This term is here employed in its widest sense, embracing hardening, case - hardening, softening, toughening, and annealing. It will be convenient first to discuss the principles advanced by different metallurgists as underlying the operation of tempering, and then to proceed to enumerate the various recipes in vogue.
The hardening of iron and steel has recently been studied in an able manner by Prof. Akerman, of Stockholm, in a paper read before the Iron and Steel Institute, of which the following is an abstract: -
The knowledge of the effects of hardening, especially on iron, is by no means so complete, and still less so generally diffused, fls is desirable. It is necessary in the first place to make ourselves acquainted with the nature of hardening. If we inquire what are the circumstances on which it depends, whether more or less of the so - called "combined" carbon in a malleable iron or steel exists as "hardening" or "cement" carbon, it appears that the latter is changed into the former by a heating to a red heat, succeeded by a violent forcing together, continued until cooling is almost complete; while hardening - carbon, on the other hand, is changed into cement. carbon by long - continued heating followed by slow cooling without extra compression. In the case of strong hardening of hard steel, we have the most powerful compression, for the rapid cooling produces a great difference of temperature between the outer and the inner layers of the piece, the more cooled exterior layers compressing the interior with greater force in proportion, partly as the latter are expanded by being more strongly heated, and partly as the limit of elasticity of the substance is high, so that there is not too great a loss of the compressing force by the extension of the exterior layers.
Again, that hammering favours the conversion of cement - carbon into hardening - carbon, or the more intimate union of the carbon with the iron in which it occurs, more than rolling, may at least occasionally to some extent be attributed to the more powerful compression exerted by the hammer, but still more to the circumstance that the iron or steel, when the rolling is ended, commonly has a far higher temperature than when it has been drawn out under the hammer. For if the iron or steel be still red - hot when the drawing is finished, a part of the carbon converted into hardening - carbon, or more intimately united with the iron during the compression to which it has been subjected, may be again changed into cement - carbon during the succeeding slow cooling. There is thus a very complete correspondence between the occurrence of hardening and cement - carbon and their mutual conversion in malleable iron and steel on the one side, and the relations of the combined carbon and the graphite in pig - iron on the other.
Proceeding to the hardening, we find that experience has sufficiently shown that its effect mainly depends upon the content of combined carbon in the iron, upon the differences of temperature between the iron or steel and the hardening fluid, and further on the rapidity of the cooling. The last - mentioned again is dependent on the quantity of the hardening fluid, its specific gravity, power of conducting heat, specific heat, boiling - point, and heat of vaporization. Of the 4 liquids, mercury, water, oil, and coal - tar, therefore, the first - named hardens much more powerfully than water, water considerably more powerfully than oil, and oil more powerfully than coal - tar. Further, the hardening power of water is altered not only by differences of temperature, but also by the addition of different substances, which change its properties in the respects just mentioned. Finally, the rapidity of cooling, so important for the degree of hardening, is also dependent on the way in which the piece is held down into the hardening fluid.
For if it be kept still in a hardening fluid of low specific gravity and small conductivity and specific heat, the quantity of the hardening fluid is not of the same importance as if the piece be unceasingly moved about in it; but in the latter case the cooling of the piece is apt to be unequal, inasmuch as by the moving about the front parts are cooled somewhat more rapidly than the back ones. This is also the case if by hardening in running water we make the quantity of the hardening fluid, so to speak, unlimited. The front part of the piece, or that which is turned up - stream, is then, of course, cooled most rapidly; and in order in such a case to attain an even hardening, it is necessary to turn round the piece rapidly and unceasingly. The layer of steam which, in the case of hardening in a substance so easily converted into vapour as water, is formed around the warm piece, is an obstacle to the contact of water with it, and thus diminishes the speed of cooling along with the degree of hardening which is dependent upon it; but if care be taken that in one way or another the steam be easily and rapidly carried away as it is formed, the rapidity of cooling, on the other hand, on account of the great heat of vaporization of water, is very considerably promoted by this conversion into vapour.
Small 3 pieces, therefore, are also very well hardened in water - dust finely distributed by means of a stream of air or steam; and the highest degree of hardening may, according to Jarolimek, be attained in this way with so moderate a quantity of water that all the water - dust which comes into contact with the warm piece is brought by it into the form of steam. These influences exerted by the formation of steam, must also be taken into consideration when, in order to attain an inferior degree of hardness, warm water is used instead of cold. It cannot accordingly be denied that there are many factors, exceedingly difficult of calculation, which exert an influence on the speed of cooling, and thereby on the degree of hardness. Nor is it much to be wondered at that mistakes are readily committed in hardening, and that great practice is required in order to be able confidently to reckon on a certain effect; and finally, that a workman accustomed to hardening considers that only a single method which he has been in the habit of employing can be used for a certain purpose, while another equally skilful workman can only attain the same result by a method essentially different.