containing about 0.4 per cent. carbon will become fine in structure.

2nd. Carburise at a temperature of about 950° C. After allowing to cool, reheat to about 900°

C. and allow to cool again. Reheat to about 750° C. and quench.

Fig. 72 shows the effect of this treatment without quenching. Compare with fig. 73, which is the same piece as carburised, and without any subsequent heat treatment. The central core of fig. 72 is quite fine, as is also the outer zone, while the region between is not rendered coarse to any appreciable extent.

These two photomicrographs demonstrate very clearly the advantage of microscopic examination in problems of this kind. They indicate admirably, in a manner not obtained by any other means, the history of the thermal treatment to which the material was subjected.

3rd. Carburise at a temperature of about 950° C. After allowing to cool, reheat to about 900° C. and quench. Reheat to about 750° C. and quench.

Quenching after each reheating retains the metal in a more homogeneous condition. If the shape of the articles presents no risk of warping and the work is of an important nature, the best results will be secured by this quenching after each reheating; indeed, for the purpose of obviating the possibility of segregation, especially if there be any free carbide of iron in the outer zone, it is advisable when possible to quench direct from the carburising temperature in addition to the other quenchings.

The advantage of quenching after each reheating

should be thoroughly appreciated by reverting to the two heat-treatment charts, figs. 36 and 37 (pp. 71 and

FIG. 73.-Before annealing. Magnified 30 diameters.

83), where it was pointed out that when steel is heated to above the critical point it becomes thoroughly homogeneous, and by hastening the cooling by quenching no time is given to allow the constituents to separate out

again, and the structure is retained in its finest possible condition.

Centre of carburised mild steel.

FIG. 74.-Coarse structure due to prolonged heating.

FIG. 75.-The same part after annealing for a few minutes. Magnified 80 diameters.

Except at the final quenching for hardening it is not necessary, indeed, it is not advisable, to allow the cooling to proceed beyond the point at which redness disappears. The metal at that temperature having passed below all

the critical points, no further change can take place in the structure, and therefore no further advantage can be gained by continuing the cooling; but by interrupting it and immediately reheating, the risk of cracks forming is considerably lessened, since there is a probability of these occurring during this reheating if the preceding quenching has been carried right out from the high temperature.

As has been said in connection with high-carbon steel, these first quenchings from the high temperature for the purpose of retaining a fine structure in the central core may be with advantage carried out in oil or hot water, cooling mediums which are less drastic in their action than cold water.

It should be noted that for reheating purposes it is always inadvisable to use an open fire or a furnace from which the air has not been entirely excluded. The oxygen which enters in the air combines with the carbon in the steel, decarburising the outer skin and thereby causing partial and sometimes complete softness on the surface.

Fig. 74 represents a central core of the same piece of steel as is shown in fig. 71. The effect of the prolonged heating during carburisation is shown in fig. 74; the same piece is shown in fig. 75 after having been reheated to about 900° C. for a few minutes: there is a wellmarked difference. These illustrations supply indisputable evidence of the effect of the heat treatment necessary to retain a soft, tough core which, as we know, is one of the fundamental objects of case-hardening.

CHAPTER X.

CONCLUSION.

IN the practical examples which I have dealt with, I have endeavoured to show the advantage of having some knowledge of arithmetic, mensuration, strength of materials, geometrical and mechanical drawing, practical mechanics, properties of iron and steel, heat, chemistry, metallurgy, and metallography. I feel sure that it will be readily admitted that a little knowledge of all these subjects should prove beneficial to young smiths and forgers. It has been said, "A little knowledge is a dangerous thing." So it is, but not in the sense in which I use the expression, and at any rate it is not half so dangerous as ignorance. The only time that it can do harm is when a man thinks and acts as if he knew everything when he only knows a very little. When I speak of a little knowledge I do not mean an imperfect knowledge of the subject, but a thorough understanding of the little in each subject that may be practically applied to smith work and forging. Of course, we shall always have with us the man who affects to despise these things, but he, more often than not, is the very man who stands most in need of them.

The spirit in which these pages has been written is that of a desire to point out to young men the kind of knowledge that may add considerably to their