An Encyclopedia of the History of Technology part 16 pps

PART ONE: MATERIALS 132 the noble metal palladium in the soluble fraction was demonstrated by Wollaston in 1803, and he announced the discovery of another noble metal, rhodium, in the following year. Ruthenium, the sixth and last member of the platinum group was identified by Claus in 1844. Wollaston’s process of platinum production was probably the first to utilize a chemical extraction and refining process so controlled that it produced pure fine metallic powder which was specifically intended for consolidation by the powder metallurgical method. He soon became well known for the high quality and ductility of the platinum he produced in this way. In the early years of the nineteenth century, large quantities of platinum were needed for the construction of items of chemical plant such as sulphuric acid boilers. Between 1803 and 1820, Wollaston produced about 7000 ounces (197kg) of ductile platinum for sulphuric acid boilers alone. Platinum deposits were discovered in the Ural mountains in Russia in 1819 and rich alluvial platinum deposits were found in 1824. In 1825, deposits at Nizhny Tagil north of Ekaterinburg (Sverdlovsk) were found to yield 100 ounces of gold and platinum per ton (3.6kg per tonne) of gravel, and Russian platinum continued to satisfy the world’s demands until 1917, when political changes encouraged the extraction of this metal from the nickel deposits at Sudbury, Ontario (see p. 98). In 1924 platinum was discovered by Dr Hans Merensky in a South African reef near Rustenburg in the Transvaal, now known to be one of the world’s richest platinum deposits. Vast quantities of copper, nickel and cobalt, and also gold and silver, are also associated with the rich metalliferous vein. The very large cupro-nickel ore deposits discovered at Jinchuan in Ganzhou Province in China in 1958 also contain considerable quantities of both gold and platinum. Rich palladium-platinum deposits have also been found in the Mitu area of Yunan Province which is only about 80km (50 miles) away from the nearest railway line and is therefore capable of being rapidly developed. Powder metallurgy in electrical engineering The first critical metallurgical requirement of the electrical engineers was a lamp filament which was more robust and could run at higher temperatures than the carbon filament of Edison (see Chapter 6). By the end of the nineteenth century it was appreciated that the light emitted by an incandescent filament varied as the twelfth power of its temperature. The most efficient lamp, therefore, was that which could operate at the highest possible temperature, and attention was concentrated upon the melting points of the most refractory metals then known. These melting points reached a peak at tungsten. Rhenium and technetium were unknown at that time and hafnium and zirconium had never been made in pure metallic form. NON-FERROUS METALS 133 Osmium, however, was known to have a high melting point and could readily be produced as an exceedingly pure finely divided powder. The use of osmium as a light filament material was first proposed in 1898 by Welsbach, ironically the inventor of the incandescent gas mantle. The ‘Osmi’ lamp, first produced in Berlin, although more economical in operation than the carbon lamp was more expensive and rather delicate. It was soon overtaken by the tantalum lamp which was introduced by Siemens and Halske of Charlottenburg in 1903. Although the melting point of osmium, 3050°C, was marginally higher than that of tantalum at 2996°C, the latter metal had the tremendous practical advantage of being ductile so that it could readily be drawn into fine wire. Ductile tantalum was first produced in 1903 by W. von Bolton. The gaseous contaminants in the nominally pure tantalum sponge were removed by melting buttons of the metal under high vacuum conditions. The prototype vacuum furnace used by von Bolton for this work is shown in Figure 1.12. These arc furnaces had water-cooled metal hearths, and were so successful that scaled-up versions were designed and used by Kroll during the Second World War for melting titanium (see p. 143). The tantalum lamp met with great and virtually instantaneous success. Between 1905 and 1911 over 103 million tantalum lamps were sold. Tungsten, which melted at 3422°C, was of course the ultimate lamp filament material, although tremendous practical difficulties were initially encountered in producing ductile tungsten wire. The first tungsten filaments, produced in 1904 by a squirting process similar to that used for the osmium filaments, produced a great deal more light than carbon filament lamps, but they were brittle and expensive. The process was developed by Just and Hanamann in 1904 at the Royal School of Technology in Vienna and lamps were produced in Britain from 1908 in the Hammersmith Osram-GEC Lamp Works, although the operation was never a commercial success. Ductile tungsten filaments were first produced by W.D.Coolidge of the US General Electric Company at Schenectady in 1909. The process he patented in 1909 is still used. Fine tungsten powder was pressed into bars about one square inch in cross-section. These bars were then sintered by heating them electrically in a pure hydrogen atmosphere to temperatures approaching their melting point. They were then reduced into wire by a process of hot fabrication within a well defined range of temperatures so that a fibrous structure was gradually developed within the rod or wire. See Figure 1.13. The Coolidge