For rock drills, cold chisels, milling and other tools it is necessary to use steel carefully tempered, so that brittleness is greatly reduced while considerable hardness and cutting power remain. Other changes of properties, as remarkable, follow upon subjecting steel to greater heat than that used for tempering. Says Professor Roberts-Austen:—“Three strips of steel identical in quality are taken. By bending one it is shown to be soft; if it is heated to redness and plunged in cold water it will become hard and will break on any attempt to bend it. The second strip, after heating and rapid cooling, if again heated to about the melting point of lead, will at once bend readily, but will spring back to a straight line when the bending force is removed. The third piece may be softened by being cooled slowly from a bright red heat, and this will bend easily and remain distorted. The metal has been singularly altered in its properties by comparatively simple treatment, and all these changes, it must be remembered, have been produced in a solid metal to which nothing has been added, and from which nothing has been taken away.”
It is the comparative slowness of cooling in oil, the greater slowness of cooling in air, that make these by far the best tempering processes, because the molecular re-arrangement, in which tempering consists, requires time. Often the critical temperature, at which a desired re-arrangement takes place, is declared by the metal losing all power of response to a magnet: this fact affords the steel-maker welcome aid; he has only to shut off heat as soon as his steel ceases to attract a magnet and plunge the steel into water in order to obtain the hardness he wishes.
The complex phenomena of heat treatment in steel manufacture are fully discussed by Professor H. M. Howe, in his “Iron, Steel and Other Alloys,” second edition, 1906.
Steel for Railroad Rails.
In another chapter of this book a word is said as to the form of rails at which Mr. P. H. Dudley has arrived as the outcome of years of experiment. He thus describes the properties which the steel should possess by virtue of due chemical composition and proper heat treatment:—
“Ductility to ensure power to resist the shock of the driving wheels, so that the steel may not break; resistance to abrasion, that it may not wear out; and high limit of elasticity, that it may not take a permanent set and be bent into a series of waves between its supporting ties, by the enormous pressures which the wheels of to-day throw upon it. The best composition is carbon 0.55 to 0.60 per cent., silicon 0.10 to 0.15, manganese 1.20, sulphur under 0.06, phosphorus under 0.06; with 50,000 to 60,000 granulations to the square inch. More granulations, or fewer, mean an increase of brittleness in the steel.”[17]
[17] Henry Marion Howe, “Iron, Steel and Other Alloys.” Second edition. Published by Albert Sauveur, Cambridge, Mass., 1906. And a note from Mr. P. H. Dudley to the author, May 2, 1906.
Invar: A Steel Invariable in Dimensions Whether Warmed or Cooled.
While the great strength of steel makes it of pre-eminent value in the arts, steel in the huge dimensions of modern roofs and bridges has the demerit of expanding with heat and contracting with cold in a troublesome degree. A notable case is that of the steel rails on the elevated railroad of New York. If this fault, common to all metals, can be materially reduced or abolished, then steel enters upon a new field of golden harvests. Here, by dint of acumen and skill the goal has been reached by M. Charles Edouard Guillaume, of the International Bureau of Weights and Measures in Paris. A few years ago he began investigating the singular magnetic qualities of nickel-steels. Then in studying expansibility by heat he discovered that when the nickel was increased to 36.2 per cent. the alloy was almost indifferent to changes of temperature, expanding but one part in one million when warmed from zero to 1° Centigrade. Because of this insensibility, the alloy at the suggestion of Professor Thury is named invar. In observations of invar which extended through six years, an elongation of one part in 100,000 was detected; subsequently its changes of length each year seemed less than one-millionth. This slight inconstancy may be overcome by further experiment; in the meantime while invar is not available for standards of length of the first order, such as those of the Bureau of Standards at Washington, there is a vast and useful field for the alloy. It offers itself for secondary standards, to be compared at intervals with primary standards at Washington or other capitals of the world.
A leading application will be in surveying. Already wires of invar have been employed by the Survey of France with the utmost success, dispensing with the burdensome apparatus formerly needed in compensating variations due to temperature. With invar wires ten men have advanced at the rate of five kilometers a day; ten years before, with ordinary steel measures, fifty men advanced one half a kilometer, that is, with but one fiftieth as much efficiency.