Let us first consider the working of iron and steel. The temperature at which the different irons and steels may be heated without injury depends upon their quality. Ordinarily iron can be heated to a higher temperature than steel. The milder the steel, that is, the smaller the percentage of combined carbon, the higher the temperature to which it can be heated without injury. In other words, the less the variation of the steel from the condition of wrought iron the higher the temperature at which it can be worked.
Iron and steel are heated for working in order to make them more plastic and easily shaped. The higher the temperature the softer the metal becomes. As the metal is heated it rises in tern-perature and changes in external appearance. It is first Mack, then blue. then a dull red, then a bright red, (usually called a cherry red), then yellow ; this yellow fades into a brilliant straw color which in turn fades into a white, at which point fusion takes place. The usual color at which wrought iron and steel are worked is that of a bright cherry red.
Where a weld is to be made, however, the metal must be heated to a white heat. The higher grades of steel cannot be heated to this temperature without being ruined. By "higher grade " is meant steels with a greater percentage of combined carbon.
Fig. 44. WEIGHT OF FLAT ROLLED IRON. Length, 12 inches.
In heating metal the fire should be burned according to the work in baud. If the three bars a, b and c of Fig. 41 are to be heated at the unshaded sections indicated by d,. e and f respec-lively, the fire must be manipulated accordingly. In the ease a where it is to be heated hut a short distance from the end, a small fire will answer. As the forge is-so constructed that a lire may Iks burned over considerable of its area, some means must be taken to limit the lire for small work. This is usually done by saturating the coal that covers the forge beyond the limits of the desired firepot with water. This is shown to have been done in Fig. 42. The end of the bar a is thrust down into the fire which is confined to the pot g by the wet coal h h. The iron should be put in that portion of the fire which is the hottest. If the end of a in Fig. 42 were to be put down against the tuyere, the cold air issuing from the same would prevent it from becoming hot. If, on the other hand, it were laid on the top of the fire it would radiate heat, so rapidly that it would not beome hot. The part to be heated must, therefore, be buried in the incandescent fuel.
In the bar h where the length to be heated is greater than in a, it may not be possible to put the bar down into the fire at an angle as in Fig. 42. A larger fire, as in Fig. 43, must be carried. The fire-pot is made larger by extending the amount of dry fuel. The bar is laid in horizontally and the fuel heaped over it.
With the bar c the fuel is also heaped over the metal as in Fig. 44. In this case the firepot is again contracted by the use of wet coal about the edges.
These are the three typical fires. They are to be used according to the work in hand.