The Validity of Additivity Rule for Pearlite Transformation in Eutectoid Steels

Abstract

The additivity rule has been widely used to predict the phase transformation behavior during continuous cooling (CCT) from the experimental isothermal phase transformation data (TTT). The calculated result of the additivity rule is the time \( (t_{\text{CCT}} ) \) which satisfies the summation of \( \Delta t/t_{\text{TTT}} \) equals unity under a specific continuous cooling rate. The \( t_{\text{TTT}} \) represents the time required to obtain the fraction X at temperature \( (T_{\text{TTT}} ), \) which is usually fitted by Johnson–Mehl–Avrami (JMA) equation. Since the TTT experiment can be performed only in a limited temperature range, we realize the formula of \( t_{\text{TTT}} \) between equilibrium and the maximum measured temperature significantly affects \( t_{\text{CCT}} \). In this study, a proper formula of \( t_{\text{TTT}} (X,T) \) is suggested by considering nucleation and growth mechanism of pearlite transformation. The parameter indicates that the extrapolation effect \( (\alpha_{\text{ex}} ) \) is defined. The allowable cooling rate range with \( \alpha_{\text{ex}} \le 0.1 \) is proposed based on the isothermal tested temperature range of Reti et al. (J Mater Eng Perfom 6:433–441 [1]). It is confirmed that allowable cooling rate range is enlarged by increasing isothermal tested temperature range to verify the validity of additivity rule. The additional isothermal test temperature is calculated which shows \( T_{\text{TTT}}^{\text{Add}} = 679.2\, {^\circ } {\text{C}} \) for eutectoid steel to determine additivity rule.