connection for 10 seconds while rocking the aluminum rod on the filter paper. MATERIALS FERROUS Ensure that the light emitting diode (LED) remains lit (indicating good electrical contact and current flow) during this period. Disconnect the wiring assembly and set it aside. Remove the filter paper and examine it to determine that a light spot appears where the connection was made. 9v battery LED Figure 1-3. Electrochemical test results, Inconel and – + stainless steel alloys. check the appropriate structural repair manual. Aircraft manufacturers design structural members to meet a specific load requirement for a particular aircraft. The methods of repairing these members, apparently similar in construction, will thus vary with different aircraft. Aluminium rod stock Alligator clip Four requirements must be kept in mind when Figure 1-2. Wiring assembly schematic. selecting substitute metals. The first and most important of these is maintaining the original Deposit one drop of 1.0 percent solution of reagent strength of the structure. The other three are: (1) grade dimethylglyoxime in ethyl alcohol on the filter maintaining contour or aerodynamic smoothness, (2) paper (same side that was in contact with the test maintaining original weight, if possible, or keeping metal). A bright, distinctly pink spot will appear added weight to a minimum, and (3) maintaining within seconds on the filter paper if the metal being the original corrosion resistant properties of the tested is Inconel. A brown spot will appear if the metal. test metal is stainless steel. Some stainless steel alloys may leave a very light pink color. However, METAL WORKING the shade and depth of color will be far less than PROCESSES would appear for Inconel. For flat surfaces, the test There are three methods of metalworking: (1) hot spot will be circular while for curved surfaces, such working, (2) cold working, and (3) extruding. The as the outside of a tube or pipe, the test spot may method used will depend on the metal involved and appear as a streak. (Refer to Figure 1-3 for sample the part required, although in some instances both test results.) This procedure should not be used in hot and cold working methods may be used to make the heat affected zone of weldments or on nickel a single part. coated surfaces. HOT WORKING SUBSTITUTION OF Almost all steel is hot worked from the ingot into AIRCRAFT METALS some form from which it is either hot or cold worked In selecting substitute metals for the repair and to the finished shape. When an ingot is stripped maintenance of aircraft, it is very important to from its mold, its surface is solid, but the interior is Module 06 - Materials and Hardware 1.7 still molten. The ingot is then placed in a soaking pit Thus, it is necessary to use a very heavy hammer which retards loss of heat, and the molten interior or to subject the part to repeated blows to ensure gradually solidifies. After soaking, the temperature complete working of the section. If the force applied is equalized throughout the ingot, then it is reduced is too weak to reach the center, the finished forged to intermediate size by rolling, making it more surface will be concave. If the center was properly readily handled. worked, the surface will be convex or bulged. The advantage of hammering is that the operator has The rolled shape is called a bloom when its section control over both the amount of pressure applied dimensions are 6 inches × 6 inches or larger and and the finishing temperature, and is able to approximately square. The section is called a billet produce small parts of the highest grade. This type when it is approximately square and less than 6 of forging is usually referred to as smith forging. It is inches × 6 inches. Rectangular sections which have used extensively where only a small number of parts a width greater than twice their thickness are called are needed. Considerable machining time and slabs. The slab is the intermediate shape from which material are saved when a part is smith forged to sheets are rolled. approximately the finished shape. Blooms, billets, or slabs are heated above the critical Steel is often harder than necessary and too brittle range and rolled into a variety of shapes of uniform for most practical uses when put under severe cross section. Common rolled shapes are sheet, bar, internal strain. To relieve such strain and reduce channel, angle, and I-beam. As discussed later in brittleness, it is tempered after being hardened. this chapter, hot rolled material is frequently finished This consists of heating the steel in a furnace to a by cold rolling or drawing to obtain accurate finish specified temperature and then cooling it in air, oil, dimensions and a bright, smooth surface. water, or a special solution. Temper condition refers to the condition of metal or