JUDGING THE HEAT OF STEEL
While the use of a pyrometer is of course the only way to have accurate knowledge as to the heat being used in either forging or hardening steels, a color chart will be of considerable assistance if carefully studied. These have been prepared by several of the steel companies as a guide, but it must be remembered that the colors and temperatures given are only approximate, and can be nothing else.
FIG. 53.—Finding hardening heats with a magnet.
The Magnet Test.—The critical point can also be determined by an ordinary horse-shoe magnet. Touch the steel with a magnet during the heating and when it reaches the temperature at which steel fails to attract the magnet, or in other words, loses its magnetism, the critical point has been reached.
Figures 53 and 54 show how these are used in practice.
The first (Fig. 53) shows the use of a permanent horse-shoe magnet and the second (Fig. 54) an electro-magnet consisting of an iron rod with a coil or spool magnet at the outer end. In either case the magnet should not be allowed to become heated but should be applied quickly.
FIG. 54.—Using electro-magnet to determine heat.
The work is heated up slowly in the furnace and the magnet applied from time to time. The steel being heated will attract the magnet until the heat reaches the critical point. The magnet is applied frequently and when the magnet is no longer attracted, the piece is at the lowest temperature at which it can be hardened properly. Quenching slightly above this point will give a tool of satisfactory hardness. The method applies only to carbon steels and will not work for modern high-speed steels.