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GENERAL

Most of the metals and alloys used in Army materiel can be welded by one or more of the processes described in this manual. This section describes the characteristics of metals and their alloys, with particular reference to their significance in welding operations.

7-2. PROPERTIES OF METALS

a. Definitions. All metals fall within two categories, ferrous or nonferrous.

(1) Ferrous metals are metals that contain iron. Ferrous metals appear in the form of cast iron, carbon steel, and tool steel. The various alloys of iron, after undergoing certain processes, are pig iron, gray cast iron, white iron, white cast iron, malleable cast iron, wrought iron, alloy steel, and carbon steel. All these types of iron are mixtures of iron and carbon, manganese, sulfur, silicon, and phosphorous. Other elements are also present, but in amounts that do not appreciably affect the characteristics of the metal.

(2) Nonferrous metals are those which do not contain iron. Aluminum, copper, magnesium, and titanium alloys are among those metals which belong to this group.

b. Physical Properties. Many of the physical properties of metals determine if and how they can be welded and how they will perform in service. Physical properties of various metals are shown in table 7-1.

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Color. Color relates to the quality of light reflected from the metal.

(2) Mass or density. Mass or density relates to mass with respect to volume. Commonly known as specific gravity, this property is the ratio of the mass of a given volume of the metal to the mass of the same volume of water at a specified temperature, usually 39F (4C). For example, the ratio of weight of one cubic foot of water to one cubic foot of cast iron is the specific gravity of cast iron. This property is measured by grams per cubic millimeter or centimeter in the metric system.

(3) Melting point. The melting point of a metal is important with regard to welding. A metal’s fusibility is related to its melting point, the temperature at which the metal changes from a solid to a molten state. Pure substances have a sharp melting point and pass from a solid state to a liquid without a change in temperature. During this process, however, there is an absorption of heat during melting and a liberation of heat during freezing. The absorption or release of thermal energy when a substance changes state is called its latent heat. Mercury is the only common metal that is in its molten state at normal room temperature. Metals having low melting temperatures can be welded with lower temperature heat sources. The soldering and brazing processes utilize low-temperature metals to join metals having higher melting temperatures.

(4) Boiling point. Boiling point is also an important factor in welding. The boiling point is the temperature at which the metal changes from the liquid state to the vapor state. Some metals, when exposed to the heat of an arc, will vaporize.

(5) Conductivity. Thermal and electrical conductivity relate to the metal’s ability to conduct or transfer heat and electricity. Thermal conductivity, the ability of a metal to transmit heat throughout its mass, is of vital importance in welding, since one metal may transmit heat from the welding area much more quickly than another. The thermal conductivity of a metal indicates the need for preheating and the size of heat source required. Thermal conductivity is usually related to copper. Copper has the highest thermal conductivity of the common metals, exceeded only by silver. Aluminum has approximately half the thermal conductivity of copper, and steels have abut one-tenth the conductivity of copper. Thermal conductivity is measured in calories per square centimeter per second per degree Celsius. Electrical conductivity is the capacity of metal to conduct an electric current. A measure of electrical conductivity is provided by the ability of a metal to conduct the passage of electrical current. Its opposite is resistivity, which is measured in micro-ohms per cubic centimeter at a standardize temperature, usually 20C. Electrical conductivity is usually considered as a percentage and is related to copper or silver. Temperature bears an important part in this property. As temperature of a metal increases, its conductivity decreases. This property is particularly important to resistance welding and to electrical circuits.

(6) Coefficient of linear thermal expansion. With few exceptions, solids expand when they are heated and contract when they are cooled. The coefficient of linear thermal expansion is a measure of the linear increase per unit length based on the change in temperature of the metal. Expansion is the increase in the dimension of a metal caused by heat. The expansion of a metal in a longitudinal direction is known as the linear expansion. The coefficient of linear expansion is expressed as the linear expansion per unit length for one degree of temperature increase. When metals increase in size, they increase not only in length but also in breadth and thickness. This is called volumetric expansion. The coefficient of linear and volumetric expansion varies over a wide range for different metals. Aluminum has the greatest coefficient of expansion, expanding almost twice as much as steel for the same temperature change. This is important for welding with respect to warpage, wapage control and fixturing, and for welding together dissimilar metals.

(7) Corrosion resistance. Corrosion resistance is the resistance to eating or wearing away by air, moisture, or other agents.

c. Mechanical Properties. The mechanical properties of metals determine the range of usefulness of the metal and establish the service that can be expected. Mechanical properties are also used to help specify and identify the metals. They are important in welding because the weld must provide the same mechanical properties as the base metals being joined. The adequacy of a weld depends on whether or not it provides properties equal to or exceeding those of the metals being joined. The most common mechanical properties considered are strength, hardness, ductility, and impact resistance. Mechanical properties of various metals are shown in table 7-2.

