METALS AND THEIR PROPERTIES
Non-Ferrous Metals
Aluminium. It is extracted from bauxite, an ore rich in aluminium oxide, which derives its name from Les Baux near Aries in France, where it was first worked commercially. Although aluminium is the most plentiful metal in the earth's crust (of which it forms 8%) it does not occur in its metallicstate, and only within the last hundred years has its production been developed.
Napoleon III of France had spoons and forks made from aluminium. The King of Siam, when he visited the French court, was delighted to receive a watch charm made from this rare metal, which was considered more precious than gold. At that time aluminium could only be manufactured by costly and labourious methods from bauxite and other salts of aluminium. . Bauxite ore is usually found near the surface and is mined in large quantities by open-cast working in Jamaica, U.S.S.R. France, Surinam, U.S.A., Guinea, Guyana, Greece, India and Australia.
Before smelting, the ore is refined by the concentration process devised by Karl Josef Bayer in 1890. In this process the almost pure aluminium oxide is dissolved in hot caustic soda while the impurities in bauxite are insoluble and are left behind. After drying the pure aluminium oxide is then converted into aluminium by electrolysis.
The electrolytic process requires a powerful electric current: for this reason aluminium reduction plants are often situated near fast flowing rivers where cheap hydro-electric power is developed. The electrolytic method was developed by Charles Hall in America and Paul Heroult in France. Although they worked independently they both arrived at the same solution to their problem.
They found that a mixture of 10% to 15% alumina in a mineral known as cryolite would become molten at i,ooo°C when an electric current was passed through it. When the mixture was kept molten by the heat from the electric current the aluminium oxide split into aluminium and oxygen.
Cryolite is. mined in Ivigtut in south-western Greenland. In recent years cryolite has been made synthetically.
Figure 8 shows an aluminium furnace.
Aluminium is an important engineering material. It is light and will not easily corrode in the atmosphere. This resistance to corrosion is due to the oxide film that rapidly forms on the surface so that the aluminium becomes insulated from further attack. Commercially pure aluminium is relatively soft but this lack of strength is overcome by alloying
WORKSHOP DISTINGUISHING TESTS FOR FERROUS METALS
| Test | Mild Steel | Carbon Steel 0-4% C to 1.5%C | Cast Iron | High Speed Steel | Wrought Iron |
| Grind on emery wheel | Bright yellow sparks with a few starlike sparks | Streams of bursting starlike sparks | Non-bursting dull red sparks close to wheel | Dull red sparks, some cling to the wheel | Bright non-bursting sparks |
| Sound— drop on anvil | Medium pitch metallic sound | Ringing high pitch sound | No ring dull note | Ringing sound not as high as carbon steel | Almost no ring. A little higher than cast iron |
| Make red hot and hammer | Works readily | Not as malleable as mild steel | Breaks up under the hammer | Does not work readily | Works readily |
| Make red hot and cool slowly | No effect | Maybecomesofter | No effect | May become brittle (depends on kind of steel) | No effect |
| Make red hot and quench | No effect | Becomes hard and brittle | May crack otherwise little change | Becomes hard and brittle | No effect |
| File | Files easily | Difficult | Filings dark, skin hard | Difficult | Files easily |
| Nick thin bar and hammer in vice | Bends then breaks. Grey crystalline fracture | Bends little then breaks. Fine silver crystalline fracture | Breaks easily. Light grey crystalline fracture | Bends little then breaks. Fine blue greycrystalline fracture | Bends well. Coarse grey fibrous fracture |
| Turn in lathe | Turns well. Swarf long and curly | Less easy than mild steel. Swarf breaks into short chips | Under hard skin, turns well. Chips crumble | Less easy than mild steel. Swarf long | Turns easily but finish is poor. Swarf long and curly |
Aluminium Alloys. Many alloys have been developed but they are used in two forms—wrought and cast. The wrought forms are those that have been forged, rolled, extruded, pressed or drawn. The cast form is made by gravity or pressure casting the molten metal in steel moulds or alternatively the metal can be poured into sand moulds which can only be used once.
