These observations are competent to explain annealing and toughening practice. A quickly quenched carbon steel is mostly martensitic which, as noted, is a solid solution of beta iron and cementite, hard and brittle. Moderate reheating or annealing changes this structure largely into troostite, which is a partly transformed martensite, possessing much of the hardness of martensite, but with a largely increased toughness and shock resistance. This toughness is the chief characteristic of the next material in the transformation series, sorbite, which is merely martensite wholly transformed into a mixture of ultramicroscopic crystals of ferrite (alpha iron) and cementite (Fe3C). "Tempering" or "drawing" should be restricted to mean moderate reheating, up to about 350 deg. C., forming troostitic steel. "Toughening" represents the practice of reheating hardened carbon steels from 350 deg. C. up to just below the lower critical, and forms sorbitic steel; while "annealing" refers to a heating for grain size at or above the transformation ranges, followed by a slow cooling. Any of these operations not only allows the transformations from austenite to pearlite to proceed, but also relieves internal stresses in the steel. Normalizing is a heating like annealing, followed by a moderately rapid quench. 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. [Illustration: 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. [Illustration: FIG. 54.--Using electro-magnet to determine heat.]