metal alloys with respect Complicated sections which cannot be rolled, or to hardness or toughness. Rolling, hammering, or sections of which only a small quantity is required, bending these alloys, or heat treating and aging are usually forged. Forging of steel is a mechanical them, causes them to become tougher and harder. working at temperatures above the critical range to At times these alloys become too hard for forming shape the metal as desired. Forging is done either and have to be re-heat treated or annealed. by pressing or hammering the heated steel until the desired shape is obtained. Metals are annealed to relieve internal stresses, soften the metal, make it more ductile, and refine Pressing is used when the parts to be forged are large the grain structure. Annealing consists of heating and heavy; this process also replaces hammering the metal to a prescribed temperature, holding it where high grade steel is required. Since a press is there for a specified length of time, and then cooling slow acting, its force is uniformly transmitted to the metal back to room temperature. To produce the center of the section, thus affecting the interior maximum softness, the metal must be cooled very grain structure as well as the exterior to give the slowly. Some metals must be furnace cooled; others best possible structure throughout. may be cooled in air. Hammering can be used only on relatively small Normalizing applies to iron base metals only. pieces. Since hammering transmits its force almost Normalizing consists of heating the part to the instantly, its effect is limited to a small depth. proper temperature, holding it at that temperature 1.8 Module 06 - Materials and Hardware until it is uniformly heated, and then cooling it in between the blades of a pair of scissors (shears). The still air. Normalizing is used to relieve stresses in shear strength is the shear force in psi at which a metals. material fails. It is the load divided by the shear area. MATERIALS FERROUS Strength, weight, and reliability are three factors Bending can be described as the deflection or which determine the requirements to be met by curving of a member due to forces acting upon it. any material used in airframe construction and The bending strength of material is the resistance repair. Airframes must be strong and yet as light it offers to deflecting forces. Torsion is a twisting weight as possible. There are very definite limits to force. Such action would occur in a member fixed which increases in strength can be accompanied at one end and twisted at the other. The torsional by increases in weight. An airframe so heavy that strength of material is its resistance to twisting. it could not support a few hundred pounds of additional weight would be of little use. The relationship between the strength of a material and its weight per cubic inch, expressed as a ratio, is All metals, in addition to having a good strength/ known as the strength/weight ratio. This ratio forms weight ratio, must be thoroughly reliable, thus the basis for comparing the desirability of various minimizing the possibility of dangerous and materials for use in airframe construction and repair. unexpected failures. In addition to these general properties, the material selected for a definite Neither strength nor weight alone can be used as application must possess specific qualities suitable a means of true comparison. In some applications, for the purpose. such as the skin of monocoque structures, thickness is more important than strength, and, in this The material must possess the strength required instance, the material with the lightest weight for a by the dimensions, weight, and use. The five basic given thickness or gauge is best. Thickness or bulk stresses which metals may be required to withstand is necessary to prevent bucking or damage caused by are tension, compression, shear, bending, and careless handling. torsion. Corrosion is the eating away or pitting of the surface The tensile strength of a material is its resistance to or the internal structure of metals. Because of the a force which tends to pull it apart. Tensile strength thin sections and the safety factors used in aircraft is measured in pounds per square inch (psi) and is design and construction, it would be dangerous to calculated by dividing the load in pounds required select a material possessing poor corrosion resistant to pull the material apart by its cross-sectional area characteristics. in square inches. Another significant factor to consider in maintenance The compression strength of a material is its and repair is the ability of a material to be formed, resistance to a crushing force which is the opposite bent, or machined to required shapes. The hardening of tensile strength. Compression strength is also of metals by cold working or