The process of welding consists of heating the faces of the two pieces to be joined until they are at the point of fusion (called the welding heat), then uniting them by blows or pressure. The temperature of iron when at a welding heat is 1900° Fahrenheit. The appearance of the metal is nearly white and when exposed to the air it will emit bright sparks. It will also give forth a slight noise similar to that produced by the escape of a small jet of steam. The theory of welding is that the semi-liquid particles of the metal are pressed into the same condition of close contact that they have in other parts of the bar. Interposition of foreign matter between the; two pieces to be joined will prevent welding. The three things that most commonly intervene to prevent welding are air, scale and dirt. Air intervenes when the two surfaces to be welded form a pocket upon being brought together. This occurs when they touch only at the edges and one or both of the surfaces are concave.
If iron is heated to any temperature above a cherry red its affinity for oxygen increases. When at a straw color or hotter the affinity is very great. This affinity results in the production of an oxide of iron known as scale. If scale forms on the surface of the metal between the time that it is drawn out of the fire and the weld made, it will prevent the union over such area as it covers. The welding of all metals is not done at the same temperature. Welding demands a certain condition of plasticity of surface. If this condition is not reached welding fails for want of contact due to excessive oxidation. The temperature of this certain condition of plasticity varies with the composition of the iron. For this reason irons having different welding points cannot be soundly welded to each other in an oxidizing flame.
In order to prevent the formation of scale and to permit the welding of irons of different compositions, a flux is used. Ah defined in "Foundry Work," page 10, a flux is, "a substance that promotes the fusing of metals".
For welding iron, clean river sand or powdered sandstone makes a good flux. Jt is useless, however, for welding steel or steel and iron. For this purpose borax is generally used. The flux is applied shortly before, the metal reaches the welding beat, no matter what the temperature of that heat may be. It melts on the surface of the metal, forming a slag. which covers the surface to be welded and prevents oxidation. When the two surfaces are brought together and subjected to a pressure or blows, the slag flows or is pressed out from between those surfaces and actual contact is secured.
Surfaces to be welded are first shaped suitably for the purpose. Ordinarily when two bars are to be welded each is beveled as shown for round bars in Fig. 50. This beveling is called scarfing, and should be so done, that when the weld is Complete there is but a slight enlargement of the newly formed bar at that point. For heavy work where the metal must remain in the fire for a long time in order to reach the welding heat, there is a different method of scarfing. This is shown in Fig. 51. One piece is opened in the form of a jaw, the other is wedge-shaped and fits the former. Where the weld II to be made between heavy pieces, the surfaces should be left con-vex as at A A in Fig. 51, This facilitates the outflow of the slag and the escape of all other impurities.
Sometimes a weld is made without scarfing; such is called a buit weld. This is best done by .slightly rounding the ends that are to be joined, as shown in Fig. 52. As the ends are forced together the metal flows outward forming a ring as in Fig. 53. This is then hammered down to the diameter of the bar by the use of the top and bottom swages, Figs. 84 and 65.
When a small bar is to be welded to a heavier piece of metal and a butt weld is used, it is said to be jumped on. Such a weld is shown at A of Fig. 54. In preparing the parts for this weld the bar should be rounded at the end as shown in Fig. 52, and the heavy piece left flat or slightly cupped. When the weld is being made the metal will flow out about the point of union. This metal should afterwards be fullered down to form a fillet as shown in Fig. 80.
The edges of the scarf will burn away to a certain extent in heating. The scarf should therefore be made enough larger that the weld may be of full size after completion. All scarfs should be slightly rounding, in order to permit the escape of air and slag. The heating can best be done in a clear fire of coal containing little gas. In malting heavy welds, heat slowly. If an attempt is made to heat large pieces too rapidly, the interior will not be heated sufficiently. Turn the metal in the fire frequently in order to heat evenly and avoid burning. Withdraw occasionally from the fire and sprinkle with flux. When welding strike light blows at first and heavy ones thereafter. As soon as the pieces are joined, strike so that the edges of the scarf will be closed. This will prevent the joint of the weld from showing when the work is completed.
Sand can be used as a flux for welding iron. For welding steel, borax should be used. To weld steel successfully the following precautions should be oberved. Clean the fire of all cinders and ashes. Put sufficient coal upon the fire so that it will be unnecessary to add more coal while taking the welding heat: Upset both pieces near the end and scarf carefully: When possible, punch a hole and rivet the two pieces together: Heat the steel to a low cherry heat and sprinkle with borax: Replace in the fire and raise to the welding heat: Clean the scarfed surfaces and strike lightly at first, followed by heavier blows. The appearance of steel when at a welding heat is a pale straw color. German steel can be heated to about the same temperature as iron.
Steel may also be welded to wrought iron. This is done in the manufacturing of edged took. The body of the tool is of iron, to which a piece of steel is welded and forms the cutting edge. This class of work is best done with a fire of anthracite coal, though cake or charcoal may be used. The fire should be burning brightly when the heating is done. Lay the iron and steel on the coal until they are red hot. Then sprinkle the surfaces of both with the flux and let it vitrify. A convenient method of doing this is to have the powdered flux (borax preferred) in a pepper-pot. As soon as the heat has changed the metals to a straw color lay them together and strike. A single blow of a drop hammer, or four or five with a light sledge will do the work. Be sure that these pieces are well covered with a flux before attempting to weld.
Two pieces of high grade steel can also he welded in the same manner.
No attempt should be made to do this class of work in an ordinary forge like that illustrated in Figs. 4 or 5. The forge or furnace should be especially built for burning anthracite coal. Such a furnace is shown in Fig. 55. This furnace is suited for hard coal or coke. It is fed from the hoppers shown at each end, and can be cleaned through the pan at the bottom. The metal to be heated is laid on top of the coals beneath the brick roof.
When burning anthracite coal, the top of the bed of fuel should not be disturbed. The poker should only be used beneath. Soft or bituminous coal fires can be raked over on the top. An anthracite fire can only be retained by banking and admitting enough draft beneath the fuel to keep it ignited. The simplest method of retaining a bituminous coal fire, when not in use, is to thrust a piece of hard wood down into the bed of coals and cover with a thin layer of ashes. To start the fire again, rake off the ashes, and put on the blast. The wood will quickly ignite and set fire to the coal.