ANNEALING, a process of heating and cooling applied to glass and various metals and alloys, with the object of relieving stresses, or of altering the properties of the material. The result is softening, due to rearrangement. In its modern form the process is effected in closed furnaces. Another class of annealing is chemical in nature, being practised in the production of mal leable iron castings. The chemical action occurs through the agency of substances, containing oxygen, packed with the castings in boxes and left for many days in the furnace. These substances extract the carbon from the castings.
Steels.—Annealing becomes necessary for a variety of reasons in numerous manufactures. An early primary reason was and is to soften steel used to make tools, so that it can be cut, filed, turned, and so on, tools having no effect on hardened steel, though it may be ground with an abrasive wheel. Hence any hard bar or tool, if it cannot be worked up or sharpened by grinding, must be softened or annealed first. Bars and tools like wise require annealing after certain processes in manufacture, as casting, rolling, forging, and machining, in order to relieve the internal stresses caused thereby, otherwise the tools made there from would crack in hardening or in use. Annealing temperatures vary from 700° to 800° C. for carbon tool-steels, and from about
to 900° C. for high-speed tool-steels.
Annealing is also used in processes of manufacturing glass and many other materials. Not only relieving the strain, the an nealing causes a rearrangement of the micro-constituents, improv ing homogeneity. An important essential is to avoid such applica tion of heat as will cause scaling of the metal or alloy. Entire protection in a box or a muffle is sometimes given, or if direct flame impinges on the article, the quality of flame is regulated suitably to avoid fierce action. What is termed bright annealing is the most remarkable attainment in this direction, whereby stamp ings and other objects of steel, copper, brass, German-silver, etc., may be annealed once or several times with entire freedom from oxidation, scaling or discolouration. Polished surfaces left from rolling, drawing or other machines thus remain clean and bright, and cleansing treatments are rendered unnecessary.
An important application of annealing is its use on chains for pulley-blocks, cranes and other hoisting tackle, as well as some sorts of couplings subject to incessant duty. These become crystal line and fatigued if kept in service too long, and they are liable to fracture; but if the chains are annealed at intervals, this danger disappears.
Furnaces for annealing are heated by coal, coke, town or producer gas or oil. The two first-mentioned fuels are not employed much for small furnaces. The smallest furnaces are but a few inches inside capacity, the largest will take steel plates, as much as 6oft. in length. Pieces of work go direct on the floor of the furnace, or in closed boxes, or the furnace is of muffle type, the flame being kept outside the heating chamber. Insertion and removal of the smaller specimens is performed by hand methods, but large units or batches of stampings, coils of wire, etc. require the help of a trolley or a crane. Special charg ing machines are also used that push the articles in by electric power, or hand gearing. In the case of the 6oft. plates referred to insertion or removal may be effected in 12 minutes, leaving no risk of overheating of the end of the plate which enters the furnace first and leaves it last.
Large quantities of articles, of small or moderate size are dealt with continuously by some form of conveyor system. For cart ridges a rotary machine is utilised, having gas-burners which im pinge on the shells whilst these are carried around automatically, and afterwards discharged down a chute. Closed furnaces have a spiral retort that passes the components through in a certain time, or a conveyor chain takes these through the length of the furnace. Bright annealing is largely done with conveyors.
In some furnaces the flames are kept away from the work, being carried round hollow walls. If the reverberatory construction is adopted, the heat descends directly on to the objects. Gas furnaces operate by natural draught or by blast, the latter giving a quicker heating up, and greater temperature. Oil may be used in such furnaces, with a difference in the form of burner. Fig. 1 may be studied to understand the arrangement of an oven type annealing furnace. The body is composed of fire-brick bound together with plates and bars, and the design is of the recuperative style ; the waste products pass in the direction of the arrows, and so heat the air-pipes, thus assist ing combustion. In another design the incoming air is pre-heated by first passing under the furnace floor. In either class there is a marked saving in fuel. The twin-chamber furnace affords another example of economy, this having an upper chamber heated by waste products from the main one below; the work consequently may be pre-heated before being put into the main chamber. A section of a Manchester furnace (fig. 2, p. 988) shows. the grate and flue arrangements; beating can be done by coal or coke or both mixed. Special recuperative flues are built in alongside the fire-box for pre-heating the air, these not in the section. Tem peratures up to I ioo° C are easily maintained. It may be men tioned that uniformity of heating is imperative for good work, while maintenance of the correct temperature (within a few degrees) should be guaranteed. See also MATERIALS, STRENGTH OF and FORGING. (F. H.)
