Properties of Metals.docx

**Properties of Metals** Of primary concern in aircraft maintenance are such general properties of metals and their alloys as hardness, malleability, ductility, elasticity, toughness, density, brittleness, fusibility, conductivity contraction and expansion, and so forth. These terms are explained t...

**Properties of Metals** Of primary concern in aircraft maintenance are such general properties of metals and their alloys as hardness, malleability, ductility, elasticity, toughness, density, brittleness, fusibility, conductivity contraction and expansion, and so forth. These terms are explained to establish a basis for further discussion of structural metals. ***Hardness*** Hardness refers to the ability of a material to resist abrasion, penetration, cutting action, or permanent distortion. Hardness may be increased by cold working the metal and, in the case of steel and certain aluminum alloys, by heat treatment. Structural parts are often formed from metals in their soft state and are then heat treated to harden them so that the finished shape is retained. Hardness and strength are closely associated properties of metals. ***Strength*** One of the most important properties of a material is strength. Strength is the ability of a material to resist deformation. Strength is also the ability of a material to resist stress without breaking. The type of load or stress on the material affects the strength it exhibits. ***Density*** Density is the weight of a unit volume of a material. In aircraft work, the specified weight of a material per cubic inch is preferred since this figure can be used in determining the weight of a part before actual manufacture. Density is an important consideration when choosing a material to be used in the design of a part to maintain the proper weight and balance of the aircraft. ***Malleability*** A metal that can be hammered, rolled, or pressed into various shapes without cracking, breaking, or leaving some other detrimental effect, is said to be malleable. This property is necessary in sheet metal that is worked into curved shapes, such as cowlings, fairings, or wingtips. Copper is an example of a malleable metal. ***Ductility*** Ductility is the property of a metal that permits it to be permanently drawn, bent, or twisted into various shapes without breaking. This property is essential for metals used in making wire and tubing. Ductile metals are greatly preferred for aircraft use because of their ease of forming and resistance to failure under shock loads. For this reason, aluminum alloys are used for cowl rings, fuselage and wing skin, and formed or extruded parts, such as ribs, spars, and bulkheads. Chrome molybdenum steel is also easily formed into desired shapes. Ductility is similar to malleability. ***Elasticity*** Elasticity is a property that enables a metal to return to its original size and shape when the force that causes the change of shape is removed. This property is extremely valuable, because it would be highly undesirable to have a part permanently distorted after an applied load was removed. Each metal has a point known as the elastic limit, beyond which it cannot be loaded without causing permanent distortion. In aircraft construction, members and parts are so designed that the maximum loads to which they are subjected do not stress them beyond their elastic limits. This desirable property is present in spring steel. ***Toughness*** A material that possesses toughness withstands tearing or shearing and may be stretched or otherwise deformed without breaking. Toughness is a desirable property in aircraft metals. ***Brittleness*** Brittleness is the property of a metal that allows little bending or deformation without shattering. A brittle metal is apt to break or crack without change of shape. Because structural metals are often subjected to shock loads, brittleness is not a very desirable property. Cast iron, cast aluminum, and very hard steel are examples of brittle metals. ***Fusibility*** Fusibility is the ability of a metal to become liquid by the application of heat. Metals are fused in welding. Steels fuse around 2,600 °F and aluminum alloys at approximately 1,100 °F. ***Conductivity*** Conductivity is the property that enables a metal to carry heat or electricity. The heat conductivity of a metal is especially important in welding, because it governs the amount of heat that is required for proper fusion. Conductivity of the metal, to a certain extent, determines the type of jig to be used to control expansion and contraction. In aircraft, electrical conductivity must also be considered in conjunction with bonding to eliminate radio interference. ***Thermal Expansion*** Thermal expansion refers to contraction and expansion that are reactions produced in metals as the result of heating or cooling. Heat applied to a metal causes it to expand or become larger. Cooling and heating affect the design of welding jigs, castings, and tolerances necessary for hot rolled material. 7-2 **Ferrous Aircraft Metals** Many different metals are required in the repair of aircraft. This is a result of the varying needs with respect to strength, weight, durability, and resistance to deterioration of specific structures or parts. In addition, the particular shape or form of the material plays an important role. In selecting materials for aircraft repair, these factors (plus many others) are considered in relation to the mechanical and physical properties. Among the common materials used are ferrous metals. The term "ferrous" applies to the group of metals having iron as their principal constituent. ***Iron*** If carbon is added to iron in percentages ranging up to approximately 1 percent, the product is vastly superior to iron alone and is classified as carbon steel. Carbon steel forms the base of those alloy steels produced by combining carbon steel with other elements known to improve the properties of steel. A base metal (such as iron) to which small quantities of other metals have been added is called an alloy. The addition of other metals changes or improves the chemical or physical properties of the base metal for a particular use. ***Steel and Steel Alloys*** To facilitate the discussion of steels some familiarity with their nomenclature is desirable. A numerical index, sponsored by the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI), is used to identify the chemical compositions of the structural steels. In this system, a four-numeral series is used to designate the plain carbon and alloy steels; five numerals are used to designate certain types of alloy steels. The first two digits indicate the type of steel, the second digit also generally (but not always) gives the approximate amount of the major alloying element, and the last two (or three) digits are intended to indicate the approximate middle of the carbon range. However, a deviation from the rule of indicating the carbon range is sometimes necessary. Small quantities of certain elements are present in alloy steels that are not specified as required. These elements are considered as incidental and may be present to the maximum amounts as follows: copper, 0.35 percent; nickel, 0.25 percent; chromium, 0.20 percent; molybdenum, 0.06 percent. The list of standard steels is altered from time to time to accommodate steels of proven merit and to provide for changes in the metallurgical and engineering requirements of industry. *\[Figure 7-1\]* Metal stock is manufactured in several forms and shapes, including sheets, bars, rods, tubing, extrusions, forgings, and castings. Sheet metal is made in a number of sizes and thicknesses. Specifications designate thicknesses in thousandths of an inch. Bars and rods are supplied in a variety of shapes, such as round, square, rectangular, hexagonal, and octagonal. Tubing can be obtained in round, oval, rectangular, or streamlined shapes. The size of tubing is generally specified by outside diameter and wall thickness. The sheet metal is usually formed cold in machines, such as presses, bending brakes, draw benches, or rolls. Forgings are shaped or formed by pressing or hammering heated metal in dies. Pouring molten metal into molds produces castings. Machining finishes the casting. Spark testing is a common means of identifying various ferrous metals. In this test, the piece of iron or steel is held against a revolving grinding stone, and the metal is identified by the sparks thrown off. Each ferrous metal has its own peculiar spark characteristics. The spark streams vary from a few tiny shafts to a shower of sparks several feet in length. (Few nonferrous metals give off sparks when touched to a grinding stone. Therefore, these metals cannot be successfully identified by the spark test.) Identification by spark testing is often inexact unless performed by an experienced person or the test pieces differ greatly in their carbon content and alloying constituents. Wrought iron produces long shafts that are straw colored as they leave the stone and white at the end. Cast iron sparks are red as they leave the stone and turn to a straw color. Low carbon steels give off long, straight shafts having a few white sprigs. As the carbon content of the steel increases, the number of sprigs along each shaft increases and the stream becomes whiter in color. Nickel steel causes the spark stream to contain small white blocks of light within the main burst. *Types, Characteristics, and Uses of Alloyed Steels* Steel containing carbon in percentages ranging from 0.10 to 0.30 percent is classed as low carbon steel. The equivalent SAE numbers range from 1010 to 1030. Steels of this grade are used for making items, such as safety wire, certain nuts, cable bushings, or threaded rod ends. This steel in sheet form is used for secondary structural parts and clamps and in tubular form for moderately stressed structural parts. Steel containing carbon in percentages ranging from 0.30 to 0.50 percent is classed as medium carbon steel. This steel is especially adaptable for machining or forging and where surface hardness is desirable. Certain rod ends and light forgings are made from SAE 1035 steel. Steel containing carbon in percentages ranging from 0.50 to 1.05 percent is classed as high carbon steel. The addition of other elements in varying quantities adds to the hardness of 7-3 **Series Designation Types** 10xx Non-sulfurized carbon steels 11xx Resulfurized carbon steels (free machining) 12xx Rephosphorized and resulfurized 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% \*Not included in the current list of standard steels **Figure 7-1.** *SAE numerical index.