forming is termed work measured in psi. When a piece of metal is cut, the hardening. If a piece of metal is formed (shaped material is subjected, as it comes in contact with the or bent) while cold, it is said to be cold worked. cutting edge, to a force known as shear. Shear is the Practically all the work an aviation mechanic does tendency on the part of parallel members to slide in on metal is cold work. While this is convenient, it opposite directions. It is like placing a cord or thread causes the metal to become harder and more brittle. Module 06 - Materials and Hardware 1.9 If the metal is cold worked too much, that is, if it which the metal is heated, the rate at which it is is bent back and forth or hammered at the same cooled, and, of course, in the final result. place too often, it will crack or break. Usually, the more malleable and ductile a metal is, the more cold The most common forms of heat treatment working it can stand. Any process which involves for ferrous metals are hardening, tempering, controlled heating and cooling of metals to develop normalizing, annealing, and casehardening. Most certain desirable characteristics (such as hardness, nonferrous metals can be annealed and many of softness, ductility, tensile strength, or refined grain them can be hardened by heat treatment. However, structure) is called heat treatment or heat treating. there is only one nonferrous metal, titanium, that With steels the term “heat treating” has a broad can be casehardened, and none can be tempered or meaning and includes such processes as annealing, normalized. normalizing, hardening, and tempering. INTERNAL STRUCTURE OF METALS Aircraft metals are subjected to both shock and The results obtained by heat treatment depend to a fatigue (vibrational) stresses. Fatigue occurs in great extent on the structure of the metal and on materials which are exposed to frequent reversals the manner in which the structure changes when of loading or repeatedly applied loads. Repeated the metal is heated and cooled. A pure metal cannot vibration or bending will ultimately cause a minute be hardened by heat treatment because there is little crack to occur at the weakest point. As vibration change in its structure when heated. On the other or bending continues, the crack lengthens until hand, most alloys respond to heat treatment since the part completely fails. This is termed shock their structures change with heating and cooling. and fatigue failure. Resistance to this condition is known as shock and fatigue resistance. It is essential An alloy may be in the form of a solid solution, a that materials used for critical parts be resistant to mechanical mixture, or a combination of a solid these stresses. solution and a mechanical mixture. When an alloy is in the form of a solid solution, the elements and HEAT TREATING compounds which form the alloy are absorbed, one Heat treatment is a series of operations involving into the other, in much the same way that salt is the heating and cooling of metals in the solid state. dissolved in a glass of water, and the constituents Its purpose is to change a mechanical property or cannot be identified even under a microscope. combination of mechanical properties so that the metal will be more useful, serviceable, and safe for When two or more elements or compounds are mixed a definite purpose. By heat treating, a metal can be but can be identified by microscopic examination, made harder, stronger, and more resistant to impact. a mechanical mixture is formed. A mechanical Heat treating can also make a metal softer and more mixture can be compared to the mixture of sand ductile. No one heat treating operation can produce and gravel in concrete. The sand and gravel are both all of these characteristics. In fact, some properties visible. Just as the sand and gravel are held together are often improved at the expense of others. In being and kept in place by the matrix of cement, the other hardened, for example, a metal may become brittle. constituents of an alloy are embedded in the matrix formed by the base metal. The various heat-treating processes are similar in that they all involve the heating and cooling of An alloy in the form of a mechanical mixture metals. They differ, however, in the temperatures to at ordinary temperatures may change to a solid 1.10 Module 06 - Materials and Hardware solution when heated. When cooled back to normal which the work is placed. If an open muffler is used, temperature, the alloy may return to its original the furnace should be designed to prevent the direct structure. On the other hand, it may remain a solid impingement of flame on the work. MATERIALS FERROUS solution or form a combination of a solid solution and mechanical mixture. An alloy which consists In furnaces heated by electricity, the heating of a combination