XVII Materials Used in Machine Construction
Cast Iron.—The essential constituents of cast iron are iron and carbon, the latter forming from 2 to 5 per cent. of the total weight. Cast iron, however, usually contains varying small amounts of silicon, sulphur, phosphorus, and manganese.
In cast iron the carbon may exist partly in the free state and partly in chemical combination with the iron.
In white cast iron the whole of the carbon is in chemical combination with the iron, while in grey cast iron the carbon is principally in the free state, that is, simply mixed mechanically with the iron. It is the free carbon which gives the grey iron its dark appearance. A mixture of the white and grey varieties of cast iron when melted produces mottled cast iron. The greater the amount of carbon chemically combined with the iron, the whiter, harder, and more brittle does it become.
The white cast iron is stronger than the grey, but being more brittle it is not so suitable for resisting suddenly applied loads. White iron melts at a lower temperature than grey iron, but after melting it does not flow so well, or is not so liquid as the grey iron. White iron contracts while grey iron expands on solidifying. The grey iron, therefore, makes finer castings than the white. Castings after solidifying contract in cooling about 1⁄8 of an inch per foot. Castings possessing various degrees of strength and hardness are produced by melting mixtures of various proportions of white and grey cast irons. White cast iron has a higher specific gravity than grey cast iron.
Cast iron gives little or no warning before breaking. The thickness of the metal throughout a casting in cast iron should be as uniform as possible, so that it may cool and therefore contract uniformly throughout; otherwise some parts may be in a state of initial strain after the casting has cooled, and will therefore be easier to fracture. Re-entrant angles should be avoided; such should be rounded out with fillets.
The presence of phosphorus in cast iron makes it more fusible, and also more brittle. The presence of sulphur diminishes the strength considerably.
The grey varieties of cast iron are called foundry irons or foundry pigs, while the white varieties are called forge irons or forge pigs, from the fact that they are used for conversion into wrought iron.
Amongst iron manufacturers the different varieties of cast iron are designated by the numbers 1, 2, 3, &c., the lowest number being applied to the greyest variety.
Chilled Castings.—When grey cast iron is melted a portion of the free carbon combines chemically with the iron; this, however, separates out again if the iron is allowed to cool slowly; but if it is suddenly cooled a greater amount of the carbon remains in chemical combination, and a whiter and harder iron is produced. Advantage is taken of this in making chilled castings. In this process the whole or a part of the mould is lined with cast iron, which, being a comparatively good conductor of heat, chills a portion of the melted metal next to it, changing it into a hard white iron to a depth varying from 1⁄8 to 1⁄2 an inch. To protect the cast-iron lining of the mould from the molten metal it is painted with loam.
Malleable Cast Iron.—This is prepared by imbedding a casting in powdered red hematite (an oxide of iron), and keeping it at a bright red heat for a length of time varying from several hours to several days according to the size of the casting. By this process a portion of the carbon in the casting is removed, and the strength and toughness of the latter become more like the strength and toughness of wrought or malleable iron.
Wrought or Malleable Iron.—This is nearly pure iron, and is made from cast iron by the puddling process, which consists chiefly of raising the cast iron to a high temperature in a reverberatory furnace in the presence of air, which unites with the carbon and passes off as gas. In other words the carbon is burned out. The iron is removed from the puddling furnace in soft spongy masses called blooms, which are subjected to a process of squeezing or hammering called shingling. These shingled blooms still contain enough heat to enable them to be rolled into rough puddled bars. These puddled bars are of very inferior quality, having less than half the strength of good wrought iron. The puddled bars are cut into pieces which are piled together, reheated, and again rolled into bars, which are called merchant bars. This process of piling, reheating, and re-rolling may be repeated several times, depending on the quality of iron required. Up to a certain point the quality of the iron is improved by reheating and rolling or hammering, but beyond that a repetition of the process diminishes the strength of the iron.
