Значение термина alloys, brass в knolik
alloys, brass - Alloys, Brass
alloys, brass - (a) Brass is perhaps the most useful and important alloy known. Its composition varies widely with the uses for which it is intended, but its constituents are copper and zinc, usually in the proportions of nearly two parts of the former to one part of the latter. Brass may also contain small quantities of tin and lead. The qualities which render this alloy so valuable may be briefly enumerated as follows: It is harder than copper, and consequently better able to resist wear and tear. It is very malleable and ductile, and therefore admits of being either rolled into thin sheets, shaped with the hammer, drawn into fine wire, or raised by stamping into objects of various forms. It is readily fusible, and therefore easily cast at a lower temperature than copper. It resists the influences of the atmosphere better than copper, although, if unprotected by lacquer or varnish, it rapidly tarnishes and blackens on exposure to the air. 'Finally, brass has a fine yellow colour, and is capable of receiving a beautiful polish.
The malleability of brass varies with its composition and with its temperature; it is also affected, to a sensible degree, by the presence, even in minute quantities, of certain other metals. Some varieties of brass are malleable only when cold, others only when hot, and others, again, are never malleable. At a temperature just below its fusing -point, brass, like copper, is brittle, and may be powdered in a. mortar. Alloys of copper and zinc present a great variety of colour, ranging between the reddish hue of the former and the bluish-white of the latter; the transition is gradual, and passes through all the intermediate stages of yellow. The table represents the intensity of colour, hardness, and fusibility possessed by these different alloys.
Brass which is required for rolling into sheets should contain no antimony, as this metal renders the alloy very brittle, and extremely liable to crack. That which has to be turned contains invariably a small proportion of lead, usually about 2 per cent.; this addition is made when the crucible containing the fused metals is taken out of the furnace. The following is an analysis of a brass which is well adapted for this purpose: Copper, 65.8; zinc, 31.8; lead, 2.15; tin, 0.25. The presence of tin was believed to be accidental. Brass required for engraving upon should always contain a little tin, in order to render it sufficiently firm. Brass laminates well in the rolling mill cold, as long as it is kept sufficiently soft; but as by lamination the metal hardens and becomes brittle, it is necessary to restore its tenacity by annealing in an oven or reverberatory furnace. The same process of annealing is necessary in the manufacture of brass wire, which is obtained by drawing it through holes in steel plates, polished carefully and adjusted in series, graduated in size, so as not to diminish too rapidly, and thus render it necessary to employ so much power for drawing as would cause the breaking of the wire. Brass is not usually so prepared as to admit of its being hammered out, as is done in the manufacture of copper utensils; but a brass-foil or Dutch metal, of the colour and approaching the thinness of gold-leaf, is manufactured by beating out thin sheets of brass with hammers worked by water-power, making 300 or 400 strokes per minute.
The copper is first placed in the crucible, and the zinc is added to it bit by bit with much caution, as soon as the former metal is in a state of incipient fusion. The ingots of copper should be heated to redness before being put into the crucible. When the mixture is well fused together, the cinders are removed, and it is poured, if required for casting, into sand-moulds; if, on the contrary, it is to be used for rolling, it is cooled in close iron ingot-moulds, previously heated, oiled, and dusted lightly over in the interior with powdered charcoal. A loss of zinc invariably occurs by volatilisation, which is always taken into consideration when weighing out the metal.
(b) The following formulae show the composition of different varieties of For button brass, an alloy of 8 parts of copper and 5 of zinc is commonly used. An alloy paler in colour, and used for the common buttons, consists of 25 of copper, 20 of zinc, 8 of lead, and 2 of tin.
For fine brass, an alloy of 2 parts of copper with 1 of zinc is the correct proportion; the metals are melted separately, poured suddenly together, and united by vigorous stirring. By raising the proportion of copper to 7 parts of copper and 3 of zinc, a bright-yellow and malleable alloy is obtained; 4 of copper and 1 of zinc yields a metal of darker colour than the last.
Brass for fine castings is an alloy of 62 parts of copper, 35 of zinc, 2 of lead, and 1 of tin; this is rather pale and brittle. An alloy used for the same purpose, and of a deep, rich colour, consists of 90 copper, 7 zinc, 2 tin, 1 lead.
For gilding, good proportions are: 64 parts copper, 32 zinc, 3 lead, 1 tin. For malleable brass, good proportions are: 33 parts copper, 25 zinc, or, 3 copper and 2 zinc. These are malleable when hot.
For soldering, an alloy of 12 parts fine brass, 6 zinc, 1 tin, melted together, is most commonly employed.
For turning, the proportions are: 98 parts fine brass, 2 lead, both melted together; or, 65 copper, 33 zinc, 2 lead.
For wire, an alloy of 72 parts copper, 28 zinc, is commonly used; this alloy must be afterwards hardened by tempering.
(c) The best plan of making brass is to melt the copper in a black-lead crucible first, dry the zinc as much as possible, and immerse the whole of the zinc into the copper when the latter is not hotter than barely to continue fluid. Drop a piece of borax the size of a walnut into the pot. When the surface of the hot metal is kept covered by fine charcoal, or by borax, it is prevented from burning, and the smallest loss of zinc is sustained.
The melting together of tin and copper is less difficult than that of zinc and copper, because tin is not so liable to evaporate as zinc, and little metal is lost. The appearance of the alloy may be improved by covering the melted metal with about 1 per cent, of dried potash; or, better still, a mixture of potash and soda. This flux has a remarkable influence on the colour, and particularly on the tenacity of the alloy. The former becomes more red, and the latter stronger. The scum forming on the surface by this addition ought to be removed before the metal is cast. Tin and copper are liable to separation in cooling: this can be prevented, at least partly, by turning the mould containing the fluid metal, and keeping it in motion until it is chilled.
Ordinary brass for castings: 20 copper, 2½ tin, 1½ zinc.
Hard brass for castings: 25 copper, 4½ tin, 2 zinc.
Red brass for fine castings: 24 copper, 5 zinc, 1 bismuth; add the bismuth last before pouring off.
Red brass for turning: (a) 24 copper, 5 zinc, 1 lead; add the lead last before pouring off. (b) 32 copper, 10 zinc, 1 lead, (c) 160 lb. copper, 50 lb. zinc, 10 lb. lead, 44 oz. antimony.
Rolled brass: 32 copper, 10 zinc, 1½ tin.
Yellow brass: (a) 70 copper, 30 zinc. (b) 20 lb. copper, 10 lb. zinc, 1 to 5 oz. lead added just before pouring.
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