Even the spray from water which is run upon roll-necks may cause these checks in steel that is passing through the rolls, so that it is better to put up a guard to deflect such water away from the body of the roll.
A hammerman may sweep a bar with a damp broom to cause the vapor to explode with violence when the hammer comes down, and so tear away all rough scale and produce a beautiful finish. A careful, skilful man may be permitted to do this, but as surely as he gets his broom too wet, so that drops of water will fall on the steel and whirl around in the spheroidal condition, just so surely will he check the steel.
The best way is to have the broom not wet enough to drip, and then to strike it up against the top die when it is ready to descend; sufficient moisture will be caught upon the die to cause a loud explosion when it strikes the hot steel; it is a violent explosion and will drive off every particle of detachable scale, leaving as beautiful a surface as that which is peculiar to Russia sheet iron.
It is common in rolling tires to run jets of water over the tire to break up the scale and produce a clean surface. Tire-makers assert that experience shows that the water does no harm. There are two reasons for this if it be true: first, the steel is of medium carbon and more inert than high steel, and it has been hammered and compacted before rolling; second, the tires are usually turned, and this would cut away any little checks that might occur on the surface.
The magnetic properties of steel are well known. Soft steel, like soft wrought iron, cannot be magnetized permanently; higher carbon steel will retain magnetism a long time, and hardened steel will retain it still longer. Hardened-steel magnets are the most permanent.
The permanency and the efficiency of a magnet increase with the quantity of carbon up to about 85 carbon; steel of higher carbon than this will not make magnets of so good permanency. The efficiency of a magnet of 85 carbon is increased largely by the addition of a little tungsten; a little less than .05% is sufficient.
It has been shown that tungsten has the property of retaining the hardness of steel up to a relatively high temperature; this additional power of retaining magnetism may indicate a close relation between the conditions set up by magnetism and by hardening.
It has been stated that maximum physical properties, except as to compression, are found at from 90 to 100 carbon; now we find maximum magnetic properties in the same region. Prof. Arnold has found by microscopic tests the same point of saturation; he fixes it at 89 carbon and deduces from it an unstable carbide of Fe₂₄C.
The magnetic maximum was found by magnet-makers by actual use in large numbers of magnets. Prof. J. W. Langley found the same maximum in a series of careful and delicate experiments undertaken to determine the best composition and the best treatment for the production of permanent magnets. Magnetism is affected by temperature, and it is found that steel becomes non-magnetic at or about the point of recalescence. This is important to electricians, as it marks the limit of temperature that is available to them. It is of interest to the scientists, as it is another indication of the importance of the changes that take place at this temperature. Later, recalescence will be found to be an equally important point to the steel-worker, especially to the temperer.
It has been stated that if a bar of steel be heated to any visible temperature and then be cooled without disturbance there will be a resulting grain or structure that is due to the highest temperature to which the bar was subjected. As a rule the highest temperature leaves a grain that appears to the eye to be the largest, or coarsest, whether the microscope shows it to be composed of larger crystals or not.