* this steel. In the fully heat-treated condition, it is very hard, withstands high shear and wear, and has little deformation. It has limited use in aircraft. SAE 1095 in sheet form is used for making flat springs and in wire form for making coil springs. The various nickel steels are produced by combining nickel with carbon steel. Steels containing from 3 to 3.75 percent nickels are commonly used. Nickel increases the hardness, tensile strength, and elastic limit of steel without appreciably 7-4 decreasing the ductility. It also intensifies the hardening effect of heat treatment. SAE 2330 steel is used extensively for aircraft parts, such as bolts, terminals, keys, clevises, and pins. Chromium steel is high in hardness, strength, and corrosion-resistant properties and is particularly adaptable for heat-treated forgings, which require greater toughness and strength than may be obtained in plain carbon steel. It can be used for articles such as the balls and rollers of antifriction bearings. Chrome-nickel or stainless steels are the corrosion resistant metals. The anticorrosive degree of this steel is determined by the surface condition of the metal, as well as by the composition, temperature, and concentration of the corrosive agent. The principal alloy of stainless steel is chromium. The corrosion resistant steel most often used in aircraft construction is known as 18-8 steel because its content is 18 percent chromium and 8 percent nickel. One of the distinctive features of 18-8 steel is that cold working may increase its strength. Stainless steel may be rolled, drawn, bent, or formed to any shape. Because these steels expand about 50 percent more than mild steel and conduct heat only about 40 percent as rapidly, they are more difficult to weld. Stainless steel can be used for almost any part of an aircraft. Some of its common applications are the fabrication of exhaust collectors, stacks and manifolds, structural and machined parts, springs, castings, tie rods, and control cables. The chrome-vanadium steels are made of approximately 18 percent vanadium and about 1 percent chromium. When heat-treated, they have strength, toughness, and resistance to wear and fatigue. A special grade of this steel in sheet form can be cold formed into intricate shapes. It can be folded and flattened without signs of breaking or failure. SAE 6150 is used for making springs; chrome-vanadium with high carbon content, SAE 6195, is used for ball and roller bearings. Molybdenum in small percentages is used in combination with chromium to form chrome-molybdenum steel, which has various uses in aircraft. Molybdenum is a strong alloying element. It raises the ultimate strength of steel without affecting ductility or workability. Molybdenum steels are tough and wear resistant, and they harden throughout when heat-treated. They are especially adaptable for welding and, for this reason, are used principally for welded structural parts and assemblies. This type steel has practically replaced carbon steel in the fabrication of fuselage tubing, engine mounts, landing gears, and other structural parts. For example, a heat-treated SAE X4130 tube is approximately four times as strong as an SAE 1025 tube of the same weight and size. A series of chrome-molybdenum steel most used in aircraft construction is that series containing 0.25 to 0.55 percent carbon, 0.15 to 0.25 percent molybdenum, and 0.50 to 1.10 percent chromium. These steels, when suitably heat treated, are deep hardening, easily machined, readily welded by either gas or electric methods, and are especially adapted to high temperature service. Inconel is a nickel-chromium-iron alloy closely resembling stainless steel (corrosion resistant steel (CRES)) in appearance. Aircraft exhaust systems use both alloys interchangeably. Because the two alloys look very much alike, a distinguishing test is often necessary. One method of identification is to use an electrochemical technique, as described in the following paragraph, to identify the nickel (Ni) content of the alloy. Inconel has nickel content greater than 50 percent, and the electrochemical test detects nickel. The tensile strength of Inconel is 100,000 pounds per square inch (psi) annealed, and 125,000 psi when hard rolled. It is highly resistant to salt water and can withstand temperatures as high as 1,600 °F. Inconel welds readily and has working qualities like those of corrosion resistant steels. **Electrochemical Test** Prepare a wiring assembly as shown in *Figure 7-2*, and prepare the two reagents (ammonium fluoride and dimethylglyoxime solutions) placing them in separate dedicated dropper solution bottles. Before testing, you must thoroughly clean the metal for the electrolytic deposit to take place. You may use nonmetallic hand scrubbing pads or 320--600 grit "crocus cloth" to remove deposits and corrosion products (thermal oxide). Connect the alligator clip of the wiring assembly to the bare metal being tested. Place one drop of a 0.05 percent reagent grade ammonium fluoride solution in deionized water on the center of a 1 inch × 1 inch sheet of filter paper. Lay the moistened filter paper over the bare metal alloy being tested. Firmly press the end of the aluminum rod over the center Aluminum rod stock 9v battery − + LED Alligator clip **Figure 7-2.** *Wiring assembly schematic.* 7-5 of the moist paper. Maintain connection for 10 seconds while rocking the aluminum rod on the filter paper. 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. Deposit one drop of 1.0 percent solution of reagent grade dimethylglyoxime in ethyl alcohol on the filter paper (same side that was in contact with the test metal). A bright, distinctly pink spot will appear within seconds on the filter paper if the metal being tested is Inconel. A brown spot will appear if the test metal is stainless steel. Some stainless-steel alloys may leave a very light pink color. However, the shade and depth of color will be far less than would appear for Inconel. For flat surfaces, the test spot will be circular while for curved surfaces, such as the outside of a tube or pipe, the test spot may appear as a streak. (Refer to *Figure 7-3* for sample test results.) This procedure should not be used in the heat-affected zone of weldments or on nickel coated surfaces. **Nonferrous Aircraft Metals** The term "nonferrous" refers to all metals that have elements other than iron as its base or principal constituent. This group includes metals, such as aluminum, titanium, copper, and magnesium, as well as alloyed metals, such as Monel and Babbitt. ***Aluminum and Aluminum Alloys*** Commercially pure aluminum is a white lustrous metal, which stands second in the scale of malleability, sixth in ductility, and ranks high in its resistance to corrosion. Aluminum combined with various percentages of other metals forms alloys, which are used in aircraft construction. Aluminum alloys with principal alloying ingredients are manganese, chromium, or magnesium and silicon show little attack in corrosive environments. Alloys with which substantial percentages of copper are more susceptible to corrosive action. The total percentage of alloying elements is seldom more than 6 or 7 percent in the wrought alloys. **Figure 7-3.** *Electrochemical test results of Inconel (In) and stainless steel (SS) alloys.* Aluminum is one of the most widely used metals in modern aircraft construction. It is vital to the aviation industry because of its high strength-to-weight ratio and its comparative ease of fabrication. The outstanding characteristic of aluminum is its lightweight. Aluminum melts at the comparatively low temperature of 1,250 °F. It is nonmagnetic and is an excellent conductor. Commercially pure aluminum has a tensile strength of about 13,000 psi, but rolling or other cold-working processes may approximately double its strength. By alloying with other metals, or by using heat-treating processes, the tensile strength may be raised to as high as 65,000 psi or to within the strength range of structural steel. Aluminum alloys, although strong, are easily worked because they are malleable and ductile. They may be rolled into sheets as thin as 0.0017 inch or drawn into wire 0.004 inch in diameter. Most aluminum alloy sheet stock used in aircraft construction range from 0.016 to 0.096 inch in thickness; however, some of the larger aircraft use sheet stock that may be as thick as 0.356 inch. The various types of aluminum may be divided into two general classes: - Casting alloys (those suitable for casting in sand, permanent mold, or die castings) - Wrought alloys (those which may be shaped by rolling, drawing, or forging). Of these two, the wrought alloys are the most widely used in aircraft construction, being used for stringers, bulkheads, skin, rivets, and extruded sections. Aluminum casting alloys are divided into two basic groups. In one, the physical properties of the alloys are determined by the alloying elements and cannot be changed after the metal is cast. In the other, the alloying elements make it possible to heat treat the casting to produce the desired physical properties. A letter preceding the alloy number identifies the casting alloys. When a letter precedes a number, it indicates a slight variation in the composition of the original alloy. This variation in composition is simply to impart some desirable quality. For example, in casting alloy 214, the addition of zinc to improve its pouring qualities is indicated by the letter A in front of the number, thus creating the designation A214. 7-6 When castings have been heat treated, the heat treatment and the composition of the casting is indicated by the letter T, followed by an alloying number. An example of this is the sand casting alloy 355, which has several different compositions and tempers and is designated by 355-T6, 355-T51, or C355-T51. Aluminum alloy castings are produced by one of three basic methods: sand mold, permanent mold, or die cast. In casting aluminum, it is important to note that in most cases different types of alloys must be used for different types of castings. Sand castings and die-castings require different types of alloys than those used in permanent molds. Sand and permanent mold castings are parts produced by pouring molten metal into a previously prepared mold, allowing the metal to solidify or freeze and then removing the part. If the mold is made of sand, the part is a sand casting; if it is a metallic mold (usually cast iron), the part is a permanent mold casting. Sand and permanent castings are produced by pouring liquid metal into the mold, the metal flowing under the force of gravity alone. The two principal types of sand casting alloys are 112 and 212\. Little difference exists between the two metals in mechanical properties, since both are adaptable to a wide range of products. The permanent mold process is a later development of the sand casting process, the major difference being in the material from which the molds are made. The advantage of this process is that there are fewer openings (called porosity) than in sand castings. The sand and the binder, which is mixed with the sand to hold it together, give off a certain amount of gas, that causes porosity in a sand casting. Permanent mold castings are used to obtain higher mechanical properties, better surfaces, or more accurate dimensions. There are two specific types of permanent mold castings: permanent metal mold with metal cores, and semi-permanent types containing sand cores. Because finer grain structure is produced in alloys subjected to the rapid cooling of metal molds, they are far superior to the sand type castings. Alloys 122, A132, and 142 are commonly used in permanent mold castings, the principal uses of which are in internal combustion engines. Die-castings used in aircraft are usually aluminum or magnesium alloy. If weight is of primary importance, magnesium alloy is used, because it is lighter than aluminum alloy. However, aluminum alloy is frequently used because it is stronger than most magnesium alloys. Forcing molten metal under pressure into a metallic die and allowing it to solidify produces a die-casting; then the die is opened and the part removed. The basic difference between permanent mold casting and die-casting is that in the permanent mold process, the metal flows into the die under gravity. In the die-casting operation, the metal is forced under great pressure. Die-castings are used where relatively large production of a given part is involved. Remember, any shape that can be forged, can be cast. Wrought aluminum and wrought aluminum alloys are divided into two general classes: non-heat-treatable alloys and heat-treatable alloys. Non-heat-treatable alloys are those in which the mechanical properties are determined by the amount of cold work introduced after the final annealing operation. The mechanical properties obtained by cold working are destroyed by any subsequent heating and cannot be restored except by additional cold working, which is not always possible. The "full hard" temper is produced by the maximum amount of cold work that is commercially practicable. Metal in the "as fabricated" condition is produced from the ingot without any subsequent controlled amount of cold working or thermal treatment. There is, consequently, a variable amount of strain hardening depending upon the thickness of the section. For heat-treatable aluminum alloys, the mechanical properties are obtained by heat treating to a suitable temperature, holding at that temperature long enough to allow the alloying constituent to enter into solid solution, and then quenching to hold the constituent in solution. The metal is left in a supersaturated, unstable state and is then age hardened either by natural aging at room temperature or by artificial aging at some elevated temperature. ***Wrought Aluminum*** Wrought aluminum and wrought aluminum alloys are designated by a four-digit index system. The system is broken into three distinct groups: 1xxx group, 2xxx through 8xxx group, and 9xxx group (which is currently unused). The first digit of a designation identifies the alloy type. The second digit indicates specific alloy modifications. Should the second number be zero, it would indicate no special control over individual impurities. Digits 1 through 9, however, when assigned consecutively as needed for the second number in this group, indicate the number of controls over individual impurities in the metal. 