Most of these alloys are classified by the British Standards Institution. They are referred to by code letters and numbers. A popular casting alloy used in the motor industry contains 7% copper, 3% zinc and 3% silicon. Another much used alloy contains 3% copper, 5% silicon and about 0-5% manganese.
Wrought components are made from an alloy containing small quantities of manganese or magnesium or magnesium and silicon, or for high strength they may contain copper or zinc.
Some alloys, containing approximately 425% copper and 0-5% magnesium, after heating and quenching in water will become harder and stronger after a few days. This is known as age hardening. Age hardened aluminium alloy is used in large quantities in the aircraft industry but because these alloys are not good corrosion resisters they are covered on each side with a thin layer of pure aluminium. This is known as cladding.
Cladding is performed by rolling a thin sheet of pure aluminium in each side of the alloy sheet. The total thickness of the pure aluminium front and back is about 10% of the thickness of the sheet.
Copper. Copper is almost without doubt the first metal to have been used for everyday articles. It is thought to have been first smelted in 3500 B.C., but even before this the metal, which can be found in the pure state (native), was hammered into useful shapes.
Copper has a reddish-gold appearance; it is malleable; it can be drawn into wire as fine as o-ooi inch diameter and rolled into thin sheets, forged, pressed, beaten or spun. It is an excellent conductor of heat and electricity and can be readily soldered, brazed or welded and it is resistant to many forms of corrosion.
Copper is crushed from ores which contain no more than 4% copper. These are ground to a fine powder and the copper bearing grains are separated by the notation method. The flotation process is worthy of note, not only because it is widely used, but because it is unusual in that the dense metal compounds are caused to float in a bath of frothed-up liquid, while the unwanted minerals are wetted and sink to the bottom. A reverberatory furnace is then used to remove more of the impurities. This leaves a mixture of copper and iron sulphide known as the matte. This is heated in a converter, which is similar to the Bessemer converter, to remove the iron and sulphur and then it is finally refined either by fire refining or by electrolytic refining.
Fire refining is a process whereby the impure copper is melted and some impurities burnt by oxidation. After the slag has been removed, hardwood poles are forced into the molten metal to remove the oxygen by combustion. This is known as poling; the copper is then poured into moulds.
Electrolytic refining produces high purity copper. In this process large pieces of impure copper (anodes) are suspended in dilute sulphuric acid. These are interleaved with thin cathodes of pure copper. By electrolytic action copper from the anodes is deposited on to the cathodes and the impurities go to the bottom of the bath.
Brass. This is the best known copper alloy consisting of copper and up to 40% zinc. Apart from zinc some brasses may contain small quantities of aluminium, lead, manganese, silicon and tin.
Cartridge Brass or Best Brass, from which cartridges and shells are made, contains 70% copper and 30% zinc (usually referred to as 70/30 brass) and is the most ductile of the cold working brasses.
Admiralty Brass is similar to cartridge brass except that it contains up to 1% tin which increases its corrosion resistance.
Gilding Metal contains copper and between 20% and 5% zinc. It derives its name from its golden colour. It is used for decorative work on buildings and for jewellery and beaten metalwork.
Muntz Metal or Yellow Metal contains 40% zinc. It is used for casting and for hot working operations such as rolling and extrusion and as a brazing alloy for steels.
Bronze. Properly speaking it is an alloy of copper and tin, but it has come to include alloys such as silicon bronze and aluminium bronze which contain no tin and others such as manganese bronze which contain only a small amount. Bronze was used for weapons as long ago as 2000 b.c. In parts of the world where tin and copper ores exist together it was probably first smelted by accident. Later copper and tin ores were smelted together in varying proportions to produce bronze for particular purposes. Bronze is harder than copper, it casts well, has good corrosion resisting qualities and good wearing qualities.
