Fig. 22 gives the form recommended for plates. In case of round specimens the meas ured length is the same; but the diameter, if pos sible, is taken as equal to one inch, or, in case any other diameter is used, certain increases or decreases are allowed in the variation of ulti mate strength, elastic limit, and percentage of elongation and reduction of area.
For further information on this subject the reader is referred to the Proceed ings of the American So ciety for Testing Materials; to "Materials of Construc tion" (J. B. Johnson), and to "Manufacture and Prop erties of Structural Steel" (H. H. Campbell).
28. Microscopic Tests. If the surface of a piece of steel is polished and then treated with certain chem icals, certain parts of its surface will remain unal tered. Pure iron will be un attacked, while compounds of iron and the metalloids will be darkened.
The result as a whole cannot be observed with the naked eye; but, if put under a microscope and magnified about 500 times, the effect is remarkable. Fig. 23 shows what abso lutely pure iron would look like. It shows the crystals closely together. When a small amount of carbon is in the iron, it does not,, as was formerly supposed, distribute itself through the total mass uniformly, but it combines in large percentages—say 0.9 per cent—with a small por
tion of the iron; and the resulting substance, which shows dark under the microscope, is found here and there between the masses of pure iron; see Plate 6 (a). As the percentage of carbon increases, larger proportions of the pure iron combine with it, and the dark places increase until the percentage of carbon rises to about 0.9 per cent, when the iron appears darkened all over; see (b), (c), and (d), Plate 6. In case other metalloids besides carbon are pres ent, the iron may take up a larger percentage of carbon than 0.9 per cent; and then, while the carbon content of the steel may be larger, it will have more of free, pure iron in it than the lower-carbon steels. Plate 6 (e) (Tnstitute of Civil Engineers, Vol. 123, 1896) shows such a case.
These views in Plate 6 are homogeneous; that is, the arrangement of the "carbon iron cement," if it may be so called, occurs with fair regularity. If, however, the steel is poor, the microscope will show irregular-sized crystals or long, dark places. At (f), Plate 6 (from "Engineering") shows some flaws.
29. Physical Properties of Metals. There are many different grades of cast iron, wrought iron, and steel; and their ultimate strengths, elastic limits, and percentages of elongation vary considerably, even in the same class of material. Table VIII shows the variation in physical properties.