of solid solution and mechanical elements are generally in the form of wire or ribbon. mixture at normal temperatures may change to a Good design requires incorporation of additional solid solution when heated. When cooled, the alloy heating elements at locations where maximum heat may remain a solid solution, return to its original loss may be expected. Such furnaces commonly structure, or form a complex solution. operate at up to a maximum temperature of about 2,000 °F. Furnaces operating at temperatures up HEAT-TREATING EQUIPMENT to about 2,500 °F usually employ resistor bars of Successful heat treating requires close control over sintered carbides. all factors affecting the heating and cooling of metals. Such control is possible only when the proper TEMPERATURE MEASUREMENT AND equipment is available and the equipment is selected CONTROL to fit the particular job. Thus, the furnace must be of Temperature in the heat-treating furnace is the proper size and type and must be so controlled that measured by a thermoelectric instrument known as temperatures are kept within the limits prescribed a pyrometer. This instrument measures the electrical for each operation. Even the atmosphere within the effect of a thermocouple and, hence, the temperature furnace affects the condition of the part being heat of the metal being treated. A complete pyrometer treated. Further, the quenching equipment and the consists of three parts—a thermocouple, extension quenching medium must be selected to fit the metal leads, and meter. and the heat treating operation. Finally, there must be equipment for handling parts and materials, for Furnaces intended primarily for tempering may cleaning metals, and for straightening parts. be heated by gas or electricity and are frequently equipped with a fan for circulating the hot air. FURNACES AND SALT BATHS There are many different types and sizes of furnaces Salt baths are available for operating at either used in heat treatment. As a general rule, furnaces are tempering or hardening temperatures. Depending designed to operate in certain specific temperature on the composition of the salt bath, heating can ranges and attempted use in other ranges frequently be conducted at temperatures as low as 325 °F to results in work of inferior quality. as high as 2,450 °F. Lead baths can be used in the temperature range of 650 °F to 1,700 °F. The rate In addition, using a furnace beyond its rated of heating in lead or salt baths is much faster in maximum temperature shortens its life and may furnaces. necessitate costly and time consuming repairs. Heat-treating furnaces differ in size, shape, capacity, Fuel fired furnaces (gas or oil) require air for proper construction, operation, and control. They may be combustion and an air compressor or blower is circular or rectangular and may rest on pedestals therefore necessary. These furnaces are usually of or directly on the floor. There are also pit-type the muffler type; that is, the combustion of the fuel furnaces, which are below the surface of the floor. takes place outside of and around the chamber in When metal is to be heated in a bath of molten salt Module 06 - Materials and Hardware 1.11 or lead, the furnace must contain a pot or crucible recording type produces a permanent record of the for the molten bath. temperature range throughout the heating operation by means of an inked stylus attached to an arm The size and capacity of a heat-treating furnace which traces a line on a sheet of calibrated paper or depends on the intended use. A furnace must be temperature chart. capable of heating rapidly and uniformly, regardless of the desired maximum temperature or the mass Pyrometer installations on all modern furnaces of the charge. An oven-type furnace should have provide automatic regulation of the temperature a working space (hearth) about twice as long and at any desired setting. Instruments of this type three times as wide as any part that will be heated are called controlling potentiometer pyrometers. in the furnace. They include a current regulator and an operating mechanism, such as a relay. Accurate temperature measurement is essential to good heat treating. The usual method is by means HEATING of thermocouples: the most common base metal The object in heating is to transform pearlite (a couples are copper-constantan (up to about 700 °F), mixture of alternate strips of ferrite and iron carbide iron-constantan (up to about 1 400 °F), and chromel- in a single grain) to austenite as the steel is heated alumel (up to about 2 200 °F). The most common through the critical range. Since this transition noble metal couples (which can be used up to about takes time, a relatively slow rate of heating must be 2 800 °F) are platinum coupled with either the alloy used. Ordinarily, the cold steel is inserted when