232 (b) They occur because right at the phase transition temperature, the driving force is not sufficient to cause the transformation to occur. The driving force is enhanced during superheating or supercooling. 10.10 We are called upon to consider the isothermal transformation of an iron- carbon alloy of eutectoid composition. (a) From Figure 10.14, a horizontal line at 550 ° C intersects the 50% and reaction completion curves 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 of composition 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 the eutectoid decreases with increasing temperature because the reaction rate is controlled by the rate of nucleation of pearlite; the rate of nucleation decreases with rising temperature because the activation energy in Equation (10.3) increases with increasing temperature.
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