David and Goliath by Osmar Schindler

Transcription

1 CHAPTER 4: ALLOYS And there came out from the camp of the Philistines a champion named Goliath, of Gath, whose height was six cubits and a span. He had a helmet of bronze on his head, and he was armed with a coat of mail, and the weight of the coat was five thousand shekels of bronze. And he had greaves of bronze upon his legs, and a javelin of bronze slung between his shoulders. 1 Samuel 17:1-6 (Revised Standard Version) David and Goliath by Osmar Schindler In the biblical story of David and Goliath, the champion of the Philistines was feared not only for his size but also for his bronze armor. David showed his faith by refusing modern technology, using only stones in battle. Bronze was the first successful metal alloy. Its properties are significantly different from the pure metals, copper and tin. This chapter describes how the lessons of the ancient alloys, bronze and steel, inspired modern metallurgists to create new alloys from a wide range of metallic elements. The ways in which properties depend on composition for alloys of two metals, aluminum and titanium, are highlighted for modern applications. Military applications continue to inspire development of alloys. 4.1 BRONZE Several different metals can be mixed with copper to make a useful alloy. Tin has been the most commonly used alloying element, but arsenic, antimony and lead have also been used. 4-1

2 Bronze with 11% tin, 50x magnification Bronze is a mixture of elements, not a compound, so in theory any proportions can be made. Bronze is harder than copper, making it useful for tools and weapons. It can be sharpened. Bronze melts at a lower temperature than pure copper. It also flows better into molds. Since it is less likely that bubbles will be trapped, the cast bronze object will be harder than cast copper. The microscopic structure of bronze explains some of the observed properties. Small amounts of tin can be incorporated into a copper lattice as substitutional impurities. Since tin atoms are larger than copper atoms, it is hard for planes of atoms to slip past them. This reduces the flexibility of the metal. Large amounts of tin in a sample will create precipitates, having a crystal structure distinct from that of the main lattice. This type of discontinuity also makes it hard for planes of atoms to slip. 10% tin in copper is a popular ratio that results in excellent properties. Due to the inhomogeneous crystal lattice, bronze alloys have lower electrical conductivity than pure copper. Bronze was the most important material in the world but was eventually superseded by iron, which was more common but more difficult to work with. Historians have theorized that the Bronze Age ended because tin supplies became scarce. 4.2 STEEL PROCESSING Iron is easily found in the earth's crust, but pure iron is not a very useful material because it rusts easily. It also has such a high melting point that it cannot easily be shaped by casting. Through a fortunate coincidence carbon, the element used to reduce iron ore, forms a very useful alloy with iron: steel. Carbon atoms are small enough to fit into the interstices of an iron lattice. 4-2

3 A blacksmith heats wrought iron in a coal fire The succesful processing of iron ore requires precise control of the proportions of carbon, iron and oxygen in the final product. In early Iron Age Europe, the furnaces were not hot enough to melt iron. When iron ore was heated with charcoal, removing the oxygen, the iron changed to a spongy solid called a bloom which still contained slag. A blacksmith could hammer out the slag, producing a purer material called wrought iron. Its final carbon content would be less than 0.5%. This nearly pure iron is malleable and soft. A blast furnace is hot enough that carbon from charcoal or coke dissolves in molten iron. When cooled the product is cast iron, with more than 2% carbon content. This material is brittle, not malleable, since the carbon atoms in the lattice block slip planes. When force is applied a cast iron object will crack rather than bending. Although Chinese metalworkers used their superior furnaces and bellows to produce cast iron objects about two thousand years ago, blast furnaces were not built in Europe until the 14th century, and industrial scale production of cast iron objects was not achieved until the 18th century. Casting is still used to make metal objects with fine details such as engine parts. In sandcasting, a pattern of the final object is made and surrounded with sand that has been mixed with a binder. The pattern is removed leaving a hollow space. Wax patterns are common; they can be melted out of the mold. Molten metal is poured into the sand mold, and allowed to cool and harden. Die casting uses a permanent mold; machines can use pressure to force molten metal into the mold. Neither cast iron nor wrought iron was an effective substitute for bronze, but eventually metalworkers in several parts of the world developed recipes for steel, an iron alloy with an intermediate carbon content, about 1%. Steel is tough and flexible, not brittle, yet hard enough to be sharpened. 4-3