the 87 percent platinum (13 percent rhodium) or the temperature in the furnace is from 300 °F to 500 °F alloy 90 percent platinum (10 percent rhodium). The below the hardening temperature. In this way, too temperatures quoted are for continuous operation. rapid heating through the critical range is prevented. The life of thermocouples is affected by the If temperature measuring equipment is not available, maximum temperature (which may frequently it becomes necessary to estimate temperatures exceed those given above) and by the furnace by some other means. An inexpensive, yet fairly atmosphere. Iron-constantan is more suited for accurate method involves the use of commercial use in reducing and chromel-alumel in oxidizing crayons, pellets, or paints that melt at various atmospheres. Thermocouples are usually encased in temperatures within the range of 125 °F to 1,600 metallic or ceramic tubes closed at the hot end to °F. The least accurate method of temperature protect them from the furnace gases. A necessary estimation is by observation of the color of the hot attachment is an instrument, such as a millivoltmeter hearth of the furnace or of the work. The heat colors or potentiometer, for measuring the electromotive observed are affected by many factors, such as the force generated by the thermocouple. In the interest conditions of artificial or natural light, the character of accurate control, place the hot junction of the of the scale on the work, and so forth. Steel begins thermocouple as close to the work as possible. to appear dull red at about 1,000 °F, and as the The use of an automatic controller is valuable in temperature increases, the color changes gradually controlling the temperature at the desired value. through various shades of red to orange, to yellow, and finally to white. A rough approximation of the Pyrometers may have meters either of the indicating correspondence between color and temperature is type or recording type. Indicating pyrometers give indicated in Figure 1-4. direct reading of the furnace temperature. The 1.12 Module 06 - Materials and Hardware It is also possible to secure some idea of the °Fahrenheit °Celsius temperature of a piece of carbon or low alloy steel, 1,500 in the low temperature range used for tempering, MATERIALS 2,700 FERROUS from the color of the thin oxide film that forms 2,600 on the cleaned surface of the steel when heated 1,400 2,500 in this range. The approximate temperature/color relationship is indicated on the lower portion of the 2,400 1,300 scale in Figure 1-4. 2,300 Color of hot body 2,200 It is often necessary or desirable to protect steel or 1,200 cast iron from surface oxidation (scaling) and loss 2,100 White of carbon from the surface layers (decarburization). 1,100 2,000 Commercial furnaces, therefore, are generally equipped with some means of atmosphere control. 1,900 Yellow 1,000 This usually is in the form of a burner for burning 1,800 controlled amounts of gas and air and directing Orange 1,700 the products of combustion into the furnace 900 muffle. Water vapor, a product of this combustion, 1,600 Light cherry is detrimental and many furnaces are equipped 1,500 800 with a means for eliminating it. For furnaces not Full cherry 1,400 equipped with atmosphere control, a variety of external atmosphere generators are available. The 1,300 700 gas so generated is piped into the furnace and one 1,200 Dark cherry generator may supply several furnaces. If no method 600 1,100 of atmosphere control is available, some degree of protection may be secured by covering the work 1,000 Dark red 500 with cast iron borings or chips. 900 800 Since the work in salt or lead baths is surrounded 400 by the liquid heating medium, the problem of 700 Temper preventing scaling or decarburization is simplified. colors 600 300 Blue Vacuum furnaces also are used for annealing steels, 500 Purple especially when a bright non-oxidized surface is a Brown prime consideration. 400 200 Straw 300 SOAKING 100 200 The temperature of the furnace must be held constant during the soaking period, since it is during this 100 period that rearrangement of the internal structure 0 of the steel takes place. Soaking temperatures for Figure 1-4. Temperature chart indicating conversion various types of steel are specified in ranges varying of Celsius to Fahrenheit or visa versa, color as much as 100 °F. (Figure 1-5) Small parts are temperature scale for hardening temperature range, soaked in the lower part of the specified range and and tempering temperature range. Module 06 - Materials and Hardware 1.13 heavy parts in the upper part of the specified range. There are many specially prepared quenching oils on The length of the soaking period depends upon the the market; their cooling rates do not vary widely. A type of steel and the size of the