7-7 The last two digits of the 1xxx group are used to indicate the hundredths of 1 percent above the original 99 percent designated by the first digit. Thus, if the last two digits were 30, the alloy would contain 99 percent plus 0.30 percent of pure aluminum, or a total of 99.30 percent pure aluminum. Examples of alloys in this group are: - 1100---99.00 percent pure aluminum with one control over individual impurities. - 1130---99.30 percent pure aluminum with one control over individual impurities. - 1275---99.75 percent pure aluminum with two controls over individual impurities. In the 2xxx through 8xxx groups, the first digit indicates the major alloying element used in the formation of the alloy as follows: - 2xxx---copper - 3xxx---manganese - 4xxx---silicon - 5xxx---magnesium - 6xxx---magnesium and silicon - 7xxx---zinc - 8xxx---other elements In the 2xxx through 8xxx alloy groups, the second digit in the alloy designation indicates alloy modifications. If the second digit is zero, it indicates the original alloy, while digits 1 through 9 indicate alloy modifications. The last two of the four digits in the designation identify the different alloys in the group. *\[Figure 7-4\]* ***Effect of Alloying Element*** 1000 series: 99 percent aluminum or higher, excellent corrosion resistance, high thermal and electrical conductivity, low mechanical properties, excellent workability. Iron and silicon are major impurities. 2000 series: Copper is the principal alloying element. Solution heat treatment, optimum properties equal to mild steel, poor corrosion resistance unclad. It is usually clad with 6000 or high purity alloy. Its best-known alloy is 2024. 3000 series: Manganese is the principal alloying element of this group, which is generally non-heat treatable. The percentage of manganese that is alloy effective is 1.5 percent. The most popular is 3003, which is of moderate strength and has good working characteristics. **Alloy Percentage of Alloying Elements Aluminum and normal impurities constitute remainder** 1100 --- --- --- --- --- --- --- --- --- 3003 --- --- 1.2 --- --- --- --- --- --- 2011 5.5 --- --- --- --- --- --- 0.5 0.5 2014 4.4 0.8 0.8 0.4 --- --- --- --- --- 2017 4.0 --- 0.5 0.5 --- --- --- --- --- 2117 2.5 --- --- 0.3 --- --- --- --- --- 2018 4.0 --- --- 0.5 --- 2.0 --- --- --- 2024 4.5 --- 0.6 1.5 --- --- --- --- --- 2025 4.5 0.8 0.8 --- --- --- --- --- --- 4032 0.9 12.5 --- 1.0 --- 0.9 --- --- --- 6151 --- 1.0 --- 0.6 --- --- 0.25 --- --- 5052 --- --- --- 2.5 --- --- 0.25 --- --- 6053 --- 0.7 --- 1.3 --- --- 0.25 --- --- 6061 0.25 0.6 --- 1.0 --- --- 0.25 --- --- 7075 1.6 --- --- 2.5 5.6 --- 0.3 --- --- **Copper Silicon Manganese Magnesium Zinc Nickel Chromium Lead Bismuth** **Figure 7-4.** *Nominal composition of wrought aluminum alloys.* 7-8 4000 series: Silicon is the principal alloying element of this group and lowers melting temperature. Its primary use is in welding and brazing. When used in welding heat-treatable alloys, this group responds to a limited amount of heat treatment. 5000 series: Magnesium is the principal alloying element. It has good welding and corrosion resistant characteristics. High temperatures (over 150 °F) or excessive cold working increases susceptibility to corrosion. 6000 series: Silicon and magnesium form magnesium silicide, which makes alloys heat treatable. It is of medium strength, good forming qualities, and has corrosion resistant characteristics. 7000 series: Zinc is the principal alloying element. The most popular alloy of the series is 6061. When coupled with magnesium, it results in heat-treatable alloys of very high strength. It usually has copper and chromium added. The principal alloy of this group is 7075. ***Hardness Identification*** Where used, the temper designation follows the alloy designation and is separated from it by a dash (i.e., 7075-T6, 2024-T4, and so forth). The temper designation consists of a letter indicating the basic temper, which may be more specifically defined by the addition of one or more digits. These designations are as follows: F---as fabricated O---annealed, recrystallized (wrought products only) H---strain hardened H1 (plus one or more digits)---strain hardened only H2 (plus one or more digits)---strain hardened and partially annealed H3 (plus one or more digits)---strain hardened and stabilized The digit following the designations H1, H2, and H3 indicates the degree of strain hardening, number 8 representing the ultimate tensile strength equal to that achieved by a cold reduction of approximately 75 percent following a full anneal, 0 representing the annealed state. ***Magnesium and Magnesium Alloys*** Magnesium, the world's lightest structural metal, is a silvery white material weighing only two-thirds as much as aluminum. Magnesium does not possess sufficient strength in its pure state for structural uses, but when alloyed with zinc, aluminum, and manganese, it produces an alloy having the highest strength-to-weight ratio of any of the commonly used metals. Magnesium is probably more widely distributed in nature than any other metal. It can be obtained from such ores as dolomite and magnesite, as well as from seawater, underground brines, and waste solutions of potash. With about 10 million pounds of magnesium in one cubic mile of seawater, there is no danger of a dwindling supply. Some of today's aircraft require more than one-half ton of this metal for use in hundreds of vital spots. Some wing panels are fabricated entirely from magnesium alloys, weigh 18 percent less than standard aluminum panels, and have flown hundreds of satisfactory hours. Among the aircraft parts that have been made from magnesium with a substantial savings in weight are nose wheel doors, flap cover skin, aileron cover skin, oil tanks, floorings, fuselage parts, wingtips, engine nacelles, instrument panels, radio masts, hydraulic fluid tanks, oxygen bottle cases, ducts, and seats. Magnesium alloys possess good casting characteristics. Their properties compare favorably with those of cast aluminum. In forging, hydraulic presses are ordinarily used, although, under certain conditions, forging can be accomplished in mechanical presses or with drop hammers. Magnesium alloys are subject to such treatments as annealing, quenching, solution heat treatment, aging, and stabilizing. Sheet and plate magnesium are annealed at the rolling mill. The solution heat treatment is used to put as much of the alloying ingredients as possible into solid solution, which results in high tensile strength and maximum ductility. Aging is applied to castings following heat treatment where maximum hardness and yield strength are desired. Magnesium embodies fire hazards of an unpredictable nature. When in large sections, its high thermal conductivity makes it difficult to ignite and prevents it from burning. It does not burn until the melting point of 1,204 °F is reached. However, magnesium dust and fine chips are ignited easily. Precautions must be taken to avoid this if possible. Should a fire occur, it could be extinguished with an extinguishing powder, such as soapstone or graphite. Water or any standard liquid or foam fire extinguisher causes magnesium to burn more rapidly and can cause explosions. Magnesium alloys produced in the United States contain varying proportions of aluminum, manganese, and zinc. A letter of the alphabet designates these alloys, with the number 1 indicating high purity and maximum corrosion resistance. Many of the magnesium alloys manufactured in the United States are produced by the Dow Chemical Company and have been given the trade name of Dow-metal™ alloys. To 7-9 distinguish between these alloys, each is assigned a letter. Thus, we have Dow-metal™ J, Dow-metal™ M, and so forth. Another manufacturer of magnesium alloys is the American Magnesium Corporation, a subsidiary of the Aluminum Company of America. This company uses an identification system like that used for aluminum alloys, with the exception that magnesium alloy numbers are preceded with the letters AM. Thus, AM240C is a cast alloy, and AM240C4 is the same alloy in the heat-treated state. AM3S0 is an annealed wrought alloy, and AM3SRT is the same alloy rolled after heat treatment. ***Titanium and Titanium Alloys*** An English priest named Gregot discovered titanium. A crude separation of titanium ore was accomplished in 1825. In 1906, enough pure titanium was isolated in metallic form to permit a study. Following this study, in 1932, an extraction process was developed and became the first commercial method for producing titanium. The United States Bureau of Mines began making titanium sponge in 1946, and 4 years later the melting process began. The use of titanium is widespread. It is used in many commercial enterprises and is in constant demand for such items as pumps, screens, and other tools and fixtures where corrosion attack is prevalent. In aircraft construction and repair, titanium is used for fuselage skins, engine shrouds, firewalls, longerons, frames, fittings, air ducts, and fasteners. Titanium is used for making compressor disks, spacer rings, compressor blades and vanes, through bolts, turbine housings and liners, and miscellaneous hardware for turbine engines. Titanium, in appearance, is like stainless steel. One quick method used to identify titanium is the spark test. Titanium gives off a brilliant white trace ending in a brilliant white burst. Also, moistening the titanium and using it to draw a line on a piece of glass can accomplish identification. This leaves a dark line similar in appearance to a pencil mark. Titanium falls between aluminum and stainless steel in terms of elasticity, density, and elevated temperature strength. It has a melting point from 2,730 °F to 3,155 °F, low thermal conductivity, and a low coefficient of expansion. It is light, strong, and resistant to stress corrosion cracking. Titanium is approximately 60 percent heavier than aluminum and about 50 percent lighter than stainless steel. Because of the high melting point of titanium, high temperature properties are disappointing. The ultimate yield strength of titanium drops rapidly above 800 °F. The absorption of oxygen and nitrogen from the air at temperatures above 1,000 °F makes the metal so brittle on long exposure that it soon becomes worthless. However, titanium does have some merit for short time exposure up to 3,000 °F where strength is not important. Aircraft firewalls demand this requirement. Titanium is nonmagnetic and has an electrical resistance comparable to that of stainless steel. Some of the base alloys of titanium are quite hard. Heat treating and alloying do not develop the hardness of titanium to the high levels of some of the heat-treated alloys of steel. It was only recently that a heat-treatable titanium alloy was developed. Prior to the development of this alloy, heating and rolling was the only method of forming that could be accomplished. However, it is possible to form the new alloy in the soft condition and heat-treat it for hardness. Iron, molybdenum, and chromium are used to stabilize titanium and produce alloys that quench-harden and age-harden. The addition of these metals also adds ductility. The fatigue resistance of titanium is greater than that of aluminum or steel. Titanium