A fire, when not properly handled becomes hollow, due to the center burning out. If iron is heated in this kind of a fire, it will become oxidized, that is to say, a dirty scale will form over the metal. Iron cannot be properly heated, and it is impossible to get the welding heat with a fire in this condition. The reason a fire becomes hollow is that it may be filled with clinkers, or too much blast may have been used, and when it comes in contact with the pieces being heated causes them to cool and oxidize. Sometimes the fire will not be directly over the hole in the tuyere; which is one cause of poor heating. This is a common fault with boys working at the forge. Always have the fire over the hole in the tuyere, and not to one side.
When the fire becomes hollow and dirty, clean it by picking out the clinkers with the poker or scraper, then move the sides of the fire towards the center of the tuyere with the shovel, keeping the well-coked inner sides near the center of the tuyere, and having the center of fire over the hole in the tuyere. Wet coal is now banked around the outer sides. Always have a thick bed of coke under the piece being heated and regulate the blast so as not to burn out the center of the fire at once. See drawing of fire with piece about on the same plane with bottom of hearth; notice dotted lines representing the wrong way to put stock in the fire. ([Fig. 20.])
Fig. 20. Section of Forge Fire.
If two pieces of iron are placed in the fire and heated, they will become gradually softer until they reach a state where the metal has become sticky. If touched together the two pieces will stick. This is what is known as welding heat. If they were taken to the anvil and hammered while in this condition they would unite and become one piece. This would be called welding. All metals cannot be welded. Iron, soft steel, low-carbon tool steel and spring steel can be welded.
A flux is used in welding steel—this excludes the air and forms a pasty surface on the metal which is squeezed out from between the surfaces of the metal when hammered. Borax and the many welding compounds are used. Very seldom is it necessary to use a flux on iron. Clean sand, which is good, is used by many. Borax or welding compound is sometimes used on very thin stock. For ordinary welding, such as is being done in school shops, borax should never be used. It is poor practice, unnecessary, and a useless waste.
In heating iron, if it is brought beyond the welding heat, it will become softer and softer until it will finally burn. This may be known by the great number of little explosive sparks coming from the fire. These little sparks are particles of iron separating from the bar and burning. As the heat gradually rises, the metal separates. If the bar were now placed on the anvil and struck a hard blow with a hammer, it would fly to pieces. Therefore, judgment must be used in striking the first blow on any welding heat—it should be light. The succeeding blows should be made gradually harder. A hard blow at the start might make the metal fly to pieces, or make the upper piece slip away from the under piece. If lighter blows were struck, the weld might be made in good shape.
The principal thing in welding is to have a clean fire. All of the clinkers must be kept out. The fire should be a well burned one, without much smoke or gas, and never any green coal near the pieces being heated. Well burned pieces of coke around the metal should always be used in raising the welding heat. In raising the welding heat very little blast should be used at first. Heat the pieces slowly so as to get them hot thruout.