Hand-Forging Steel.—In the main this does not differ materially from the same work in iron. Special care must be exercised to have the fire clear of sulphur, hence charcoal is the best fuel to use. In cases where the use of bituminous coal cannot be avoided, the fire should be blowed up for several minutes before putting in the steel, to drive off the sulphur.
Steel to be forged should not be heated to so high a degree as is employed for iron; for ordinary light work a little above a cherry-red is enough. It does not work well under a high degree of heat; and, to make amends, it can be worked much colder than iron. In fact, it is always best to hammer it with light blows until the red color of the heat has entirely disappeared, as this improves its texture by adding decidedly to the closeness of the grain.
Welding Steel.—The common method employed for welding iron to iron is often resorted to for welding steel to steel, but a great deal more care is necessary to success in the latter than in the former case. There must be much precision so far as relates to the rate of heat, as the margin for variation is extremely small. If the temperature is not high enough there will be no adhesion, of course; and if it attains to only a few degrees above what is actually necessary, the steel either “runs” and is ruined, or is ruined by going into an unworkable condition known as “burnt.” It sometimes becomes necessary to weld steel and iron together; this may be effected by the same process as that employed in welding steel to steel. None but workmen of thorough experience would be apt to succeed in either case, on the old plan of proceeding the same as in welding iron to iron.
But steel may be more easily welded than on the old plan by the employment of certain welding compositions. One of them consists of half a pound of saltpetre dissolved in half a pound of oil of vitriol, and afterwards added to two gallons of soft water. Heat the pieces to a cherry-red, then plunge them into this composition; after which proceed to reheat and weld in the usual way. At the welding the strokes of the hammer should be quick and light.
Another composition is made by pulverizing together ten parts of borax and one part of sal-ammoniac. Thoroughly melt the composition so made in an iron pot, then pour out upon some level surface to cool. When cooled grind to a fine powder. Heat the pieces of steel and sprinkle this welding powder over them; then return to the fire, and again heat up, and it is ready to go together under the hammer.
Some smiths claim to weld steel successfully by dusting over the heated pieces a powder composed of clear white sand, 2 lbs., and plaster of Paris, 1 lb.; then reheating and proceeding in the usual way.
In welding steel to iron the foregoing processes are employed the same as if both pieces were steel.
Tempering.—Heat the steel to a bright cherry-red, and plunge it at once into cold water. It will then be as hard as fire and water could make it, and too hard for anything except hardened bearings for machinery, or for some kind of implements necessary to be extremely hard, as tools for cutting glass, and the like. In this condition it is almost as brittle as glass itself, and hence would not stand for most of the uses to which tempered steel is applied. Its great degree of hardness must, therefore, be reduced to the proper standard, depending upon what it is to be used for. This is done by heating and closely observing the resulting colors as they appear upon the metal. If the piece under process is an edge-tool of considerable bulk, only the cutting-edge, and a little back of it, is plunged into the water at the hardening, the rest of the implement being left still hot. It is then held into the light and observed closely, when the different colors, indicating the different degrees of hardness, will be seen moving slowly, one after the other, down towards the edge, driven by the heat still left in the part of the metal not plunged. When the color wanted has reached the edge, the entire piece is plunged into the slack-tub, which stops further action of the heat, and establishes the required degree of hardness exactly where it is desired. But very light articles and implements cannot be tempered in this way, as they will not retain sufficient heat to drive the colors; it will be necessary to reheat them gradually in some way to make the colors move. Very light pieces, as drills and the like, are best tempered in a spirit or alcohol lamp; after having been hardened they should be held in the flame of the lamp a little back of the point or cutting-edge, which will enable the operator to note the movement of the colors. In this case his actions, so far as the colors are concerned, will be governed the same as in the other. Small articles to be tempered alike all over may be placed upon a bit of sheet-iron, after hardening, and the iron held over the fire of the forge, or directly over the flame of the lamp, until the required color has appeared, when they must be quickly plunged into the water. On large articles the colors will be often so strongly marked as to be readily seen on the surface of the metal, rough, just as it came from the hammer, but in small articles they will be somewhat faint; hence it is best to give small articles a slight polish before exposing them to heat for drawing the temper. Nine shades of color will present themselves one after the other as a piece of thoroughly-hardened steel is exposed to gradually-increasing heat. They are:
1. Very faint yellow, appearing at a temperature of 430° Fahrenheit. If slacked at this color, the piece will be very hard, having a temper admirably suited to drills for working in hard metals or hard stone.