CHAPTER IV. THE THERMAL TREATMENT OF METALS. Annealing, Hardening, and Tempering.-The mechanical properties of metals are often, in a great measure, dependent on the thermal treatment to which they have been subjected. The effect of heat on zinc has already been noticed. There can be no question that the application of heat to a metal may produce a remarkable molecular change in its structure, the nature of the change depending on that of the metal or alloy, and on the treatment it has undergone. It will be well, therefore, to consider carefully what happens when metals are submitted to the three principal operations involving thermal treatment, which are known respectively as annealing, hardening, and tempering. Usually all three are intimately related. Annealing may be defined as the release of strain in metals which may itself have been produced by mechanical treatment, such as hammering, rolling, or wiredrawing, or by either rapid or slow cooling from a more or less elevated temperature. As an example of the former, it may be mentioned that metals and alloys which have been rendered excessively hard by rolling are heated usually to bright redness and allowed to cool slowly. In the case of copper, it does not appear to be important whether the cooling is slow or rapid, and in recent years much experimental evidence has been accumulated, which tends to show that in the case of certain metals which have been hardened, a more or less prolonged exposure to a low temperature under 100° will sensibly anneal them. On the other hand, the rapidity with which the cooling is effected is very important. Bronze containing about 20 per cent. of tin* is rendered very malleable by rapid cooling, and so are certain alloys of iron with more than 7 per cent. of manganese. It is, however, in the case of iron and steel that thermal treatment is especially important. Steel, it must be remembered, is modified iron. The name iron is, in fact, a comprehensive one, for the mechanical behaviour of the metal is so singularly changed by influences acting from within * Riche, Ann. de Chim. et de Phys., vol. xxx. (1873), p. 417. and without its mass as to lead many to think, with Paracelsus, that iron and steel must be two distinct metals, their properties being so different. Pure iron may be prepared in a form pliable and soft as copper, steel can readily be made sufficiently hard to scratch glass; and notwithstanding this extraordinary variance in the physical properties of iron and certain kinds of steel, the chemical difference between them is comparatively very small, and would hardly secure attention if it were not for the importance of the results to which it gives rise. It is necessary to consider the nature of the transformations which iron can sustain, and to see how it differs from steel, of which an old writer has said:* "Its most useful and advantageous property is that of becoming extremely hard when ignited and plunged in cold water, the hardness produced being greater in proportion as the steel is hotter and the water colder. The colours which appear on the surface of steel slowly heated direct the artist in tempering or reducing the hardness of steel to any determinate standard." There is still so much confusion between the words "temper," 99 66 'tempering," and "hardening," in the writings of even very eminent authorities, that it is well to keep these old definitions carefully in mind. Hardening is the result of rapidly cooling a strongly heated mass of steel. Tempering consists in re-heating the hardened steel to a temperature far short of that to which it was raised before hardening; this heating may or may not be followed by rapid cooling. Annealing, as applied to steel, consists in heating the mass to a temperature higher than that used for tempering, and allowing it to cool slowly. This may be shown experimentally in the following manner :— Three strips of steel of identical quality may be taken. It can be shown by bending one that it is soft; but if it is heated to redness and plunged in cold water it will become hard and will break on any attempt to bend it. The second strip may, after heating and rapid cooling, be again heated to about the melting point of lead, when it will bend readily, but will spring back to a straight line when the bending force is removed. The third piece may be softened by being cooled slowly from a bright red heat, and this will bend easily and will remain distorted. The metal has been singularly altered in its properties by comparatively simple treatment, and all these changes, it must be remembered, have been produced in a solid metal to which nothing has been added, and from which nothing material has been taken. * The First Principles of Chemistry, by W. Nicholson, London, 1760, p. 312. The theory of the operation, described above, has been laboriously built up, and its consideration introduces many questions of great interest, both in the history of science and in our knowledge of molecular physics. History. First, as regards the history of the subject. The knowledge that steel might be hardened must have been derived from remote antiquity. Copper hardened with tin was its only predecessor, and it continued to be used very long after it was known that steel might be hardened. It would, moreover, appear that a desire to appreciate the difficulties of a people, to whom cutting instruments of hard steel were unknown, seems to have induced experimenters in quite recent times to fashion implements of bronze, and a trustworthy authority states that "Sir Francis Chantry formed an alloy containing about 16 parts of copper, 2 of zinc, and 2 of tin, of which he had a razor made, and even shaved with it."