For attaining this end is sometimes used a less powerful hardening fluid, and sometimes a warm instead of a cold fluid, and sometimes the piece is held only a short time in the hardening fluid, and is taken out while it is vet warm in its interior and allowed finally to cool in the air. Further, the material may, for this purpose, be heated more gently, but it must be kept in mind in connection with this that a less heat than a gentle red heat (cherry - red) in general does not induce any proper hardening; and, on the other hand, that tool steel cannot in most cases be heated to a higher temperature than that just indicated without running the risk of becoming, by hardening, too brittle. It is thus properly only for soft steel and iron that the degree of heating can be varied to a greater extent, but it holds good specially for the latter, and above all for weld - iron, that the temperature must be considerably higher than for hard steel, if the proper action of hardening is to be attained. The more strongly and the longer the iron or steel after hardening is again heated, with slow cooling supervening, the more completely are the effects of hardening removed; and care ought therefore to be taken in tempering; but here we have a good help in the different colours of tempering which follow one after the other.

On the appearance of fracture also, the hardening has an influence, the grain becoming finer.

Causes of Hardening - Having discussed in detail the effects of hardening on the different varieties of iron and steel, we shall endeavour to ascertain the intimate causes of these effects. That they increase with the rapidity of cooling, and thus are a consequence thereof, has been already shown; but the question is, how the rapidity of cooling can produce such effects. The hypothesis that is still most common is that which assumes that a rapid cooling gives us the status quo, or the state into which the substance was brought by the heating which immediately preceded the rapid cooling, and the action, of the hardening would in such a case only be a result of the heating itself, inasmuch as the rapid cooling only, so to say, fixed the warm condition, or, in other words, made it possible for the substance, even in a cold state, to show itself as it was during the heating. During slow cooling, on the other hand, the molecules would have opportunity to group them-selves in a more crystalline manner, and hence the coarser grain.

But if the molecules can move about in this way during the cooling, they may well do so to a still greater extent at the highest temperature to which the substance has been heated, for this grouping of the molecules is rendered possible just by the softening to a greater or less degree which the heating causes in the iron, and the softening is naturally greater the higher the temperature is. That, first of all, a violent compression must in such a case take place is self - evident, for we have now to do with a body heated from without, which therefore, at least when the heating has not been of all the longer duration, is apt to be warmer in the outer than in the inner layers. When now this body, by dipping in a hardening fluid, or in some other way, is exposed to a rapid cooling acting from without, the outer layers are cooled first, and the difference of temperature between the outer and the inner layers is greater the whole way through in the same proportion as the method of cooling is more powerful, and the cooling is accompanied by compressing or forcing together, and the more the outer layers have been cooled in proportion to the inner, with the greater compressing force must the former work upon the latter, which by their resistance react upon the outer layers.

The compressing force is, however, by no means exclusively dependent on the rapidity of cooling, but also on the compactness of the material; for the smaller this is, the more readily does the material allow itself to be compressed, and the less accordingly becomes the resistance which the interior develops against a certain compressing force, so that no great resistance is ever experienced in such a case. In this way is explained the fact, which has already been pointed out, that the effect of hardening is greater on the compact ingot - iron than on the weld-iron, which is looser in its structure. Further, the compressing force is naturally in a very high degree dependent on the limit of elasticity of the material; and the smaller this is, the more easily are the outer layers stretched by the resistance of the inner, And the smaller, therefore, is the portion of the contracting force which can be made available as actually compressing. All substances which in iron increase its limit of elasticity, ought therefore to have an influence on its power of hardening; a fact which has also been confirmed by experience, inasmuch as not only carbon, but also manganese, silicon, and phosphorus, have shown themselves to have some influence in this respect.

The action of the other substances, however, upon the degree of hardening is limited in comparison with that of carbon; and the explanation of this appears, as has already been pointed out, to lie mainly in the more intimate union between iron and carbon which a violent compression produces. As the union between these substances becomes more intimate, the influence of the content of carbon on the iron also becomes greater, and it is just an increased exertion of the influence of the carbon on the iron that is attained by hardening. If we now compare with this the influence of hardening as stated above, it appears that it only still further increases the degree of the properties just mentioned as dependent on a certain content of combined carbon, quite as if the content of combined carbon had been increased by the hardening; and in most complete correspondence with this stands the fact that the ultimate tensile strength is not continuously increased by the hardening; but if steel with a large content of carbon be strongly hardened, the limit of the increase of tensile strength is exceeded. The correspondence between the action of hardening and of a larger content of carbon is thus manifest in this case also.