part. Naturally, straight mineral oil with a Saybolt viscosity of about heavier parts require longer soaking to ensure equal 100 at 100 °F is generally used. Unlike brine and heating throughout. As a general rule, a soaking water, the oils have the greatest cooling velocity at period of 30 minutes to 1 hour is sufficient for the a slightly elevated temperature—about 100–140 average heat-treating operation. °F—because of their decreased viscosity at these temperatures. COOLING The rate of cooling through the critical range When steel is quenched, the liquid in immediate determines the form that the steel will retain. contact with the hot surface vaporizes; this vapor Various rates of cooling are used to produce the reduces the rate of heat abstraction markedly. desired results. Still air is a slow cooling medium, Vigorous agitation of the steel or the use of a pressure but is much faster than furnace cooling. Liquids are spray quench is necessary to dislodge these vapor the fastest cooling media and are therefore used in films and thus permit the desired rate of cooling. hardening steels. There are three commonly used The tendency of steel to warp and crack during the quenching liquids— brine, water, and oil. Brine is quenching process is difficult to overcome because the strongest quenching medium, water is next, and certain parts of the article cool more rapidly than oil is the least. Generally, an oil quench is used for others. alloy steels, and brine or water for carbon steels. The following recommendations will greatly reduce QUENCHING MEDIA the warping tendency: Quenching solutions act only through their ability 1. Never throw a part into the quenching bath. to cool the steel. They have no beneficial chemical By permitting it to lie on the bottom of the action on the quenched steel and in themselves bath, it is apt to cool faster on the top side than impart no unusual properties. Most requirements on the bottom side, thus causing it to warp or for quenching media are met satisfactorily by water crack. or aqueous solutions of inorganic salts, such as table 2. Agitate the part slightly to destroy the coating salt or caustic soda, or by some type of oil. The rate of vapor that could prevent it from cooling of cooling is relatively rapid during quenching in evenly and rapidly. This allows the bath to brine, somewhat less rapid in water, and slow in oil. dissipate its heat to the atmosphere. 3. Immerse irregular shaped parts so that the Brine usually is made of a 5 to 10 percent solution heavy end enters the bath first. of salt (sodium chloride) in water. In addition to its greater cooling speed, brine has the ability to QUENCHING EQUIPMENT “throw” the scale from steel during quenching. The The quenching tank should be of the proper size to cooling ability of both water and brine, particularly handle the material being quenched. Use circulating water, is considerably affected by their temperature. pumps and coolers to maintain approximately Both should be kept cold—well below 60 °F. If the constant temperatures when doing a large amount volume of steel being quenched tends to raise the of quenching. To avoid building up a high temperature of the bath appreciably, add ice or use concentration of salt in the quenching tank, make some means of refrigeration to cool the quenching provisions for adding fresh water to the quench tank bath. used for molten salt baths. 1.14 Module 06 - Materials and Hardware Tank location in reference to the heat-treating rapid, as by quenching in oil or water, the carbon furnace is very important. Situate the tank to permit precipitates as a cloud of very fine carbide particles, rapid transfer of the part from the furnace to the and the steel is hard. The fact that the carbide MATERIALS FERROUS quenching medium. A delay of more than a few particles can be dissolved in austenite is the basis of seconds will, in many instances, prove detrimental the heat treatment of steel. The temperatures at to the effectiveness of the heat treatment. When which this transformation takes place are called heat treating material of thin section, employ guard the critical points and vary with the composition of sheets to retard the loss of heat during transfer to the steel. The percentage of carbon in the steel has the quench tank. Provide a rinse tank to remove all the greatest influence on the critical points of heat salt from the material after quenching if the salt is treatment. not adequately removed in the quenching tank. HARDENING HEAT TREATMENT OF FERROUS Pure iron, wrought iron, and extremely low carbon METALS steels cannot be appreciably hardened by heat The first important consideration in the heat treatment treatment, since they contain no hardening element. of a steel part is to know its chemical composition. Cast iron can be hardened, but its heat treatment is