*' The Greek alchemical MSS., which have been so carefully examined by M. Berthelot, give various receipts, from which it is evident that in the early days the nature of the quenching fluid was considered to be all important. There were certain rivers the waters of which were supposed to be specially efficacious. Pliny, who says that the difference between waters of various rivers can be recognised by workers in steel, also knew that oil might be used with advantage for hardening certain varieties of the metal. It is sad to think how many of the old recipes for hardening and tempering have been lost. Theophilus, writing in the eleventh century, gives very quaint instructions in the art of hardening steel. The belief, however, in the efficacy of curious nostrums and solutions for hardening steel could hardly have been firmer in the third century B.C. than in the sixteenth of our era. Pure cold water is now usually employed for hardening, but it was far too simple a material for many a sixteenth-century artificer to employ, as is shown by the quaint recipes contained in one of the earliest books of trade secrets, which, by its title, showed the existence of the belief that the "right use of alchemy" was to bring chemical knowledge to bear upon industry. The earliest edition was published in 1531,† and the first English translation in 1583, from which the following extracts may be of interest. "Take snayles, and first drawn water of a red die of which water being taken *Engines of War, by H. Wilkinson, 1841, p. 194. + Rechter Gebrauch d. Alchimei, 1531. There were many English editions. "A profitable boke declaring dyuers approoued remedies," &c., London, 1583. See Prof. Ferguson's learned paper " On some Early Treatises on Technological Chemistry," Phil. Soc. Glasgow, Jan. 1886. in the two first monthes of haruest when it raynes," boil it with the snails," then heate your iron red hot and quench it therein, and it shall be hard as steele." "Ye may do the like with the blood of a man of xxx years of age and of a sanguine complexion, being of a merry nature and pleasant distilled in the middst of May." This may seem trivial enough, but the belief in the efficacy of such solutions survived into the present century, for in a work published in 1810 the artist is prettily directed* "to take the root of blue lilies, infuse it in wine and quench the steel in it," and the steel will be hard; on the other hand, he is told that if he "takes the juice or water of common beans and quenches iron or steel in it, it will be soft as lead." As must always be the case when the practice of an art is purely empyrical, such procedure was often fantastic, but it is by no means obsolete, for probably at the present day there is hardly a workshop in which some artificer could not be found with a claim to possess a quaint nostrum for hardening steel. Even the use of absurdly compounded baths was supported by theoretical views. Otto Tachen,† for instance, writing of steel in about the year 1666, says that steel when it is "quenched in water acquires strength, because the light alcaly in the water is a true comforter of the light acid in the iron, and cutlers do strengthen it with the alcaly of animals," hence the use of snails. Again, Lemery‡ explains in much the same way the production of steel by heating iron in the presence of horns of animals. The These points have been dwelt upon in order to bring out clearly the fact that the early workers attached great importance to the nature of the fluid in which hot steel was quenched, and they were right, though their theories may have been wrong. degree of rapidity with which heat is abstracted from the steel during the operation of hardening is as important at the present day as it ever was. Roughly speaking, if steel has to be made glass hard, ice-cold water, brine, or mercury is used; if it has only to be made slightly hard, hot water or oil may be employed ;while, as Thomas Gill § suggested in 1818, both "hardening" and "tempering" may be united in a single operation by plunging the hot metal in a bath of molten lead or other suitable metal, which will, of course, abstract the heat more slowly. The use of lead and of other metals in hardening steel has, * The Laboratory; or, School of Arts, sixth edition, 1799, p. 228. There is a later edition of 1810. + Key to the Ancient Hippocratical Learning, London, 1690, p. 68. |