4 Clockwise from left: cast bronze, iron, and gold objects Some blacksmiths learned to steel one side of a wrought iron blade by leaving it in contact with very hot charcoal for a certain length of time. This would be used to make sharp cutting edges. Additional processing techniques were discovered to improve the properties of steel objects. If steeled iron is cold hammered it gains additional strength. Quenching hot metal in cold water makes the metal harder and more brittle. If quenched steel is reheated to about 700 C it loses some of the hardness but regains some flexibility. This process is called tempering. The hardness of steel weapons made them superior to bronze. Several techniques were developed that could consistently produce steel. In China, cast iron was the starting material. Iron was melted in a large crucible, then puddled: stirred so oxygen from the air could react with the excess carbon. In India batches of wootz steel were prepared by mixing wrought iron, wood and leaves in a clay crucible. It was heated in a charcoal pit for several hours, until the metal melted and dissolved carbon from the plant materials. The liquid steel was poured into stone molds to form flat ingots that could be shipped to the Mediterranean countries. "Damascus steel" made in Damascus and Toledo was based on this procedure. This technique was not widely shared, and the secret was lost in the tenth century. In England in the 1700 s progress was made on preparing batches of steel. Manufacturers would pile charcoal and iron, cover it with sand, then apply heat. Carbon diffused into surface layer of the iron, so a layer of steel could be hammered off. These bits of steel were hammered together to make a brittle laminate steel. Benjamin Huntsman designed a furnace crucible (based on glass making procedures) that could achieve higher temperatures. Chips of laminate steel melted into a homogeneous fluid, solidifying with a uniform composition. A shortage of wood for charcoal in England led Abraham Darby to substitute coke in the 4-4

5 Chinese puddling technique and Bessemer steel converter reduction of iron ore. Coke was made by charring coal to remove volatile elements (such as sulfur), leaving the carbon. England had large supplies of coal and iron ore so was able to dominate the production of iron. By the mid-1800 s, the crucible method was unable to meet demand. Henry Bessemer developed a large scale puddling process, blasting air at molten cast iron. Oxygen in the air combined with carbon in the cast iron, leaving as carbon dioxide and carbon monoxide gases. This reaction released a lot of energy; metalworkers watched the flame size and color to determine when to halt the process. Railway transportation improved, since Bessemer steel rails did not have to be repaired as often as those made of wrought iron. Some scientists think that the 1912 sinking of the Titanic was due to the use of iron rivets instead of steel ones. It was known that steel rivets were stronger, and could be installed by machines rather than by hand, but the shipbuilders had difficulty obtaining enough rivets for the huge project. Steel rivets were used on the central hull, but the stern and bow, where the iceberg hit, had iron rivets. Analysis of salvaged iron rivets showed a high proportion of slag, which would have made the material brittle. Car companies created a new market for steel in the 20th century. Early automobiles were built on wooden frames. Steel mills sent heated bars of steel through rollers to produce sheets, which could be attached to the frames. This combination had certain advantages. The stiffness of the wood was better for the frame, and woodworking was such a common skill that the shapes of cars could be changed easily. The steel was more flexible and lighter than wooden 4-5