This, in turn, determines its upper critical point. limited. When cast iron is cooled rapidly, it forms When the upper critical point is known, the next white iron, which is hard and brittle. When cooled consideration is the rate of heating and cooling to be slowly, it forms gray iron, which is soft but brittle used. Carrying out these operations involves the use under impact. of uniform heating furnaces, proper temperature controls, and suitable quenching mediums. In plain carbon steel, the maximum hardness depends almost entirely on the carbon content of BEHAVIOR OF STEEL DURING the steel. As the carbon content increases, the ability HEATING AND COOLING of the steel to be hardened increases. However, this Changing the internal structure of a ferrous metal is increase in the ability to harden with an increase in accomplished by heating to a temperature above its carbon content continues only to a certain point. In upper critical point, holding it at that temperature for practice, that point is 0.85 percent carbon content. a time sufficient to permit certain internal changes to When the carbon content is increased beyond 0.85 occur, and then cooling to atmospheric temperature percent, there is no increase in wear resistance. under predetermined, controlled conditions. For most steels, the hardening treatment consists At ordinary temperatures, the carbon in steel exists of heating the steel to a temperature just above in the form of particles of iron carbide scattered the upper critical point, soaking or holding for throughout an iron matrix known as “ferrite.” The the required length of time, and then cooling it number, size, and distribution of these particles rapidly by plunging the hot steel into oil, water, or determine the hardness of the steel. At elevated brine. Although most steels must be cooled rapidly temperatures, the carbon is dissolved in the for hardening, a few may be cooled in still air. iron matrix in the form of a solid solution called Hardening increases the hardness and strength of “austenite,” and the carbide particles appear only the steel but makes it less ductile. after the steel has been cooled. If the cooling is slow, the carbide particles are relatively coarse and few. When hardening carbon steel, it must be cooled to In this condition, the steel is soft. If the cooling is below 1,000 °F in less than 1 second. Should the Module 06 - Materials and Hardware 1.15 time required for the temperature to drop to 1,000 medium on the areas to be hardened. This also is °F exceed 1 second, the austenite begins to transform accomplished by the induction and flame hardening into fine pearlite. This pearlite varies in hardness, procedures previously described, particularly on but is much harder than the pearlite formed by large production jobs. annealing and much softer than the martensite desired. After the 1,000 °F temperature is reached, Shallow hardening steels, such as plain carbon and the rapid cooling must continue if the final structure certain varieties of alloy steels, have such a high is to be all martensite. critical cooling rate that they must be quenched in brine or water to effect hardening. In general, When alloys are added to steel, the time limit for intricately shaped sections should not be made of the temperature drop to 1,000 °F increases above shallow hardening steels because of the tendency the 1 second limit for carbon steels. Therefore, a of these steels to warp and crack during hardening. slower quenching medium will produce hardness in Such items should be made of deeper hardening alloy steels. steels capable of being hardened by quenching in oil or air. Because of the high internal stresses in the “as quenched” condition, steel must be tempered just TEMPERING before it becomes cold. The part should be removed Tempering reduces the brittleness imparted by from the quenching bath at a temperature of hardening and produces definite physical properties approximately 200 °F, since the temperature range within the steel. Tempering always follows, never from 200 °F down to room temperature is the precedes, the hardening operation. In addition to cracking range. reducing brittleness, tempering softens the steel. Tempering is always conducted at temperatures Hardening temperatures and quenching mediums below the low critical point of the steel. In for the various types of steel are listed in Figure 1-5. this respect, tempering differs from annealing, normalizing, or hardening, all of which require HARDENING PRECAUTIONS temperatures above the upper critical point. When A variety of different shapes and sizes of tongs hardened steel is reheated, tempering begins at for handling hot steels is necessary. It should be 212 °F and continues as the temperature increases remembered that cooling of the area contacted by toward the low critical