While it is true that there are many variations of the principal processes of welding and joining metals, they may safely be placed in one of four general classes: smith or forge welding, electric welding, gas or hot-flame welding, and chemical welding. Included with these are such allied operations as soldering, brazing, and riveting for joining metals; so some reference will also be made to them, although the latter cannot be considered as welding in any sense of the term. Riveting is treated of here because it is one of the processes being superseded by the modern welding systems for some classes of manufacture, and the student should have some knowledge of the process and its relative value.
Smith welding, or forging, is the general process of forming or joining metals by hammering or pressing the pieces into the desired shape and may be done either hot or cold, depending upon circumstances. When joining two or more pieces of metal, especially iron or steel, it is done hot and is one of the oldest of the useful arts. It is the most common of all of the welding processes but depends more upon the skill of the operator than any other process of welding; hence it is gradually being superseded by them. It is also rather expensive and slow and is not so suitable for large or heavy work as some of the other systems.
Electric welding has been used as a laboratory process for a number of years but has recently been developed commercially to such an extent that it is rapidly coming to the front as the most important of all of the welding processes. Five different systems have been developed for using electrically generated heat for welding purposes, each of which has been named in honor of the inventor. These are the Thomson, Zerener, Benardos, Slavianoff, and LaGrange-Hoho systems, of which the Thomson has two forms, viz., "butt welding" and "spot welding," the names indicating the kinds of joint formed.
The Thomson system requires the use of alternating current whereas the other systems use direct current for welding. The Thomson system and its latter day modifications consist in bringing together the two pieces to be welded, in a special machine, passing an alternating current through the point of contact until the parts are heated sufficiently to be soft and then squeezing them together until they unite. The heating is due to the resistance of the joint to the passing of the current before the parts are soft enough to weld.
The LaGrange-Hoho system is also based on resistance and consists in placing the pieces of metal in a bath of electrolyte, causing the current to flow to them from a positive electrode and heating them until soft enough to weld. The actual weld is made by hammering, the same as smith welding; so this is not at present a very important process commercially.
The three other processes use the electric arc as the source of heat, and do their work by filling in additional material to join the pieces. The Zerener system is based upon using two pieces of carbon in a suitable holder, causing the arc or flame to be deflected toward the work by means of a magnetic field and using the arc to melt the filling material. The Bervardos system consists in using a piece of carbon or graphite as one electrode for the arc, drawing the arc between the carbon and the work (which is the other electrode) and using the heat of the arc to melt the filling material required for joining the pieces.
The Slavianoff system also consists in drawing an arc between the work and an electrode, but a piece of the filling material is used as one of the electrodes and melts directly into place on the job. This is the most important of the electric welding processes, although it is the most recently developed to a commercially practical point. It is applicable to practically every class of welding and for nearly all metals, and is the simplest of all welding processes. It will probably be in universal use within a very few years.
Gas welding, or hot-flame welding, is at present next in importance to smith welding and is applicable to many kinds of work which cannot be done by forging. The three most important processes commercially are known as the Oxy~acetylene, Oxy-hydro-gen, and Blau-gas processes. All of these processes consist in using oxygen and another gas to give a flame of sufficiently high tempera-ture and heating capacity to melt the material to be welded, the gas used with the oxygen being indicated by the name of the process. In all cases the oxygen and other gas are mixed in a suitable burner and the flame directed on the work in such a way as to cause the metals to flow together and, when no extra material is added to form the weld, it is said to be "autogenous" or self-forming. Electric-arc welding is also autogenous to the same extent, although the term is not strictly correct. The details of these systems will be described later.
Chemical welding is exemplified today almost exclusively by the process known as "thermit welding" and consists in igniting a mixture of oxide of iron and aluminum so as to set up a chemical reaction which evolves an intense heat. This is done by placing the mixture in a suitable mold and igniting, causing the aluminum to reduce the iron from its oxide, thus evolving the heat required and forming what is called "thermit-steel". The molten steel is allowed to run out of the mold and into and around the part to be welded, thus forming a "cast-weld" of considerable strength. A suitable mold must also be formed about the part worked upon in order to retain the metal until cooled; so this process is comparatively slow and expensive. Several other chemical processes have been developed but as they are not of very great importance commercially at this time, they will not be described here.
Brazing and soldering are processes which approach welding so closely in some of their applications that they are worthy of serious consideration as a part of that subject. Brazing consists in joining metal by fusing a filling material called "spelter" into the joint by heating, first preparing the surfaces of the joint with a suitable flux. When brass is brazed, it really becomes a welding process because brass is the principal constituent of spelter. The heat is produced by a gas flame and the work is done at a comparatively high temperature.
Soldering, or "metallic gluing", as it has been called, is done by melting a soft alloy into the space between the parts to be joined. It can be done with a gas flame or a heated soldering copper and at comparatively low temperatures. Soldering is a comparatively old process, is cheap, easily learned, and in wide use, but it should not be used for any joint requiring much strength. It is not suitable for joints in iron or steel or in several of the alloys.
Riveting consists in joining plates, sheets, or other shapes of metals by means of small pieces of metal which pass through the parts and are headed over on both sides. This process is also old and will be described in detail later in order to bring out the comparison between this and the newer methods of joining materials.
Several other processes of joining metals are being advocated by their inventors, among which may be mentioned the "Ferrofix brazing process" and the "Laffitte welding plate", although neither of them is in very extended use at present. Ferrofix brazing consists in cementing together two pieces of iron by means of a thin film of brass in such a way that the brass alloys with the iron and forms a joint that is stronger than the iron.
The necessary heat is supplied by a gas flame of suitable temperature. Laffitte welding plate consists of a special chemical preparation molded over a sheet of wire gauze and it is used by placing the plate between the surfaces to be welded. The parts are heated to a cherry red, the plate inserted, and the pressure applied while the reaction takes place. The preparation of the plate is such that it supplies the flux, the reagent, and the filling material and makes a joint of relatively high strength. Both of these processes have been introduced into the United States but recently and their relative values have still to be determined; they will be described in detail later.