The Metallurgy of Iron and Steel (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1908 edition. Excerpt: ...of these lines is reached, will continue progressively as the excess metal separates out. The line D E is often called the 'lower freezing-point curve, ' because this line represents the temperature of freezing of the eutectic of the series, and we have already shown that every alloy in the series automatically forms a eutectic by 'selective' precipitation; therefore every alloy in the series will not be entirely solid until it reaches the temperature at which the eutectic solidifies,1 which is always the same. Cooling Curves.--There are certain thermal changes which accompany the chemical changes I have outlined in the preceding paragraphs. These thermal changes are of importance, because it is by means of them that we are usually able to obtain the first evidence of the precipitation of excess metal, the formation and solidification of a eutectic, etc. Consider the alloy containing 83 per cent. of lead and 17 per cent. of tin, at 300 C., and let us observe by means of a thermometer or pyrometer the rate of 1 Some metallurgists prefer not to draw the line D E, but to represent tb freezing of the eutectic merely by the point B. cooling. At first the thermometer will fall pretty fast, but when we reach 275, where the line A B is met, the rate of fall is suddenly retarded. It thus becomes evident to us that some event counteracts the fall in temperature. What this event is we learn from microscopic evidence, and, as has already been explained, it is the precipitation of lead. This explanation might have been expected, because the precipitation of lead at a temperature below its normal freezing-point would of course be accompanied by freezing and, during the freezing, the metal would liberate its latent heat of fusion and thus oppose...