If the forging is to extend to a material change in the shape of the iron, the rate of heat must be much higher; it must be increased to what smiths call the “white flame heat.” The hammering must be much heavier, of course; if the piece is large a sledge-hammer must be brought into requisition. But the gunsmith will seldom have work heavy enough to demand the aid of an assistant with a sledge-hammer.
Welding.—The “welding” or “sparkling heat” is required in this process, which is a higher degree of heat than either named in the forgoing. In securing this heat, the metal is brought nearly to a state of fusion; which condition is made known by its sparkling, and presenting the appearance of being covered with a glaze, or a fresh coat of varnish. So soon as the two pieces of iron to be welded together have both attained to this necessary degree of heat, they are taken from the fire with the utmost dispatch, the scales or dirt which would hinder their incorporation scraped off, placed in contact at the heated point, and hammered until a union has been effected, and no seam or fissure remains visible. If the first effort fails to unite them sufficiently, they must be reheated and rehammered until the desired end is secured.
The fire for welding should be free from sulphur; and the iron, while heating, should be taken out now and then and sprinkled over at the point of greatest heat with powdered glass, or with powdered borax. A small proportion of sand or powdered clay is sometimes mixed with the borax. These applications tend to prevent the iron from running or burning, and they are supposed to assist the adhesion when the two pieces are brought together in the act of welding.
Hardening Iron by Hammering.—Iron may be hardened to the character of a pretty fair spring metal by simply hammering it thoroughly while in a cold state. Many of the cheap spiral springs in use, as those attached to small bells for the purpose of imparting a vibratory motion, are hardened or stiffened in this way. They are first cut from soft sheet-iron and then hammered into the required hardness. Some heat to a cherry red and hammer to and after entire coldness.
Case-Hardening.—The various parts of gun mountings, such as guards, heel plates, etc., and the different parts of locks, such as hammers, tumblers, triggers and plates, as received by the gunsmith from the manufacturer or dealer in such articles, are generally in the rough or partially finished condition. Many gunsmiths, particularly those in the country, where there is more or less a class of cheap work, finish up these parts with a file and a little hand polishing, and when the work is put together hand it over to the customer. Not only tumblers and triggers, but even sears and tubes are finished up in this manner. As these parts are almost always made of soft iron, the result is they soon wear and have to be repaired.
The gunsmith who does good work will thoroughly case-harden the parts when they are fitted and finished, and by so doing will turn out a really good piece of work that will wear as well as hardened steel. Why the majority of the trade do not case-harden their work cannot very well be explained, unless they are ignorant of the process or do not care to be put to the trouble of doing it. It is true it may be made a tedious job or a quick and easy one.
Some gunsmiths, when such work is finished, heat it red hot, smear it with prussiate of potash or cyanide of potassium, and while hot, plunge it into cold water, letting it chill. This produces a superficially hardened surface that is not “skin deep,” and as soon as this surface becomes abraded will wear away rapidly.
If the case-hardening of the expert manufacturer be examined, it will be observed that the surface of such work has a fine grayish appearance, and in many places mottled with colored tints that are pleasing and beautiful to the mechanical eye. It will be further observed that the hardening is of such depth that it will wear for a long time. In fact it will wear better than hardened steel. The condition of the material is that of a hardened steel surface stretched over and shrunk upon the iron body of the work. It is stronger than steel, for it has the tenacity of iron for its interior. It has the advantage of steel, inasmuch as it may be bent when cold to a limited degree, and when so hardened will not break as readily as steel. This property of bending is not confined to all articles, as they may case-harden entirely through, and then they will be very brittle and easily broken, but by drawing them to temper after hardening, in the same manner as a tool is drawn to temper, they may be of any hardness desired.
A good way to Case-Harden.—The easiest and perhaps the best way to case-harden gun work is to have a number of short pieces of common gas pipe, such as will be adapted to the size or quantity of the work, and have one end of these pieces securely plugged or closed. One way will be to heat the pipe and close by flattening the end with a hammer on the anvil, but it is a “slouchy” way of doing it. A neater way is to have a gas fitter cut a thread in the pipe and then screw in a plug, such as are used to close ends of gas pipe; if such cannot be obtained, drive in a cast-iron plug and upset the end of the pipe so that it will not readily come out. In these pieces of pipe place the work, packing it well with good, fine bone-dust, such as is used by farmers for fertilizing land. Be careful to so pack that the different pieces of work will not touch each other. Stop the open end of the pipe with a cover, but in such a manner as to be readily opened; place the pipe and its contents in a good fire, letting it remain at a red heat for fifteen minutes or more, dependent upon the thickness of the articles or the depth they should be hardened. Remove from the fire and quickly empty the contents of the pipe into a pail of cold water.
If pieces of gas pipe cannot very well be obtained, thimbles from old carriage hubs may be used instead. Plug up the small end, fit a cover to the large end and use as if it were gas pipe. As these thimbles are made of cast iron they will not bear the rough usage nor the heat that wrought iron will withstand. Common cast malleable iron makes the best receptacles to contain work for case-hardening.