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(1) Tensile strength. Tensile strength is defined as the maximum load in tension a material will withstand before fracturing, or the ability of a material to resist being pulled apart by opposing forces. Also known as ultimate strength, it is the maximum strength developed in a metal in a tension test. (The tension test is a method for determining the behavior of a metal under an actual stretch loading. This test provides the elastic limit, elongation, yield point, yield strength, tensile strength, and the reduction in area.) The tensile strength is the value most commonly given for the strength of a material and is given in pounds per square inch (psi) (kiloPascals (kPa)). The tensile strength is the number of pounds of force required to pull apart a bar of material 1.0 in. (25.4 mm) wide and 1.00 in. (25.4 mm) thick (fig. 7-1).metal15.gif (22407 bytes)

(2) Shear strength. Shear strength is the ability of a material to resist being fractured by opposing forces acting of a straight line but not in the same plane, or the ability of a metal to resist being fractured by opposing forces not acting in a straight line.

 

3) Fatigue strength. Fatigue strength is the maximum load a material can withstand without failure during a large number of reversals of load. For example, a rotating shaft which supports a weight has tensile forces on the top portion of the shaft and compressive forces on the bottom. As the shaft is rotated, there is a repeated cyclic change in tensile and compressive strength. Fatigue strength values are used in the design of aircraft wings and other structures subject to rapidly fluctuating loads. Fatigue strength is influenced by microstructure, surface condition, corrosive environment, and cold work.

(4) Compressive strength. Compressive strength is the maximum load in compression a material will withstand before a predetermined amount of deformation, or the ability of a material to withstand pressures acting in a given plane . The compressive strength of both cast iron and concrete are greater than their tensile strength. For most materials, the reverse is true.

 

(5) Elasticity. Elasticity is the ability of metal to return to its original size, shape, and dimensions after being deformed, stretched, or pulled out of shape. The elastic limit is the point at which permanent damage starts. The yield point is the point at which definite damage occurs with little or no increase in load. The yield strength is the number of pounds per square inch (kiloPascals) it takes to produce damage or deformation to the yield point.

(6) Modulus of elasticity. The modulus of elasticity is the ratio of the internal stress to the strain produced.

(7) Ductility. The ductility of a metal is that property which allows it to be stretched or otherwise changed in shape without breaking, and to retain the changed shape after the load has been removed. It is the ability of a material, such as copper, to be drawn or stretched permanently without fracture. The ductility of a metal can be determined by the tensile test by determining the percentage of elongation. The lack of ductility is brittleness or the lack of showing any permanent damage before the metal cracks or breaks (such as with cast iron).

(8) Plasticity. Plasticity is the ability of a metal to be deformed extensively without rupture. Plasticity is similar to ductility.

(9) Malleability. Malleability is another form of plasticity, and is the ability of a material to deform permanently under compression without rupture. It is this property which allows the hammering and rolling of metals into thin sheets. Gold, silver, tin, and lead are examples of metals exhibiting high malleability. Gold has exceptional malleability and can be rolled into sheets thin enough to transmit light.

(10) Reduction of area. This is a measure of ductility and is obtained from the tensile test by measuring the original cross-sectional area of a specimen to a cross-sectional area after failure.

(11) Brittleness. Brittleness is the property opposite of plasticity or ductility. A brittle metal is one than cannot be visibly deformed permanently, or one that lacks plasticity.

(12) Toughness. Toughness is a combination of high strength and medium ductility. It is the ability of a material or metal to resist fracture, plus the ability to resist failure after the damage has begun. A tough metal, such as cold chisel, is one that can withstand considerable stress, slowly or suddenly applied, and which will deform before failure. Toughness is the ability of a material to resist the start of permanent distortion plus the ability to resist shock or absorb energy.

(13) Machinability and weldability. The property of machinability and weldability is the ease or difficulty with which a material can be machined or welded.

(14) Abrasion resistance. Abrasion resistance is the resistance to wearing by friction.

(15) Impact resistance. Resistance of a metal to impacts is evaluated in terms of impact strength. A metal may possess satisfactory ductility under static loads, but may fail under dynamic loads or impact. The impact strength of a metal is determined by measuring the energy absorbed in the fracture.

