Granite is composed of the minerals muscovite (K2O. 3A120, . 6SiO2), quartz (SiO2), and orthoclase (K2O . Al2O3 . 6SiO2). The components are silica, alumina, and potash; the phases are the three solid minerals; hence the system is univariant, and it is in equilibrium.

A. Findlay's The Phase Rule and its Applications, London, 1903, is a text-book devoted to this branch of chemistry.

EXERCISES.—(1) Find the percentage of pearlite and excess ferrite in medium carbon steels containing 0·3 and 0.7 per cent. of carbon respectively. Ans. 33 and 67; and 78 and 22.

(2) An alloy containing a high percentage of graphite is required. If the castings, owing to their thinness, or from the nature of the mould used, are expected to cool rapidly, why should the percentage of silicon be increased? Ans. To stimulate the formation of graphite, since rapid cooling retards the separation of graphite.

(3) Trace the changes which steels containing 0·4, 0·8, and 1.6 per cent. of carbon undergo as they cool slowly from the molten condition.

(4) Find the total ferrite, and total cementite in a low carbon steel containing 0.2 per cent. of carbon. Ans. 97 and 3.

THE HARDENING, ANNEALING, AND

TEMPERING OF STEEL

§ 23. General Properties of Hypo- and Hypereutectic Steels

THE mechanical properties of iron-carbon alloys are closely connected with the relative amounts of the two elements. The relation between the percentage of carbon in an alloy and the tenacity in tons per square inch is indicated 1 in the following table:

Per cent. of carbon = 0.05 0.1 0.2 0.4 0.6 0.8 1.0 1.3 Tenacity . 25.00 260 310 360 43.0 580 600 44.0

The gradual increase in the tenacity of the metal as the amount of carbon approaches the eutectic proportions is brought out very clearly. The results are shown graphically in Fig. 31.

Cementite is a very brittle substance, harder than glass, while ferrite is as soft and as ductile as copper. The relative proportions and the distribution of these two constituents in any alloy must affect its mechanical properties. In hypoeutectic steels the presence of an excess of ferrite renders the metal ductile and tenacious. On the other hand, in hypereutectic

1 H. M. Howe, Eng. and Mining Journ., 241, 1887.

steels, the presence of an excess of cementite diminishes the ductility and tenacity of the metal.

In a chisel which has to be subjected to blows, there should be no structurally free cementite. Chisel steels, in consequence, contain approximately 0.9 per cent. of carbon. A cutting file which has to withstand

but little shock may have a little free cementite—say, 5 per cent. excess cementite-or 1.2 per cent. of carbon; while a razor which has no shock to meet, and wants essentially a keen cutting edge, may have as much as 10 per cent. of excess cementite, or 1.5 per cent. of carbon. The brittleness of such a steel is not a serious objection.

When the metal has 2 or more per cent. of carbon, the resulting alloys are called cast irons. White cast iron, for example, has so much cementite that it cannot be filed or drilled with ordinary tools. In white cast iron the carbon is principally in the form of cementite. By heating white cast iron to a suitable

1 H. M. Howe, Metallographist, 4. 177, 1901; 6. 203, 1903; Encyc. Brit. 29. 570, 1902.