becomes softer as the degree of purity is increased. It is not practical to distinguish between the various grades of commercially pure or unalloyed titanium by chemical analysis; therefore, the grades are determined by mechanical properties. *Titanium Designations* The A-B-C classification of titanium alloys was established to provide a convenient and simple means of describing all titanium alloys. Titanium and titanium alloys possess three basic types of crystals: A (alpha), B (beta), and C (combined alpha and beta). Their characteristics are: - A (alpha)---all-around performance; good weld ability; tough and strong both cold and hot; and resistant to oxidation. - B (beta)---bendability; excellent bend ductility; strong both cold and hot, but vulnerable to contamination. - C (combined alpha and beta for compromise performances)---strong when cold and warm, but weak when hot; good bendability; moderate contamination resistance; excellent forge ability. Titanium is manufactured for commercial use in two basic compositions: commercially-pure titanium and alloyed titanium. A-55 is an example of commercially-pure titanium. It has yield strength of 55,000 to 80,000 psi and is a general-purpose grade for moderate to severe forming. It is sometimes used for nonstructural aircraft parts and for all types of corrosion-resistant applications, such as tubing. Type A-70 titanium is closely related to type A-55 but has yield strength of 70,000 to 95,000 psi. It is used where higher strength is 7-10 required, and it is specified for many moderately stressed aircraft parts. For many corrosion applications, it is used interchangeably with type A-55. Both type A-55 and type A-70 is weldable. One of the widely-used titanium base alloys is designated as C-110M. It is used for primary structural members and aircraft skin, has 110,000 psi minimum yield strength, and contains 8 percent manganese. Type A-110AT is a titanium alloy that contains 5 percent aluminum and 2.5 percent tin. It also has high minimum yield strength at elevated temperatures with the excellent welding characteristics inherent in alpha-type titanium alloys. *Corrosion Characteristics* The corrosion resistance of titanium deserves special mention. The resistance of the metal to corrosion is caused by the formation of a protective surface film of stable oxide or chemi-absorbed oxygen. Film is often produced by the presence of oxygen and oxidizing agents. Corrosion of titanium is uniform. There is little evidence of pitting or other serious forms of localized attack. Normally, it is not subject to stress corrosion, corrosion fatigue, intergranular corrosion, or galvanic corrosion. Its corrosion resistance is equal or superior to 18-8 stainless steel. Laboratory tests with acid and saline solutions show titanium polarizes readily. The net effect, in general, is to decrease current flow in galvanic and corrosion cells. Corrosion currents on the surface of titanium and metallic couples are naturally restricted. This partly accounts for good resistance to many chemicals; also, the material may be used with some dissimilar metals with no harmful galvanic effect on either. ***Copper and Copper Alloys*** Copper is one of the most widely distributed metals. It is the only reddish-colored metal and is second only to silver in electrical conductivity. Its use as a structural material is limited because of its great weight. However, some of its outstanding characteristics, such as its high electrical and heat conductivity, in many cases overbalance the weight factor. Because it is very malleable and ductile, copper is ideal for making wire. It is corroded by salt water but is not affected by fresh water. The ultimate tensile strength of copper varies greatly. For cast copper, the tensile strength is about 25,000 psi, and when cold rolled or cold drawn, its tensile strength increases to a range of 40,000 to 67,000 psi. In aircraft, copper is used primarily in the electrical system for bus bars, bonding, and as lock wire. Beryllium copper is one of the most successful of all the copper base alloys. It is a recently developed alloy containing about 97 percent copper, 2 percent beryllium, and sufficient nickel to increase the percentage of elongation. The most valuable feature of this metal is that the physical properties can be greatly stepped up by heat treatment, the tensile strength rising from 70,000 psi in the annealed state to 200,000 psi in the heat-treated state. The resistance of beryllium copper to fatigue and wear makes it suitable for diaphragms, precision bearings and bushings, ball cages, and spring washers. Brass is a copper alloy containing zinc and small amounts of aluminum, iron, lead, manganese, magnesium, nickel, phosphorous, and tin. Brass with a zinc content of 30 to 35 percent is very ductile, but that containing 45 percent has relatively high strength. Muntz metal is a brass composed of 60 percent copper and 40 percent zinc. It has excellent corrosion-resistant qualities in salt water. Its strength can be increased by heat treatment. As cast, this metal has an ultimate tensile strength of 50,000 psi, and it can be elongated 18 percent. It is used in making bolts and nuts, as well as parts that come in contact with salt water. Red brass, sometimes termed "bronze" because of its tin content, is used in fuel and oil line fittings. This metal has good casting and finishing properties and machines freely. Bronzes are copper alloys containing tin. The true bronzes have up to 25 percent tin, but those with less than 11 percent are most useful, especially for such items as tube fittings in aircraft. Among the copper alloys are the copper aluminum alloys, of which the aluminum bronzes rank very high in aircraft usage. They would find greater usefulness in structures if it were not for their strength-to-weight ratio as compared with alloy steels. Wrought aluminum bronzes are almost as strong and ductile as medium carbon steel, and they possess a high degree of resistance to corrosion by air, salt water, and chemicals. They are readily forged, hot or cold rolled, and many react to heat treatment. These copper base alloys contain up to 16 percent of aluminum (usually 5 to 11 percent), to which other metals, such as iron, nickel, or manganese, may be added. Aluminum bronzes have good tearing qualities, great strength, hardness, and resistance to both shock and fatigue. Because of these properties, they are used for diaphragms, gears, and pumps. Aluminum bronzes are available in rods, bars, plates, sheets, strips, and forgings. Cast aluminum bronzes, using about 89 percent copper, 9 percent aluminum, and 2 percent of other elements, have high strength combined with ductility and are resistant to 7-11 corrosion, shock, and fatigue. Because of these properties, cast aluminum bronze is used in bearings and pump parts. These alloys are useful in areas exposed to salt water and corrosive gases. Manganese bronze is an exceptionally high strength, tough, corrosion-resistant copper zinc alloy containing aluminum, manganese, iron, and occasionally, nickel or tin. This metal can be formed, extruded, drawn, or rolled to any desired shape. In rod form, it is generally used for machined parts for aircraft landing gears and brackets. Silicon bronze is a more recent development composed of about 95 percent copper, 3 percent silicon, and 2 percent manganese, zinc, iron, tin, and aluminum. Although not a bronze in the true sense because of its small tin content, silicon bronze has high strength and great corrosion resistance. *Monel* Monel, the leading high nickel alloy, combines the properties of high strength and excellent corrosion resistance. This metal consists of 68 percent nickel, 29 percent copper, 0.2 percent iron, 1 percent manganese, and 1.8 percent of other elements. It cannot be hardened by heat treatment. Monel, adaptable to casting and hot or cold working, can be successfully welded. It has working properties like those of steel. When forged and annealed, it has a tensile strength of 80,000 psi. This can be increased by cold working to 125,000 psi, sufficient for classification among the tough alloys. Monel has been successfully used for gears and chains to operate retractable landing gears and for structural parts subject to corrosion. In aircraft, Monel is used for parts demanding both strength and high resistance to corrosion, such as exhaust manifolds and carburetor needle valves and sleeves. *K-Monel* K-Monel is a nonferrous alloy containing mainly nickel, copper, and aluminum. Adding a small amount of aluminum to the Monel formula produces it. It is corrosion resistant and capable of being hardened by heat treatment. K-Monel has been successfully used for gears and structural members in aircraft, which are subjected to corrosive attacks. This alloy is nonmagnetic at all temperatures. Both oxyacetylene and electric arc welding have successfully welded K-Monel sheet. ***Nickel and Nickel Alloys*** There are basically two nickel alloys used in aircraft: Monel and Inconel. Monel contains about 68 percent nickel and 29 percent copper, plus small amounts of iron and manganese. Nickel alloys can be welded or easily machined. Some of the nickel Monel, especially the nickel Monels containing small amounts of aluminum, are heat-treatable to similar tensile strengths of steel. Nickel Monel is used in gears and parts that require high strength and toughness, such as exhaust systems that require high strength and corrosion resistance at elevated temperatures. Inconel alloys of nickel produce a high strength, high temperature alloy containing approximately 80 percent nickel, 14 percent chromium, and small amounts of iron and other elements. The nickel Inconel alloys are frequently used in turbine engines because of their ability to maintain their strength and corrosion resistance under extremely high-temperature conditions. Inconel and stainless steel are similar in appearance and are frequently found in the same areas of the engine. Sometimes it is important to identify the difference between the metal samples. A common test is to apply one drop of cupric chloride and hydrochloric acid solution to the unknown metal and allow it to remain for 2 minutes. At the end of the soak period, a shiny spot indicates the material is nickel Inconel, and a copper-colored spot indicates stainless steel. **Substitution of Aircraft Metals** In selecting substitute metals for the repair and maintenance of aircraft, it is very important to check the appropriate structural repair manual. Aircraft manufacturers design structural members to meet a specific load requirement for an aircraft. The methods of repairing these members, apparently similar in construction, vary with different aircraft. Four requirements must be kept in mind when selecting substitute metals. The first and most important of these is maintaining the original strength of the structure. The other three are maintaining contour or aerodynamic smoothness; maintaining original weight, if possible, or keeping added weight to a minimum; and maintaining the original corrosion-resistant properties of the metal. **Metalworking Processes** There are three methods of metalworking: hot working, cold working, and extruding. The method used depends on the metal involved and the part required, although in some instances both hot and cold working methods may be used to make a single part. ***Hot Working*** Almost all steel is hot worked from the ingot into some form from which it is either hot or cold worked to the finished shape. When an ingot is stripped from its mold, its surface is solid, but the interior is still molten. The ingot is then placed 7-12 in a soaking pit, which retards loss of heat, and the molten interior gradually solidifies. After soaking, the temperature is equalized throughout the ingot, then it is reduced to intermediate size by rolling, making it more readily handled. The rolled shape is called a bloom when its section dimensions are 6 inches × 6 inches or larger and square. The section is called a billet when it is square and less than 6 inches × 6 inches. Rectangular