Supplement to the Fourth, Fifth, and Sixth Editions of the Encyclopædia Britannica/Annealing
ANNEALING. The nature and use of this process in Glass-making, has been already explained in the body of the work; it remains to add some details in regard to its application to metals. In the manufactures, in which the malleable metals are employed, annealing is used to soften a metal after it has been rendered hard by the hammer, and also to soften cast-iron, which is rendered very hard and brittle by rapid cooling.
In the manufacture of steel goods, which are first formed by the hammer, and require to be filed or otherwise treated, and in which softness and flexibility are essential to the change, annealing is absolutely necessary. This is particularly the case in making files and scissors; that the metal may be left sufficiently soft for cutting the teeth, and for filing off those parts which cannot be ground. Annealing is not less necessary in the drawing of wire, whether iron, copper, brass, silver, or gold. The operation of drawing soon gives the wire a degree of hardness and elasticity, which, if not removed from time to time by annealing, would prevent the extension of the wire, and render it extremely brittle. The same operation is also necessary in rolling or flatting those metals which are in a cold state, such as brass, silver, gold, &c. The Brazier who forms vessels of copper and brass by the hammer, can work upon it only for a little time, before he is obliged to anneal it.
The common methods employed for annealing iron and steel are very injudicious, and materially injure the latter, when it is used for making cutting instruments. After they have been formed by the hammer, they are generally piled up in an open fire, slowly raised to red heat, and then allowed as gradually to cool. By this method, the surface of the steel will be found considerably scaled, from the action of the oxygen of the atmosphere. When it is remembered, that steel consists of iron joined to carbon, it will be evident, that the steel immediately under the scaly oxyd will be deprived of its carbon, which has been carried off by the attraction of the oxygen; and in consequence, will lose the property, of acquiring that degree of hardness necessary to a cutting instrument.
Nothing, therefore, can be more obvious, than that steel particularly should be annealed in close vessels, to prevent that effect. For this purpose, the goods should be placed in a trough or recess made of fire-stone, or fire-brick, and stratified with ashes, or clean sand, and finally covered with a thick stratum of the same; but if the size of the vessel be small, it may have a cover of its own materials. This oven or trough must now be heated by the flame of a furnace passing under and round it till the whole is of a red heat. It must then be suffered to cool, without letting in the air. The goods so treated, will be much softer than by the common method. The surface, instead of becoming scaled, will have acquired a metallic whiteness, from the presence of a small quantity of carbonaceous matter contained in the ashes in which they were imbedded. They will become so flexible also, as to allow them to bend considerably without breaking, which is very far from being the case before the operation. The fracture, before annealing, will be smooth and short; but afterwards it will be rough, exhibiting bright parts of a crystalline appearance. Wire, especially that of iron and steel, should be treated in a similar way, when it is annealed. The wire used for some purposes, requires to be soft, and is sold in that state. If the wire after finishing, when it is bright and clean, were to be annealed in contact with oxygen, it would not only lose all its lustre and smoothness, but much of its tenacity. The process above-mentioned will therefore be particularly necessary in annealing finished wire, as well as in softening if from time to time during the drawing.
Copper and brass suffer much less than iron and steel from annealing in the open air, and do not require to be heated above a low red heat. If, however, the lustre is to be preserved, a close vessel would be desirable. The latter metals, after annealing, although much discoloured by the oxygen of the atmosphere, may be cleansed, by immersion in a hot liquor composed of water and a small quantity of sulphuric or nitric acid. Very small brass or copper wire is frequently annealed, by exposing it to the flame of hay or straw. In casting minute pieces of pig-iron, which is generally done in wet sand, the metal possesses the property of steel to such a degree, as to assume, by the rapid cooling, a degree of hardness equal to hardened steel; at the same time, that the articles are so brittle as to break by falling on the ground. When, however, these goods are treated in the way above directed, they acquire a degree of softness which renders them penetrable by the file, and at the same time capable of bending. In this state, they are much less tenacious than steel, but still so much so, as to have been sold in the form of cutlery for steel.
The change which metals undergo by annealing, is not yet thoroughly understood. Most of the malleable metals are susceptible of two distinct forms, one called the crystalline form, which they assume by slow cooling; and the other, the fibrous, which is acquired by hammering or rolling. When this, however, is carried beyond a certain point, the metal becomes so hard, that it is not capable of being bent far without breaking. All the malleable metals in the ingot or in their cast state are brittle, and exhibit a crystalline fracture. By hammering or rolling, they become more tenacious, and break with difficulty, exhibiting what is called a fibrous fracture. At the same time, they become stiffer and more elastic. They lose the latter properties by annealing, but become more malleable. If the annealing, however, be long continued, the malleability diminishes, and they again have a crystalline fracture. Zinc by wire-drawing becomes very flexible, and possesses a degree of tenacity not inferior to that of copper. But, if it be kept in boiling water for a length of time, it will resume its original brittleness, and show a crystalline appearance when broken. This proves that the particles of metals can change their arrangement, without losing their solid form;—which is still more strongly confirmed by the fact, that brass wire loses its tenacity by exposure to the fumes of acids, and even by the presence of a damp atmosphere. This is not caused by the moisture, but by the action of air upon the moistened surface. The manufacturers of common pins are obliged to keep their wire in a dry atmosphere, or immerged in water. If the wire be first moistened, and then exposed to the air, it will assume the brittle state much sooner. In this condition it breaks with a crystalline fracture, similar to that exhibited by an ingot. When a steel plate, such as a watch-spring, has been once tempered, the operation of simply rubbing it bright, will render it soft and elastic. The same change is brought about by slightly hammering it. It, however, resumes its elastic state, by being carefully heated till it becomes of a blue colour. If the heat be continued to redness, particularly in a close vessel, it becomes perfectly annealed. (T.)