A simple method of ascertaining what heat a steel will stand before beginning to burn is as follows: Heat the steel to its burning point, and at various stages, beginning at the yellow stage, lay it on the anvil and give it a few blows with the hand hammer. Repeat the process until a heat is reached that will cause the steel, when struck with the hammer, to give off sparks like small fireworks. When this stage is reached it will show that the burning point is arrived at.
Careful observations of these points will enable the smith to know just when to begin to use the flux for welding, which, by the way, must be just before the steel reaches the burning point.
Another important point is the formation of the scarfs. These should not be fullered down so thin or left in the same form as the scarfs as when joining iron, and should be shorter (compare A and B, Fig. 66).
[Illustration: Fig. 66.--Scarfs for Iron and Steel]
In bringing the steel up to its welding point, care must be taken to have it at a uniform heat throughout at the part for welding; and, to get this, the blast should not be forced at the start, but used gently. In some cases the blast should be stopped occasionally, to allow the steel to soak. Then restart the blast, and gradually force it when beginning to use the flux, continuing so until the welding point is reached. Only light blows should be given at the start, just to cause the two parts to stick together; but when stuck, the harder the blow the better the weld.
An idea seems prevalent that the flux has a certain influence on the steel, and converts it into a form that makes it more weldable. The real use of a flux in this case is simply to retard the heat, and form a coat or shell on the steel, and so counteract and prevent the burning action which takes place when heating steel in an ordinary blast fire.
The reason why different fluxes are required for different brands of steel is no doubt due to the differences in manufacture. As a rule, the greater the heat the steel will stand before burning, the less it requires a flux to protect it, so that in a great many cases a flux consisting of some clean, sharp sand is all that is required; but the steels that will burn at a lower heat require something more than sand to protect them. Hence arises the necessity of adding burnt borax, crushed glass, powdered marble, etc.