tungsten wire process was the first to make extensive use of the newly developed rotary swageing machine. Swageing was continued until the bar had been reduced to rod about 0.75mm in diameter. This was then hot drawn to wire through diamond dies. By 1914, over 100 million lamps had been sold in the United States alone, and manufacturing licences had been granted by the US General Electric Company to most of the developed world. The trade name Osram, first used by the Osram Lamp Works in Berlin, derives from the words osmium and PART ONE: MATERIALS 134 wolfram. After 1909 it became associated with those lamps using drawn tungsten filaments made in accordance with GE patents. After about 1913 ductile tungsten, and then ductile molybdenum, became the dominant materials of the electrical industry. When tungsten contacts were introduced the reliability of motor-car ignition systems improved considerably. The Coolidge X-ray tube, introduced in 1913, had a tungsten target and was the first to permit the long exposures required by the new technique of X- ray crystallography. Figure 1.12: The vacuum arc furnace first used in 1902 for the production of ductile tantalum. The pressed powder pellet, weighing 80–100g was melted on a water cooled nickel hearth by a direct current arc of 50–300 amperes. During melting, pressure in the furnace chamber increased from 5 × 10 -3 to 5 × 10 -2 mm of Hg. This process was devised by Dr W. von Bolton of the Charlottenburg lamp factory of Siemens and Halske. He was assisted in this development by the engineer Otto Archibald Simpson and also by Dr M.Pirani who later invented the thermal conductivity vacuum gauge. Between 1903 and 1912 more than 60 million tantalum lamps were produced at Charlottenburg. For this about one ton of vacuum melted tantalum was processed. The furnace diagram shown here was first published by Dr Pirani in 1952. Courtesy Vacuum. NON-FERROUS METALS 135 Between the wars, most of the serious metallurgical research in Great Britain was undertaken by firms such as Metropolitan Vickers, GEC, Telcon, Standard Telephones and the major cable companies. Bell Telephones, the US General Electric Company and Westinghouse exercised a similar influence in the United States. The influence of Siemens and Heraeus in Germany, and the Philips Laboratories at Eindhoven was equally profound. The healthiest and most vigorous offspring of this marriage between metallurgy and electrical technology was the sintered carbide industry which must, by any standards, be regarded as one of the major metallurgical innovations of the twentieth century. SINTERED CARBIDE CUTTING TOOLS Tungsten wire had to be hot drawn, and the deficiencies of the steel dies which were first used to reduce the diameter of the hot swaged tungsten rod to the dimensions of wire which could be handled by diamond dies soon became apparent. Tungsten carbide seemed to fulfil most of the characteristics of the material required. The extremely hard carbide W 2 C had first been prepared by Moissan in 1893 when he fused tungsten with carbon in his electric furnace. Sintered carbides were first produced in 1914 by the German firm of Voigtländer and Lohmann. These compacts, which were rather brittle, were produced by sintering mixtures of WC and W 2 C at temperatures close to their melting point. As Moissan had shown, tungsten carbide could be melted and cast from an electric arc furnace. Cast tungsten carbides dies were produced by this method before 1914 by the Deutsche Gasglühlicht Gesellschaft, although the cast product had a very coarse grain size and the dies were again very brittle. By 1922 Schröter of the Osram Lamp Works in Berlin had shown that tungsten carbide, sintered in the presence of a liquid cement could be very tough. The three metals, iron, cobalt and nickel all provided a satisfactory molten cement, although cobalt provided the best combination of hardness and toughness. The new sintered alloy was rapidly adopted in Germany for the manufacture of wire drawing dies and between 1923 and 1924 was used and sold by the Osram Group under the trade name Osram Hartmetall for wire drawing and also for cutting tools. Friedrich Krupp AG of Essen were granted a manufacturing licence for this exciting new sintered product in 1925 and introduced the first successful sintered cutting tools to the world in 1927. Widia (wie Diamant =like diamond) consisted of tungsten carbide powder sintered together with a cement consisting of about 6 per cent cobalt. It was exhibited in 1927 at the Leipzig Fair, where its ability to machine cast iron at unheard-of speeds was demonstrated. A.C.Wickman of Coventry acquired the sole rights to import Widia and sell it in the United Kingdom. In association with Wickman, Krupps started to manufacture tungsten carbide in Britain in 1931. PART ONE: MATERIALS 136 Figure 1.13 NON-FERROUS METALS 137 Krupp, originally the major shareholder in the Tool Manufacturing Company, eventually became the sole owner. When the Second World War started in 1939, A.C.Wickman took over control of the factory and changed its name to Hard Metal Tools Ltd. Manufacturing licences to produce sintered carbides had by the mid-1930s been granted by Krupp to British firms such as BTH Ltd, Metro Cutanit, Firth Brown Tools and Murex Ltd. The American General Electric Company acquired the sole American rights to the Widia process in 1928 and issued many manufacturing licences. From this operation emerged nearly all the well-known proprietary grades of carbide. In order to circumvent