232(b) They occur because right at the phase transition temperature, the driving force is notsufficient to cause the transformation to occur. The driving force is enhanced duringsuperheating or supercooling.10.10 We are called upon to consider the isothermal transformation of an iron- carbon alloy ofeutectoid composition.(a) From Figure 10.14, a horizontal line at 550°C intersects the 50% and reaction completioncurves at about 2.5 and 6 seconds, respectively; these are the times asked for in the problem.(b) The pearlite formed will be fine pearlite. From Figure 10.22(a), the hardness of an alloy ofcomposition 0.76 wt% C that consists of fine pearlite is about 265 HB (27 HRC).10.11 The reaction rate for the austenite-to-pearlite transformation at temperatures just below theeutectoid decreases with increasing temperature because the reaction rate is controlled by therate of nucleation of pearlite; the rate of nucleation decreases with rising temperature becausethe activation energy in Equation (10.3) increases with increasing temperature.
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Austenite, Ferrite, pearlite, spheroidite, fine pearlite