sections, which have a width greater than twice their thickness, are called slabs. The slab is the intermediate shape from which sheets are rolled. Blooms, billets, or slabs are heated above the critical range and rolled into a variety of shapes of uniform cross section. Common rolled shapes are sheet, bar, channel, angle, and I-beam. As discussed later in this chapter, hot-rolled material is frequently finished by cold rolling or drawing to obtain accurate finish dimensions and a bright, smooth surface. Complicated sections, which cannot be rolled, or sections of which only a small quantity is required, are usually forged. Forging of steel is a mechanical working at temperatures above the critical range to shape the metal as desired. Forging is done either by pressing or hammering the heated steel until the desired shape is obtained. Pressing is used when the parts to be forged are large and heavy; this process also replaces hammering where high-grade steel is required. Since a press is slow acting, its force is uniformly transmitted to the center of the section, thus affecting the interior grain structure, as well as the exterior to give the best possible structure throughout. Hammering can be used only on relatively small pieces. Since hammering transmits its force almost instantly, its effect is limited to a small depth. Thus, it is necessary to use a very heavy hammer or to subject the part to repeated blows to ensure complete working of the section. If the force applied is too weak to reach the center, the finished forged surface is concave. If the center was properly worked, the surface is convex or bulged. The advantage of hammering is that the operator has control over both the amount of pressure applied and the finishing temperature and can produce small parts of the highest grade. This type of forging is usually referred to as smith forging. It is used extensively where only a small number of parts are needed. Considerable machining time and material are saved when a part is smith forged to approximately the finished shape. Steel is often harder than necessary and too brittle for most practical uses when put under severe internal strain. To relieve such strain and reduce brittleness, it is tempered after being hardened. This consists of heating the steel in a furnace to a specified temperature and then cooling it in air, oil, water, or a special solution. Temper condition refers to the condition of metal or metal alloys with respect to hardness or toughness. Rolling, hammering, or bending these alloys, or heat treating and aging them, causes them to become tougher and harder. At times, these alloys become too hard for forming and must be re-heat treated or annealed. Metals are annealed to relieve internal stresses, soften the metal, make it more ductile, and refine the grain structure. Annealing consists of heating the metal to a prescribed temperature, holding it there for a specified length of time, and then cooling the metal back to room temperature. To produce maximum softness, the metal must be cooled very slowly. Some metals must be furnace cooled; others may be cooled in air. Normalizing applies to iron base metals only. Normalizing consists of heating the part to the proper temperature, holding it at that temperature until it is uniformly heated, and then cooling it in still air. Normalizing is used to relieve stresses in metals. Strength, weight, and reliability are three factors that determine the requirements to be met by any material used in airframe construction and repair. Airframes must be strong and yet as lightweight as possible. There are very definite limits to which increases in strength can be accompanied by increases in weight. An airframe so heavy that it could not support a few hundred pounds of additional weight would be of little use. All metals, in addition to having a good strength-to­weight ratio, must be thoroughly reliable, thus minimizing the possibility of dangerous and unexpected failures. In addition to these general properties, the material selected for a definite application must possess specific qualities suitable for the purpose. The material must possess the strength required by the dimensions, weight, and use. The five basic stresses that metals may be required to withstand are tension, compression, shear, bending, and torsion. The tensile strength of a material is its resistance to a force, which tends to pull it apart. Tensile strength is measured in pounds per square inch (psi) and is calculated by dividing the load in pounds required to pull the material apart by its cross-sectional area in square inches. The compression strength of a material is its resistance to a crushing force, which is the opposite of tensile strength. Compression strength is also measured in psi. When a piece of metal is cut, the material is subjected, as it comes in contact with the cutting edge, to a force known as shear. Shear is the tendency on the part of parallel members to slide in opposite 7-13 directions. It is like placing a cord or thread between the blades of a pair of scissors (shears). The shear strength is the shear force in psi at which a material fails. It is the load divided by the shear area. Bending can be described as the deflection or curving of a member due to forces acting upon it. The bending strength of material is the resistance it offers to deflecting forces. Torsion is a twisting force. Such action would occur in a member fixed at one end and twisted at the other. The torsional strength of material is its resistance to twisting. The relationship between the strength of a material and its weight per cubic inch, expressed as a ratio, is known as the strength-to-weight ratio. This ratio forms the basis for comparing the desirability of various materials for use in airframe construction and repair. Neither strength nor weight alone can be used as a means of true comparison. In some applications, such as the skin of monocoque structures, thickness is more important than strength. In this instance, the material with the lightest weight for a given thickness or gauge is best. Thickness or bulk is necessary to prevent bucking or damage caused by careless handling. Corrosion is the eating away or pitting of the surface or the internal structure of metals. Because of the thin sections and the safety factors used in aircraft design and construction, it would be dangerous to select a material possessing poor corrosion-resistant characteristics. Another significant factor to consider in maintenance and repair is the ability of a material to be formed, bent, or machined to required shapes. The hardening of metals by cold working or forming is termed work hardening. If a piece of metal is formed (shaped or bent) while cold, it is said to be cold worked. Practically all the work an aviation mechanic does on metal is cold work. While this is convenient, it causes the metal to become harder and more brittle. If the metal is cold worked too much, that is, if it is bent back and forth or hammered at the same place too often, it will crack or break. Usually, the more malleable and ductile a metal is, the more cold working it can stand. Any process that involves controlled heating and cooling of metals to develop certain desirable characteristics (such as hardness, softness, ductility, tensile strength, or refined grain structure) is called heat treatment or heat-treating. With steels, the term "heat-treating" has a broad meaning and includes processes such as annealing, normalizing, hardening, and tempering. In the heat treatment of aluminum alloys, only two processes are included: the hardening and toughening process and the softening process. The hardening and toughening process is called heat-treating, and the softening process is called annealing. Aircraft metals are subjected to both shock and fatigue (vibrational) stresses. Fatigue occurs in materials that are exposed to frequent reversals of loading or repeatedly applied loads, if the fatigue limit is reached or exceeded. Repeated vibration or bending ultimately causes a minute crack to occur at the weakest point. As vibration or bending continues, the crack lengthens until the part completely fails. This is termed "shock and fatigue failure." Resistance to this condition is known as shock and fatigue resistance. It is essential that materials used for critical parts be resistant to these stresses. Heat treatment is a series of operations involving the heating and cooling of metals in the solid state. Its purpose is to change a mechanical property, or combination of mechanical properties, so that the metal is 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. No one heat-treating operation can produce all these characteristics. In fact, some properties are often improved at the expense of others. In being hardened, for example, a metal may become brittle. The various heat-treating processes are similar in that they all involve the heating and cooling of metals. They differ, however, in the temperatures to which the metal is heated, the rate at which it is cooled, and, of course, in the result. The most common forms of heat treatment for ferrous metals are hardening, tempering, normalizing, annealing, and casehardening. Most nonferrous metals can be annealed and many of them can be hardened by heat treatment. However, there is only one nonferrous metal, titanium, that can be casehardened, and none can be tempered or normalized. ***Internal Structure of Metals*** The results obtained by heat treatment depend on the structure of the metal and on the way the structure changes when the metal is heated and cooled. A pure metal cannot be hardened by heat treatment, because there is little change in its structure when heated. On the other hand, most alloys respond to heat treatment since their structures change with heating and cooling. An alloy may be in the form of a solid solution, a mechanical mixture, or a combination of a solid solution and a mechanical mixture. When an alloy is in the form of a solid solution, the elements and compounds that form the alloy are absorbed, one into the other, in much the same way that salt is dissolved in a glass of water, and the constituents cannot be identified even under a microscope. 7-14 When two or more elements or compounds are mixed but can be identified by microscopic examination, a mechanical mixture is formed. A mechanical mixture can be compared to the mixture of sand and gravel in concrete. The sand and gravel are both visible. Just as the sand and gravel are held together and kept in place by the matrix of cement, the other constituents of an alloy are embedded in the matrix formed by the base metal. An alloy in the form of a mechanical mixture at ordinary temperatures may change to a solid solution when heated. When cooled back to normal temperature, the alloy may return to its original structure. On the other hand, it may remain a solid solution or form a combination of a solid solution and mechanical mixture. An alloy, which consists of a combination of solid solution and mechanical mixture at normal temperatures, may change to a solid solution when heated. When cooled, the alloy may remain a solid solution, return to its original structure, or form a complex solution. ***Heat-Treating Equipment*** Successful heat treating requires close control over all factors affecting the heating and cooling of metals. Such control is possible only when the proper equipment is available and the equipment is selected to fit the job. Thus, the furnace must be of the proper size and type and must be controlled so that temperatures are kept within the limits prescribed for each operation. Even the atmosphere within the furnace affects the condition of the part being heat-treated. Further, the quenching equipment and the quenching medium must be selected to fit the metal and the heat-treating operation. Finally, there must be equipment for handling parts and materials, for cleaning metals, and for straightening parts. *Furnaces and Salt Baths* There are many different types and sizes of furnaces used in heat treatment. As a rule, furnaces are designed