【正文】 les a wide range of properties to be achieved through heat treatment. To begin to understand these processes, consider a steel of the eutectoid position, % carbon, being slow cooled along line xx’ in . At the upper temperatures, only austenite is present, the % carbon being dissolved in solid solution with the iron. When the steel cools to 727℃(1341℉), several changes occur simultaneously.The iron wants to change from the FCC austenite structure to the BCC ferrite structure, but the ferrite can only contain % carbon in solid solution. The rejected carbon forms the carbonrich cementite intermetallic with position Fe3C. In essence, the net reaction at the eutectoid is austenite %C→ferrite %C+cementite %C.Since this chemical separation of the carbon ponent occurs entirely in the solid state, the resulting structure is a fine mechanical mixture of ferrite and cementite. Specimens prepared by polishing and etching in a weak solution of nitric acid and alcohol reveal the lamellar structure of alternating plates that forms on slow cooling.This structure is posed of two distinct phases, but has its own set of characteristic properties and goes by the name pearlite, because oits resemblance to mother of pearl at low magnification.Steels having less than the eutectoid amount of carbon (less than %) are known as hypoeutectoid steels. Consider now the transformation of such a material represented by cooling along line yy’ in . At high temperatures, the material is entirely austenite, but upon cooling enters a region where the stable phases are ferrite and austenite. Tieline and levellaw calculations show that lowcarbon ferrite nucleates and grows, leaving the remaining austenite richer in carbon.At 727℃(1341℉), the austenite is of eutectoid position (% carbon) and further cooling transforms the remaining austenite to pearlite. The resulting structure is a mixture of primary or proeutectoid ferrite (ferrite that formed above the eutectoid reaction) and regions of pearlite.Hypereutectoid steels are steels that contain greater than the eutectoid amount of carbon. When such steel cools, as shown in zz’ of the process is similar to the hypoeutectoid case, except that the primary or proeutectoid phase is now cementite instead of ferrite.As the carbonrich phase forms, the remaining austenite decreases in carbon content, reaching the eutectoid position at 727℃(1341℉). As before, any remaining austenite transforms to pearlite upon slow cooling through this temperature.It should be remembered that the transitions that have been described by the phase diagrams are for equilibrium conditions, which can be approximated by slow cooling. With slow heating, these transitions occur in the reverse manner. However, when alloys are cooled rapidly, entirely different results may be obtained, because sufficient time is not provided for the normal phase reactions to occur, in such cases, the phase diagram is no longer a useful tool for engineering analysis. Hardening Hardening is the process of heating a piece of steel to a temperature within or above its critical range and then cooling it rapidly. If the carbon content of the steel is known, the proper temperature to which the steel should be heated may be obtained by reference to the ironiron carbide phase diagram. However, if the position of the steel is unknown, a little preliminary experimentation may be necessary to determine the range. A good procedure to follow is to heatquench a number of small specimens of the steel at various temperatures and observe the result, either by hardness testing or by microscopic examination. When the correct temperature is obtained, there will be a marked change in hardness and other properties. In any heattreating operation the rate of heating is important. Heat flows from the exterior to the interior of steel at a definite rate. If the steel is heated too fast, the outside bees hotter than the interior and uniform structure cannot be obtained. If a piece is irregular in shape, a slow rate is all the more essential to eliminate warping and cracking. The heavier the section, the longer must be the heating time to achieve uniform results.Even after the correct temperature has been reached, the piece should be held at that temperature for a sufficient period of time to permit its thickest section to attain a uniform temperature.The hardness obtained from a given treatment depends on the quenching rate, the carbon content, and the work size. In alloy steels the kind and amount of alloying element influences only the hardenability (the ability of the workpiece to be hardened to depths) of the steel and does not affect the hardness except in unhardened or partially hardened steels.Steel with low carbon content will not respond appreciably to hardening treatment. As the carbon content in steel increases up to around %, the possible hardness obtainable also increases. Above this point the hardness can be increased only slightly, because steels above the eutectoid point are made up entirely of pearlite and cementite in the annealed state. Pearlite responds best to heattreating operations。 and steel posed mostly of pearlite can be transformed into a hard steel. As the size of parts to be hardened increases, the surface hardness decreases somewhat even though all other conditions have remained the same. There is a limit to the rate of heat flow through steel. No matter how cool the quenching medium may be, if the heat inside a large piece cannot escape faster than a certain critical rate, there is a definite limit to the inside hardness. However, brine or water quenching is capable of rapidly bringing the surface of the quenched part to its own temperature and maintaining it at or close to this temperature.Under these circumstances there would always be some finite depth of surface hardening regardless of size. This is not true in oil quenching, w