Improved Method to Verify the Additivity Rule for Pearlite Transformation in Eutectoid Steel

Abstract

The additivity rule is beneficial when kinetics between the isothermal and non-isothermal (usually continuous cooling) phase transformation is interrelated. When isothermal transformation (Time Temperature Transformation (TTT)) follows the Johnson-Mehl-Avrami equation (X = 1 - exp (-k(TTT)tau(n)(TTT)(TTT))) even though the reaction exponent is a function of temperature, if n 1 is larger than 1.0, as happens in pearlite transformation, it is confirmed that isothermal kinetics is derived from continu- ous cooling kinetics by the same approach proposed by Ozawa, if the additivity rule is applicable. In this case the Continuous Cooling Transformation (CCT) with cooling rate q, is described by X = 1 - exp {-[J/q](n)(TTT)}, where J = integral(T)(T)(eq) k(TTT)(1)/n(TTT) dT. It is suggested that the additivity rule is applicable when the difference between J(T) from TTT (i.e. J(TTT)(T)) and J(T) from CCT (i.e. J(TTT <- CCT)(T)) is within experimental error. Both TTT and CCT are measured for austenite decomposition to pearlite in eutectoid steel. Considering the error of both CCT and TTT experiments, pearlite transformation is proved to be partially additive. When temperatures are from 610 to 640 degrees C, pearlite transformation is additive. However, when the temperature is 650 degrees C or above, pearlite transformation is non-additive due to a change of the transformation mechanism, which is confirmed by the isokinetic condition using the Master Curve Method.