to operate in certain specific temperature ranges and attempted use in other ranges frequently results in work of inferior quality. In addition, using a furnace beyond its rated maximum temperature shortens its life and may necessitate costly and time-consuming repairs. Fuel-fired furnaces (gas or oil) require air for proper combustion, and an air compressor or blower is therefore necessary. These furnaces are usually of the muffler type; that is, the combustion of the fuel takes place outside of and around the chamber in which the work is placed. If an open muffler is used, the furnace should be designed to prevent the direct impingement of flame on the work. In furnaces heated by electricity, the heating elements are generally in the form of wire or ribbon. Good design requires incorporation of additional heating elements at locations where maximum heat loss may be expected. Such furnaces commonly operate at up to a maximum temperature of about 2,000 °F. Furnaces operating at temperatures up to about 2,500 °F usually employ resistor bars of sintered carbides. *Temperature Measurement and Control* A thermoelectric instrument, known as a pyrometer, measures temperature in the heat-treating furnace. This instrument measures the electrical effect of a thermocouple and, hence, the temperature of the metal being treated. A complete pyrometer consists of three parts: a thermocouple, extension leads, and meter. Furnaces intended primarily for tempering may be heated by gas or electricity and are frequently equipped with a fan for circulating the hot air. Salt baths are available for operating at either tempering or hardening temperatures. Depending on the composition of the salt bath, heating can be conducted at temperatures as low as 325 °F to as high as 2,450 °F. Lead baths can be used in the temperature range of 650 °F to 1,700 °F. The rate of heating in lead or salt baths is much faster in furnaces. Heat-treating furnaces differ in size, shape, capacity, construction, operation, and control. They may be circular or rectangular and may rest on pedestals or directly on the floor. There are also pit-type furnaces, which are below the surface of the floor. When metal is to be heated in a bath of molten salt or lead, the furnace must contain a pot or crucible for the molten bath. The size and capacity of a heat-treating furnace depends on the intended use. A furnace must be capable of heating rapidly and uniformly, regardless of the desired maximum temperature or the mass of the charge. An oven-type furnace should have a working space (hearth) about twice as long and three times as wide as any part that is heated in the furnace. Accurate temperature measurement is essential to good heat-treating. The usual method is by means of thermocouples: the most common base metal couples are copper-constantan (up to about 700 °F), iron-constantan (up to about 1,400 °F), and chromel-alumel (up to about 2,200 °F). The most common noble metal couples (which can be used up to about 2,800 °F) are platinum coupled with either the alloy 87 percent platinum (13 percent rhodium) or the alloy 90 percent platinum (10 percent rhodium). The temperatures quoted are for continuous operation. The life of thermocouples is affected by the maximum temperature (which may frequently exceed those given 7-15 above) and by the furnace atmosphere. Iron-constantan is more suited for use in reducing and chromel-alumel in oxidizing atmospheres. Thermocouples are usually encased in metallic or ceramic tubes closed at the hot end to protect them from the furnace gases. A necessary attachment is an instrument, such as a millivoltmeter or potentiometer, for measuring the electromotive force generated by the thermocouple. In the interest of accurate control, place the hot junction of the thermocouple as close to the work as possible. The use of an automatic controller is valuable in controlling the temperature at the desired value. Pyrometers may have meters either of the indicating type or recording type. Indicating pyrometers give direct reading of the furnace temperature. The recording type produces a permanent record of the temperature range throughout the heating operation by means of an inked stylus attached to an arm, which traces a line on a sheet of calibrated paper or temperature chart. Pyrometer installations on all modern furnaces provide automatic regulation of the temperature at any desired setting. Instruments of this type are called controlling potentiometer pyrometers. They include a current regulator and an operating mechanism, such as a relay. ***Heating*** The object in heating is to transform pearlite (a mixture of alternate strips of ferrite and iron carbide in a single grain) to austenite as the steel is heated through the critical range. Since this transition takes time, a relatively slow rate of heating must be used. Ordinarily, the cold steel is inserted when the temperature in the furnace is from 300 °F to 500 °F below the hardening temperature. In this way, too rapid heating through the critical range is prevented. If temperature-measuring equipment is not available, it becomes necessary to estimate temperatures by some other means. An inexpensive, yet accurate method involves the use of commercial crayons, pellets, or paints that melt at various temperatures within the range of 125 °F to 1,600 °F. The least accurate method of temperature estimation is by observation of the color of the hot hearth of the furnace or of the work. The heat colors observed are affected by many factors, such as the conditions of artificial or natural light, the character of the scale on the work, and so forth. Steel begins to appear dull red at about 1,000 °F, and as the temperature increases, the color changes gradually through various shades of red to orange, to yellow, and finally to white. A rough approximation of the correspondence between color and temperature is indicated in *Figure 7-5*. It is also possible to secure some idea of the temperature of a piece of carbon or low alloy steel, in the low temperature range used for tempering, from the color of the thin oxide film that forms on the cleaned surface of the steel when heated in this range. The approximate temperature/color relationship is indicated on the lower portion of the scale in *Figure 7-5*. It is often necessary or desirable to protect steel or cast iron from surface oxidation (scaling) and loss of carbon from the surface layers (decarburization). Commercial furnaces, therefore, are generally equipped with some means of atmosphere control. This usually is in the form of a burner for burning controlled amounts of gas and air and directing the products of combustion into the furnace muffle. Water vapor, a product of this combustion, is detrimental and many furnaces are equipped with a means for eliminating it. For furnaces not equipped with atmosphere control, a variety of external atmosphere generators are available. The gas so generated is piped into the furnace and one generator may supply several furnaces. If no method of atmosphere control is available, some degree of protection may be secured by covering the work with cast iron borings or chips. Since the liquid heating medium surrounds the work in salt or lead baths, the problem of preventing scaling or decarburization is simplified. Vacuum furnaces also are used for annealing steels, especially when a bright non-oxidized surface is a prime consideration. ***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. Soaking temperatures for various types of steel are specified in ranges varying as much as 100 °F. *\[Figure 7-6\]* Small parts are soaked in the lower part of the specified range and heavy parts in the upper part of the specified range. The length of the soaking period depends upon the type of steel and the size of the part. Naturally, heavier parts require longer soaking to ensure equal heating throughout. As a general rule, a soaking period of 30 minutes to 1 hour is sufficient for the average heat-treating operation. ***Cooling*** The rate of cooling through the critical range determines the form that the steel retains. 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 are therefore used in hardening steels. There are three commonly used quenching liquids: brine, water, and oil. Brine is the strongest quenching medium, water is next, and oil is the least. Generally, an oil quench is used for alloy steels and brine or water for carbon steels. 7-16 **°Fahrenheit °Centigrade °F °C °F °C Color of Hot Body Temper Colors** 2,700 2,600 **2,500** 2,400 2,300 2,200 2,100 **2,000** 1,900 1,800 1,700 1,600 **1,500** 1,400 1,300 1,200 1,100 **1,000** 900 800 700 600 **500** 400 300 200 100 **1,500** 1,400 1,300 1,200 1,100 **1,000** 900 800 700 600 **500** 400 300 200 100 **0 Straw 220 430 Dark Blue 290 550 Purple 270 520 Yellow Brown 250 460 White 1200 2192 Light Yellow 1100 2012 Yellow 1050 1922 Light Orange 980 1796 Orange 930 1706 Light Red 870 1598 810 Light cherry 1490 Cherry 760 1400 Dark Cherry 700 1292 Blood Red 650 1202 Brown Red 600 1112 Approximate Temperature Colors Figure 7-5.** *Temperature chart indicating conversion of Centigrade to Fahrenheit or vice versa, color temperature scale for hardening temperature range, and tempering temperature range.* ***Quenching Media*** Quenching solutions act only through their ability to cool the steel. They have no beneficial chemical action on the quenched steel and in themselves impart no unusual properties. Most requirements for quenching media are met satisfactorily by water or aqueous solutions of inorganic salts, such as table salt or caustic soda, or by some type of oil. 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. Their temperature considerably affects the cooling ability of both water and brine, particularly water. Both should be kept cold---well below 60 °F. If the volume of steel being quenched tends to raise the temperature of the bath appreciably, add ice or use some means of refrigeration to cool the quenching bath. There are many specially prepared quenching oils on the market; their cooling rates do not vary widely. A straight mineral oil with a Saybolt viscosity of about 100 at 100 °F is generally used. Unlike brine and water, the oils have the greatest cooling velocity at a slightly elevated temperature--- about 100--140 °F---because of their decreased viscosity at these temperatures. When steel is quenched, the liquid in immediate contact with the hot surface vaporizes; this vapor reduces the rate of heat abstraction markedly. Vigorous agitation of the steel or the use of a pressure spray quench is necessary to dislodge these vapor films and thus permit the desired rate of cooling. The tendency of steel to warp and crack during the quenching process is difficult to overcome because certain parts of the article cool more rapidly than others. The following recommendations greatly reduce the warping tendency. 1. Never throw a part into the quenching bath. By permitting it to lie on the bottom of the bath, it is apt to cool faster on the topside than on the bottom side, thus causing it to warp or crack. 2. Agitate the part slightly to destroy the coating of vapor that could prevent it from cooling evenly and rapidly. This allows the bath to dissipate its heat to the atmosphere. 3. Immerse irregular shaped parts so that the heavy end enters the bath first. 