6 panels would have been. Although aluminum was available, it was more expensive and harder to paint than steel. In the 1910's and 1920's automobile manufacturers developed techniques for mass production; machines were created to stamp or press body panels out of sheet steel, reducing labor costs. Some designers tried making body panels out of papier mache or plaster coated wire, even colorful fabrics, but steel was the standard for decades. A new method for forming metals, powder metallurgy, developed to meet the needs of the automobile industry. Metal powders are mixed in the desired proportions, put into a die (a precision mold), and pressed. The shaped material is ejected from the die and put into a furnace. To allow metallic bonds to form within the sample, the object is sintered: heated at a temperature just below its melting point. The atmosphere in the furnace is controlled to limit exposure to oxygen and other chemicals that could contaminate the sample. Sintered objects can have a very smooth surface with precise details. Heat treatments such as forging increase the strength of steel. The grains of the metal sample are aligned; a preferred orientation can be selected to maximize strength in the direction that will be stressed. Since iron and steel have low coefficients of thermal expansion, the size of the object does not change much. Modern steels receive more sophisticated heat treatments than those possible at a blacksmith's forge. Furnaces provide precise temperatures for precise lengths of time. Cooling rates (quenching with water or cold air) are also controlled to achieve desired properties. Pieces of steel may be connected by welding. Electric current or heat melts the metal at the interface of the two pieces. Upon cooling they merge into one object. Since the crystal structures are disturbed during the melting and cooling, welds tend to be weak points. Another concern is that impurities such as oxygen could be acquired from the surroundings during the welding process. VARIATION OF PROPERTIES WITH COMPOSITION It has already been observed that the properties of steel are very sensitive to its carbon composition. Small amounts of other metals can also be added to steel to modify its properties. Henry Ford used vanadium steel for the 1908 Model T; adding only 1% vanadium made the steel four times stronger, so less metal was needed; the car was lighter and cheaper. Stainless steel is a very common alloy with approximately 10-20% chromium, and less than 1% nickel. The chromium protects the steel from oxidation. Stainless steel can be sterilized in high pressure steam without deforming, making it very useful for food and medical applications. Tungsten carbide steel is one of the hardest materials known. A common recipe includes 18% tungsten. It can handle very high temperatures without softening or losing sharpness, probably because tungsten itself has a very high melting point. This alloy is used to make machine tools, pieces of steel that are used to cut other metals. They generate a lot of friction, and heat, while operating at high speeds. Tungsten carbide tools will not wear down as quickly as other materials. Tool steel objects are formed using powder metallurgy. HSLA (high strength low alloy) steels contain much less carbon than a typical steel, perhaps 0.5%. Up to 1.5% manganese plus small amounts of other transition metals including vanadium, chromium, nickel, copper, and molybdenum can be used to provide strength, toughness, and corrosion resistance. Modern steelmaking has adapted to recycle scrap metal, and to work with low grade iron 4-6

7 ores (containing compounds in addition to iron oxide). Since nonmetallic elements (other than carbon) can diminish the properties of steel, refineries adjust their chemical reactions to control the composition of the final product. One example is the Basic Oxygen Steelmaking Process, designed to handle ores with phosphorus or sulfur. Liquid pig iron (from a blast furnace) can be transferred to a vessel, along with scrap iron and steel. Oxygen is blown onto the hot metal where it reacts with carbon and silicon. These reactions produce heat, melting the scrap metal. Carbon monoxide gas will escape through the air; silicon dioxide forms an insoluble slag floating on the liquid metal. The "basic" part of the process involves a special lining, made of MgO. The lining is gradually worn away by hot metal, and reacts with any P or S in the molten iron to make it part of the slag. 4.3 ALUMINUM ALLOYS Aluminum's natural abundance and low density inspire designers to find many uses for it. Adding small amounts of other metals makes it hard enough to be a structural material. For many years aircraft manufacturers have used aluminum. In addition to having a light weight, the materials should be stiff and be able to handle temperature extremes. Lithium, the lightest metallic element, can act as a substitutional impurity in an aluminum crystal. Copper has been found to increase the strength of aluminum-lithium alloys. Magnesium can also be used as an alloying element instead of lithium. The alloys are not quite as resistant to corrosion. Aluminum oxide forms on the surface of pure aluminum, making a protective coating, but its formation is interrupted by the presence of impurities. Simply mixing lithium or magnesium with aluminum is not enough to make a stiff metal. In bronze, the large tin atoms make it difficult for copper atoms to slide along their slip planes. In steel, the small carbon atoms fill interstices in the iron lattice, preventing the iron planes from slipping. Lithium and magnesium are not larger than aluminum, and if they are at lattice sites they will not stop atoms from slipping. A process called precipitation hardening reduces the malleability of an aluminum alloy. The material is heated to a temperature below the melting point. This increases the amount of the impurity that will dissolve in aluminum. The alloy is allowed to cool at a particular rate, and allowed to age for several days. Although a lithium or magnesium atom fits in the aluminum lattice, it is not as strongly bonded to the surrounding aluminum atoms as a matching aluminum atom would be. There is some strain on the lattice, distorting it. Heat allows the impurity atoms to drift around in the crystal. When a few lithium atoms meet each other they will bond and form a precipitate, a clump of atoms with a different (smaller) lattice pattern. The precipitates block the slip planes in the aluminum lattice. The material with precipitates is significantly harder. Alloy with a precipitate 4-7

