A slight modification of this plan is used in hardening armor-plates, where many jets are used to insure even quenching of the large surface. This plan is supposed to be patented, or, more properly, it is patented; but as it is very old and well known the patent should not be allowed to disturb anybody.
Water only has been mentioned so far as a quenching medium, because it is the simplest and the cheapest generally. Oil is used frequently where extreme hardness is not necessary and toughness is desirable. Oil gives a good hardness with toughness, and it is used almost universally for springs, and it is sometimes used to toughen railroad axles and similar work. The oil acts more slowly than water and leaves the piece in more nearly a tempered condition; it is neither so hard nor so brittle as it would be if quenched in water. Straits fish-oil is good and cheap; lard-oil gives greater hardness than fish-oil; mineral oil is too fiery to use safely; but there are mixed oils in the market made expressly for hardening which are cheap and efficient.
If it is desired to get the greatest hardness, brine will harden harder than fresh water; and mercury will give the greatest hardness of all. It is a rather expensive cooling medium.
Acid added to water increases its hardening power; but those who know the effects of acids will be very chary of using them.
As to heating, too much emphasis cannot be given to the importance of even temperature throughout the mass. The illustration of the painted piece mentioned in connection with heating for forging applies more forcibly here. Every piece that is to be quenched should look as if it were covered with a perfectly even coat of paint of the exact tint necessary to give the best result.
All hardening should be done on a rising temperature, because then the grain and strains cannot be greater than those due to the highest heat, and this maximum heat can be watched and kept within limits. If a piece be quenched from a falling temperature, the grain and strains will be those due to the highest temperature, modified slightly by the distance through which it has cooled, and always coarser and more brittle than if quenched at the same heat produced by rising temperature. If by accident a piece gets too hot to be quenched, it should be allowed to go entirely cold, and then be heated again to the right color.
After a piece of steel is hardened it is usually tempered to relieve some of the strain, reduce brittleness, and increase the toughness.
This is done by heating; usually the piece is held over the fire, or in contact with a large piece of steel or iron heated for the purpose, until it takes on a certain color which indicates the degree of tempering that is wanted.
Where great numbers of pieces are to be tempered, a bath is very convenient. Boiling in water produces only a slight tempering sufficient for some purposes. Steaming under given pressure will produce even heating and uniform tempering.
When pieces are quenched in oil, they can be tempered easily and nicely by watching the oil that adheres to them. When the oil is dried off and begins to char, the tempering is good, about right for saw-teeth. If the heat is run up until the oil flashes, the tempering is pretty thorough and is about right for good springs. If the oil be all burned off, there will be little temper left except in very high steel. High steel becomes much harder when quenched than low steel; consequently very high hardened steel may be heated until it begins to show color and still retain considerable hardness or temper, whereas a milder steel, under 90 or 100 carbon, when heated to such a degree will retain no temper, it will be soft.