7-17 **Steel Number Quenching Medium (n) Temperatures Tempering (drawing) Temperature for Tensile Strength (psi)** 1020 1,650--1,750 1,600--1,700 1,575--1,675 Water --- --- --- --- --- 1022 (x1020) 1,650--1,750 1,600--1,700 1,575--1,675 Water --- --- --- --- --- 1025 1,600--1,700 1,575--1,650 1,575--1,675 Water (a) --- --- --- --- 1035 1,575--1,650 1,575--1,625 1,525--1,600 Water 875 --- --- --- --- 1045 1,550--1,600 1,550--1,600 1,475--1,550 Oil or water 1,150 --- --- (n) --- 1095 1,475--1,550 1,450--1,500 1,425--1,500 Oil (b) --- 1,100 850 750 2330 1,475--1,525 1,425--1,475 1,450--1,500 Oil or water 1,100 950 800 --- --- 3135 1,600--1,650 1,500--1,550 1,475--1,525 Oil 1,250 1,050 900 750 650 3140 1,600--1,650 1,500--1,550 1,475--1,525 Oil 1,325 1,075 925 775 700 4037 1,600 1,525--1,575 1,525--1,575 Oil or water 1,225 1,100 975 --- --- 4130 (x4130) 1,600--1,700 1,525--1,575 1,525--1,625 Oil (c) (d) 1,050 900 700 575 4140 1,600--1,650 1,525--1,575 1,525--1,575 Oil 1,350 1,100 1,025 825 675 4150 1,550--1,600 1,475--1,525 1,550--1,550 Oil --- 1,275 1,175 1,050 950 4340 (x4340) 1,550--1,625 1,525--1,575 1,475--1,550 Oil --- 1,200 1,050 950 850 4640 1,675--1,700 1,525--1,575 1,500--1,550 Oil --- 1,200 1,050 750 625 6135 1,600--1,700 1,550--1,600 1,575--1,625 Oil 1,300 1,075 925 800 750 6150 1,600--1,650 1,525--1,575 1,550--1,625 Oil (d)(e) 1,200 1,000 900 800 6195 1,600--1,650 1,525--1,575 1,500--1,550 Oil (f ) --- --- --- --- NE8620 --- --- 1,525--1,575 Oil --- 1,000 --- --- --- NE8630 1,650 1,525--1,575 1,525--1,575 Oil --- 1,125 975 775 675 NE8735 1,650 1,525--1,575 1,525--1,575 Oil --- 1,175 1,025 875 775 NE8740 1,625 1,500--1,550 1,500--1,550 Oil --- 1,200 1,075 925 850 30905 --- (g)(h) (i) --- --- --- --- --- --- 51210 1,525--1,575 1,525--1,575 1,775--1,825 (j) Oil 1,200 1,100 (k) 750 --- 51335 --- 1,525--1,575 1,775--1,850 Oil --- --- --- --- --- 52100 1,625--1,700 1,400--1,450 1,525--1,550 Oil (f ) --- --- --- --- Corrosion resisting --- --- --- --- (m) --- --- --- --- (16-2)(1) Silicon chromium --- --- 1,700--1,725 Oil --- --- --- --- --- (for springs) **Normalizing Air Cool (°F) Annealing (°F) Hardening (°F) 100,000 (°F) 125,000 (°F) 150,000 (°F) 180,000 (°F) 200,000 (°F) NOTES:** (a) Draw at 1,150 °F for tensile strength of 70,000 psi. (b) For spring temper draw at 800--900 °F. Rockwell hardness C-40--45. (c) Bars or forgings may be quenched in water from 1,500--1,600 °F. (d) Air cooling from the normalizing temperature produces a tensile strength of approximately 90,000 psi. (e) For spring temper draw at 850--950 °F. Rockwell hardness C-40--45. (f ) Draw at 350--450 °F to remove quenching strains. Rockwell hardness C-60--65. (g) Anneal at 1,600--1,700 °F to remove residual stresses due to welding or cold work. May be applied only to steel containing titanium or columbium. (h) Anneal at 1,900--2,100 °F to produce maximum softness and corrosion resistance. Cool in air or quench in water. (i) Harden by cold work only. (j) Lower side of range for sheet 0.06 inch and under. Middle of range for sheet and wire 0.125 inch. Upper side of range for forgings. (k) Not recommended for intermediate tensile strengths because of low impact. (l) AN-QQ-S-770---It is recommended that, prior to tempering, corrosion-resisting (16 Cr-2 Ni) steel be quenched in oil from a temperature of 1,875--1,900 °F, after a soaking period of 30 minutes at this temperature. To obtain a tensile strength at 115,000 psi, the tempering temperature should be approximately 525 °F. A holding time at these temperatures of about 2 hours is recommended. Tempering temperatures between 700 °F and 1,100 °F is not approved. (m) Draw at approximately 800 °F and cool in air for Rockwell hardness of C-50. (n) Water used for quenching shall be within the temperature range of 80--150 °F. **Figure 7-6.** *Heat treatment procedures for steels.* 7-18 ***Quenching Equipment*** The quenching tank should be of the proper size to handle the material being quenched. Use circulating pumps and coolers to maintain approximately constant temperatures when doing a large amount of quenching. To avoid building up a high concentration of salt in the quenching tank, make provisions for adding fresh water to the quench tank used for molten salt baths. Tank location in reference to the heat-treating furnace is very important. Situate the tank to permit rapid transfer of the part from the furnace to the quenching medium. A delay of more than a few seconds, in many instances, proves detrimental to the effectiveness of the heat treatment. During transfer to the quench tank, employ guard sheets to retard the loss of heat when heat treating material of thin section. Provide a rinse tank to remove all salt from the material after quenching if the salt is not adequately removed in the quenching tank. **Heat Treatment of Ferrous Metals** The first important consideration in the heat treatment of a steel part is to know its chemical composition. This, in turn, determines its upper critical point. When the upper critical point is known, the next consideration is the rate of heating and cooling to be used. Carrying out these operations involves the use of uniform heating furnaces, proper temperature controls, and suitable quenching mediums. ***Behavior of Steel During Heating and Cooling*** Changing the internal structure of a ferrous metal is accomplished by heating to a temperature above its upper critical point, holding it at that temperature for a time sufficient to permit certain internal changes to occur, and then cooling to atmospheric temperature under predetermined, controlled conditions. At ordinary temperatures, the carbon in steel exists in the form of particles of iron carbide scattered throughout an iron matrix known as "ferrite." The number, size, and distribution of these particles determine the hardness of the steel. At elevated temperatures, the carbon is dissolved in the iron matrix in the form of a solid solution called "austenite," and the carbide particles appear only after the steel has been cooled. If the cooling is slow, the carbide particles are relatively coarse and few. In this condition, the steel is soft. If the cooling is rapid, as by quenching in oil or water, the carbon precipitates as a cloud of very fine carbide particles, and the steel is hard. The fact that the carbide particles can be dissolved in austenite is the basis of the heat treatment of steel. The temperatures at which this transformation takes place are called the critical points and vary with the composition of the steel. The percent of carbon in the steel has the greatest influence on the critical points of heat treatment. ***Hardening*** Pure iron, wrought iron, and extremely low carbon steels cannot be appreciably hardened by heat treatment, since they contain no hardening element. Cast iron can be hardened, but its heat treatment is limited. When cast iron is cooled rapidly, it forms white iron, which is hard and brittle. When cooled slowly, it forms gray iron, which is soft but brittle under impact. In plain carbon steel, the maximum hardness depends almost entirely on the carbon content of the steel. As carbon content increases, the ability of steel to harden also increases. However, this increase in the ability to harden with an increase in carbon content continues only to a certain point. In practice, that point is 0.85 percent carbon content. When the carbon content is increased beyond 0.85 percent, there is no increase in wear resistance. For most steels, the hardening treatment consists of heating the steel to a temperature just above the upper critical point, soaking or holding for the required length of time, and then cooling it rapidly by plunging the hot steel into oil, water, or brine. Although most steels must be cooled rapidly for hardening, a few may be cooled in still air. Hardening increases the hardness and strength of the steel but makes it less ductile. Carbon steel must be cooled to below 1,000 °F in less than 1 second when hardening. Should the time required for the temperature to drop to 1,000 °F exceed 1 second, the austenite begins to transform into fine pearlite. This pearlite varies in hardness, but is much harder than the pearlite formed by annealing and much softer than the martensite desired. After the 1,000 °F temperature is reached, the rapid cooling must continue if the final structure is to be all martensite. The time limit for the temperature drop to 1,000 °F increases above the 1 second limit for carbon steels when alloys are added to steel. Therefore, a slower quenching medium produces hardness in alloy steels. Because of the high internal stresses in the "as quenched" condition, steel must be tempered just before it becomes cold. The part should be removed from the quenching bath at a temperature of approximately 200 °F, since the temperature range from 200 °F down to room temperature is the cracking range. Hardening temperatures and quenching mediums for the various types of steel are listed in *Figure 7-6*. ***Hardening Precautions*** A variety of different shapes and sizes of tongs for handling hot steels is necessary. It should be remembered that cooling of the area contacted by the tongs is retarded and that such 7-19 areas may not harden, particularly if the steel being treated is very shallow hardening. Small parts may be wired together or quenched in baskets made of wire mesh. Special quenching jigs and fixtures are frequently used to hold steels during quenching in a manner to restrain distortion. When selective hardening is desired, covering with alundum cement or some other insulating material may protect portions of the steel. Selective hardening may be accomplished by using water or oil jets designed to direct quenching medium on the areas to be hardened. This also is accomplished by the induction and flame hardening procedures previously described, particularly on large production jobs. Shallow hardening steels, such as plain carbon and certain varieties of alloy steels, have such a high critical cooling rate that they must be quenched in brine or water to effect hardening. In general, intricately-shaped sections should not be made of shallow hardening steels because of the tendency of these steels to warp and crack during hardening. Such items should be made of deeper hardening steels capable of being hardened by quenching in oil or air. ***Tempering*** Tempering reduces the brittleness imparted by hardening and produces definite physical properties within the steel. Tempering always follows, never precedes, the hardening operation. In addition to reducing brittleness, tempering softens the steel. Tempering is always conducted at temperatures below the low critical point of the steel. In this respect, tempering differs from annealing, normalizing, or hardening, all of which require temperatures above the upper critical point. When hardened steel is reheated, tempering begins at 212 °F and continues as the temperature increases toward the low critical point. By selecting a definite tempering temperature, the resulting hardness and strength can be predetermined. Approximate temperatures for various tensile strengths are listed in *Figure 7-6*. The minimum time at the tempering temperature should be 1 hour. If the part is over one inch in thickness, increase the time by 1 hour for each additional inch of thickness. Tempered steels used in aircraft work have from 125,000 to 200,000 psi ultimate tensile strength. Generally, the rate of cooling from the tempering temperature has no effect on the resulting structure; therefore, the steel is usually cooled in still air after being removed from the furnace. ***Annealing*** Annealing of steel produces a fine-grained, soft, ductile metal without internal stresses or strains. In the annealed state, steel has its lowest strength. In general, annealing is the opposite of hardening. Heating the metal to just above the upper critical point, soaking at that temperature, and cooling very slowly in the furnace accomplishes annealing of steel. (Refer to *Figure 7-6* for recommended temperatures.) Soaking time is approximately 1 hour per inch of thickness of the material. To produce maximum softness in steel, the metal must be cooled very slowly. Slow cooling is obtained by shutting off the heat and allowing the furnace and metal to cool together to 900 °F or lower, then removing the metal from the furnace and cooling in still air. Another method is to bury the heated steel in ashes, sand, or other substance that does not conduct heat readily. ***Normalizing*** The normalizing of steel removes the internal stresses set up by heat treating, welding, casting, forming, or machining. Stress, if not controlled, leads to failure. Because of the better physical properties, aircraft steels are often used in the normalized state, but seldom, if ever, in the annealed state. One of the most important uses of normalizing in