(16) Hardness. Hardness is the ability of a metal to resist penetration and wear by another metal or material. It takes a combination of hardness and toughness to withstand heavy pounding. The hardness of a metal limits the ease with which it can be machined, since toughness decreases as hardness increases.metal16.gif (84061 bytes)

a) Brinell hardness test. In this test, a hardened steel ball is pressed slowly by a known force against the surface of the metal to be tested. The diameter of the dent in the surface is then measured, and the Brinell hardness number (bhn) is determined by from standard tables.

(b) Rockwell hardness test. This test is based upon the difference between the depth to which a test point is driven into a metal by a light load and the depth to which it is driven in by a heavy load. The light load is first applied and then, without moving the piece, the heavy load is applied. The hardness number is automatically indicated on a dial. The letter designations on the Rockwell scale, such as B and C, indicate the type of penetrator used and the amount of heavy load. The same light load is always used.

(c) Scleroscope hardness test. This test measures hardness by letting a diamond-tipped hammer fall by its own weight from a fixed height and rebound from the surface; the rebound is measured on a scale. It is used on smooth surfaces where dents are not desired.

a. General. It is necessary to know the composition of the metal being welded in order to produce a successful weld. Welders and metal workers must be able to identify various metal products so that proper work methods may be applied. For Army equipment, drawings (MWOs) should be available. They must be examined in order to determine the metal to be used and its heat treatment, if required. After some practice, the welder will learn that certain parts of machines or equipment are always cast iron, other parts are usually forgings, and so on.

b. Tests. There are seven tests that can be performed in the shop to identify metals. These should be supplemented  which present physical and mechanical properties of metal, and table 7-3, which presents hardness data. These tests are as follows:

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(1) Appearance test. The appearance test includes such things as color and appearance of machined as well as unmachined surfaces. Form and shape give definite clues as to the identity of the metal. The shape can be descriptive; for example, shape includes such things as cast engine blocks, automobile bumpers, reinforcing rods, I beams or angle irons, pipes, and pipe fittings. Form should be considered and may show how the part was rode, such as a casting with its obvious surface appearance and parting mold lines, or hot rolled wrought material, extruded or cold rolled with a smooth surface. For example, pipe can be cast, in which case it would be cast iron, or wrought, which would normally be steel. Color provides a very strong clue in metal identification. It can distinguish many metals such as copper, brass, aluminum, magnesium, and the precious metals. If metals are oxidized, the oxidation can be scraped off to determine the color of the unoxidized metal. This helps to identify lead, magnesium, and even copper. The oxidation on steel, or rust, is usually a clue that can be used to separate plain carbon steels from the corrosion-resisting steels.

(2) Fracture test. Some metal can be quickly identified by looking at the surface of the broken part or by studying the chips produced with a hammer and chisel. The surface will show the color of the base metal without oxidation. This will be true of copper, lead, and magnesium. In other cases, the coarseness or roughness of the broken surface is an indication of its structure. The ease of breaking the part is also an indication of its ductility of lack of ductility. If the piece bends easily without breaking, it is one of the more ductile metals. If it breaks easily with little or no bending, it is one of the brittle metals.

(3) Spark test. The spark test is a method of classifying steels and iron according to their composition by observing the sparks formed when the metal is held against a high speed grinding wheel. This test does not replace chemical analysis, but is a very convenient and fast method of sorting mixed steels whose spark characteristics are known. When held lightly against a grinding wheel, the different kinds of iron and steel produce sparks that vary in length, shape, and color. The grinding wheel should be run to give a surface speed of at least 5000 ft (1525 m) per minute to get a good spark stream. Grinding wheels should be hard enough to wear for a reasonable length of time, yet soft enough to keep a free-cutting edge. Spark testing should be done in subdued light, since the color of the spark is important. In all cases, it is best to use standard samples of metal for the purpose of comparing their sparks with that of the test sample.

(a) Spark testing is not of much use on nonferrous metals such as coppers, aluminums, and nickel-base alloys, since they do not exhibit spark streams of any significance. However, this is one way to separate ferrous and nonferrous metals.

