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220                      METALLURGY   OF   CAST   IRON.
Average cast iron, when sound, "stretches about .00018, or one part in 5,555 of its length; or J/% inch in 57.9 feet for every ton of tensile strength per square inch up to its elastic limit, which is at about one-half its break strength. The extent of stretching, however, varies much with the quality of the iron, as in wrought iron.'' * For further information on the stretching qualities of cast iron, see Chapter LV., page 422.
Toughness may be defined as strength, but applies more properly to that quality permitting cast iron to bend before it breaks, and in transverse testing, such is called "deflection."
Strength of cast Iron is its ability to resist transverse, tensile crushing, and impact blows or strains, and, in a sense, includes tenacity, elasticity and toughness. It is very rare that castings are designed to resist other than transverse or crushing loads. For this reason transverse tests are the forms of testing mainly used to obtain knowledge of the strength of cast iron, as in securing the transverse strength of test bars, we can also note the " deflection," a quality which tells us of the ductility and toughness of iron better than any other present method can. Deflection also to a great degree informs us of the softness of iron.
Brittleness is that quality adverse to strength and it-greatest in " white " or " chilled " grades of cast iron, also high-silicon or phosphorus mixtures.
Chill is that quality producing a "white" or crystalline body in iron. It can be produced by rapid cooling or by having high sulphur or low silicon, which produce, in the carbon, a state opposite that of graph-
* Trautwine.arbon and free from sulphur, phosphorus,teel Institute Journal, 1893, Vol. i, p. 168.ion of manga j: which was added being found in the form of oxide in the so-The philosophical explanation of this extraordinary effect i my opinion, to be found in the fact that the f erro-manganesu .ğR. C. Hindiey, M. Hoskins, Harvard College, Havemeyer University, Henry Hiels Chemical Co., Isabella Furnace, Iron Gate Furnace, Iroquois Iron Co., Illinois Steel Co., Jefferson Iron Co., Kittan-ning Iron & Steel Co., C. A. Kelly Plow Co., Lebanon Furnace, Longdale Iron Co., Lackawanna Iron & Steel Co., Logan Iron........................   16,720     "                                     t-