If a piece of steel or iron is heated, the metal becomes softer as the temperature is raised. Finally a heat is reached, called the welding heat, at which the metal is so soft that, if two pieces similarly heated are placed in contact, they will stick. If the pieces so heated are placed together and hammered, they may be joined in one piece. This process is known as welding.
The greatest difficulty in welding is to heat properly, which must be done evenly and cleanly. If the temperature is raised too high, the iron will burn, throwing off bright star-like sparks. If the temperature is too low, the pieces will not stick to each other. The proper heat can only be determined by experimenting, which may be easily done by doubling over a piece of scrap iron for 2 or 3 inches and welding into a solid piece.
Fig. 26. Work Supported by Jib Crane.
As the welding heat is reached, in heating wrought iron and mild steel, small particles of the metal are melted and blown upward from the fire by the blast, and as these small particles come in contact with the air, they burn and form small explosive sparks like little white stars. Whenever these sparks are seen coming from the fire, it is a sure indication that the iron is burning. They are sometimes used as a sort of an indication of the welding heat, but the only sure way of determining the heat is by the appearance of the heated iron, which might be described as sort of creamy white. The welding heat is sometimes described as a white heat. This is not correct, because iron or steel is never raised to a white heat even when melted, as may be easily proved by comparing a piece of wrought iron at welding heat, with an ordinary arc lamp.
When two pieces of metal are welded together there must be nothing between them. Heated iron or steel is always covered with scale (iron oxide). This scale, if allowed to stay on the surfaces to be joined, will prevent a good weld. It is necessary when welding, to heat the iron or steel to a high enough temperature to melt this scale and when the two pieces are put together, if the joint or scarf is properly made, most of this melted scale is easily forced from between the two pieces, leaving the clean surfaces of the metal in contact. This scale only melts at a very high heat, much higher than the heat at which it would be possible to weld the iron if it could be kept free from scale.
Fluxes are used to lower the melting point of the scale. The flux is sprinkled on the surfaces to be joined just before the metal reaches the welding heat. The metal is then put back into the fire, raised to the welding heat and the weld made as usual. The scale is acted upon by the flux and melts at a lower heat than when no flux is used. As the flux melts it spreads or runs over the hot metal and forms a sort of protective covering, which, by keeping out the air, prevents to a large extent the formation of more scale. The flux in no way acts as a cement or glue to stick the pieces together, but merely helps to melt off the scale already formed, and prevents the formation of more.
These substances are common fluxes. Sand may be used when welding wrought iron and machine steel; borax is substituted for sand for fine work and when welding tool steel.
Borax is a better flux, as it melts at a lower temperature than sand, and thus makes welding possible at a lower heat. Borax and sal ammoniac (ammonium-chloride) are sometimes mixed and used as a welding compound or flux, the proportion being about 4 parts borax to 1 part sal ammoniac. This mixture is also a good flux for brazing. Borax contains a large amount of water which makes it boil and foam when melting and in this condition is very liable to drop away from the heating metal. If borax is heated red hot and allowed to cool, the water is driven off and the borax is left in a glass-like condition. Borax treated this way and then powdered is the best for welding, as it melts and sticks to the metal without any boiling.
These are fluxes serving the same purpose as sand or borax. Some of the better ones use borax as a basis. Some of these compounds are first class for their purpose and others are not so good, being simply intended as cheap substitutes for borax.
For most welding the ends of the pieces to be joined must be so shaped that when welded they make a smooth joint. This shaping of the ends is known as scarfing, and the shaped end is called a scarf. The scarfed ends should not fit tightly before welding but should be so shaped that they touch in the center of the joint, leaving the sides somewhat open. In this way, when the weld is made, the melted scale is forced from between the pieces. If the scarfs were made to touch on the edge of the joint, leaving the center hollow, the scale not having a chance to escape would be held in the center of the joint, leaving a weak place, and making a bad weld.