(b) The spark resulting from the test should be directed downward and studied. The color, shape, length, and activity of the sparks relate to characteristics of the material being tested. The spark stream has specific items which can be identified. The straight lines are called carrier lines. They are usually solid and continuous. At the end of the carrier line, they may divide into three short lines, or forks. If the spark stream divides into more lines at the end, it is called a sprig. Sprigs also occur at different places along the carrier line. These are called either star or fan bursts. In some cases, the carrier line will enlarge slightly for a very short length, continue, and perhaps enlarge again for a short length. When these heavier portions occur at the end of the carrier line, they are called spear points or buds. High sulfur creates these thicker spots in carrier lines and the spearheads. Cast irons have extremely short streams, whereas low-carbon steels and most alloy steels have relatively long streams. Steels usually have white to yellow color sparks, while cast irons are reddish to straw yellow. A 0.15 percent carbon steel shows sparks in long streaks with some tendency to burst with a sparkler effect; a carbon tool steel exhibits pronounced bursting; and a steel with 1.00 percent carbon shows brilliant and minute explosions or sparklers. As the carbon content increases, the intensity of bursting increases.

(c) One big advantage of this test is that it can be applied to metal in, all stages, bar stock in racks, machined forgings or finished parts. The spark test is best conducted by holding the steel stationary and touching a high speed portable grinder to the specimen with sufficient pressure to throw a horizontal spark stream about 12.00 in. (30.48 cm) long and at right angles to the line of vision. Wheel pressure against the work is important because increasing pressure will raise the temperature of the spark stream and give the appearance of higher carbon content. The sparks near and around the wheel, the middle of the spark stream, and the reaction of incandescent particles at the end of the spark stream should be observed. Sparks produced by various metals are shown in figure 7-4.

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CAUTION

 

 

The torch test should be used with discretion, as it may damage the part being tested. Additionally, magnesium may ignite when heated in the open atmosphere.

 

(4) Torch test. With the oxyacetylene torch, the welder can identify various metals by studying how fast the metal melts and how the puddle of molten metal and slag looks, as well as color changes during heating. When a sharp corner of a white metal part is heated, the rate of melting can be an indication of its identity. If the material is aluminum, it will not melt until sufficient heat has been used because its high conductivity. If the part is zinc, the sharp corner will melt quickly, since zinc is not a good conductor. In the case of copper, if the sharp comer melts, it is normally deoxidized copper. If it does not melt until much heat has been applied, it is electrolytic copper. Copper alloys, if composed of lead, will boil. To distinguish aluminum from magnesium, apply the torch to filings. Magnesium will burn with a sparkling white flame. Steel will show characteristic colors before melting.

(5) Magnetic test. The magnetic test can be quickly performed using a small pocket magnet. With experience, it is possible to judge a strongly magnetic material from a slightly magnetic material. The nonmagnetic materials are easily recognized. Strongly magnetic materials include the carbon and low-alloy steels, iron alloys, pure nickel, and martensitic stainless steels. A slightly magnetic reaction is obtained from Monel and high-nickel alloys and the stainless steel of the 18 chrome 8 nickel type when cold worked, such as in a seamless tube. Nonmagnetic materials include copper-base alloys, aluminum-base alloys, zinc-base alloys, annealed 18 chrome 8 nickel stainless, the magnesium, and the precious metals.

(6) Chisel test. The chip test or chisel test may also be used to identify metals. The only tools required are a banner and a cold chisel. Use the cold chisel to hammer on the edge or corner of the material being examined. The ease of producing a chip is an indication of the hardness of the metal. If the chip is continuous, it is indicative of a ductile metal, whereas if chips break apart, it indicates a brittle material. On such materials as aluminum, mild steel and malleable iron, the chips are continuous. They are easily chipped and the chips do not tend to break apart. The chips for gray cast iron are so brittle that they become small, broken fragments. On high-carbon steel, the chips are hard to obtain because of the hardness of the material, but can be continuous.

(7) Hardness test. Refer to table 7-3 for hardness values of the various metals, and to the above information on the three hardness tests that are commonly used. A less precise hardness test is the file test. A summary of the reaction to filing, the approximate Brinell hardness, and the possible type of steel is shown in table 7-6. A sharp mill file must be used. It is assumed that the part is steel and the file test will help identify the type of steel. metal20.gif (119200 bytes)metal21.gif (32227 bytes)

 

(8) Chemical test. There are numerous chemical tests than can be made in the shop to identify some material. Monel can be distinguished form Inconel by one drop of nitric acid applied to the surface. It will turn blue-green on Monel, but will show no reaction on Inconel. A few drops of a 45 percent phosphoric acid will bubble on low-chromium stainless steels. Magnesium can be distinguished from aluminum using silver nitrate, which will leave a black deposit on magnesium, but not on aluminum. These tests can become complicated, and for this reason are not detailed further here.

c. Color Code for Marking Steel Bars. The Bureau of Standards of the United States Department of Commerce has a color code for making steel bars. The color markings provided in the code may be applied by painting the ends of bars. Solid colors usually mean carbon steel, while twin colors designate alloy and free-cutting steel.

d. Ferrous Metal. The basic substance used to make both steel and cast iron (gray and malleable) is iron. It is used in the form of pig iron. Iron is produced from iron ore that occurs chiefly in nature as an oxide, the two most important oxides being hematite and magnetite. Iron ore is reduced to pig iron in a blast furnace, and the impurities are removed in the form of slag (fig. 7-5). Raw materials charged into the furnace include iron ore, coke, and limestone. The pig iron produced is used to manufacture steel or cast iron.

