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Watchmakers' Hand Book

Part II,
Page 3


      67. It is unnecessary to observe that as we are writing for the use of practical watch and clockmakers, we shall make no reference to those elaborate systems of testing that form a remarkable feature of the engineering of the present day, but shall confine ourselves to the tests which workmen can apply for themselves.

      When the grain is fine, close and homogeneous, the fracture in even curved lines and the surface of a uniform grey aspect, the metal is considered to be of good quality. It is, nevertheless, unsafe to rely too much on such indications, for a steel that has been hammered until cold willexhibit a fine close grain, whereas the grain of the same metal will be coarse and open if it was still red hot when the hammering or rolling was concluded. The grain of hardened steel, moreover, depends on the degree of heat to which it has been subjected.

      When of equal quality, even if from the same maker, the grain will be finer in bars of small dimensions than in those that are larger.

      Cast steel even, especially in large pieces, is not always perfectly homogeneous, as can be easily perceived on applying the file, or better, in the lathe; an object can with difficulty be turned perfectly round, and loses its shape in the smoothing. As a general rule it is impossible to form a reliable opinion on a specimen of steel until after it has been worked, filed, turned, and tested according to the particular use to which it is to be applied; for a steel that is excellent for making, say, a spring or a cutter, may be quite unsuitable and give most unsatisfactory results if used for making staffs or fine pivots, or converse may be equally true.

      As a preliminary test, break a piece of the metal; after having examined the fracture, form a cutting edge, harden to what seems to be a convenient degree, then sharpen and employ for cutting small pieces of iron. If the edge is in no way damaged by the iron, this may be taken as a good indication that the metal possesses body and tena ity, and that it can be hardened as much as is required for such purposes.

      The following are methods of obtaining more complete information as to the homogeneity, the character of grain, the degree of hardness a given sample can attain, as well as its malleability, body, elasticity, etc.

      68. Homogeneity. Place drops of dilute sulphuric acid equidistant along the surface of a bar. If the metal is homogeneous all drops will impart the same color.

      Cold hammer or hammer-harden, and then fire-harden part of the bar. Break off the extremity in order to study the fracture; if the result of this examination is satisfactory, polish the hardened end carefully, and with the aid of a glass again examine into the homogeneity of the metal. the polish will be the better and more uniform according as the steel is more homogeneous.

      69. Grain, hardness and temperature of hardening. All varieties of steel do not harden to the same degree, and each requires to be heated to its own particular temperature for hardening; morover, the character of the metal, its color, and the size of its grains vary according to the degree of heat to which it is subjected. It would be difficult to draw any exact conclusions as to the character of grain and the hardness without the following practical method, which we owe to Reaumur:

      At a temperature not exceeding a dull redness, forge a piece of the steel to be tested into the form of a razor blade, that is to say, leaving a thick on one edge and thin on the other, in the direction of its length. Then heat the blade, holding it by one end, and when the other end has reached a bright red heat, plunge the whole into water. Part of the steel will then be hard hardened.

      Along the entire length of the thinner edge carefullu break off the metal with a hammer or otherwise, and examine the character of grain at different points of the exposed thicker edge thus left.

      As the form, color, etc., of the grain depend on the degree of heat to which each portion was subjected, it follows that we shall observe four types of grains: (1) Large, white, sparkling grains; (2) Grains that are medium sized and intermixed, some being white and sparkling, others white and dull; (3) Fine dull grains; and (4) Grains that are medium sized, but dull and ill-defined.

      According as the third class of grains is observed to be more numerous than the second class, so is the fineness of the steel greater, and conversely.

      This method of testing possesses, moreover, the advantage that the workman can experiment on his blade of steel with a view to determine the temperature best suited for hardening.

      Grain No. 1 corresponds to a white-red heat for hardening.

      Grain No. 2 corresponds to an orange-red heat for hardening.

      Grain No. 3 corresponds to some shade of cherry-red heat for hardening.

      Grain No. 4 corresponds to a dull red heat for hardening.

      As there is some difficulty in remembering the exact points at which these several temperatures are reached we will complete this account of the mode of testing by the following:

      70. On a thick plate of metal, maintained at an orange-red heat, place three fragments of the steel under examination, previously marking them, so as to observe the order in which they are immersed in water; and have three vessels of water standing near. As soon as one of the pieces reaches a dull cherry-red heat, allow it to slide into one of the vessels; heat the second to a medium cherry, and the third to a bright cherry-red, introducing them in succession into the water-vessels. If these pieces be examined as to the resistance thy offer to breaking, and the fracture of each be compared with the fracture at different points of the razor blade, the file being used to test hardness, we shall have sufficient evidence as to the most suitable temperature for the hardening of this particular variety of steel.

      71. Body. A steel that possesses body is not brittle. This quality may be tested in the following manner: Several bars of different kinds of steel, of equal dimensions and hardened as uniformly as possible, are bent until the breaking point is reached. If all the conditions or the trial are identical, those bars that stand the greatest angular deflection without breaking have the most body.

      Steel that posses body does not break as easily as that which is wanting in body. Its fracture will be, as it were, beveled off like the mouth-piece of a whistle. If soft and fibrous, it will be characterized by parts being torn asunder.

      72. Elasticity. The elasticity may be tested in a similar manner. It is proportional to the curvature or to the weight that a rod or blade of the metal under examination can support without failing to return to its initial position as soon as the weight or obstacle is removed. A sample of steel that is distorted by a weight that another specimen resists, is evidently inferior in elasticity.

      73. Malleability, tenacity, ductility. A cold hammering, if carefully managed, will suffice to indicate the limits between which steel will support frequently repeated percussion, without breaking, cracking or flying.

      Forge a piece, introducing it a number of times into the fire in order to ascertain whether it loses its distinctive characteristics rapidly, and whether it can be successfully forged.

      It is more difficult to forge, according as it is harder and more "steely."

      The degree of malleability is indicated by the manner in which it supports hammering and rolling, and by the smallness of the dimensions to which it can be brought by such operations.

      Passing the metal through a draw-plate with smooth holes, or tapping it in a screw plate will give usefull indications as to degree of ductility and tenacity.

      A metal is said to be malleable when it can be easily spread out under the hammer or in the laminating rolls. It is called ductile when it can be reduced to very thin wire by passing through a draw-plate.

      It would seem that these two properties, ductility and malleability, should always exist together, to the same degree, in any given metal, but such is not the case. Iron can be drawn into very fine wire, but cannot be reduced to sheets of relatively equal thinness; tin and lead give leaves of relatively equal thinness, but cannot be drawn out into very fine wires; gold and silver are bothe very malleable and ductile, and they rank highest as regards the possession of these properties.

      Steel is more fusible and malleable than iron, but less ductile.

      74. Observations. Formerly the makers of cylinder escapements preferred forged steel, and their cylinders often cracked after the hardening: modern makers employ drawn steel, and it is much preferable. At the same time they do not appear to recognize the principal reason for the difference in the two varieties of steel. It seems to us to be as follows: forged steel is malleable; drawn steel, which has passed well through the draw-plate, is ductile, and, therefore, tenacious. But tenacity in a metal is nothing else than an exceptional resistance, opposed by its particles to a fracture or separation; it follows, then that drawn steel will crack with less facility than the other.

Submitted by: Samuel Kirk (##)

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