In the operation of rolling, hammering, or drawing. metals become hard and brittle; and to avoid fracture in further working the sheets or plates, it is essential that these should be softened, or annealed, as it is called.
Sheet iron or steel is made by passing pieces of the metal, in almost a white-hot condition, backwards and forwards through powerful rolls and rolling down to the required thickness. After rolling, the sheets are very hard, and have to be kept in an annealing furnace for several hours to bring them back to the soft state. The length of time they are in the furnace, and the slowness of cooling, more or less determine the degree of softness of the sheet. When the sheets are placed in batches in the furnace and heated in an uncovered state, they are said to be open-annealed. For some purposes, however, batches of sheets are placed in iron boxes and annealed without coming in contact with the atmosphere or the furnace flames; these are called close-annealed sheets. The open-annealed sheets have more scale on them than the close-annealed sheets, the latter, of course, having a much smoother surface. To obtain a good smooth surface, sheets are sometimes run, when cold, through smooth rolls after they are close-annealed; and this quality of iron is called cold-rolled-close-annealed.
In stretching the edge of an article, throwing off a flange, or in raising, hollowing, stamping, or spinning, some judgment must be exercised as to the suitable times for annealing. One kind of a job may only require to be softened once, whilst others may have to be annealed several times before the sheet metal can be worked with safety up to the required shape. In any case, care ought always to be taken against working a metal up to the splitting or cracking point for the want of annealing.
In annealing iron or steel the highest degree of softness is obtained when the sheet or plate is allowed to remain red hot as long as possible and to cool out very slowly. In thin sheet metal care should be taken that the edge of the sheet is not "burnt" or over-annealed. Even if a piece is not burnt out the edge may be got to a white heat, and this part will break away when being hammered.
Copper becomes soft when made red hot and allowed to cool out slowly in the air or plunged into water. When cooled out in water there is the additional advantage that the surface of the sheet is cleaned in the process by the removal of the scale in the water. This is especially the case if the surface of the copper is sprinkled with common salt before the sheet is made red hot.
Brass is annealed by gradually heating, and then being allowed to cool out slowly.
Zinc gets rather brittle at low temperatures. This is well known to those who work sheet zinc during winter in a cold workshop. For safe working during cold weather, sheet zinc should be warmed so that it can just bo handled, and this is especially so if any sharp bends or edges have to be made.
In working upon any part of a sheet or plate that is to be used in a pipe or vessel that is to be subjected to a pressure, the greatest care must be taken that no part of the metal is left in a stressed condition, either through hammering or local heating. Serious results sometimes happen through want of thought in this direction. The metal can generally be brought to a proper condition by careful heating with the blowpipe or furnace, not only the parts that have been worked, but also the surrounding metal that may have been affected.
Every time a piece of metal is made red hot, whilst in contact with the atmosphere, fresh scale forms on its surface. This is due to the oxygen in the air combining with the metal to form an oxide. It is therefore evident if we require a metal not to scale or waste during annealing, it must be kept out of contact with the atmosphere, and this is in many cases an exceedingly difficult thing to do. Small articles in iron may be covered with rust or oxide, and copper may be buried in ash dust. Furnaces for large work are now being constructed in which metals can be heated out of contact with the atmosphere.
Theory of Annealing. Why metals become hard when worked, or why they become soft under heat treatment, are difficult questions to answer. Or, again, why a metal like steel becomes hard when plunged into water, or copper under the same treatment becomes soft, is no easy task to solve. Sufficient to say that these matters are now being carefully investigated, and at the no distant future a full scientific explanation will be forthcoming. We can, however, imagine that under hammering or rolling, the particles of the metal become pushed or crushed into unnatural positions, and then the metal is strained or hard. When heated, and whilst the metal is in the soft state, we may suppose that the particles then assume their natural position, and the metal comes back to its normal condition of softness.