point. By selecting a definite the tongs is retarded and that such areas may not tempering temperature, the resulting hardness harden, particularly if the steel being treated is and strength can be predetermined. Approximate very shallow hardening. Small parts may be wired temperatures for various tensile strengths are listed together or quenched in baskets made of wire mesh. in Figure 1-5. The minimum time at the tempering Special quenching jigs and fixtures are frequently temperature should be 1 hour. If the part is over 1 used to hold steels during quenching in a manner to inch in thickness, increase the time by 1 hour for restrain distortion. each additional inch of thickness. Tempered steels used in aircraft work have from 125,000 to 200,000 When selective hardening is desired, portions of the psi ultimate tensile strength. steel may be protected by covering with alundum cement or some other insulating material. Selective Generally, the rate of cooling from the tempering hardening may be accomplished also by the use temperature has no effect on the resulting structure; of water or oil jets designed to direct quenching therefore, the steel is usually cooled in still air after 1.16 Module 06 - Materials and Hardware being removed from the furnace. The more rapid quenching obtained by air cooling, as compared to furnace cooling, results in a harder and ANNEALING stronger material than that obtained by annealing. MATERIALS FERROUS Annealing of steel produces a fine grained, soft, Recommended normalizing temperatures for the ductile metal without internal stresses or strains. In various types of aircraft steels are listed in Figure the annealed state, steel has its lowest strength. In 1-5. general, annealing is the opposite of hardening. CASE HARDENING Annealing of steel is accomplished by heating the Case hardening produces a hard wear-resistant metal to just above the upper critical point, soaking surface or case over a strong, tough core. Case at that temperature, and cooling very slowly in the hardening is ideal for parts which require a wear- furnace. (Refer to Figure 1-5 for recommended resistant surface and, at the same time, must be temperatures.) Soaking time is approximately 1 tough enough internally to withstand the applied hour per inch of thickness of the material. To loads. The steels best suited to case hardening are produce maximum softness in steel, the metal must the low carbon and low alloy steels. If high carbon be cooled very slowly. Slow cooling is obtained steel is case hardened, the hardness penetrates the by shutting off the heat and allowing the furnace core and causes brittleness. In case hardening, and metal to cool together to 900 °F or lower, then the surface of the metal is changed chemically by removing the metal from the furnace and cooling introducing a high carbide or nitride content. The in still air. Another method is to bury the heated core is unaffected chemically. steel in ashes, sand, or other substance that does not conduct heat readily. When heat treated, the surface responds to hardening while the core toughens. The common NORMALIZING forms of case hardening are carburizing, cyaniding, The normalizing of steel removes the internal and nitriding. Since cyaniding is not used in aircraft stresses set up by heat treating, welding, casting, work, only carburizing and nitriding are discussed forming, or machining. Stress, if not controlled, will in this section. lead to failure. CARBURIZING Because of the better physical properties, aircraft Carburizing is a case hardening process in which steels are often used in the normalized state, but carbon is added to the surface of low carbon steel. seldom, if ever, in the annealed state. One of the Thus, a carburized steel has a high carbon surface most important uses of normalizing in aircraft work and a low carbon interior. When the carburized is in welded parts. Welding causes strains to be set steel is heat treated, the case is hardened while the up in the adjacent material. In addition, the weld core remains soft and tough. itself is a cast structure as opposed to the wrought structure of the rest of the material. These two types A common method of carburizing is called “pack of structures have different grain sizes, and to refine carburizing.” When carburizing is to be done by this the grain as well as to relieve the internal stresses, all method, the steel parts are packed in a container welded parts should be normalized after fabrication. with charcoal or some other material rich in carbon. The container is then sealed with fire clay, placed Normalizing is accomplished by heating the steel in a furnace, heated to approximately 1,700 °F, and above the upper critical point and cooling in still air. soaked at that temperature for several hours. As the Module 06 - Materials and Hardware 1.17