The weldings which are alluded to here are such as are used by practical blacksmiths in their general work without any special appliances or apparatus whatever. The majority of the exercises on welding in this book require the use of iron; for this reason this preliminary consideration of metals need not have any further special attention.
61. Heating.—When the word “fuel” is used here, either coal or coke may be meant. Coal is the original in either case, for coke is formed from it by the removal of gaseous substances. It is better that the coal first be converted into coke, and that only the coke should come in direct contact with the heating metals.
Fig. 42.—Sectional View of a Blacksmithing Fire.
Figure [42] shows a sectional view of a blacksmithing fire: d is the bed of hot coke; c is the dampened and unburned coal which surrounds the fire, continually forming more coke as it is needed and also holding the fire in a compact form; a shows the proper way of placing the metal in the fire, b, the improper way because the metal is too near the entrance of the blast. As heating is such an important operation, a thorough understanding of what causes imperfect heats, as well as how to prevent them, is necessary.
The best fire for perfect heating is a reducing one, that is, one in which the combustion of the fuel is rapid enough to use entirely the oxygen in the air which is supplied. An oxidizing fire is one that does not use all the oxygen in the blast for the combustion of the fuel. The surplus oxygen will produce, on the surface of the metal, oxide of iron, or a black scale, which is extremely injurious. This scale will prevent welding, so all possible precautions should be taken to avoid its forming.
A reducing fire can be maintained, and an oxidizing one avoided, by having plenty of fuel surrounding the metal, equally, and allowing the entrance of only sufficient air or blast to provide the necessary heating.
If a piece of metal is left in a fixed position while heating, the lower side will become the hottest. For that reason, all metals to be welded are placed with scarfs downward. If the required heat is to be a penetrating and thorough one, the metal is turned frequently to bring all surfaces in contact with the most intense point of heat.
Even though every possible precaution is taken in all other steps of the welding, the pieces cannot be joined perfectly if the heating is carelessly done.