Heat treatment of steel is a matter of the utmost importance and is absolutely essential for making the best materials. It is fortunate that the mechanical shaping of steel is done commonly at a temperature which gives the material good properties when finished. Thus, in the making of things like steel beams or steel rails, the heating and shaping have been carried out together, and, as the metal cools off, it is suitable for use. The lower the carbon content of a steel, the less the heat treatment will affect its final condition.
These general statements are good for materials requiring no unusual properties. Thus rail steel needs to have only the normal structure for a steel of the right manganese and carbon content to give normal material. If the manganese is about right, if the carbon is medium - that is from 0.50 to 0.75 per cent - and if the material has been rolled properly, there results a good material for rails. The same in general holds for structural steel, while in the case of many sheets which are of lower carbon steel, they are suitable for use as they come from the rolling mill. In the great tonnage of. steel production it is essential only that the chemical composition shall be right approximately and that the mechanical working shall have been done at a suitable temperature with normal cooling.
But there are many more cases, not so much on a tonnage basis as for small units for special uses, where the normal material would be utterly inapplicable. These in particular are where objects are to be of hardened steel, and where they should be of tempered steel. We can get hard materials by the right chemical composition and with slight attention to any heat treatment, but, by having the carbon content exactly of the right amount and by cooling the material suddenly, very hard steels can be produced without further alloying.
All such hardened steels are apt to be unduly brittle and suitable for use usually only as the cutting or wearing edge of an instrument or tool. But by taking this hardened material and heating it carefully to such a temperature that the austenitic structure will begin to break up into the structures more stable at lower temperatures, we are able to get materials which will be intermediate and have not only considerable hardness but much increased strength and toughness. These in general are the tempered steels; they are the steels in which great tensile strength is required and which are especially desirable for innumerable uses, since, with the great strength, the bulk or size of the object can be kept small. The tempered steels are particularly useful for all sorts of tools and implements, and also are used widely in all classes of machines. Most of the working parts of the distinctively modern machines, such as flying machines, automobiles, and submarines, as well as innumerable locomotive and stationary-engine parts are made of such materials.
The temperature at which metal is to be worked should be graded entirely by the finishing temperature which well may be about 700° C. The more mechanical working there is to be done, the higher must be the initial temperature; on the other hand, an initial temperature of over 1150° C. hardly is to be desired; accordingly, with much working to do, this means that reheating is the logical conclusion.
Again, the most quickly chilled steel will have an austenitic structure, and a steel just annealed to full softness will have a very finely pearlitic structure. The various stages through the changes from this first to this second stage correspond to the decomposition stages of the solid solution and to the formation of the pearlite; they are designated martensite, troostite, osmondite, and sorbite. Tempering is the production of one or more of these special structures; it makes no difference how the structure is obtained.
Elements Present Other Than Iron (Per Cent)
In practice, tempering may be a chilling somewhat slower than a real quenching, as in molten lead or in oil; or it may be a chilling in a water or a brine solution, and a subsequent reheating until the proper structure is developed, which will be made permanent by chilling from the second drawing temperature. The cutting edge of a tool which has been quenched is reheated by withdrawing it and letting the heat from behind follow down until the drawing is exactly right, after which the entire head of the tool is quenched.
In the high-speed steels these structures are spread out and highly differentiated, and are obtainable with much precision. In the straight carbon steels it is difficult thus to separate them, so that the most expert knowledge is required to get a thick piece of the same tempered structure throughout.
In studying such an arrangement as that in Table VI, it must be kept in mind that the chemical specification is no more than one of many factors determining the quality and the properties of the material. The method of manu-facture, whether crucible, Bessemer, or open-hearth furnace, or the acid or the basic process; the manner of shaping; the heat treatment; the physical properties, such as tensile strength, are all often just as important as chemical composition.