the German patents, firms such as Fansteel attempted in 1932 to introduce tantalum carbide tools which were sintered with nickel rather than cobalt. Such materials were inferior in toughness to the cobalt bonded composites and were never very successful. Other powder metallurgical innovations Other products of the powder metallurgy industry, which began to develop very rapidly after 1910, are too numerous to describe exhaustively. Sintered porous self-lubricated bronze bearings were introduced by the American General Electric Company in 1913, and this was followed by sintered metallic filters, first produced by the Bound Brook Oilless Bearing Company in 1923. Powder metallurgy was also used to produce a variety of magnetic alloys, since it was found that fine powders of metals such as iron, cobalt and nickel could be cheaply produced in a high state of purity by chemical methods. Such powders could be fully or partly consolidated into any desired shape by powder metallurgical methods without the contamination inevitably associated with conventional melting and alloying procedures. Permalloy, a very soft ironnickel magnetic powder, was developed in 1928 by the Bell Telephone Laboratories. In the 1940s when the domain theory of magnetization suggested that very fine powders could be used to produce permanent magnets of very high coercive force some very elegant powder metallurgical techniques were developed. Figure 1.13: The ductile tungsten wire, first produced by Coolidge of the General Electric Company of Schenectady in 1909 was obtained from bars of tungsten sintered in apparatus similar to that illustrated. After a preliminary low temperature sintering process, pressed bars of tungsten powder about in square crosssection were gripped between water-cooled copper electrodes. The lower electrode floated on a bath of mercury to allow for the considerable shrinkage which occurred as the bar sintered. The apparatus was then capped by the water cooled copper bell shown, and all air purged from the vessel by a flowing current of hydrogen. Sintering was accomplished by passing a heavy alternating current through the bar so that its temperature was raised to about 3300°C. Bars thus sintered were then hot worked by rotary swageing and finally drawn to wire in diamond dies. PART ONE: MATERIALS 138 Powder metallurgy also made it possible to produce, merely by mixing the appropriate powders, a whole range of composite materials which could have been manufactured in no other way. In this way a whole generation of new electrical contact materials was developed between the wars. These included, for example, composite contacts which incorporated insoluble mixtures of silver and nickel, silver and graphite, silver-tungsten, copper tungsten and, probably the most important, silver-cadmium oxide. The last formulation is still widely employed because it combines the low contact resistance of silver with the ability of cadmium oxide to quench any arcs formed when the contacts open under load. The compaction of powdered metals As in the time of Wollaston, metal powders are still compacted in steel dies since this is generally the cheapest and most convenient method of producing large quantities of components on a routine basis. The technical limitations imposed by die wall friction are generally accepted and various expedients have been devised to obtain more uniformly pressed components and to reduce the incidence of interior cracking caused by concentrated internal stresses in the compact. Considerable difficulties were encountered, however, in the early years of the powder metallurgy industry when the production of larger and more complex components was attempted. It was then appreciated that such products would most effectively be compacted under a pure isostatic pressure but it was not until 1930 that F.Skaupy devised a method of hydrostatic pressing which was simple, cheap and industrially acceptable. Figure 1.14 illustrates a typical arrangement which was used for pressing relatively thin-walled tungsten carbide tubes, an operation which would not have been feasible in a conventional steel die. The carbide powders were contained in a sealed rubber bag, which was then subjected to hydrostatic pressure from a fluid pumped into the pressure vessel shown. The shape and configuration of the powder compact was ensured by the use of polished steel tubes which were so arranged that they allowed the working fluid to act on the compact from all directions. Compacts with high pressed densities and low internal stresses were thereby obtained. With arrangements of this type, compaction pressures of the order of 9250 bar (60 tons per square inch) could be safely utilized. Pressures in a steel die were usually limited to about 2300 bar (15 tons per square inch) to avoid the formation of internal cracks in the compact. In the years immediately after the Second World War it was established by organizations such as the Nobel Division of ICI that large metallic compacts could be effectively compacted by surrounding them with a uniform coating of high explosive which was then detonated within a large bath of water. This technique was among those investigated for the consolidation of large ingots of NON-FERROUS METALS 139 titanium from the sponge then being produced by the Metals Division of ICI. The explosive approach, though versatile, is expensive and not well suited for routine manufacture, although its value in the research and development area has been amply demonstrated. Isostatic pressing is