Heat Treatment
When molten steel solidifies, austenite is formed. As further cooling takes place, the critical range is reached and the austenite goes through a transition until at the lower critical limit the familiar pearlite with excess ferrite or cementite, depending upon the carbon content is formed. The transition from austenite to pearlite through the critical range is normally a slow operation. It has been found that the transition can be arrested if this operation is speed up by such means as dropping austenitic steel just above the critical range in cold water or oil. By this means a structure can be produced at atmospheric temperature with physical characteristics different from those which normally would be obtained with slow cooling. This operation is so severe that an extremely hard, brittle material with shrinkage strains is obtained. By reheating the metal below the critical range the normal transition in the critical range is allowed to proceed a little further and shrinkage strains are reduced, thus creating a useful condition of moderate hardness and strength. The important consideration in the heat treatment of a piece of steel is to know its chemical composition in turn, determines its critical range. When the critical temperature is known the next consideration is the rate of heating and cooling to be employed to insure completion of transition or retardation of transition as the case may be. The aim in heating is to transform pearlite to austenite as critical range is passed through. This transition takes time; so a relatively slow rate of heating is employed. It is customary to insert the cold steel in the furnace when it is from 300 deg. to 500 deg.F. below the hardening temperature. In this way too rapid heating of the cold steel through the critical range is prevented. It is cheaper to keep a furnace up to the hardening temperature and remove heated steel and insert new cold steel periodically without permitting the temperature to drop several hundred degrees before inserting the new cold work. This is sometimes done where the work is not extremely important, but it does not guaranteed complete and thorough transition to austenite. There is also the possibility of cracking, depending on the shape of the material, due to rapid heating and expansion. Several types of furnace are employed in heating. The common type is a “dry heat” furnace and is fired by oil, gas or electricity. A uniform temperature must be maintained through out the furnace, and the work must be properly placed to insure uniform heating. The work must not be placed too close to the wall of the furnace; otherwise radiated heat from the wall will heat one face of the work beyond the rest, with resultant uneven heating. In a dry furnace it is desirable to maintain a neutral atmosphere, so that the heated steel will neither oxidize nor decarburize. Practically however, this condition is difficult to realize, and considerable scaling of the work results. In this respect the electric furnace is most the satisfactory because only a slight amount of scaling takes place. An atmosphere free of oxygen is maintained in one type of electric furnace by feeding a carbon vapor into it during heating operations. The carbon vapor is generated by “cracking” an oil in a smaller subsidiary furnace. There is a practically no scaling of the work in this type of furnace. Special paint coatings, such as “Galvo Anti-Scale,” are sometimes used to minimize scaling during the heating operation when atmospheric control is not available.