Pearlite is a definite conglomerate of ferrite and cementite containing about six parts of the former to one of the latter. When pure, it has a carbon content of about 0.95 per cent. It represents the complete transformation of the eutectoid austenite accomplished by slow-cooling of an iron-carbon alloy through the transformation range. (See Fig. 46.)
FIG. 50.—Quenched high-carbon steel, polished, etched and viewed at 100 magnifications. This structure is called martensite and is desired when maximum hardness is essential. Photograph by H. S. Rawdon.
| FIG. 51.—Martensite (light needles) passing into troosite (dark patches). 130 X. From a piece of eutectoid steel electrically welded. | FIG. 52.—Sorbite (dark patches) passing into pearlite (wavy striations). Light Areas are Patches of Ferrite. 220 X. From a piece of hypo-eutectoid steel electrically welded. |
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° C., forming troostitic steel. "Toughening" represents the practice of reheating hardened carbon steels from 350° 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.
