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CAST-IRON AND IRON CASTINGS.
The Brands of Iron used in foundries for ordinary castings are Nos. 1 2, 3, and 4, which are grey cast-irons. The quality of the iron can be judged by inspecting the fracture. When the colour of the fracture is a uniform dark grey with high metallic lustre, the iron is tough; but when the colour is dark grey, mottled, and without lustre, it is very weak. When the colour is lightish grey, with high metallic lustre, the iron is tough and hard; but when the colour is light grey, without metallic lustre, it is hard and brittle. When the colour is dull white, the iron is harder and more brittle than the last named one. When the colour is greyish white, with small radiating crystals, the iron is extremely hard and brittle, No. 1 has a dark grey fracture, with high metallic lustre; it is more fusible and more fluid than the others; but being deficient in hardness and strength, it is only suitable for very light castings. Nos. 2 and 3 are used for ordinary castings, the colour being a lighter grey, with a less degree of lustre than No. 1. The Brands used for the manufacture of wrought-iron are Nos. 4, 5, 6—grey forge-iron; No. 7 is a mottled iron; and No. 8 is a white castiron. Strength of Cast-iron.—The average strength of cast-iron to resist a crushing or breaking strain of compression is 42 tons per square inch of section, and its safe working strength in compression free from flexure is:– for cast-iron pillars, girders, and similar castings carrying dead weights, oth the breaking strain, or 7 tons: for pillars and machinery subject to vibration, *th, or 5} tons; and for cast-iron arches, Toth of the breaking strain, or 3 tons per square inch of section. The average tensile strength of cast-iron, is 6 tons per square inch of section, and its safe working strength in tension, is #th the breaking strain, or 1% tons per square inch of section. Testing Cast-iron.—A bar of good cast-iron, 1 inch square x 3 feet 6 inches long, placed upon supports 3 feet apart, should bear a gradually applied weight of 7 cwt. In contracts for castings, it is usual to specify the weight which a test-bar, cast from the same metal as the castings, shall carry, the usual stipulation being that a test-bar of cast-iron, 3 feet 6 inches long x 2 inches deep × 1 inch thick, placed upon supports 3 feet apart, shall bear in the middle a gradually applied weight of from 27 to 30 cwt., which will cause a deflection of about $ inch. The permanent set, caused by the deflection, is not taken notice of. These test-bars generally break, when a weight of from 31% to 32 cwt. is applied in the middle. The average breaking strain is usually taken of several test-bars, to guard against the effect of flaws in the castings. Cast-iron should be twice run, of fine grain, uniform, and of even grey colour, easily filed, and soft enough to be slightly indented when struck with a hammer. Castings.-The mixtures of cast-iron, found in practice to be most suitable for different kinds of work, are given in the following table.
Table 73.—Mixtures of METAL For various CAST-IRON CASTINGs.
Very tough and hard ) Hematite, No. 3 . - - . I part. cast-iron, for anvils, Pontypool, No. 4 . - - . . . , , for steam hammers, [ Clyde, No. 4 • I or and similar work . .) | Monkland, No. 8 I , , Chilled cast - iron Hematite, No. 5 . - - - . 5 parts. rolls, a mixture || 1 is jji. C. B 5 which chills about \| Ci. hit." - . . . - - 4 -# inches deep - - - - yo Chilled cast - o: #. §"; - - - - - o parts. rolls, a mixture ... ... . - - - - xx which chills about o . . . . . . . !, x 1} inches deep . . Pontypool white 4 Chilled cast - iron Hematite, mottled I part. rolls, a mixture Hematite, No. 5 . - - - which chills **) Blaenavon or Pontypool, C. B. . . 1 ,,
The strength of cast-iron is increased by remelting, up to Io meltings.
GUN-METAL AND BRASS CASTINGS.
Brass-Furnace.—A simple and effective brass furnace is shown in Fig. 147. It is 15 inches square x 28 inches deep inside. Hole for flue, 7 inches x Io inches. Chimney, Io inches square inside by not less than 15 feet high; the furnace to be built of brick, lined with firebrick; the front fire-bar bearer is moveable, and slides forward to let the fire-bars drop down, when required. This furnace will melt about 80 lb. of metal quickly and easily. A. shows the tongs for pouring the metal, and B. the tongs for iifting the crucible off the fire.
Brass-Melting.—The process of melting may be briefly described thus. After the fire is lighted, the crucible is placed over it, upside
down, until properly heated, when it is put in its place with its bottom resting on a firebrick, to keep it off the bars. Coke is then filled round to steady it. Copper cut into small pieces is then placed in the crucible and melted. Afterwards tin is added, melted and mixed. When the metal comes to a proper heat for casting, if a piece of zinc be dropped into the crucible, it will immediately flare up; if it does not flare up, the metal is not at its proper casting heat. When ready, the rubbish is skimmed of the top, and the metal is poured into the moulds. The
moulding-boxes are opened as soon as the metal is poured, and the
per square inch is about 60,000 lbs., and consists of various proportions, one of which is: copper, 6o parts; zinc, 35 parts; tin, 2 parts; wroughtiron, 3 parts. Muntz Metal consists of: copper, 3 parts: zinc, 2 parts. It is used for sheathing ships. Tenacity, 49,000 lbs. per square inch. Malleable Brass can be forged either hot or cold. Consists of: copper, 56 parts; zinc, 42 ; wrought-iron, 2 parts. Phosphor-bronze is a superior metal for bearings, wheels, and other castings, where great strength, toughness, and durability are required. The tenacity per square inch of the toughest quality is about 56,000 lbs. : great care is required in the production of castings from this alloy. Unlike ordinary bronze, it can be remelted without injuring its quality. A steel journal well fitted into phosphor-bronze bearings is much less liable to heat than any other known materials that can be used for such a purpose for heavy work. A Non-corrosive Bronze is manufactured by the Phosphor Bronze Company, in sheets, rods, and tubes, and also in wire for overhead telegraph and telephone-wires and springs. Its tenacity when rolled and drawn into wire is from 100,000 to 150,000 lbs. per square inch. Silicium-Bronze is a new special alloy, manufactured by the Phosphor Bronze Co. for electric conducting wire. It can be made to possess the strength of best iron wire with the conductivity of pure copper, or the strength of steel wire with twice its conductivity. Compressed Bronze.—The compression of the metal while in a fluid state, by closing the blow-holes, caused by the formation of gas, increases the density and tenacity of the metal. The tenacity of compressed bronze is about 65,000 lbs. per square inch. Ormolu is a metal used for ornaments of stoves and artistic metal work. It can be got up by finishing to a brilliant gold-like surface. It consists of from 2% to 3 parts of copper, according to the depth of colour required, to 1 part of zinc. The castings after being polished are dipped in acid, and then brightened by means of a wire scratch-brush, and finally lacquered to prevent tarnishing. Rolled and Wire-Drawn Brass is stronger than cast brass. The metal during these processes becomes dense and hard, and requires to be frequently annealed, which is effected by heating the metal and allowing it to cool slowly. The tenacity of the best quality of brass wire is 80,000 lbs. per square inch.
Brass Work.-The proportions of a variety of the alloys found in practice to be most suitable for different kinds of work, are given in the following table, containing too different alloys.