Hardness
المؤلف:
GEORGE A. HOADLEY
المصدر:
Essentials of Physics
الجزء والصفحة:
P-24
2025-10-21
53
is a relative property; there is no such thing as an absolutely hard or soft body. A body that can scratch or wear another is the harder of the two. Glass is harder than wax but softer than the diamond. The diamond is the hardest of all natural substances, and diamond dust is used to cut other stones. Brittleness must not be mistaken for hardness. Steel, which is hard, is tough; while glass, which is also hard, is brittle.
Steel is rendered very hard and brittle by being heated to a red heat and then being plunged into water. In order to render it serviceable for cutting tools, or for springs, it is reheated slowly until it has the desired degree of hardness, which is indicated by its color, when it is again plunged into water or oil. This process is called tempering.
water or oil. This process is called tempering. Iron may be rendered soft, or annealed, by cooling it gradually and evenly from a high temperature. This renders iron wire pliable, and if the same process is applied to glass, the strains are taken out and it is much less liable to crack on being heated.
on being heated. A swiftly moving body will cut one that is at rest even if the latter is harder, as in the case of a soft iron disk rotating at a high speed, which is sometimes used to cut off hard steel bars. The cutting power of an emery or carborundum wheel depends both upon the hardness of the material and the high speed at which it is run. A buffing wheel used for polishing metals is an example of this action.
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