The art of joining metals is one of the oldest known to man. Ever since the first cave dweller or half-monkey man hammered his first piece of iron or copper between two stones, the development of the art has gone forward, and will probably continue to develop as long as men use metals. The welding of iron is apparently as old as the production of that metal by man, for there is, in a temple yard of the ancient city of Delhi, India, an iron pillar nearly two thousand years old, which shows unmistakable evidences of having been welded. The shaft projects 22 feet above the surface of the ground, extends over 40 feet into the earth, and is about 16 inches in diameter. It was apparently welded into one piece from blooms of about 70 pounds weight each; the joints are as perfect as could be made with our most modern equipment, and yet they were forged by hand.
Strictly speaking, "welding" is the uniting or joining of two pieces of metal by hammering them together while they are hot enough to be plastic and the application of the term would thereby be limited almost entirely to work done in a blacksmith shop. But modern methods of obtaining high temperatures by gases and electricity have made possible the development of other and better methods of joining metals, many of which cannot be welded by hammering. Custom has applied the term "welding" to these also, although some of them are really brazed or soldered with metals of high melting temperatures. Any process by which cohesion between the molecules of the pieces to be joined is brought about may be called "welding".
Metals are most easily welded when in that degree of plasticity between the molten and the solid states; hence, those metals which remain plastic the longest while cooling are the easiest to weld. Originally welding was limited to such metals as iron, platinum, nickel, and gold but the recent development of high temperature systems has extended the field greatly. In fact, it is safe to say that every metal may be welded by some one or other of the modern methods. So recently as five years ago it was considered impossible to weld aluminum; but this is being done successfully today with the electric-arc system, using the graphite electrode, and practically every kind of alloy is also being welded by both the electric-arc and the gas systems.
Successful welding by any method depends almost entirely upon three factors: flow, cohesion, and temperature. The metal must tend to flow under great pressure, even if but to a slight degree. The surfaces of the pieces to be welded must tend to "wet" each other or cohere to an appreciable extent. The working temperature must be that at which the foregoing conditions are most prominent The best welding condition for iron and steel exists within a limited range of temperature only and, when in this condition and at this temperature, they possess the property of expanding when cooled and contracting when heated. Therefore, modern welding systems are designed to take advantage of these well-known laws in the best and most economical manner, or else they are not considered commercially desirable and will, therefore, soon be abandoned.
It is for the purpose of describing in a general way these systems and their applications that this book has been compiled and it is our desire that all students of the art of welding may profit to the highest degree through their study of this subject. The field is expanding rapidly and great financial gains should be possible to the well informed during the next few years.