now widely employed for the manufacture of smaller components: vast quantities of pressings such as the alumina insulators of sparking plugs are economically produced in this way. In 1958 the Battelle Memorial Institute of Columbus, Ohio, began to modify and improve the isostatic pressing process so that it could be used to consolidate metal powders at high temperatures. See Figure 1.15. Laboratory gas pressing equipment was developed which allowed metals, alloys and ceramics to be consolidated in metal capsules at temperatures up to 2225°C and pressures up to 3900 bar (25 tons per square inch). The commercial units Figure 1.14: The first satisfactory method of consolidating metal powders under a hydrostatic pressure devised by F.Skaupy in 1930. PART ONE: MATERIALS 140 available in 1964 operated at similar temperatures, although routine operating pressures were limited to about 1100 bar (7 tons per square inch). Pressing was accomplished in a cold wall autoclave, with the furnace used for heating the compact enclosed within this chamber and thermally insulated from it so that the chamber walls remained cold. The unit was pressurized with an inert gas such as argon. HIP (hot isostatic pressing) is now extensively used to compact either one very large component or a multiplicity of relatively small specimens. Units with an ability to consolidate components up to 22 inches in diameter and 108 inches long (56cm×274cm) were being used in the USA in 1964 and Figure 1.15: The technique of hot isostatic pressing, introduced in 1960 by the Battelle Memorial Institute is shown in the second diagram. In this approach the metal or alloy powders are poured into shaped sheet metal cans which are subsequently evacuated and sealed by welding. The can and its contents are then subjected to the combined effects of heat and an external pressure of an inert gas in a cold-walled autoclave. The hot isostatic pressing ‘HIP’ process is now widely employed, particularly for consolidation of complex shapes from superalloy powders. NON-FERROUS METALS 141 much larger facilities are now providing routine service throughout the world. ‘Hipping’, as a metallurgical technique, is no longer confined to powder metallurgy. Many critical and expensive castings such as those used for gas turbine blading are routinely processed in this way to seal up and eliminate any blowholes or other microscopic defects which might possibly be present. Higher operating pressures are now available; the economics of the process are largely dictated by the fatigue life of the pressure vessel. TITANIUM AND THE NEWER METALS Titanium was first produced in metallic form in 1896 by Henri Moissan, who obtained it by electric furnace melting. His product, being heavily contaminated by oxygen, nitrogen and carbon, was very brittle. In the early years of the twentieth century ferro-titanium alloys were widely used for deoxidizing and scavenging steel. Metal containing less than 1 per cent of impurities had been made, however, and such material had a specific gravity of only 4.8g/cm 3 . The metal had a silvery-white colour and was hard and brittle when cold, although some material was forgeable at red heat. Widely varying melting points were initially reported, although work by C.W.Waidner and G.K.Burgess at the US Bureau of Standards indicated that the true melting point was probably between 1795° and 1810°C. The extraordinarily high affinity of titanium for nitrogen was noted in 1908 and this led to a number of proposals that the metal could be used for the atmospheric fixation of nitrogen. It was claimed that the titanium nitrides reacted with steam or acids to produce ammonia. Pure ductile titanium was first produced at the Schenectady Laboratories of the General Electric Company in 1910 by M.A.Hunter, who reduced titanium tetrachloride with sodium in an evacuated steel bomb. This approach was used for a considerable time for producing small quantities of relatively pure titanium for experimental purposes. Titanium of very high purity which was completely ductile even at room temperature was first made in 1925 by Van Arkell and De Boer who prepared pure titanium iodide from sodium reduced titanium and subsequently decomposed this volatile halide on a heated tungsten filament. It had a density of 4.5g/cm 3 , and an elastic modulus which was comparable to that of steel. For the first time titanium was seen as a promising new airframe material which was free from many of the disadvantages inherent to beryllium. W.J.Kroll began to work on titanium in his private research laboratory in Luxembourg in the decade before the Second World War, using an approach which was identical in principle with that employed by Wöhler in his work on aluminium in 1827 (see p. 102). He found that the metal obtained by reducing titanium tetrachloride with calcium was softer and more ductile than that obtained from a sodium reduction, and applied for a German patent for this . sheet metal cans which are subsequently evacuated and sealed by welding. The can and its contents are then subjected to the combined effects of heat and an external pressure of an inert gas in. BTH Ltd, Metro Cutanit, Firth Brown Tools and Murex Ltd. The American General Electric Company acquired the sole American rights to the Widia process in 1928 and issued many manufacturing licences the United States. The influence of Siemens and Heraeus in Germany, and the Philips Laboratories at Eindhoven was equally profound. The healthiest and most vigorous offspring of this marriage between