Phosphor Bronze contains 3.75% to 12% tin and between o.1% to 0.5% phosphorus. This is a good bearing material but as the phosphorous content increases so the ductility decreases.
Lead Bronze containing about 30% lead has been used with great success on aero engines. The lead in the bearings acts as a metallic lubricant when the oil film breaks down. Small amounts of lead (about 1 %) are added to bronze to improve its machinability.
Aluminium Bronze, containing 5% to 10% aluminium, is produced in strip, wire, rod and tube forms. This alloy is used as a substitute for steel where strength and non-magnetic properties are required. It is also used for ships' fittings and components which need to be corrosive resistant.
Gunmetal was used, in early times, for casting cannon. It is a bronze which contains a small amount of zinc and is excellent for casting. Admiralty gun metal contains 88% copper, 10% tin and 2% zinc and is widely used for marine purposes. A gun metal which contains 85% copper and 5% each of tin, zinc and lead (85-5-5-5) is a much used casting material in foundries.
Tin. It is smelted from cassiterite which is a tin oxide known also as tinstone. It is mined in Malaya, Indonesia and Bolivia. The tin ore is washed out from the tin deposits, most of which are alluvial. The ore is then crushed and roasted in a rever-beratoiy furnace to remove the sulphur and arsenic present. Then it is washed and mixed with slaked lime and anthracite and smelted in a revcrberatory furnace. The crude tin is then poured into moulds. Later this is purified in another rever-beratory furnace in which most of the impurities are skimmed off as slag.
Tin is a shiny white metal which is very soft and weak and has a low melting point (232°C) and has excellent corrosion resisting properties. The "cry of tin" is the name given to the sound a piece of tin makes when it is bent backwards and forwards.
The most common use of tin is in tinplate, which consists of steel sheets coated on each side with a thin layer of tin from which containers are made to store food, paints, lubricants and many other commodities.
Almost 50% of the tin produced goes into the making of tin-plate. The mild steel sheets before being coated with tin are hot rolled to 007 inch thick, then cleaned by pickling in dilute sulphuric acid, then cold rolled to about o-oi inch thick. After annealing it is rolled again which gives the sheet the correct surface finish and degree of hardness. The prepared sheet is then tinned either by electro-deposition or by hot dipping. The former process accounts for about three-quarters of the world production.
The electric process is carried out in large automatic plants which can make tinplate at between 500 and 1,700 feet per minute; the tin being in long strips which are coiled. In the hot dipping process the sheet steel is cut into small pieces which are passed through a bath of molten tin equipped with rollers which remove the excess tin on the surface. The thickness of the tin coating by this process is between 60 and 100 millionthsofaninch thick and by the electric process between 15 and 60 millionths.
Tin Lead Solders (see Chapter 4 on joining metals).
White Metal Alloys. These are alloys chiefly of tin, lead, copper and antimony. The best known white metal is Babbits metal which consists of 88% tin, 4% copper and 8% antimony. This is used for bearings. Apart from bearing metal the term white metal includes pewter, printers' alloys and solders.
Type Metal is an alloy used for printing type and consists of 50% to 85% lead, up to 25% tin and between 10% and 30% antimony. This has the unusual property of expanding on cooling, thus it forces itself into every corner of the type mould giving an excellent casting.
Pewter is a white metal and consists, today, of approximately 94% tin, 4% to 5% antimony and 1 to 2% copper. This alloy is also known as Britannia metal. It can be cast, rolled or spun and is suitable for food vessels. Ancient pewter was composed of lead and tin and was used for utensils and coins. Between the fourteenth and eighteenth centuries it was widely used for plates and other vessels. In this country the pewterers had their own guild and "touch mark", which was a hallmark and ensured a certain standard of workmanship and quality of material.
Zinc. This is a blue-grey metal which is obtained from zinc blend (a sulphide) mined chiefly in North America and Australia. The metal does not occur in its native state but is obtained either by electrolysis or distillation. Zinc was first made in Sumatra and China and was not commercially produced in England until 1740.
