alloys melting

Значение термина alloys melting в knolik

alloys melting - Alloys Melting
alloys melting - (a) This operation may be carried on in an earthenware crucible, when small quantities are being operated upon; but when large masses of metal have to be dealt with, as in the case of statues, etc., a reverberatory furnace must be employed to effect the melting. As a rule, the least fusible metal is placed in the crucible first, unless it be in very small quantity, and will dissolve readily in the other metal, in which case it goes in last; and if, as in the case of zinc, the volatilisation of the metal be extremely rapid, it is introduced only the moment before the fused mass is ready to be poured into the mould or other receptacle. The order in which the metals are melted has a material effect upon the nature of the resulting alloy, for it has been proved by experiment that the latter often possesses different properties when the mixing has taken place in a different order. The fused metals should be kept thoroughly well stirred up until the mixture is complete; otherwise the heaviest metal will sink to the bottom of the mass, and the alloy will not be of uniform composition. This contingency is sometimes avoided by melting the mass a second time. When three metals have to be united together, they should first be melted in pairs, and afterwards together.

(b) Guettier gives the following suggestions on the subject of fusing the metals: (1) The melting-pot should be red-hot (a white heat is better), and those metals first placed in it which require the most heat to fuse them. (2) Put the metals in the melting-pot in strict order, following exactly the different fusing points from the highest degree of temperature required down to the lowest, in regular sequence, and being especially careful to refrain from adding the next metal until those already in the pot are completely melted. (3) When the metals fused together in the crucible require very different temperatures to melt them, a layer of charcoal should be placed upon them, or if there is much tin in the alloy, a layer of sand should be used. (4) The molten mass should be vigorously stirred with a stick, and even while pouring it into another vessel the stirring should not be relaxed. (5) Use a little old alloy in making new, if there is any on hand. (6) Make sure that the melting-pots are absolutely clean and free from traces of former operations.

(c) Workmen who are unaccustomed to mixing or treating metals while in a liquid state will generally melt such metal upon a blacksmith's forge by applying heat so rapidly that the ladle will become red-hot before the metal within it begins to melt. When it has melted, a dross rises to the surface, and is skimmed off by the workmen and thrown away. The skimming process is kept up as long as the ladle remains on the fire. Now, such a course is wrong, because, by applying heat too suddenly, the metals which fuse at lower degrees of heat, sweat out, and are burned before those which melt at higher temperatures become fluid. The dross, as it is commonly called, which rises to the surface, is in many cases, the antimony, or hardening property of the alloy, and should not be thrown away. The surface of the melted metal should be kept covered with fine charcoal, which will prevent oxidation. A small lump of sal-ammoniac should also be kept upon the surface of the metal. The metal should always be stirred before pouring otherwise the heaviest metals will separate and sink to the bottom of the ladle, and a constantly varying quality of metal will be the result. By melting the metal slowly, and keeping it properly fluxed as described, it will run sharp; each casting will be found uniform throughout, and the metal will be of equal hardness. In observing these simple precautions, much of the dissatisfaction now experienced in using antifriction alloys will disappear.

(d) Manganese Alloys. - In practice, the copper should be first melted in a crucible in the ordinary manner, and the spiegel-eisen or ferro-manganese, either with or without the addition of wrought-iron scrap, should at the same time be melted in a separate smaller high temperature furnace, in a plumbago (graphite) crucible, under powdered charcoal; when it is completely fused, the copper also fused and at a boiling heat, the ferro-manganese should be poured into the copper, and the two well mixed together by stirring with an iron rod previously made red hot; the tin, zinc, or both should then be added in the usual way, and in the requisite proportions according to the kind of alloy it is required to produce. After the tin and zinc are added, the metal should be again well stirred with a red-hot rod, and skimmed; it may then either be poured into ingot moulds for future use, or it can at once be cast into moulds to produce any articles required.

While most experimenters have succeeded in combining manganese and copper by simultaneous reduction from their respective oxides - Heusler Brothers, of Dillenburg, recognised a greater advantage in reducing metallic manganese from pure pyrolusite for itself, and afterward alloying it in any required proportion with other metals. The reduction takes place in large plumbago (graphite) crucibles, with an admixture of carbon and of very basic materials, by which after 6 hours' smelting in a powerful coke fire, "crude manganese" is obtained, this containing 90 to 92 per cent, manganese, 6 to 6.5 carbon, 0.5 to to 1.5 iron, and 0.5 to 1.2 silicon. The crude metal can be refined to contain 94 to 95 per cent, manganese, when it is remelted with a suitable flux, and this metal contains only combined carbon, while in its crude state graphitic carbon also is almost always present. The refined metal is white, with crystalline fracture, and it oxidizes slowly when exposed to damp air; it is, therefore, soon combined with copper, thus forming "manganese copper," with 70 parts copper and 30 manganese. The alloy is cast either in ingots or shot, and becomes a commercial article in this state; its fracture is of steel-grey colour and very close, and it is not difficult to combine it in any proportion with other metals or alloys, such as brass, bronze, gun-metal, bell-metal, yellow-metal, and others. The same combination has been found a very powerful "physic" in refining copper, because the manganese will take up all oxygen which is absorbed by the bath of refined copper in the refining furnace before it is made tough, either by an addition of lead or by an insertion of a pole of green wood.

(e) In the formation of alloys in which one of the metals is more fusible than the other, the less fusible metal should be fused first, and the more fusible metal added either in the molten or solid state. As the fusible metals are added, the temperature of the alloy should be reduced, to prevent oxidation or burning away of the fusible metals; for this reason, it is better to add the more fusible metals in the solid state, as by so doing the temperature of the metal is decreased. Alloys are always more fusible than the less fusible metals of which they are composed, and in some cases are more fusible than the most fusible metal they contain, as is the case of alloys of tin, lead, and bismuth. Some founders, in order to have the metal thoroughly united, first fuse the metals together, cast them into ingots, and remelt them for use. This practice is bad, for in the after-fusion there is always more or less of the more fusible metal burned away; and ft is hard to determine the proportions of the alloy, or to have any certainty as to the quality of the castings. In melting ingots or scrap-alloys, they should be fused as rapidly as possible, and at the lowest available temperature, so as to avoid oxidation.

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