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Plain carbon steel consists of iron and carbon. Carbon is the hardening element. Tougher alloy steel contains other elements such as chromium, nickel, and molybdenum. Cast iron is nothing more than basic carbon steel with more carbon added, along with silicon. The carbon content range for steel is 0.03 to 1.7 percent, and 4.5 percent for cast iron.

Steel is produced in a variety of melting furnaces, such as open-hearth, Bessemer converter, crucible, electric-arc, and induction. Most carbon steel is made in open-hearth furnaces, while alloy steel is melted in electric-arc and induction furnaces. Raw materials charged into the furnace include mixtures of iron ore, pig iron, limestone, and scrap. After melting has been completed, the steel is tapped from the furnace into a ladle and then poured into ingots or patterned molds. The ingots are used to make large rectangular bars, which are reduced further by rolling operations. The molds are used for castings of any design.

Cast iron is produced by melting a charge of pig iron, limestone, and coke in a cupola furnace. It is then poured into sand or alloy steel molds. When making gray cast iron castings, the molten metal in the mold is allowed to become solid and cool to room temperature in open air. Malleable cast iron, on the other hand, is made from white cast iron, which is similar in content to gray cast iron except that malleable iron contains less carbon and silicon. White cast iron is annealed for more than 150 hours at temperatures ranging from 1500 to 1700F (815 to 927C). The result is a product called malleable cast iron. The desirable properties of cast iron are less than those of carbon steel because of the difference in chemical makeup and structure. The carbon present in hardened steel is in solid solution, while cast iron contains free carbon known as graphite. In gray cast iron, the graphite is in flake form, while in malleable cast iron the graphite is in nodular (rounded) form. This also accounts for the higher mechanical properties of malleable cast iron as compared with gray cast iron.Iron ore is smelted with coke and limestone in a blast furnace to remove the oxygen (the process of reduction) and earth foreign matter from it. Limestone is used to combined with the earth matter to form a liquid slag. Coke is used to supply the carbon needed for the reduction and carburization of the ore. The iron ore, limestone, and coke are charged into the top of the furnace. Rapid combustion with a blast of preheated air into the smelter causes a chemical reaction, during which the oxygen is removed from the iron. The iron melts, and the molten slag consisting of limestone flux and ash from the coke, together with compounds formed by reaction of the flux with substances present in the ore, floats on the heavier iron liquid. Each material is then drawn off separately metal23.gif (101815 bytes)

All forms of cast iron, steel, and wrought iron consist of a mixture of iron, carbon, and other elements in small amounts. Whether the metal is cast iron or steel depends entirely upon the amount of carbon in it. Table 7-7 shows this principle. metal24.gif (30456 bytes)

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STANDARD DESIGNATION SYSTEM FOR STEEL

a. Numbers are used to designate different chemical compositions. A four-digit number series designates carbon and alloying steels according to the types and classes shown in table 7-8. This system has been expanded, and in some cases five digits are used to designate certain alloy steels.

b. Two letters are often used as a prefix to the numerals. The letter C indicates basic open hearth carbon steels, and E indicates electric furnace carbon and alloy steels. The letter H is sometimes used as a suffix to denote steels manufactured to meet hardenability limits.

c. The first two digits indicate the major alloying metals in a steel, such as manganese, nickel-chromium, and chrome-molybdenum.

d. The last digits indicate the approximate middle of the carbon content range in percent. For example, 0.21 indicates a range of 0.18 to 0.23 percent carbon. In a few cases, the system deviates from this rule, and some carbon ranges relate to the ranges of manganese, sulfur, phosphorous, chromium, and other elements.

e. The system designates the major elements of a steel and the approximate carbon range of the steel. It also indicates the manufacturing process used to produce the steel. The complete designation system is shown in table 7-9. metal26.gif (32712 bytes)

 

f. The number 2340 by this system indicates a nickel steel with approximately 3 percent nickel and 0.40 percent carbon. The number 4340 indicates a nickel-chrome-molybdenum metal with 0.40 percent carbon.