59. Welding, the most difficult operation in the art of forging, is the process of joining two or more pieces of metal into one solid mass.
All the previous operations allow some time for thought; in welding, the worker must determine instantly where each blow is to be delivered, as the welding heat of the metal vanishes rapidly; therefore, he is compelled to think and act very quickly.
A scientific analysis of a perfect weld shows that it consists of several processes, and that each one must be perfectly executed. If any of these operations are improperly done, the result will be a partial failure; if they are essential ones, the weld may readily be considered as totally unfit.
60. The Material for Welding.—This must be considered, because there are different qualities in each metal to be operated upon, and some metals can be worked more easily than others.
A cross section of a bar of iron viewed through the microscope is seen to be made up of a great number of layers or fibers, called laminæ, resembling the grain or fiber in wood. These were cemented together in the process of rolling or welding in the mill where the iron was manufactured, and are continuous through its length. This makes the bar of uniform quality throughout.
In welding, these fibers are joined diagonally at the ends, consequently the strength of the weld depends entirely on how closely or perfectly this cohesion is made. Careful hammering at the proper heat brings the fibers in as close contact as possible, squeezes out the slag and scale, and therefore greatly assists in strengthening the weld.
Iron is an easy metal to weld. To prove this, place two pieces of iron in a clean, non-oxidizing fire, allowing them to attain a white or welding heat; then place them in contact and notice how readily they stick together, proving that iron is easily welded at the proper temperature. But in order to make the contact thorough, the pieces must be hammered. This shows that hammering is a secondary operation, and that iron cannot be joined by either heating or hammering alone.
By a similar experiment with soft steel, you will notice that the pieces do not adhere like iron. If borax is applied while they are heating, then slight indications of adhesion will be noticeable. This shows that borax, sand, or something of a like nature must be used in welding steel. In this case hammering is not a secondary operation, but an essential one.
A higher carbon or tool steel may be experimented upon, with nearly the same result. The noticeable difference between the lower and higher qualities of steel proves that the greater the quantity of carbon, the harder will be the welding, and if the experiments were extended to still higher carbon steels, it would be discovered that they could not be joined except by the use of a specially prepared flux. There are indeed some high carbon steels that cannot be welded.
If a forging is to be made of a special quality of material, it is frequently advisable to avoid welds, because two pieces that are welded can hardly be considered so strong as a piece of the same material that has not been welded.