In the electrolytic process concentrates of zinc are dissolved in sulphuric acid and after purification deposited electro-lytically on aluminium sheets from which it is later stripped and cast into slabs. This method produces an almost pure metal. The distillation or thermal process is one in which a briquetted mixture of roasted ores and bituminous coal is heated in a vertical retort made from silicon carbide bricks, in which reduction can proceed continuously. The zinc forms as a vapour and is caught as a liquid metal in condensers.
A large proportion of zinc produced is used for making brass. Another important use is for coating iron—galvanising—which gives excellent protection against rust. It is also used in the form of zinc-base alloys for die casting.
Lead. It is a heavy bluish-grey metal (specific gravity 11-3) which is easily fusible and lacks strength. It is obtained chiefly from the lead ore, galena. After the ores arc crushed they are separated from the impurities by the flotation process and after roasting they are reduced in a blast furnace.
Lead was used in antiquity; the Romans made extensive use of it for lining baths and making water pipes, the seams of which they fused together by pouring hot lead along the prepared joint. This was known as "lead burning". "Plumbum" was the name they used for lead and our present-day workers in lead arc known as plumbers. Lead has a low melting point (327°C) and casts easily from the molten state. It is used for roof covering and water pipes and for fonts and rainwater heads. It resists acid well and is used for containers for these corrosive liquids. A more recent use is for shielding against radiation in' atomic energy establishments. It is estimated that at present 15,000 tons of lead per year are used for this purpose in Britain.
Nickel. This is a silvery white metal which takes a good polish and is resistant to corrosion and is magnetic. It can be cast, forged, welded, brazed and hard soldered; and it is strong both at low and high temperatures. Its melting point is 1,454°C, nearly as high as iron. Eighty-five per cent of the world's output of nickel is obtained from ores mined in Sudbury, Ontario.
The ores usually contain copper and iron and sometimes precious metals. The name "nickel" is German and is derived from "Kupfer-Nickell" which might be translated as "devil's copper". It was so called by miners in the Hartz Mountains
when they produced a nickel-copper alloy when struggling to obtain copper. The original process for separating nickel from its ores was developed in 1751. This process consisted of melting metallic sulphides with nitre cake which, when cooled, formed two separate layers, the top one containing copper and the bottom nickel. Since then nickel production has been greatly improved. Nickel is used chiefly as an alloy in ferrous and non-ferrous metals and as a pure metal in plating.
Nickel Silver. In China, two thousand years before nickel was isolated, an alloy called "paktong" was made by melting a mixture of copper-nickel ore and zinc ore. A number of alloys of this type were made in Europe in the early part of this century and were known as German silver.
Nickel silver with 5% to 35% nickel, 50% to 60% copper and 15% to 35% zmc 's used, because of its colour and resistance to corrosion, for tableware which is silver plated and known as E.P.N.S. (Electro Plated Nickel Silver). It is widely used for contact springs in telephone exchanges.
Gold. This is the precious metal of antiquity. Its malleability has been mentioned, but it is also the most ductile metal: one grain Troy can be drawn into wire 1 1/2 miles long. The pure metal is too soft for general use so it is hardened by alloying. The term "carat" used in connection with gold means a 24th part: e.g., 18 carat gold has 18 parts by weight pure gold and 6 parts alloying elements. The alloying elements are often 3 parts copper and 1 part silver.
Silver. It is the best conductor of heat and electricity; it is malleable and ductile and its pleasant white colour makes it a pleasure to handle. Sterling or standard silver has contained, since 1696, 92-5% pure silver. Silver can be cast or wrought and it is available in sheet, strip and tube form. Apart from its use in silver-smithing it is used in large quantities for making silver solders.
Platinum. This is white in colour and is malleable and ductile. It is superior to gold in its resistance to corrosion. Because of its colour and strength it is used for making the fine settings in jewellery for precious stones, particularly diamonds.
