(1) Chemical composition, i.e., the relative amounts of other elements present;

(2) Distribution of constituents, i.e., the relative proportions of ferrite, cementite, etc., present; (3) Size of grains.

Here are a few tests, by Sauveur, on the relation between the size of the grain and the physical properties of the same piece of steel:1

The relation between the average area and the tenacity is

Tenacity=75·5 – 0·004 A.

An experienced man can generally give a surprisingly accurate guess of the temperature to which the steel has been heated, from the fracture, or from the microstructure of the metal. Tschernoff and Brinell have observed that the higher the temperature of annealing the larger the size of the grain. This will be evident on examination of Campbell's diagrams, Figs. 41 to 43, which represent the appearance of soft steel when heated to the temperatures indicated: Fig. 41 to about 900°; Fig. 42 to 1200°; and Fig. 43 to

1 N. Ljamin, Chem. Zeit., 21. 205, 1899; Baumaterialien, 3. 105, 1899, finds the tenacity in different steels varies directly as the size of the pearlite grains-at the same finishing temperature; H. Jüptner von Jonstorff, Metallographist, 2. 222, 1899; Stahl und Eisen, 19. 237, 278, 1899; F. Osmond, Annales des Mines, [9], 8. 153, 1900.

about 1300°. A similar result has been observed during the crystallization of brass.1 The following are the more important generalizations which have been made:

I. The higher the temperature above the Ac1 point from which steel cools the larger the size of the grains, while if the Ac1 point is the highest temperature attained, then the steel will have the finest possible structure which it can assume. Howe and Sauveur 2 seem to believe that there is a definite relation between the size of the grain and the highest temperature to which the steel has been heated. For a steel containing 1.1 per cent. of carbon, this relation can be represented by the formula

T = 680 + 281,250 A

where A denotes the actual area of the grain in square millimetres; T is the highest temperature reached in the annealing furnace. Tschernoff's experiments do not quite tally with this formula at the more elevated temperatures, say 1400°, although at lower temperatures the agreement is satisfactory. The more exact law is given by the curve in Fig. 44. Let us illustrate

1 A. H. Coote, Technics, 2. 290, 1904; H. M. Howe, Iron, Steel, and other Alloys, Boston, 250, 1904.

2 H. M. Howe and A. Sauveur, Eng. and Mining Journ., 60. 537, 1895; A. Sauveur, Trans. Amer. Inst. Mining Eng., 26. 863, 1896; Metallographist, 2. 264, 1899; H. Jüptner von Jonstorff, Stahl und Eisen, 19. 237, 278, 1899; I. A. Brinell, Journ. Iron and Steel Inst., 29. i. 365, 1886; Metallographist, 2. 129, 1899; H. Fay and S. Badlam, ibid., 4. 31, 1901; J. E. Stead, Journ. Iron and Steel Inst., 53. i. 145, 1898; 54. ii. 147, 1898; Metallographist, 1. 289, 1898; 2. 85, 1899; C. H. Risdale, ibid., 3. 64, 1900; Journ. Iron and Steel Inst., 53. i. 220, 1898; 56. ii. 102, 1899; E. J. Ball, ibid., 37. i. 85, 1890; 39. i. 103, 1891; J. O. Arnold, Proc. Inst. Civil Eng., i. 123, 1895; D. Tschernoff, Proc. Inst. Mech. Eng., 152, 286, 1880.

FIG. 43.-Grain Size acquired at 1300°. (The Iron Age.)

[To face p. 72.

its application to the specimen of steel for which the curve Ac1P was determined; SP, that is OM, represents the size of the grain at the temperature OS.

If the size of the grain is smaller than it should be at any given temperature, then the grain will grow in size until it reaches the normal size indicated by the above law. On the other hand, if the grain is larger than that which is characteristic of any particular temperature, the grain will not shrink to its normal size. For example, if the steel has the grain size SP, Fig. 44, at the temperature OR, the size of the grain will remain OM, and will not shrink to ON. The slope of the curve will be different for different steels; the steel represented in Fig. 44 contained 11 per cent. of carbon, and traces of silicon and manganese.

The average maximum size of the grain at any temperature is the average size of the grain when the full sectional area is presented. In some places only part, or one corner of the grain is shown. Sauveur 1

1 A. Sauveur, Trans. Amer. Inst. Mining Eng., 22. 546, 1893; Journ. Iron and Steel Inst., 56. ii. 195, 1899; Metallographist, 2. 264, 1899; R. G. Morse, ibid., 3. 130, 1900; Trans. Amer. Inst. Mining Eng., 29, 1900.