The process of piling and rolling gives wrought iron a fibrous structure. When subjected to vibrations for a long time, the structure becomes crystalline and the iron brittle. The crystalline structure induced in this way may be removed by the process of annealing, which consists in heating the iron in a furnace, and then allowing it to cool slowly.
Forging and Welding.—The process of pressing or hammering wrought iron when at a red or white heat into any desired shape is called forging. If at a white heat two pieces of wrought iron be brought together, their surfaces being clean, they may be pressed or hammered together, so as to form one piece. This is called welding, and is a very valuable property of wrought iron.
Steel.—This is a compound of iron with a small per-centage of carbon, and is made either by adding carbon to wrought iron, or by removing some of the carbon from cast iron.
In the cementation process, bars of wrought iron are imbedded in powdered charcoal in a fireclay trough, and kept at a high temperature in a furnace for several days. The iron combines with a portion of the carbon to form blister steel, so named because of the blisters which are found on the surface of the bars when they are removed from the furnace.
The bars of blister steel are broken into pieces about 18 inches long, and tied together in bundles by strong steel wire. These bundles are raised to a welding heat in a furnace, and then hammered or rolled into bars of shear steel.
To form cast steel the bars of blister steel are broken into pieces and melted into crucibles.
In the Siemens-Martin process for making steel, cast and wrought iron are melted together on the hearth of a regenerative gas-furnace.
Bessemer steel is made by pouring melted cast iron into a vessel called a converter, through which a blast of air is then urged. By this means the carbon is burned out, and comparatively pure iron remains. To this is added a certain quantity of ‘spiegeleisen,’ which is a compound of iron, carbon, and manganese.
Hardening and Tempering of Steel.—Steel, if heated to redness and cooled suddenly, as by immersion in water, is hardened. The degree of hardness produced varies with the rate of cooling; the more rapidly the heated steel is cooled, the harder does it become. Hardened steel is softened by the process of annealing, which consists in heating the hardened steel to redness, and then allowing it to cool slowly. Hardened steel is tempered, or has its degree of hardness lowered, by being heated to a temperature considerably below that of a red heat, and then cooling suddenly. The higher the temperature the hardened steel is raised to, the lower does its ‘temper’ become.
Case-hardening.—This is the name given to the process by which the surfaces of articles made of wrought iron are converted into steel, and consists in heating the articles in contact with substances rich in carbon, such as bone-dust, horn shavings, or yellow prussiate of potash. This process is generally applied to the articles after they are completely finished by the machine tools or by hand. The coating of steel produced on the article by this process is hardened by cooling the article suddenly in water.
Copper.—This metal has a reddish brown colour, and when pure is very malleable and ductile, either when cold or hot, so that it may be rolled or hammered into thin plates, or drawn into wire. Slight traces of impurities cause brittleness, although from 2 to 4 per cent. of phosphorus increases its tenacity and fluidity. Copper is a good conductor of heat and of electricity. Copper is largely used for making alloys.
Alloys.—Brass contains two parts by weight of copper to one of zinc. Muntz metal consists of three parts of copper to two of zinc. Alloys consisting of copper and tin are called bronze or gun-metal. Bronze is harder the greater the proportion of tin which it contains; five parts of copper to one of tin produce a very hard bronze, and ten of copper to one of tin is the composition of a soft bronze. Phosphor bronze contains copper and tin with a little phosphorus; it has this advantage over ordinary bronze, that it may be remelted without deteriorating in quality. This alloy also has the advantage that it may be made to possess great strength accompanied with hardness, or less strength with a high degree of toughness.
Wood.—In the early days of machines wood was largely used in their construction, but it is now used to a very limited extent in that direction. Beech and hornbeam are used for the cogs of mortise wheels. Yellow pine is much used by pattern-makers. Box, a heavy, hard, yellow-coloured wood, is used for the sheaves of pulley blocks, and sometimes for bearings in machines. Lignum-vitæ is a very hard dark-coloured wood, and remarkable for its high specific gravity, being 11⁄3 times the weight of the same volume of water. This wood is much used for bearings of machines which are under water.