aircraft work is in welded parts. Welding causes strains to be set up in the adjacent material. In addition, the weld itself is a cast structure as opposed to the wrought structure of the rest of the material. These two types of structures have different grain sizes, and to refine the grain as well as to relieve the internal stresses, all welded parts should be normalized after fabrication. Heating the steel above the upper critical point and cooling in still air accomplish normalizing. The more rapid quenching obtained by air-cooling, as compared to furnace cooling, results in a harder and stronger material than that obtained by annealing. Recommended normalizing temperatures for the various types of aircraft steels are listed in *Figure 7-6*. ***Casehardening*** Casehardening produces a hard, wear-resistant surface or case over a strong, tough core. Casehardening is ideal for parts that require a wear-resistant surface and, at the same time, must be tough enough internally to withstand the applied loads. The steels best suited to casehardening are the low carbon and low-alloy steels. If high-carbon steel is casehardened, the hardness penetrates the core and causes brittleness. In casehardening, the surface of the metal is changed chemically by introducing a high carbide or nitride content. The core is unaffected chemically. When heat-treated, the surface responds to hardening while the core toughens. The common forms of casehardening are carburizing, cyaniding, and nitriding. Since cyaniding is 7-20 not used in aircraft work, only carburizing and nitriding are discussed in this section. *Carburizing* Carburizing is a casehardening process in which carbon is added to the surface of low-carbon steel. Thus, carburized steel has a high-carbon surface and a low-carbon interior. When the carburized steel is heat-treated, the case is hardened while the core remains soft and tough. A common method of carburizing is called "pack carburizing." When carburizing is to be done by this method, the steel parts are packed in a container with charcoal or some other material rich in carbon. The container is then sealed with fire clay, placed in a furnace, heated to approximately 1,700 °F, and soaked at that temperature for several hours. As the temperature increases, carbon monoxide gas forms inside the container and, being unable to escape, combines with the gamma iron in the surface of the steel. The depth to which the carbon penetrates depends on the length of the soaking period. For example, when carbon steel is soaked for 8 hours, the carbon penetrates to a depth of about 0.062 inch. In another method of carburizing, called "gas carburizing," a material rich in carbon is introduced into the furnace atmosphere. The carburizing atmosphere is produced by using various gases or by the burning of oil, wood, or other materials. When the steel parts are heated in this atmosphere, carbon monoxide combines with the gamma iron to produce practically the same results as those described under the pack carburizing process. A third method of carburizing is that of "liquid carburizing." In this method, the steel is placed in a molten salt bath that contains the chemicals required to produce a case comparable with one resulting from pack or gas carburizing. Alloy steels with low-carbon content, as well as low-carbon steels, may be carburized by any of the three processes. However, some alloys, such as nickel, tend to retard the absorption of carbon. Thus, the time required to produce a given thickness of case varies with the composition of the metal. *Nitriding* Nitriding is unlike other casehardening processes in that, before nitriding, the part is heat-treated to produce definite physical properties. Thus, parts are hardened and tempered before being nitrided. Most steels can be nitrided, but special alloys are required for best results. These special alloys contain aluminum as one of the alloying elements and are called "nitralloys." In nitriding, the part is placed in a special nitriding furnace and heated to a temperature of approximately 1,000 °F. With the part at this temperature, ammonia gas is circulated within the specially constructed furnace chamber. The high temperature cracks the ammonia gas into nitrogen and hydrogen. The ammonia, which does not break down, is caught in a water trap below the regions of the other two gases. The nitrogen reacts with the iron to form nitride. The iron nitride is dispersed in minute particles at the surface and works inward. The depth of penetration depends on the length of the treatment. Soaking periods (as long as 72 hours) are frequently required to produce the desired thickness during nitriding. Nitriding can be accomplished with a minimum of distortion, because of the low temperature at which parts are casehardened and because no quenching is required after exposure to the ammonia gas. **Heat Treatment of Nonferrous Metals** ***Aluminum Alloys*** In the wrought form, commercially-pure aluminum is known as 1100. It has a high degree of resistance to corrosion and is easily formed into intricate shapes. It is relatively low in strength and does not have the properties required for structural aircraft parts. The process of alloying generally obtains high strengths. The resulting alloys are less easily formed and, with some exceptions, have lower resistance to corrosion than 1100 aluminum. Alloying is not the only method of increasing the strength of aluminum. Like other materials, aluminum becomes stronger and harder as it is rolled, formed, or otherwise cold worked. Since the hardness depends on the amount of cold working done, 1100 and some wrought aluminum alloys are available in several strain-hardened tempers. The soft or annealed condition is designated O. If the material is strain hardened, it is said to be in the H condition. The most widely used alloys in aircraft construction are hardened by heat treatment rather than by cold work. These alloys are designated by a somewhat different set of symbols: T4 and W indicate solution heat treated and quenched but not aged, and T6 indicates an alloy in the heat-treated, hardened condition. - W---solution heat treated, unstable temper - T---treated to produce stable tempers other than F, O, or H - T2---annealed (cast products only) - T3---solution heat treated and then cold worked - T4---solution heat treated - T5---artificially aged only 7-21 - T6---solution heat treated and then artificially aged - T7---solution heat treated and then stabilized - T8---solution heat treated, cold worked, and then artificially aged - T9---solution heat treated, artificially aged, and then cold worked - T10---artificially aged and then cold worked Additional digits may be added to T1 through T10 to indicate a variation in treatment, which significantly alters the characteristics of the product. Aluminum-alloy sheets are marked with the specification number on approximately every square foot of material. If for any reason this identification is not on the material, it is possible to separate the heat-treatable alloys from the non-heat-treatable alloys by immersing a sample of the material in a 10 percent solution of caustic soda (sodium hydroxide). The heat-treatable alloys turn black due to the copper content, whereas the others remain bright. In the case of clad material, the surface remains bright, but there is a dark area in the middle when viewed from the edge. ***Alclad Aluminum*** The terms "Alclad and Pureclad" are used to designate sheets that consist of an aluminum-alloy core coated with a layer of pure aluminum to a depth of approximately 5^1^⁄2 percent on each side. The pure aluminum coating affords a dual protection for the core, preventing contact with any corrosive agents, and protecting the core electrolytically by preventing any attack caused by scratching or from other abrasions. There are two types of heat treatments applicable to aluminum alloys: solution heat treatment and precipitation heat treatment. Some alloys, such as 2017 and 2024, develop their full properties as a result of solution heat treatment followed by about 4 days of aging at room temperature. Other alloys, such as 2014 and 7075, require both heat treatments. The alloys that require precipitation heat treatment (artificial aging) to develop their full strength also age to a limited extent at room temperature; the rate and amount of strengthening depends upon the alloy. Some reach their maximum natural or room temperature aging strength in a few days, and are designated as --T4 or --T3 temper. Others continue to age appreciably over a long period of time. Because of this natural aging, the --W designation is specified only when the period of aging is indicated, for example, 7075--W (^1^⁄2 hour). Thus, there is considerable difference in the mechanical and physical properties of freshly quenched (--W) material and material that is in the --T3 or --T4 temper. The hardening of an aluminum alloy by heat treatment consists of four distinct steps: 1. Heating to a predetermined temperature. 2. Soaking at temperature for a specified length of time. 3. Rapidly quenching to a relatively low temperature. 4. Aging or precipitation hardening either spontaneously at room temperature, or because of a low temperature thermal treatment. The first three steps above are known as solution heat treatment, although it has become common practice to use the shorter term, "heat treatment." Room temperature hardening is known as natural aging, while hardening done at moderate temperatures is called artificial aging, or precipitation heat treatment. ***Solution Heat Treatment*** *Temperature* The temperatures used for solution heat treating vary with different alloys and range from 825 °F to 980 °F. As a rule, they must be controlled within a very narrow range (±10 °F) to obtain specified properties. If the temperature is too low, maximum strength is not obtained. When excessive temperatures are used, there is danger of melting the low melting constituents of some alloys with consequent lowering of the physical properties of the alloy. Even if melting does not occur, the use of higher than recommended temperatures promotes discoloration and increases quenching strains. *Time at Temperature* The time at temperature, referred to as soaking time, is measured from the time the coldest metal reaches the minimum limit of the desired temperature range. The soaking time varies, depending upon the alloy and thickness, from 10 minutes for thin sheets to approximately 12 hours for heavy forgings. For the heavy sections, the nominal soaking time is approximately 1 hour for each inch of cross-sectional thickness. *\[Figure 7-7\]* Choose the minimum soaking time necessary to develop the required physical properties. The effect of an abbreviated soaking time is obvious. An excessive soaking period aggravates high-temperature oxidation. With clad material, prolonged heating results in excessive diffusion of copper and other soluble constituents into the protective cladding and may defeat the purpose of cladding. ***Quenching*** After the soluble constituents are in solid solution, the material is quenched to prevent or retard immediate re-precipitation. Three distinct quenching methods are employed. The one to 7-22 be used in any instance depends upon the part, the alloy, and the properties desired. **Thickness (inch) Time (minutes)** Up to 0.032 30 0.032 to ˜⁄˛ 30 ˜⁄˛ to ¼ 40 Over ¼ 60 NOTE: Soaking time starts when the metal (or the molten bath) reaches a temperature within the range speciÿed above. **Figure 7-7.** *Typical soaking times for heat treatment.* *Cold Water Quenching* Parts produced from sheet, extrusions, tubing, small forgings, and similar type material are generally quenched in a cold-water bath. The temperature of the water before quenching should not exceed 85 °F. Using a sufficient quantity of water keeps the temperature rise under 20 °F. Such a drastic quench ensures maximum resistance to corrosion. This is particularly important when working with alloys, such as 2017, 2024, and 7075. This is the reason a drastic quench is preferred, even though a slower que