【正文】 te, which serve as additional crystallization nuclei. Quasieutectoid We have discussed the austenite to pearlite transformation in steels whose position is close to eutectoid. If the content of carbon in steel differs from the eutectoid value, the pearlite transformation will be preceded with the precipitation of ferrite or cementite (as follows from the iron carbon constitutional diagram). In hypoeutectoid steels, the transformation of austenite begins with the formation of ferrite and the saturation of the remaining solution with carbon, and in hypereutectoid steels, with the precipitation of cementite and depletion of the austenite of carbon. Under equilibrium a condition, 6 the deposition of austenite into ferrite and cementite (pearlite transformation) begins when the content of carbon in austenite, remained upon precipitation of excess ferrite or cementite, corresponds to % carbon. The eutectoid which forms from undercooled austenite and has a concentration differing from the eutectoid value is called quasieutectoid in hypereutectoid steels contains more percent carbon and that in hypoeutectoid steels, less than percent, the deviation from this value being greater at lower temperature of transformation. Therefore, the lower the temperature of transformation, the less the excess ferrite (or cementite) precipitates before the pearlite transformation begins. At temperature near the bend of C curve and at lower temperature, deposition of austenite begins without precipitation of excess phases. If we take a hypereutectoid steel instead of hypoeutectoid, the deposition of austenite at small degrees of undercooling will be preceded with precipitation of cementite. Martensite Transformation If the cooling rate is higher, the transformation has no time to proceed in the upper temperature range. The austenite will be undercooled to a low temperature and will transformation martensite. Such a cooling will result in hardening. Therefore, to harden steel, it should be cooled at a high rate so that austenite has no time depose in the upper temperature range. The lowest cooling rate needed to undercool austenite up to martensite transformation is called the critical rate of hardening. If steel is to be hardened, it should be cooled at a rate not less than the critical rate. The critical rate is lower for steels whose curve of the beginning of transformation passes farther to the right. In other words, with a lower rate of austenite to pearlite transformation, it is easier to undercool the austenite to the temperature of martensite transformation and the critical rate of hardening will be lower. If cooling is done at a rate slightly below the critical rate, the austenite will undergo only a partial transformation in the upper temperature range and the structure will consist of the products of transformation in the upper temperature range (troostite) and martensite. The critical rate of hardening can be determined from the diagram of isothermal deposion of austenite. This analysis shows that a simple superposition of cooling curve on the isothermal diagram of austenite deposition can give only an approximate quantitative estimation of a transformation occurring in continuous cooling. 鋼的熱處理原則 在現(xiàn)代機械工程中，熱處理的作用不能被過高估計 ，但是， 熱處理使金屬性能發(fā)生變化 7 仍 是具有重要意義的。 heating rate h? and cooling rate c? . If heating (or cooling) is made at a constant rate, the temperaturetime relationship will be described by a straight line with a respective angle of incline. With a varying heating (or cooling) rate, the actual rate should be attributed to the given temperature, more strictly, to an infinite change of temperature and time: that is the first derivative of temperature in time: ?? ddtact /? . Heat treatment may be a plex process, including multiple heating stages, interrupted or stepwise heating (cooling), cooling to subzero temperature, etc. Any process of heat treatment can be described by a diagram in temperaturetime coordinates. Formation of Austenite The transformation of