8 Aluminum alloys may also be cold worked. Rolling and pressing distorts grains, and reduces their size. 4.4 TITANIUM Lockheed's SR-71 "Blackbird" Titanium has a lower density than steel. Its stiffness is less than steel but significantly greater than aluminum,. These properties make it ideal for many applications. High performance bicycle frames and golf clubs are made of titanium. Titanium with 4% aluminum is popular for surgical implants such as artificial joints since it is well tolerated by the body's tissues, resists corrosion, is strong and not too stiff. Another special property is that it can handle very high temperatures. The SR-71 "Blackbird" spy plane, built out of titanium alloy, flies so fast that its surface temperatures can reach nearly 1000 F. This actually succeeds in annealing the metal. Although titanium is a very common metal, alloy development didn't begin until the late 1940's. The common ore, TiO 2, could not simply be reduced with carbon because carbides were produced. An indirect chemical route was discovered. First titanium oxide reacts with chlorine gas to produce titanium chloride, with carbon carrying away the oxygen. Then the titanium chloride is reduced with magnesium. High temperatures are required for these reactions. TiO Cl C TiCl CO TiCl 4 + 2Mg 2 MgCl 2 + Ti Even after the metal was available, there were still obstacles to its application. Molten titanium metal will react with atmospheric oxygen or nitrogen, so new processing methods in inert atmospheres were needed to replace existing forging and machining techniques. Large amounts of oxygen make titanium brittle, more like the ore, but titanium with 0.3% oxygen is strong, harder to bend than pure titanium. Bicycle frames have used that alloy, as well as one that contains 3% aluminum and 2.5% vanadium. Slightly higher amounts of those metals result in a hard metal suitable for jet engines. In 1959 the Soviet Union decided to build a titanium submarine. Compared to steel it had higher strength, lower weight, resisted corrosion. For identical hull weights, a titanium hull 4-8

9 could withstand higher pressures, allowing deeper dives. Since titanium was non-magnetic it could also avoid magnetic mines and detection devices. 4.5 GOLD Although pure gold is extremely valuable, it is too soft to be a useful material. Adding metals of other sizes creates lattice defects, making the alloy less malleable. The purity of a gold sample is reported in karats. 24 karat gold is the highest purity, at least 99.9% gold. 14 karat gold, commonly used for jewelry, is 14/24 or 58.5% gold. Silver is a common alloying element. A red colored alloy is produced by the addition of copper. White gold can be prepared by adding nickel, palladium, platinum or rhodium. 4.6 MAGNETS Materials are magnetic if their valence electrons align in a special way. This is most likely to happen in transition metals, since they have many loosely held valence electrons. Iron, cobalt and nickel are often magnetic. The earth's iron core makes it a giant magnet, and the terms north and south are used to describe the two directions of a magnetic field. The north pole of a magnet is attracted to the earth's North Pole. Compounds can also be magnetic. An iron ore with the formula Fe 3 O 4 found in Magnesia, Turkey was called magnetite, and its name because associated with the unusual property. In medieval times the rock was called lodestone (since it will "lead" north) and it was used for navigation. People discovered that iron or steel needles could be rubbed on the rock to acquire magnetization. These needles were made into navigational compasses. Ceramic magnets such as iron, cobalt, and chromium oxides are manufactured by powder metallurgy. Small particles can be mixed with polymers to make flexible refrigerator magnets, or coated on plastic strips to make audio and video recording tapes. Strong magnetic fields are applied during processing to align the fields of the particles. Deposition of thin films in a vacuum chamber is used for computer hard drives. Alloys can make very strong magnets. The first successful combination, aluminumnickel-cobalt, was discovered in the 1930's. Alloys containing rare earth elements are even more successful. Samarium cobalt and neodymium-iron-boron are two common combinations. In the early 1800's it was discovered that electric current running through coils of wire creates a magnetic field. Electromagnets are used in electric transformers. Magnetism may be lost when a material is heated. Upon heating the electrons gain energy and can reorient, losing their special alignment. 4-9

10 Learning Goals for Chapter 4 After studying this chapter you should be able to: composition & structure List combinations of elements found in common alloys. Compare the structures of pure metals with alloys. Distinguish the chemical compositions of wrought iron and cast iron. Describe the compositions of metallic and ceramic magnets. properties Explain which physical properties change when metals are made into alloys. Compare chemical properties of titanium s ore and metal to those of iron. Explain why some materials are magnetic. processing Describe processes used in steel manufacturing. Describe the methods used to prepare metallic and ceramic magnets. performance Describe impacts of steel manufacturing on society. Identify properties of alloys particularly useful for aircraft, bicycles and cars. Compare the performances of metallic and ceramic magnets. Vocabulary bloom casting die casting magnetic north pole powder metallurgy puddling quenching sandcasting sintering south pole steel tempering welding 4-10